Huan Liu
email: liu@alumni.ubc.ca

Guangji Hu
email: guangji.hu@ubc.ca

Ibrahim Alper Basar
email: alperbasar@alumni.ubc.ca

Jianbing Li
email: jianbing.li@unbc.ca

Nathalie Lyczko
email: lyczko@mines-albi.fr

Ange Nzihou
email: ange.nzihou@mines-albi.fr

Cigdem Eskicioglu
email: cigdem.eskicioglu@ubc.ca

Phosphorus recovery from municipal sludge-derived ash and hydrochar through wet-chemical technology: A review towards sustainable waste management

Keywords: Resources recovery Thermochemical treatment Phosphorus extraction Crystallization Waste valorization Fertilizer ACP, amorphous calcium phosphate, Al-P, aluminum phosphate, AP, apatite phosphate, Ca-P, calcium phosphates, CER, cation exchange resin, DCP, dicalcium phosphate, EDTA, ethylenediaminetetraacetic acid, EDTMP, ethylene diamine tetramethylene phosphonate, EU, European Union, Fe-P, iron phosphates, HAP, hydroxyapatite, HMs, heavy metals, HTC, hydrothermal carbonization, HTL, hydrothermal liquefaction, IAP, ion -apatite inorganic phosphate, ND, non-detected, NMR, nuclear magnetic resonance, NTA, nitrilotriacetic acid, OCP, octacalcium phosphate, OP, organic phosphorus, P, phosphorus, PEE, phosphorus extraction efficiency, PPE, phosphorus precipitation efficiency, Ref., references, RSM, response surface methodology, SI, saturation index, SMT, the Standards, Measurements, and Testing Protocol, SS, secondary sludge, t tricalcium phosphate, TS, total solids, XANES, X-ray absorption near edge structure, XRD, X-ray diffraction

Phosphorus (P) is a non-renewable resource, and its recovery and recycling are necessary for meeting future P demands and environmental conservation. Ash and hydrochar from incineration and hydrothermal liquefaction (HTL) of municipal sludge, respectively, represent promising sources for efficient P recovery. The full-scale application of wet-chemical techniques has proven their technical feasibility for P recovery from sludgederived ash. HTL for sludge treatment has received significant attention for producing biocrude with netpositive energy recovery. P recovery from the solid by-product of sludge HTL, hydrochar, is a critical step in holistic sustainable sludge management. This review aims to guide P recovery from sludge-derived ash and hydrochar by presenting recent advances in wet-chemical extraction and precipitation. By comparing their characteristics, ash and hydrochar derived from municipal sludge exhibit similar feasibilities and challenges for P recovery. Extraction is recognized as the critical step for P recovery. The advantages and disadvantages of various P extraction approaches are provided. Acidic extraction achieves high efficiency (up to 100%) but requires removal of co-extracted heavy metals. Alkaline extraction prevents metals contamination but shows low efficiency (<70%). Sequential extraction is first identified as an important advance in this review, with promising P extraction efficiencies (70-91%) and low metal interferences. Three possible fertilizers (struvite, calcium phosphates, and vivianite) are recommended through the final precipitation, given their pros and cons of recovery. Ultimately, this review highlights the need for a wastewater biorefinery integrating wastewater treatment, HTL (energy recovery), and nutrient recovery for sustainable sludge management.

Introduction

Phosphorus (P) is a fundamental nutrient that sustains all life on earth. It forms the basis of living organisms, such as phosphodiester bridges in nucleotide chains, adenosine triphosphate for energy transfer, phospholipids in cellular membranes and proteins, and calcium phosphates (Ca-P) in teeth and bones [START_REF] Wu | Potentials and challenges of phosphorus recovery as vivianite from wastewater: A review[END_REF]. Thus, P deficiencies would highly inhibit cell growth and other functions (e.g., crop yields). Plants take up P from soil while animals acquire it from plants or animals at lower trophic levels [START_REF] Peng | A comprehensive review of phosphorus recovery from wastewater by crystallization processes[END_REF]. However, a recent global meta-analysis suggested that nearly half of aboveground plant production suffered from a significant P limitation in natural terrestrial ecosystems [START_REF] Hou | Global metaanalysis shows pervasive phosphorus limitation of aboveground plant production in natural terrestrial ecosystems[END_REF]. Due to the limited amount of bioavailable P (i.e., orthophosphate) in natural soil, P is primarily extracted from mined phosphate rock (containing 11-15% of P) and 80-90% is used as fertilizer [START_REF] Kroiss | Phosphorus in Water Quality and Waste Management[END_REF].

The global amount of phosphate rock exploration has quadrupled during the last 80 years due to the increasing agricultural demand for P fertilizers [START_REF] Mayer | Total Value of Phosphorus Recovery[END_REF]. Phosphate rock is non-renewable as it takes tens of millions of years to form the principal resources (sedimentary marine phosphorites and igneous deposits) through natural P cycle [START_REF] Weigand | RecoPhos: Fullscale fertilizer production from sewage sludge ash[END_REF]. Phosphate rock is also highly unevenly distributed across the globe despite the abundant reserve (approximately 69 billion tonnes according to current data) [7]. As shown in Fig. 1a, over 70% of phosphate rock is located in Morocco and Western Sahara, which have<1% of the world population. However, with the world's largest population, China only shares 4.6% of phosphate reserve, while India nearly has no domestic resources [START_REF] Kroiss | Phosphorus in Water Quality and Waste Management[END_REF]. Based on the current data of phosphate rock reserves in Canada, China, Russia, and the United States (US) [START_REF] Murray | Hybrid life-cycle environmental and cost inventory of sewage sludge treatment and end-use scenarios: A case study from China[END_REF][START_REF] Hou | Global metaanalysis shows pervasive phosphorus limitation of aboveground plant production in natural terrestrial ecosystems[END_REF]200, 600, and 1,000 million tonnes, respectively) and their mine production rates (0.8, 110, 14, and 23 million tonnes/year, respectively), phosphate reserves in those geometrically large countries could be exhausted within a short period (29-43 years) [7,[START_REF]Mineral Commodity Summaries: Phosphate Rock[END_REF]. Many countries with limited geological P sources must rely on imports either partially or entirely, which causes political and economic risks. The imbalanced distribution of P resources and uncertainty of markets have triggered global food insecurity with a 14-month price spike (700% upward) in 2008 [START_REF] Weigand | RecoPhos: Fullscale fertilizer production from sewage sludge ash[END_REF]. During that period, >40 countries suffered from food prices sparked riots, which was at least partially caused by jumping fertilizer prices [START_REF] Mayer | Total Value of Phosphorus Recovery[END_REF]. Unlike fossil fuels, P cannot be replaced or synthesized when it is scarce or expensive. On the other hand, the lost P ends up as the primary contributor to aquatic eutrophication, posing severe threats to ecosystem and water supplies [START_REF] Huang | Transformation of phosphorus during (hydro)thermal treatments of solid Biowastes: Reaction mechanisms and implications for P reclamation and recycling[END_REF]. Collectively, P recovery and recycling strategy has been recognized as a necessary approach to meet future P demands, sustain food production, and preserve the environment.

Municipal sludge as one of the major urban waste solids has been recognized as a complementary P sink [START_REF] Shi | Hydrothermal conversion of dewatered sewage sludge: Focusing on the transformation mechanism and recovery of phosphorus[END_REF]. As shown in Fig. 1b, unlike unevenly distributed phosphate rock, municipal sludge is readily available in most countries, with an annual production rate of 1.4-38.7 kg dry sludge per capita. However, the recovery efficiency of P directly from municipal sludge is low (<50% of total input) with high costs due to its large volume but low concentrations of P [START_REF] Egle | Phosphorus recovery from municipal wastewater: An integrated comparative technological, environmental and economic assessment of P recovery technologies[END_REF]. On the contrary, sludge requires proper treatment for the increasing amount and potential concerns of contaminants (e.g., pathogens, heavy metals, persistent organic pollutants, and micropollutants) [START_REF] Kor-Bicakci | Recent developments on thermal municipal sludge pretreatment technologies for enhanced anaerobic digestion[END_REF]. Every year, a significant amount of municipal sludge requires proper disposal, such as 12.7, 7.8, and 4.0 million dry tonnes in the United States, China, and India, respectively (Fig. 2). Conventional approaches (e.g., land application and landfilling) of municipal sludge treatment are getting increasingly difficult. Land application is considered the best practical option, but it is associated with raised public concerns about pollutants in sludge. Landfilling is widely used in many countries (e.g., Greece, Italy, Iceland, and Malta), while it is restricted due to the concerns of over methane generation and it does not provide opportunities for recovery of organic matters and nutrients in municipal sludge [START_REF] Donatello | Recycling and recovery routes for incinerated sewage sludge ash (ISSA): A review[END_REF]. Thermochemical processes, such as incineration and hydrothermal liquefaction (HTL) are the major alternatives to conventional disposal routes. They have significant advantages, such as deodorization, high efficiency, substantial waste volume reduction (~90%), organic pollutants destruction, and pathogen deactivation [START_REF] Fang | Feasibility of wet-extraction of phosphorus from incinerated sewage sludge ash (ISSA) for phosphate fertilizer production: A critical review[END_REF][START_REF] Moreno | Chapter 10 Environmental and techno-economic assessment of thermochemical treatment systems for sludge[END_REF]. More importantly, incineration and HTL generated solid residues (i.e., ash and hydrochar, respectively) are concentrated with P (9-13% and 4-13%, respectively) comparable to lowgrade phosphate rock [START_REF] Marrone | Bench-scale evaluation of hydrothermal processing technology for conversion of wastewater solids to fuels[END_REF][START_REF] Jupp | Phosphorus recovery and recyclingclosing the loop[END_REF][START_REF] Ovsyannikova | Feedstock-dependent phosphate recovery in a pilot-scale hydrothermal liquefaction bio-crude production[END_REF][START_REF] Shah | Hydrothermal liquefaction of high ash containing sewage sludge at sub and supercritical conditions[END_REF]. A comprehensive assessment considering technological, environmental, and economic aspects has shown that sludge ash could be a more promising source than wastewater and municipal sludge for P recovery (60-90% of total wastewater input) [START_REF] Egle | Phosphorus recovery from municipal wastewater: An integrated comparative technological, environmental and economic assessment of P recovery technologies[END_REF]. Therefore, using ash or hydrochar as potential P resources will alleviate the pressing shortage of P globally.

Incineration of municipal sludge is mostly applied in some European countries (e.g., France, United Kingdom, Germany, Denmark, Netherlands, and Switzerland), Japan, and Hongkong in China because of the health concerns and scarcity of land [START_REF] Donatello | Recycling and recovery routes for incinerated sewage sludge ash (ISSA): A review[END_REF][START_REF] Fang | Feasibility of wet-extraction of phosphorus from incinerated sewage sludge ash (ISSA) for phosphate fertilizer production: A critical review[END_REF]. However, an estimation of average costs for sludge management in European countries showed that incineration was the highest (US $382/dry tonne), compared to direct land application (US $194-254/dry tonne) and landfilling (US $309/dry tonne) [START_REF] Kacprzak | Sewage sludge disposal strategies for sustainable development[END_REF]. Due to the large infrastructural investments (account for half of the total cost) and massive energy demand (primarily for sludge drying), incineration is not an attractive solution in other countries where conventional disposal is still permissible [START_REF] Jupp | Phosphorus recovery and recyclingclosing the loop[END_REF][START_REF] Murray | Hybrid life-cycle environmental and cost inventory of sewage sludge treatment and end-use scenarios: A case study from China[END_REF][START_REF] Đurđević | Jurić, Energy Recovery from Sewage Sludge: The Case Study of Croatia[END_REF]. Although incineration facilities are well-established and reliable, HTL has attracted extensive interest from academics and industries recently for several advantages. First, it converts wet sludge (5-20% total solids) into energy-dense biocrude for fuel production by processing at a moderate temperature (250-374 • C) and autogenous pressure (4-22 MPa) within a short period (1-60 min) [START_REF] Seiple | Municipal wastewater sludge as a renewable, cost-effective feedstock for transportation biofuels using hydrothermal liquefaction[END_REF]. Second, without drying feedstocks, it is able to generate net-positive energy recovery due to significantly reduced energy inputs and high performance of heat exchanger (up to 80% heat recovery) [START_REF] Conti | Valorization of animal and human wastes through hydrothermal liquefaction for biocrude production and simultaneous recovery of nutrients[END_REF][START_REF] Anastasakis | Continuous hydrothermal liquefaction of biomass in a novel pilot plant with heat recovery and hydraulic oscillation[END_REF]. An economic analysis by Pacific Northwest National Laboratory indicated that deploying HTL into wastewater treatment plants in the US could save biosolids management costs by US $1.43 billion/year and total liability of US $3.46 billion/year based on current sludge disposal practices (i.e., land application, landfilling, or incineration). Third, compared to high incineration temperatures (e.g., 900 • C), reaction temperatures of HTL are much lower and thus decrease the risk of reactor corrosion. Based on our previous literature review [START_REF] Basar | A review on key design and operational parameters to optimize and develop hydrothermal liquefaction of biomass for biorefinery applications[END_REF], the current state of HTL systems is still at pilot-scale requiring further demonstrations, and thus studies related to HTL process will continue to be popular in the next decades. With the ever-increasing stress of depleting fossil fuel and resources (particularly P), building an energy and P recovery system (i.e., coupling HTL and P recovery) is a crucial and sustainable approach.

Ash and hydrochar derived from municipal sludge are accumulated with heavy metals, which could constrain their direct reuse as fertilizers [START_REF] Luyckx | Closing the phosphorus cycle: Multicriteria techno-economic optimization of phosphorus extraction from wastewater treatment sludge ash[END_REF][START_REF] Wang | Hydrothermal treatment coupled with mechanical expression at increased temperature for excess sludge dewatering: Heavy metals, volatile organic compounds and combustion characteristics of hydrochar[END_REF]. Besides, the bioavailability of P in ash and hydrochar is restricted due to the bound with minerals [START_REF] Meng | A review of phosphorus recovery from different thermal treatment products of sewage sludge[END_REF][START_REF] Fei | Feasibility of sewage sludge derived hydrochars for agricultural application: Nutrients (N, P, K) and potentially toxic elements (Zn, Cu, Pb, Ni, Cd)[END_REF]. Therefore, removing heavy metals and enhancing the P bioavailability are the principal goals of P recovery. The commonly developed P recovery methods are thermochemical and wet-chemical processes. Thermochemical treatment involves the volatilization of P with dosed sodium, potassium, or magnesium salts at a reducing atmosphere by heating (typically 900-950 • C for 15-20 min) for producing bioavailable Ca-P [START_REF] Jupp | Phosphorus recovery and recyclingclosing the loop[END_REF]. However, this process requires high energy inputs and equipment investment, and inorganic chlorides may be needed to improve heavy metals removal. Wet-chemical technology is more promising than thermochemical processes for its significant advantages: Higher cost-effectiveness, simpler setup, and higher potential for large-scale application [START_REF] Fang | Feasibility of wet-extraction of phosphorus from incinerated sewage sludge ash (ISSA) for phosphate fertilizer production: A critical review[END_REF][START_REF] Luyckx | Closing the phosphorus cycle: Multicriteria techno-economic optimization of phosphorus extraction from wastewater treatment sludge ash[END_REF]. It generally involves three steps: Extracting P from solids to liquid, separating P from hazardous metals, and producing plant-available P precipitates. The extraction process is mainly classified into two types based on the extractants: Acidic and alkalic. Acids extract P by breaking metal-P bonds that co-leaches heavy metals, although the P extraction efficiency is high (up to 100% for pH < 2). Thus, removal of metals from acidic extracts is often required. Alkalic solutions only extract P bound Fig. 1. The distribution estimates of a) global phosphate rock reserves (totally 69 billion tonnes, grey color donates < 0.1% of total) [7] and b) annual sludge generation per capita in medium-and high-income countries (grey color donates unknown) [START_REF] Wei | Development, current state and future trends of sludge management in China: Based on exploratory data and CO2-equivaient emissions analysis[END_REF][START_REF] Eurostat | Sewage sludge production and disposal from urban wastewater (in dry substance (d.s))[END_REF][START_REF] Mateo-Sagasta | Global wastewater and sludge production, treatment and use[END_REF][START_REF] Singh | Estimation of energy recovery potential of sewage sludge in India: Waste to watt approach[END_REF][START_REF] Marrone | Bench-scale evaluation of hydrothermal processing technology for conversion of wastewater solids to fuels[END_REF].

to amphoteric aluminum and iron, and barely any heavy metals leach under alkalic conditions. Alkalic extraction saves the separation step and downstream processing costs, while the P extraction efficiency is limited (mostly < 70%). P in both acidic and alkalic extracts is subsequently precipitated and recovered as fertilizers (e.g., struvite, Ca-P, and vivianite).

P recovery from incinerated sludge ash by wet-chemical techniques has been well studied and applied on the industrial scale (e.g., EcoPhos® and Fertilizer Industry) [START_REF] Weigand | RecoPhos: Fullscale fertilizer production from sewage sludge ash[END_REF][START_REF] Egle | Phosphorus recovery from municipal wastewater: An integrated comparative technological, environmental and economic assessment of P recovery technologies[END_REF]. Associated with significant research interests on HTL, an increasing number of studies have focused on P recovery from hydrochar recently [START_REF] Ovsyannikova | Experimental and thermodynamic studies of phosphate behavior during the hydrothermal carbonization of sewage sludge[END_REF]. Previous literature has reviewed the necessity and benefits of P recovery [START_REF] Mayer | Total Value of Phosphorus Recovery[END_REF][START_REF] Jupp | Phosphorus recovery and recyclingclosing the loop[END_REF], potential of P recovery from thermal treatment products [START_REF] Meng | A review of phosphorus recovery from different thermal treatment products of sewage sludge[END_REF], transformation of P species during thermal and hydrothermal treatments [START_REF] Huang | Transformation of phosphorus during (hydro)thermal treatments of solid Biowastes: Reaction mechanisms and implications for P reclamation and recycling[END_REF], feasibility of wetextraction of P from sludge ash [START_REF] Fang | Feasibility of wet-extraction of phosphorus from incinerated sewage sludge ash (ISSA) for phosphate fertilizer production: A critical review[END_REF], P precipitation processes [START_REF] Wu | Potentials and challenges of phosphorus recovery as vivianite from wastewater: A review[END_REF][START_REF] Peng | A comprehensive review of phosphorus recovery from wastewater by crystallization processes[END_REF][START_REF] Li | Phosphorus recovery through struvite crystallisation: Recent developments in the understanding of operational factors[END_REF][START_REF] Tansel | Struvite formation and decomposition characteristics for ammonia and phosphorus recovery: A review of magnesiumammonia-phosphate interactions[END_REF][START_REF] Kataki | Phosphorus recovery as struvite: Recent concerns for use of seed, alternative Mg source, nitrogen conservation and fertilizer potential[END_REF][START_REF] Kumar | Assessing the feasibility of N and P recovery by struvite precipitation from nutrient-rich wastewater: a review[END_REF][START_REF] Li | Phosphorous recovery through struvite crystallization: Challenges for future design[END_REF], and full-scale P recovery technologies [START_REF] Desmidt | Global phosphorus scarcity and full-scale P-recovery techniques: A review[END_REF]. To the best of our knowledge, this is the first review about P recovery from hydrochar. The important advances in the wet-chemical method, sequential extraction for P recovery, is first identified from the literature. Considering the similar characteristics of ash and hydrochar, the insights of P recovery from ash are likely to support the development of recovery approaches from sludge-derived hydrochar. This review compares the physicochemical properties of ash and hydrochar derived from municipal sludge and summarizes the fundamentals of wet-chemical technologies, providing guidance on P recovery. It also reveals the feasibility, knowledge gaps, challenges, and recent advances of wetchemical recovery of P from these two solid residues. Lastly, it emphasizes the construction of sustainable waste management beyond Pbearing ash and hydrochar.

Thermochemical treatment of municipal sludge

Incineration has been implemented in full scale due to the high calorific value of municipal sludge (8-18 MJ/kg dry matter), which neutralizes pathogens and organic matters by complete combustion at 750-900 • C with a sludge residence time of seconds [START_REF] Jupp | Phosphorus recovery and recyclingclosing the loop[END_REF][START_REF] Đurđević | Jurić, Energy Recovery from Sewage Sludge: The Case Study of Croatia[END_REF]. As shown in Fig. 3, incineration mainly involves sludge drying to achieve selfsustained combustion (sludge total solids content over 30% by weight), combustion of sludge in an incinerator, heat generation from incineration, ash collection, and cleaning of flue gases (e.g., CO, CO 2 , Fig. 2. Annual production of municipal sludge in medium-and high-income countries. Data . adapted from [START_REF] Wei | Development, current state and future trends of sludge management in China: Based on exploratory data and CO2-equivaient emissions analysis[END_REF][START_REF] Eurostat | Sewage sludge production and disposal from urban wastewater (in dry substance (d.s))[END_REF][START_REF] Mateo-Sagasta | Global wastewater and sludge production, treatment and use[END_REF][START_REF] Singh | Estimation of energy recovery potential of sewage sludge in India: Waste to watt approach[END_REF][START_REF] Marrone | Bench-scale evaluation of hydrothermal processing technology for conversion of wastewater solids to fuels[END_REF] Fig. 3. Overview of the hydrothermal liquefaction and incineration processes [START_REF] Marrone | Bench-scale evaluation of hydrothermal processing technology for conversion of wastewater solids to fuels[END_REF][START_REF] Donatello | Recycling and recovery routes for incinerated sewage sludge ash (ISSA): A review[END_REF][START_REF] Đurđević | Jurić, Energy Recovery from Sewage Sludge: The Case Study of Croatia[END_REF][START_REF] Van De Velden | The distribution of heavy metals during fluidized bed combustion of sludge (FBSC)[END_REF]. TS = total solids in municipal sludge by weight.

H 2 O, NO x , SO x , and fine particles) [START_REF] Donatello | Recycling and recovery routes for incinerated sewage sludge ash (ISSA): A review[END_REF][START_REF] Đurđević | Jurić, Energy Recovery from Sewage Sludge: The Case Study of Croatia[END_REF]. With the wide use of fluidized-bed incinerator on a large scale, sludge ash residues are mostly (>99%) collected as fly ash by the electrostatic precipitator or fabric filter [START_REF] Van De Velden | The distribution of heavy metals during fluidized bed combustion of sludge (FBSC)[END_REF]. Due to the significant volume reduction (>90%), most P is concentrated in ash from municipal sludge incineration. However, this process is unlikely to be widely applied for the benefit of P recovery only due to its high investments in incineration facilities, while it is more attractive in countries where land application is restricted, such as the Netherlands and Switzerland [START_REF] Jupp | Phosphorus recovery and recyclingclosing the loop[END_REF].

HTL is a hydrothermal process that particularly suits wet biomass waste with high moisture content since it saves the high cost of drying the feedstock as required by other thermochemical methods, e.g., incineration, pyrolysis, and gasification. It works under subcritical conditions with intermediate temperatures and pressures below the critical point of water (374.3 • C and 22.1 MPa) to maintain the reactions in a liquid state [START_REF] Seiple | Municipal wastewater sludge as a renewable, cost-effective feedstock for transportation biofuels using hydrothermal liquefaction[END_REF]. In an enclosed system and the absence of air or oxygen, HTL converts sludge biomass into biocrude that is a liquid mixture of slightly oxygenated hydrocarbons, with a higher heating value of 30-38 MJ/kg, which can be further upgraded and refined as liquid fuel [START_REF] Marrone | Bench-scale evaluation of hydrothermal processing technology for conversion of wastewater solids to fuels[END_REF][START_REF] Silva Thomsen | Hydrothermal liquefaction of sewage sludge; energy considerations and fate of micropollutants during pilot scale processing[END_REF]. As illustrated in Fig. 3, four phases are generated from the HTL of municipal sludge after separation: Biocrude, aqueous phase, hydrochar (solid residue), and gas (mostly CO 2 ). A bench-scale continuous-flow HTL treatment achieved a significant solids reduction of 94-98% by weight for primary, secondary, and digested sludge [START_REF] Marrone | Bench-scale evaluation of hydrothermal processing technology for conversion of wastewater solids to fuels[END_REF]. The remaining solid phase hydrochar is mainly comprised of mineral ash, char, and some unreacted biomass, where over 80% of P from feedstock can be concentrated [START_REF] Shi | Hydrothermal conversion of dewatered sewage sludge: Focusing on the transformation mechanism and recovery of phosphorus[END_REF]. Techno-economic analyses have shown the profitable potentials of applying HTL to municipal sludge treatment [START_REF] Seiple | Municipal wastewater sludge as a renewable, cost-effective feedstock for transportation biofuels using hydrothermal liquefaction[END_REF][START_REF] Do | Techno-economic analysis of bio heavy-oil production from sewage sludge using supercritical and subcritical water[END_REF]. Nutrient recovery from HTL waste streams, such as hydrochar, can add value and improve the environmental and financial sustainability of sludge treatment, and promote the implementation of full-scale HTL into a wastewater treatment plant.

Characteristics of sludge-derived ash and hydrochar

Following incineration or HTL of municipal sludge, P recovery is affected by the physicochemical properties of the derived ash or hydrochar, such as concentration of P, contents and species co-existing metals/metalloids, P transformation (speciation) during a thermochemical treatment, and predominant reactions between phosphate and metals/metalloids.

Overview of chemical compositions

The recovery potential of P from a waste stream is largely dependent on its chemical compositions. First, the value and benefits of waste materials as P sources mainly rely on their P concentrations. Second, the relative abundance of metals (especially those affined with P) determines P species and the complexity of P recovery processes. Fig. 4 summarizes chemical compositions of ash and hydrochar derived from municipal sludge reported in recent studies. As a comparison, typical compositions of phosphate rock mined from various countries are also presented. As shown in Fig. 4, phosphate rock mainly consists of P (P 2 O 5 ) and calcium (CaO), with small amounts of other metals (e.g., Fe and Si), while sludge-derived ash and hydrochar have much more complex compositions. Due to the diverse constituents of municipal sludge and different thermal treatment techniques, the compositions of generated ash and hydrochar vary significantly. Fig. 4b andd present the major elements (mean values) of ash and hydrochar produced in different countries, respectively. Unlike the consistent contents of phosphate rock, mineral concentrations in ash and hydrochar vary considerably in different regions. Therefore, regional-specific P recovery methods should be developed to suit various compositions and avoid likely operational issues (e.g., low efficiency) by directly applying existing processes for phosphate rock. Most studies (80% of collected data, 10-90 percentile range) have found that sludge-derived ash is mainly composed of Si (SiO 2 , 21.2-44.6%), P (P 2 O 5 , 13.5-25.7%), Al (Al 2 O 3 , 5.6-21.1%), Ca (CaO, 8.4-20.8%), and Fe (Fe 2 O 3 , 4.3-20%), whereas other components (e.g., MgO, Na 2 O, and K 2 O) are general-ly<5% (Fig. 4c). Similarly, the main elements in sludge-derived hydrochar are also Si (SiO 2 ), P (P 2 O 5 , 5.2-13.6%), Al (Al 2 O 3 , 0.6-17%), Ca (CaO, 1.4-9.2%), and Fe (Fe 2 O 3 , 1.8-13.7%), but in much lower levels compared to ash (note that the content of silica here is not considered representative due to limited data and large variance, Fig. 4e). The smaller concentrations in hydrochar are because ash only contains inorganic matters and little residual carbon (0.2-6%) for complete combustion, while a large portion of organics (12-52% of volatile matter) and carbon (7-58%) may remain in hydrochar depending on reaction conditions due to incomplete conversion [START_REF] Fang | Feasibility of wet-extraction of phosphorus from incinerated sewage sludge ash (ISSA) for phosphate fertilizer production: A critical review[END_REF][START_REF] Ekpo | A comparison of product yields and inorganic content in process streams following thermal hydrolysis and hydrothermal processing of microalgae, manure and digestate[END_REF][START_REF] He | Products evolution during hydrothermal conversion of dewatered sewage sludge in sub-and near-critical water: Effects of reaction conditions and calcium oxide additive[END_REF]. If sludge is from enhanced P removal wastewater treatment process, the precipitation salts (alum, lime, or FeCl 3 ) used will increase the contents of Al, Ca, or Fe, respectively, in ash and hydrochar [START_REF] Donatello | Recycling and recovery routes for incinerated sewage sludge ash (ISSA): A review[END_REF][START_REF] Bunce | A review of phosphorus removal technologies and their applicability to small-scale domestic wastewater treatment systems[END_REF]. The contents of these minerals (e.g., Al, Ca, and Fe) will eventually determine the P species in sludge-derived ash and hydrochar, thus affecting the proper P extraction methods.

Trace elements (particularly heavy metals) in sludge-derived ash and hydrochar are much varied, strongly related to the industrial activities of wastewater sources. Fig. 5 illustrates the concentration distribution of heavy metals present in ash and hydrochar. Among tested samples, most (80% of collected data, 10-90 percentile range) ash contains abundant Cr (70-1,031 mg/kg), Cu (492-2,500 mg/kg), Ni (44-148 mg/kg), Pb (64-627 mg/kg), and Zn (993-3,810 mg/kg), whereas the concentrations of As (5-54 mg/kg), Cd (2-13 mg/kg) and Hg (0.1-3.9 mg/kg) are comparable to phosphate rock (Table 1). A one-year monitoring study found that during sludge incineration most heavy metals (e.g., As, Cd, Cr, Cu, Ni, Pb, and Zn) were collected in the electrostatic precipitator as fly ash, except that 80% of Hg was lost (in the forms of effluent water, stack gas, and others) [START_REF] Van De Velden | The distribution of heavy metals during fluidized bed combustion of sludge (FBSC)[END_REF]. Although As, Cd, Hg, and Pb are anticipated to volatilize during combustion, they tend to condense on ash surfaces when temperature falls in the collector [START_REF] Donatello | Recycling and recovery routes for incinerated sewage sludge ash (ISSA): A review[END_REF]. Another study reported that most P (nearly 89%) was distributed in bottom ash when the incineration temperature was controlled below 950 • C [START_REF] Li | Potential recovery of phosphorus during the fluidized bed incineration of sewage sludge[END_REF]. Sludgederived hydrochar is also found to be concentrated with Cr (78-971 mg/kg), Cu (162-2,510 mg/kg), Ni (32-600 mg/kg), Pb (37-121 mg/ kg), Zn (500-3,147 mg/kg), as well as Hg (0.2-72 mg/kg), plus some As (2.7-29 mg/kg) and Cd (1.1-4.4 mg/kg). Previous studies have proven that most heavy metals, including Hg, accumulate in hydrochar during the hydrothermal conversion of municipal sludge [START_REF] Shi | Hydrothermal conversion of dewatered sewage sludge: Focusing on the transformation mechanism and recovery of phosphorus[END_REF][START_REF] Wang | Hydrothermal treatment coupled with mechanical expression at increased temperature for excess sludge dewatering: Heavy metals, volatile organic compounds and combustion characteristics of hydrochar[END_REF][START_REF] Fei | Feasibility of sewage sludge derived hydrochars for agricultural application: Nutrients (N, P, K) and potentially toxic elements (Zn, Cu, Pb, Ni, Cd)[END_REF][START_REF] Zhang | A two-step process for sewage sludge treatment: Hydrothermal treatment of sludge and catalytic hydrothermal gasification of its derived liquid[END_REF][START_REF] Ma | Gasification performance of the hydrochar derived from co-hydrothermal carbonization of sewage sludge and sawdust[END_REF][START_REF] Ren | Effect of sewage sludge hydrochar on soil properties and Cd immobilization in a contaminated soil[END_REF][START_REF] Leng | Bio-char derived from sewage sludge by liquefaction: Characterization and application for dye adsorption[END_REF][START_REF] Li | Quantitative evaluation of heavy metals in solid residues from sub-and super-critical water gasification of sewage sludge[END_REF][START_REF] Yue | The change of heavy metals fractions during hydrochar decomposition in soils amended with different municipal sewage sludge hydrochars[END_REF]. The high concentrations of heavy metals are the primary concerns when developing P extraction methods for ash and hydrochar. Since the above-mentioned heavy metals are regulated by fertilizer ordinance in various countries, minimizing their co-extraction could reduce the subsequent processing costs (e.g., separation of P from heavy metals).

For some heavy metals (e.g., Cr and As), chemical states are more important than total concentrations in determining their toxicity, raising more concerns in using the produced fertilizers [START_REF] Chen | Immobilization of chromium contaminated soil by co-pyrolysis with rice straw[END_REF][START_REF] Shao | Speciation and migration of heavy metals in sediment cores of urban wetland: bioavailability and risks[END_REF]. Studies suggested that more stable and less toxic Cr(III) was dominant in sludgederived ash, while the toxic Cr(VI) was only 0.1 mg/kg, even less than that in phosphate rock (0.5 mg/kg) and triple superphosphate fertilizer (0.7 mg/kg) [START_REF] Vogel | Chemical state of chromium, sulfur, and iron in sewage sludge ash based phosphorus fertilizers[END_REF][START_REF] Takaoka | Chemical states of trace elements in sewage sludge incineration ash by using x-ray absorption fine structure[END_REF][START_REF] Vogel | Chromium (VI) in phosphorus fertilizers determined with the diffusive gradients in thin-films (DGT) technique[END_REF]. Takaoka et al. also found that more mobile and hazardous As(III) was completely transformed into As(V) in sludgederived ash after incineration [START_REF] Takaoka | Chemical states of trace elements in sewage sludge incineration ash by using x-ray absorption fine structure[END_REF]. However, Zhao et al. found that As(III) accounts for over 50% of As in fly ash from combustion of municipal sludge at 900 • C, with a concentration comparable to that in phosphate rock (2.1-5.5 mg/kg) [START_REF] Zhao | Speciation transformation of arsenic during municipal sewage sludge incineration with cotton stalk as additive[END_REF][START_REF] Zhao | Effect of the addition of biomass on the fate and speciation of arsenic during sewage sludge combustion[END_REF][START_REF] Macedo | Determination of total arsenic and arsenic (III) in phosphate fertilizers and phosphate rocks by HG-AAS after multivariate optimization based on Box-Behnken design[END_REF]. To date, the chemical states of these heavy metals in sludge-derived hydrochar remain unknown. Heavy metal removal is necessary to produce public-acceptable P fertilizers from ash, hydrochar, and phosphate rock. The advantage of using sludge-derived ash and hydrochar is that they are readily available in all countries. Fig. 4. Chemical compositions of phosphate rock and sludge-derived ash and hydrochar: (a) typical compositions of phosphate rock from various countries [START_REF] Weigand | RecoPhos: Fullscale fertilizer production from sewage sludge ash[END_REF][START_REF] Gorazda | Fertilisers production from ashes after sewage sludge combustion -A strategy towards sustainable development[END_REF]; (b) mean values and (c) violin distribution of ash compositions [START_REF] Weigand | RecoPhos: Fullscale fertilizer production from sewage sludge ash[END_REF][START_REF] Luyckx | Closing the phosphorus cycle: Multicriteria techno-economic optimization of phosphorus extraction from wastewater treatment sludge ash[END_REF][START_REF] Gorazda | Fertilisers production from ashes after sewage sludge combustion -A strategy towards sustainable development[END_REF][START_REF] Li | Transformation of apatite phosphorus and non-apatite inorganic phosphorus during incineration of sewage sludge[END_REF][START_REF] Li | Heavy metal removal and speciation transformation through the calcination treatment of phosphorus-enriched sewage sludge ash[END_REF][START_REF] Lim | Selective acidic elution of Ca from sewage sludge ash for phosphorus recovery under pH control[END_REF][START_REF] Petzet | On wet chemical phosphorus recovery from sewage sludge ash by acidic or alkaline leaching and an optimized combination of both[END_REF][START_REF] Zin | Struvite production from food processing wastewater and incinerated sewage sludge ash as an alternative N and P source: Optimization of multiple resources recovery by response surface methodology[END_REF][START_REF] Thomsen | Changes imposed by pyrolysis, thermal gasification and incineration on composition and phosphorus fertilizer quality of municipal sewage sludge[END_REF][START_REF] Xu | Recovery of phosphorus as struvite from sewage sludge ash[END_REF][START_REF] Takahashi | Technology for recovering phosphorus from incinerated wastewater treatment sludge[END_REF][START_REF] Vogel | Chemical state of chromium, sulfur, and iron in sewage sludge ash based phosphorus fertilizers[END_REF][START_REF] Takaoka | Chemical states of trace elements in sewage sludge incineration ash by using x-ray absorption fine structure[END_REF][START_REF] Donatello | EU landfill waste acceptance criteria and EU Hazardous Waste Directive compliance testing of incinerated sewage sludge ash[END_REF][START_REF] Cheeseman | Properties and microstructure of lightweight aggregate produced from sintered sewage sludge ash[END_REF][START_REF] Franz | Phosphate fertilizer from sewage sludge ash (SSA)[END_REF][START_REF] Liang | A comparison between sulfuric acid and oxalic acid leaching with subsequent purification and precipitation for phosphorus recovery from sewage sludge incineration ash[END_REF][START_REF] Donatello | Production of technical grade phosphoric acid from incinerator sewage sludge ash (ISSA)[END_REF][START_REF] Fang | Phosphorus recovery and leaching of trace elements from incinerated sewage sludge ash (ISSA)[END_REF][START_REF] Herzel | Sewage sludge ash -A promising secondary phosphorus source for fertilizer production[END_REF][START_REF] Kleemann | Comparison of phosphorus recovery from incinerated sewage sludge ash (ISSA) and pyrolysed sewage sludge char (PSSC)[END_REF][START_REF] Lee | Identification of phosphorus forms in sewage sludge ash during acid pre-treatment for phosphorus recovery by chemical fractionation and spectroscopy[END_REF]; (d) mean values and (e) violin distribution of hydrochar compositions [START_REF] Marrone | Bench-scale evaluation of hydrothermal processing technology for conversion of wastewater solids to fuels[END_REF][START_REF] Shi | Hydrothermal conversion of dewatered sewage sludge: Focusing on the transformation mechanism and recovery of phosphorus[END_REF][START_REF] Ovsyannikova | Feedstock-dependent phosphate recovery in a pilot-scale hydrothermal liquefaction bio-crude production[END_REF][START_REF] Shah | Hydrothermal liquefaction of high ash containing sewage sludge at sub and supercritical conditions[END_REF][START_REF] Conti | Valorization of animal and human wastes through hydrothermal liquefaction for biocrude production and simultaneous recovery of nutrients[END_REF][START_REF] Wang | Hydrothermal treatment coupled with mechanical expression at increased temperature for excess sludge dewatering: Heavy metals, volatile organic compounds and combustion characteristics of hydrochar[END_REF][START_REF] Ekpo | A comparison of product yields and inorganic content in process streams following thermal hydrolysis and hydrothermal processing of microalgae, manure and digestate[END_REF][START_REF] Zhang | Efficient phosphorus recycling and heavy metal removal from wastewater sludge by a novel hydrothermal humification-technique[END_REF][START_REF] Zhang | The hydrochar characters of municipal sewage sludge under different hydrothermal temperatures and durations[END_REF][START_REF] Zhuang | Influences of microstructural alternations and inorganic catalysis on the thermochemical conversion of biowaste-derived hydrochar[END_REF][START_REF] Feng | Phosphorus transformation in hydrothermal pretreatment and steam gasification of sewage sludge[END_REF][START_REF] Zhang | A two-step process for sewage sludge treatment: Hydrothermal treatment of sludge and catalytic hydrothermal gasification of its derived liquid[END_REF][START_REF] Marin-Batista | Energy and phosphorous recovery through hydrothermal carbonization of digested sewage sludge[END_REF][START_REF] Breulmann | Process conditions of pyrolysis and hydrothermal carbonization affect the potential of sewage sludge for soil carbon sequestration and amelioration[END_REF][START_REF] Wang | Pyrolysis study of hydrothermal carbonization-treated digested sewage sludge using a Py-GC/MS and a benchscale pyrolyzer[END_REF]. EU = European Union (data from Belgium, Denmark, Germany, Poland, Spain, Sweden, and Switzerland). [START_REF] Weigand | RecoPhos: Fullscale fertilizer production from sewage sludge ash[END_REF][START_REF] Luyckx | Closing the phosphorus cycle: Multicriteria techno-economic optimization of phosphorus extraction from wastewater treatment sludge ash[END_REF][START_REF] Gorazda | Fertilisers production from ashes after sewage sludge combustion -A strategy towards sustainable development[END_REF][START_REF] Li | Heavy metal removal and speciation transformation through the calcination treatment of phosphorus-enriched sewage sludge ash[END_REF][START_REF] Lim | Selective acidic elution of Ca from sewage sludge ash for phosphorus recovery under pH control[END_REF][START_REF] Petzet | On wet chemical phosphorus recovery from sewage sludge ash by acidic or alkaline leaching and an optimized combination of both[END_REF][START_REF] Zin | Struvite production from food processing wastewater and incinerated sewage sludge ash as an alternative N and P source: Optimization of multiple resources recovery by response surface methodology[END_REF][START_REF] Thomsen | Changes imposed by pyrolysis, thermal gasification and incineration on composition and phosphorus fertilizer quality of municipal sewage sludge[END_REF][START_REF] Xu | Recovery of phosphorus as struvite from sewage sludge ash[END_REF][START_REF] Vogel | Chemical state of chromium, sulfur, and iron in sewage sludge ash based phosphorus fertilizers[END_REF][START_REF] Takaoka | Chemical states of trace elements in sewage sludge incineration ash by using x-ray absorption fine structure[END_REF][START_REF] Donatello | EU landfill waste acceptance criteria and EU Hazardous Waste Directive compliance testing of incinerated sewage sludge ash[END_REF][START_REF] Cheeseman | Properties and microstructure of lightweight aggregate produced from sintered sewage sludge ash[END_REF][START_REF] Franz | Phosphate fertilizer from sewage sludge ash (SSA)[END_REF][START_REF] Liang | A comparison between sulfuric acid and oxalic acid leaching with subsequent purification and precipitation for phosphorus recovery from sewage sludge incineration ash[END_REF][START_REF] Donatello | Production of technical grade phosphoric acid from incinerator sewage sludge ash (ISSA)[END_REF][START_REF] Fang | Phosphorus recovery and leaching of trace elements from incinerated sewage sludge ash (ISSA)[END_REF][START_REF] Herzel | Sewage sludge ash -A promising secondary phosphorus source for fertilizer production[END_REF][START_REF] Kleemann | Comparison of phosphorus recovery from incinerated sewage sludge ash (ISSA) and pyrolysed sewage sludge char (PSSC)[END_REF][START_REF] Lee | Identification of phosphorus forms in sewage sludge ash during acid pre-treatment for phosphorus recovery by chemical fractionation and spectroscopy[END_REF][START_REF] Liu | Design and optimization of a reactive crystallization process for high purity phosphorus recovery from sewage sludge ash[END_REF][START_REF] Ottosen | Extracting phosphorous from incinerated sewage sludge ash rich in iron or aluminum[END_REF] and (b) sludge-derived hydrochar [START_REF] Marrone | Bench-scale evaluation of hydrothermal processing technology for conversion of wastewater solids to fuels[END_REF][START_REF] Shi | Hydrothermal conversion of dewatered sewage sludge: Focusing on the transformation mechanism and recovery of phosphorus[END_REF][START_REF] Shah | Hydrothermal liquefaction of high ash containing sewage sludge at sub and supercritical conditions[END_REF][START_REF] Conti | Valorization of animal and human wastes through hydrothermal liquefaction for biocrude production and simultaneous recovery of nutrients[END_REF][START_REF] Wang | Hydrothermal treatment coupled with mechanical expression at increased temperature for excess sludge dewatering: Heavy metals, volatile organic compounds and combustion characteristics of hydrochar[END_REF][START_REF] Ekpo | A comparison of product yields and inorganic content in process streams following thermal hydrolysis and hydrothermal processing of microalgae, manure and digestate[END_REF][START_REF] Zhang | Efficient phosphorus recycling and heavy metal removal from wastewater sludge by a novel hydrothermal humification-technique[END_REF][START_REF] Zhang | The hydrochar characters of municipal sewage sludge under different hydrothermal temperatures and durations[END_REF][START_REF] Zhang | A two-step process for sewage sludge treatment: Hydrothermal treatment of sludge and catalytic hydrothermal gasification of its derived liquid[END_REF][START_REF] Breulmann | Process conditions of pyrolysis and hydrothermal carbonization affect the potential of sewage sludge for soil carbon sequestration and amelioration[END_REF][START_REF] Wang | Pyrolysis study of hydrothermal carbonization-treated digested sewage sludge using a Py-GC/MS and a benchscale pyrolyzer[END_REF][START_REF] Leng | Bio-char derived from sewage sludge by liquefaction: Characterization and application for dye adsorption[END_REF][START_REF] Escala | Hydrothermal carbonization as an energy-efficient alternative to established drying technologies for sewage sludge: A feasibility study on a laboratory scale[END_REF][START_REF] Liu | Effect of hydrothermal carbonization on migration and environmental risk of heavy metals in sewage sludge during pyrolysis[END_REF][START_REF] Wang | Influence of pyrolysis temperature on characteristics and environmental risk of heavy metals in pyrolyzed biochar made from hydrothermally treated sewage sludge[END_REF][START_REF] Xu | The fate of heavy metal during subcritical and supercritical water gasification of sewage sludge[END_REF].

Transformation of P species

The Standards, Measurements, and Testing (SMT) Protocol has been widely used to determine P species [START_REF] González Medeiros | Analytical phosphorus fractionation in sewage sludge and sediment samples[END_REF][START_REF] Ruban | Harmonized protocol and certified reference material for the determination of extractable contents of phosphorus in freshwater sediments -A synthesis of recent works[END_REF]. P in solids can exist as inorganic P (IP) and organic P (OP). By extracting with HCl or NaOH solutions, IP can be classified as apatite phosphate (AP) and non-apatite inorganic phosphate (NAIP), respectively. NAIP is mainly in the form of Al, Fe, Mg, and Mn oxides and hydroxides, while AP is primarily bound to Ca [START_REF] Liang | A comparison between sulfuric acid and oxalic acid leaching with subsequent purification and precipitation for phosphorus recovery from sewage sludge incineration ash[END_REF]. Fig. 6a and b display P speciation in sludge-derived ash and hydrochar under various incineration and hydrothermal conditions, respectively. Ash only contains IP because of complete combustion, whereas hydrochar may retain negligible OP from municipal sludge. Although the concentration of AP in ash slightly increases with increasing temperature, the changes of AP and NAIP are not significant except at higher temperatures (e.g., 950 • C). The dramatic loss of NAIP at 950 • C may be caused by its high volatilization, while AP is more stable at high incineration temperature [START_REF] Li | Transformation of apatite phosphorus and non-apatite inorganic phosphorus during incineration of sewage sludge[END_REF][START_REF] Li | Potential recovery of phosphorus during the fluidized bed incineration of sewage sludge[END_REF]. The AP amount in hydrochar also rises gradually with enhanced hydrothermal temperature, but it seems that the P species are largely affected by other treatment conditions (e.g., residence time, solids content, and liquid chemistry) [START_REF] Huang | Transformation of phosphorus during (hydro)thermal treatments of solid Biowastes: Reaction mechanisms and implications for P reclamation and recycling[END_REF]. Shi et al. found that AP and NAIP were equal in hydrochar produced at 320 • C for 0.5 h [START_REF] Shi | Hydrothermal conversion of dewatered sewage sludge: Focusing on the transformation mechanism and recovery of phosphorus[END_REF], whereas Zhai et al. reported almost only AP was present in hydrochar obtained at 350 and 400 • C for 0.5 h [START_REF] Zhai | Recovery of phosphorus from sewage sludge in combination with the supercritical water process[END_REF]. The content of NAIP implies the maximum extractable P by direct alkalic extraction for P recovery. Higher reaction temperatures for incineration and HTL are likely to transform NAIP to AP in ash and hydrochar, which reduces the feasibility of alkalic extraction method.

The molecular moieties of P in sludge-derived ash and hydrochar are relatively simple according to the 31 P nuclear magnetic resonance (NMR) spectra. Qian and Jiang found that various P species (orthophosphate monoesters, orthophosphate diesters, and pyrophosphate) in municipal sludge were all transformed into orthophosphate in ash when combustion temperature was over 600 • C [START_REF] Qian | Migration of phosphorus in sewage sludge during different thermal treatment processes[END_REF]. Nanzer et al. investigated four types of sludge-derived ash samples from mono-combustion plants and only observed the presence of orthophosphate [START_REF] Nanzer | The Molecular environment of phosphorus in sewage sludge ash: implications for bioavailability[END_REF]. Due to the dehydration of M 2 (HPO 4 ) x or M(H 2 PO 4 ) x (Mmetals) and degradation of polyphosphates in sludge, a considerable amount of pyrophosphate might be formed in ash produced at medium temperature ranges (300-600 • C) [START_REF] Qian | Migration of phosphorus in sewage sludge during different thermal treatment processes[END_REF][START_REF] Nanzer | The Molecular environment of phosphorus in sewage sludge ash: implications for bioavailability[END_REF][START_REF] Huang | Speciation dynamics of phosphorus during (hydro)thermal treatments of sewage sludge[END_REF]. However, it was reported that high incineration temperatures (700-900 • C) could prohibit the formation of pyrophosphates [START_REF] Qian | Migration of phosphorus in sewage sludge during different thermal treatment processes[END_REF]. For hydrochar, Li et al. and Shi et al. also reported that orthophosphate was dominant (>90% of total P), and other forms of P (e.g., orthophosphate monoester and pyrophosphate) gradually disappeared with increasing hydrothermal temperature at 260 • C (4 h) and 320 • C (0.5 h), respectively [START_REF] Shi | Hydrothermal conversion of dewatered sewage sludge: Focusing on the transformation mechanism and recovery of phosphorus[END_REF][START_REF] Li | Phosphorus species transformation and recovery without apatite in FeCl3-assisted sewage sludge hydrothermal treatment[END_REF]. Even at a low hydrothermal temperature (225 • C for 24 h), pyrophosphate can be totally converted into orthophosphate [START_REF] Huang | Speciation dynamics of phosphorus during (hydro)thermal treatments of sewage sludge[END_REF]. The improved purity of orthophosphate and inhibition of pyrophosphate formation by controlling reaction temperatures are beneficial for precipitation recovery after wetchemical extraction.

P complexation and mineralogy

Due to the predominance of Al, Ca and Fe in sludge-derived ash and hydrochar, P exists in the forms of solid precipitates (e.g., metal phosphate salts) and mineral-adsorbed phases [START_REF] Huang | Evolution of phosphorus complexation and mineralogy during (hydro)thermal treatments of activated and anaerobically digested sludge: Insights from sequential extraction and P K-edge XANES[END_REF]. P K-edge X-ray absorption near edge structure (XANES) is an advanced technique that has been applied to determine the complexation and mineralogy of P. For sludge-derived ash, one study found the P speciation varied depending on the abundance of Al, Ca, and Fe: Crystalline or amorphous variscite (AlPO 4 ⋅2H 2 O) was dominant (87% of total P) in Al-rich ash; P was mainly β-tricalcium phosphate [Ca 3 (PO 4 ) 2 , 65% of total P] or sorbed to calcite (20% of total P) in Ca-rich ash; however, no ash sample showed the presence of iron phosphates (Fe-P) even in Fe-rich ash, which was also confirmed by the 31 P NMR analysis [START_REF] Nanzer | The Molecular environment of phosphorus in sewage sludge ash: implications for bioavailability[END_REF]. Vogel et al. identified Fe 3 (PO 4 ) 2 as the major iron form in a Fe-rich (19% by weight) ash using the Fe K-edge XANES spectra [START_REF] Vogel | Chemical state of chromium, sulfur, and iron in sewage sludge ash based phosphorus fertilizers[END_REF]. For sludge-derived hydrochar, various P fractions could be detected. Shi et al. reported that Ca-P, such as hydroxyapatite [Ca 5 (PO 4 ) 3 OH] and octacalcium phosphate [Ca 8 H 2 (PO 4 ) 6 ⋅5H 2 O], became more dominant (57% to 80% of total P) in hydrochar with increasing hydrothermal temperature from 170 to 320 • C, while other forms, e.g., FePO 4 , Mg 2 (PO 4 ) 3 and AlPO 4 , reduced from 43% to 20% [START_REF] Shi | Hydrothermal conversion of dewatered sewage sludge: Focusing on the transformation mechanism and recovery of phosphorus[END_REF]. The hydrochar samples were also found to be Al/Ca/Fe/Mg abundant (Ca > Fe > Mg > Al) [START_REF] Shi | Hydrothermal conversion of dewatered sewage sludge: Focusing on the transformation mechanism and recovery of phosphorus[END_REF]. From a waste activated sludge with 3.6% Fe and 2.4% Al by weight, AlPO 4 was the major form (43% of total P) in hydrochar (treated at 225 • C for 4 h), whereas both aluminum phosphate (Al-P: AlPO 4 and alumina-adsorbed, 38% of total P) and ferrihydrite-adsorbed P (27% of total P) became the majority in hydrochar derived from digested sludge with a much higher Fe content (9.1%) [START_REF] Huang | Evolution of phosphorus complexation and mineralogy during (hydro)thermal treatments of activated and anaerobically digested sludge: Insights from sequential extraction and P K-edge XANES[END_REF]. Another study observed the dominance of strengite (FePO 4 ⋅2H 2 O, 42% of total P), Al-P (AlPO 4 and aluminaadsorbed, 34% of total P), and Ca-P (hydroxyapatite and octacalcium phosphate, 21% of total P) in hydrochar obtained at 185 • C for 4 h from digested sludge [START_REF] Wang | Effect of interstage hydrothermal treatment on anaerobic digestion of sewage sludge: Speciation evolution of phosphorus, iron, and sulfur, ACS Sustain[END_REF]. It was also found that higher temperature (>155 • C) promoted the oxidation of Fe 3 (PO 4 ) 2 to FePO 4 in hydrochar [START_REF] Wang | Effect of interstage hydrothermal treatment on anaerobic digestion of sewage sludge: Speciation evolution of phosphorus, iron, and sulfur, ACS Sustain[END_REF].

X-ray diffraction (XRD) has been widely used to analyze the crystallized phases in sludge-derived ash and hydrochar. Most studies found that major crystalline compounds in sludge-derived ash were quartz (SiO 2 ), hematite (Fe 2 O 3 ), and whitlockite or whitlockite-like compounds [Ca 9 X(PO 4 ) 7 , X = Al/Fe/Mg] [START_REF] Luyckx | Closing the phosphorus cycle: Multicriteria techno-economic optimization of phosphorus extraction from wastewater treatment sludge ash[END_REF][START_REF] Gorazda | Fertilisers production from ashes after sewage sludge combustion -A strategy towards sustainable development[END_REF][START_REF] Li | Heavy metal removal and speciation transformation through the calcination treatment of phosphorus-enriched sewage sludge ash[END_REF][START_REF] Lim | Selective acidic elution of Ca from sewage sludge ash for phosphorus recovery under pH control[END_REF][START_REF] Petzet | On wet chemical phosphorus recovery from sewage sludge ash by acidic or alkaline leaching and an optimized combination of both[END_REF][START_REF] Donatello | EU landfill waste acceptance criteria and EU Hazardous Waste Directive compliance testing of incinerated sewage sludge ash[END_REF][START_REF] Fang | Phosphorus recovery and leaching of trace elements from incinerated sewage sludge ash (ISSA)[END_REF][START_REF] Kleemann | Comparison of phosphorus recovery from incinerated sewage sludge ash (ISSA) and pyrolysed sewage sludge char (PSSC)[END_REF][START_REF] Petzet | Recovery of phosphorus and aluminium from sewage sludge ash by a new wet chemical elution process (SESAL-Phos-recovery process)[END_REF][START_REF] Li | Change in re-use value of incinerated sewage sludge ash due to chemical extraction of phosphorus[END_REF][START_REF] Donatello | Effect of milling and acid washing on the pozzolanic activity of incinerator sewage sludge ash[END_REF][START_REF] Adam | Thermochemical treatment of sewage sludge ashes for phosphorus recovery[END_REF]. Whitlockite can be readily dissolvable in both organic and inorganic acids but can be barely dissolved by alkalic and chelating agents (e.g., EDTA) [START_REF] Petzet | On wet chemical phosphorus recovery from sewage sludge ash by acidic or alkaline leaching and an optimized combination of both[END_REF][START_REF] Fang | Phosphorus recovery and leaching of trace elements from incinerated sewage sludge ash (ISSA)[END_REF][START_REF] Li | Change in re-use value of incinerated sewage sludge ash due to chemical extraction of phosphorus[END_REF]. In Al-rich ash, AlPO 4 (crystallographically disordered) was also commonly detected by XRD, which is alkaline-soluble [START_REF] Nanzer | The Molecular environment of phosphorus in sewage sludge ash: implications for bioavailability[END_REF][START_REF] Petzet | Recovery of phosphorus and aluminium from sewage sludge ash by a new wet chemical elution process (SESAL-Phos-recovery process)[END_REF][START_REF] Adam | Thermochemical treatment of sewage sludge ashes for phosphorus recovery[END_REF]. Nanzer et al. identified AlPO 4 , whitlockite, and hydroxyapatite as the major crystalline P phase in Al-rich, Fe-rich, and Carich ashes, respectively [START_REF] Nanzer | The Molecular environment of phosphorus in sewage sludge ash: implications for bioavailability[END_REF]. It was also found that more Ca 3 (PO 4 ) 2 and less AlPO 4 could be detected with increasing incineration temperature (from 675 to 950 • C) [START_REF] Li | Transformation of apatite phosphorus and non-apatite inorganic phosphorus during incineration of sewage sludge[END_REF][START_REF] Li | Potential recovery of phosphorus during the fluidized bed incineration of sewage sludge[END_REF]. For sludge-derived hydrochar, quartz (SiO 2 ) has also been found as the main crystalline compound [START_REF] Ovsyannikova | Feedstock-dependent phosphate recovery in a pilot-scale hydrothermal liquefaction bio-crude production[END_REF][START_REF] Xu | Migration and transformation of phosphorus in municipal sludge by the hydrothermal treatment and its directional adjustment[END_REF][START_REF] Zheng | Enhanced transformation of phosphorus (P) in sewage sludge to hydroxyapatite via hydrothermal carbonization and calcium-based additive[END_REF][START_REF] Acelas | Supercritical water gasification of sewage sludge: Gas production and phosphorus recovery[END_REF][START_REF] Yu | Effects of three macroelement cations on P mobility and speciation in sewage sludge derived hydrochar by using hydrothermal treatment[END_REF][START_REF] Zheng | Migration and Transformation of Phosphorus during Hydrothermal Carbonization of Sewage Sludge: Focusing on the Role of pH and Calcium Additive and the Transformation Mechanism[END_REF][START_REF] Wang | Feedwater pH affects phosphorus transformation during hydrothermal carbonization of sewage sludge[END_REF]. However, crystalline P phases in hydrochar vary depending on the sludge properties and hydrothermal conditions. Acelas et al. observed the presence of whitlockite, AlPO 4, and hydroxyapatite in hydrochar generated under 600 • C for 1 h [START_REF] Acelas | Supercritical water gasification of sewage sludge: Gas production and phosphorus recovery[END_REF]. Yu et al. found that P minerals were highly associated with the predominance of Al, Ca, and Fe in hydrochar [START_REF] Yu | Effects of three macroelement cations on P mobility and speciation in sewage sludge derived hydrochar by using hydrothermal treatment[END_REF]. Xu et al. only detected Ca 7 Mg 2 (PO 4 ) 6 as the crystallized P in Ca-rich hydrochar (260 • C for 4 h) [START_REF] Xu | Migration and transformation of phosphorus in municipal sludge by the hydrothermal treatment and its directional adjustment[END_REF]. Zheng et al. reported AlPO 4 as the main fraction in hydrochar (280 • C for 1 h), with the identification of other types (Ca-P and Fe-P) [START_REF] Zheng | Enhanced transformation of phosphorus (P) in sewage sludge to hydroxyapatite via hydrothermal carbonization and calcium-based additive[END_REF][START_REF] Zheng | Migration and Transformation of Phosphorus during Hydrothermal Carbonization of Sewage Sludge: Focusing on the Role of pH and Calcium Additive and the Transformation Mechanism[END_REF]. Another study also showed the dominance of AlPO 4 regardless of the hydrothermal Fig. 6. P species in ash (a) and hydrochar (b) derived from municipal sludge (MS) under various incineration/hydrothermal conditions. Bottom labels (up to down) represent treatment conditions (temperature and/or residence time) and data sources, respectively. SS = secondary sludge, MPS = mixed primary and secondary sludge, AP = apatite P, NAIP = non-apatite inorganic P, OP = organic P, inorganic P = AP + NAIP. Data sources: Li1 [START_REF] Li | Potential recovery of phosphorus during the fluidized bed incineration of sewage sludge[END_REF], Li2 [START_REF] Li | Transformation of apatite phosphorus and non-apatite inorganic phosphorus during incineration of sewage sludge[END_REF], Liang [START_REF] Liang | A comparison between sulfuric acid and oxalic acid leaching with subsequent purification and precipitation for phosphorus recovery from sewage sludge incineration ash[END_REF], Li3 [START_REF] Li | Phosphorus species transformation and recovery without apatite in FeCl3-assisted sewage sludge hydrothermal treatment[END_REF], Shi [START_REF] Shi | Hydrothermal conversion of dewatered sewage sludge: Focusing on the transformation mechanism and recovery of phosphorus[END_REF], Xu [START_REF] Xu | Migration and transformation of phosphorus in municipal sludge by the hydrothermal treatment and its directional adjustment[END_REF], Zhai [START_REF] Zhai | Recovery of phosphorus from sewage sludge in combination with the supercritical water process[END_REF], Zhang [START_REF] Zhang | Hydrothermal co-carbonization of sewage sludge and pinewood sawdust for nutrient-rich hydrochar production: Synergistic effects and products characterization[END_REF], and Zheng [START_REF] Zheng | Enhanced transformation of phosphorus (P) in sewage sludge to hydroxyapatite via hydrothermal carbonization and calcium-based additive[END_REF].

conditions (pH = 3-11 at 200-260 • C for 2 h) due to much higher Al contents than other metals [START_REF] Wang | Feedwater pH affects phosphorus transformation during hydrothermal carbonization of sewage sludge[END_REF]. Due to the low concentrations in sludge-derived ash and hydrochar, most studies did not report the detection of crystallized heavy metals by XRD. Both thermochemical processes (i.e., incineration and hydrothermal conversion) cause the formation of more stable and insoluble P, with the final P complexes closely associated with the speciation/abundance of Pbinding metals/minerals (e.g., Al, Ca, Fe, and Mg). In Al-and Ca-rich ash/hydrochar, AlPO 4 and Ca-P (e.g., β-tricalcium phosphate and hydroxyapatite) tend to predominate P-binding compounds, respectively. FePO 4 could be the primary form in Fe-rich hydrochar. Increasing incineration or hydrothermal temperatures also favor the formation of more stable Ca-P (i.e., apatite) while reducing the formation of Al-P and Fe-P in sludge-derived ash and hydrochar.

Wet-chemical extraction of P

The wet-chemical extraction for P recovery can be distinguished as acidic and alkalic extraction methods. Due to the high concentrations of metals and complex compositions of sludge-derived ash and hydrochar, it is likely to cause operational issues or suboptimal extraction efficiencies using conventional extraction methods. The following sections summarize the optimization processes and recent advances in P extraction.

Acidic extraction

Acidic extraction is the most used wet-chemical method for P recovery due to its high extraction efficiency. Both inorganic and organic acids have been used for P recovery from sludge-derived ash and hydrochar. Inorganic acids include hydrochloric acid (HCl, K a = 1.3 × 10 6 ), sulfuric acid (H 2 SO 4 , K a1 = 1 × 10 3 ), nitric acid (HNO 3 , K a = 24), and phosphoric acid (H 3 PO 4 , K a1 = 7.1 × 10 -3 ), while organic acids involve oxalic acid (C 2 H 6 O 6 , K a1 = 5.9 × 10 -2 ), citric acid (C 6 H 8 O 7 , K a1 = 8.4 × 10 -3 ), gluconic acid (C 6 H 12 O 7 , K a1 = 2.5 × 10 -4 ), formic acid (CH 2 O 2 , K a = 1.8 × 10 -4 ), and acetic acid (C 2 H 4 O 2 , K a = 1.8 × 10 -5 ). These inorganic acids have been proven for their high P extraction capacity by dissolving alkali-metal oxides and leaching P contained in most phases, while organic acids facilitate the release of metals/metalloids and metal-bound P by inducing chelating effects [START_REF] Fang | Phosphorus recovery and leaching of trace elements from incinerated sewage sludge ash (ISSA)[END_REF]. Considering that P in ash and hydrochar dominantly exists as Ca-P, Al-P, and Fe-P, the potential reactions (simplified) during P extraction by acids are listed in Eqs. ( 1)-( 6) [START_REF] Luyckx | Closing the phosphorus cycle: Multicriteria techno-economic optimization of phosphorus extraction from wastewater treatment sludge ash[END_REF][START_REF] Petzet | On wet chemical phosphorus recovery from sewage sludge ash by acidic or alkaline leaching and an optimized combination of both[END_REF]. However, it should be noted that P compounds are often present in more complex forms (e.g., whitlockite). Among these acids, H 2 SO 4 is mostly employed at a commercial scale for its low cost.

AlPO 4 + 3H + ⇌Al 3+ + H 3 PO 4 , K sp = 9.8 × 10 -21
(1)

FePO 4 + 3H + ⇌Fe 3+ + H 3 PO 4 , K sp = 1.3 × 10 -22 (2)
Ca 3 (PO 4 ) 2 + 6H + ⇌3Ca 2+ + 2H 3 PO 4 , K sp = 2.1 × 10 -33 (3) Fe 3 (PO 4 ) 2 + 6H + ⇌3Fe 2+ + 2H 3 PO 4 , K sp = 1.6 × 10 -36 (4) Ca 3 (PO 4 ) 2 + 3H 2 SO 4 ⇌3CaSO 4 ↓ + 2H 3 PO 4 (5) Ca 3 (PO 4 ) 2 + 3H 2 C 2 O 4 ⇌3CaC 2 O 4 ↓ + 2H 3 PO 4 (6)
With the use of H 2 SO 4 and H 2 C 2 O 4 , hardly soluble gypsum (CaSO 4 , K sp = 4.9 × 10 -5 ) and calcium oxalate (CaC 2 O 4 , K sp = 2.3 × 10 -9 ) have been identified on the particle surfaces of acid-extracted sludge ash, respectively [START_REF] Li | Change in re-use value of incinerated sewage sludge ash due to chemical extraction of phosphorus[END_REF]. These reactions (Eqs. ( 5) and ( 6)) could enhance the dissolution of Ca-P. On the other hand, P leaching efficiency might be decreased with time due to the physical barrier formed by the deposition of CaSO 4 and CaC 2 O 4 on ash particles [START_REF] Luyckx | Closing the phosphorus cycle: Multicriteria techno-economic optimization of phosphorus extraction from wastewater treatment sludge ash[END_REF]. Besides, the formation of CaSO 4 could increase the waste volume and constrain the possible reuse of acid-washed ash as a construction material, since gypsum would affect the early hydration chemistry and setting time and reduce the strength development of Portland cement [START_REF] Li | Change in re-use value of incinerated sewage sludge ash due to chemical extraction of phosphorus[END_REF][START_REF] Semerci | Comparison of dried sludge and sludge ash for phosphorus recovery with acidic and alkaline leaching[END_REF]. On the contrary, CaC 2 O 4 is not expected to negatively affect cement properties as organic fraction shall be combusted in a cement kiln [START_REF] Luyckx | Closing the phosphorus cycle: Multicriteria techno-economic optimization of phosphorus extraction from wastewater treatment sludge ash[END_REF].

Based on the stoichiometric calculations of Eqs. ( 1)-( 6), if the molar ratio of H + /P equals 3, P should be completely extracted by acid solutions. However, due to the presence of other acid-soluble compounds, such as oxides of alkali and alkaline earth metals, a greater amount of acid is required to extract almost all P from ash and hydrochar [START_REF] Petzet | On wet chemical phosphorus recovery from sewage sludge ash by acidic or alkaline leaching and an optimized combination of both[END_REF]. Fig. 7a shows the low correlation (adjusted R 2 = 0.29) between P extraction efficiency and H + /P molar ratio for acidic extraction, indicating that using H + /P is inadequate to estimate the required amount of acid due to varied characteristics of ash and hydrochar. A stoichiometric equation (Eq. ( 7)) based on the mineral compositions (%) of phosphate rock was proposed by the US Department of Agriculture [START_REF]Superphosphate: Its History[END_REF], and it can be used to more accurately estimate the required amount of 100% sulfuric acid (kg) per 100 kg of dry material (ash or hydrochar). It should be noted that the estimation assumes that fluorine does not interfere with acid extraction. As demonstrated by Fig. 7b, the predicted P extraction efficiency based on the required amount of H 2 SO 4 (calculated by Eq. ( 7)) is linearly correlated (adjusted R 2 = 0.82) with the actual extraction efficiency. Therefore, Eq. ( 7) can be used to estimate the acid demands when using H 2 SO 4 for P extraction from sludge-derived ash and hydrochar.

H 2 SO 4 (kg/100kg dry material) = 1.749(CaO%) + 0.962(Al 2 O 3 %) + 0.614(Fe 2 O 3 %) + 2.433(MgO%) + 1.582(Na 2 O%) + 1.041(K 2 O%) -0.691(P 2 O 5 %) -1.225(SO 3 %) (7)

Influential factors for P extraction

Along with the chemical dosage, many process parameters can affect P extraction efficiency. These factors include compositions of ash and hydrochar, type and concentration of extractant, solution pH, liquid to solid (L/S) ratio, contact time, agitation, extraction temperature, and upstream treatment variables (e.g., incineration and HTL conditions). It has been reported that although increasing aqueous temperature from 30 to 70 • C could slightly (<1%) enhance P extraction efficiency, it leached more metals (e.g., Ca, Cu, Fe, and Mn) from ash by 0.05 M H 2 SO 4 at an L/S ratio of 150 mL/g for 4 h. It was determined that elevated temperature was not suitable for P extraction as it caused high solubility of metals [START_REF] Biswas | Leaching of phosphorus from incinerated sewage sludge ash by means of acid extraction followed by adsorption on orange waste gel[END_REF]. Semerci et al. also compared the impacts of extraction temperature [START_REF] Shah | Hydrothermal liquefaction of high ash containing sewage sludge at sub and supercritical conditions[END_REF][START_REF] Basar | A review on key design and operational parameters to optimize and develop hydrothermal liquefaction of biomass for biorefinery applications[END_REF], and 70 • C) and found that temperature was neither effective nor economical on P extraction [START_REF] Semerci | Comparison of dried sludge and sludge ash for phosphorus recovery with acidic and alkaline leaching[END_REF]. Most other studies only investigated acidic extraction under room temperature (20-25 • C) and atmospheric pressure. More comprehensive studies are necessary to determine the interactive effects between extraction temperature and other parameters, along with the cost-effectiveness.

Table 2 summarizes the percentage of extracted P and leached heavy metals under various conditions from the literature. It is generally agreed that HCl, H 2 SO 4 , HNO 3 , and oxalic acid result in a good P extraction for their strong acidity under identical extraction conditions (acid normality, L/S ratio, and contact time). However, the liquid pH during extraction is required to be maintained ≤ 2 for a high extraction efficiency (up to 100%) from sludge-derived ash and hydrochar. Exceptions apply to H 2 SO 4 , citric acid, and oxalic acid, which can still extract most P (>96%) at a slightly higher pH (2.1-2.6) under the extraction conditions of 20 mL/g for 2 h [START_REF] Fang | Phosphorus recovery and leaching of trace elements from incinerated sewage sludge ash (ISSA)[END_REF]. H 2 SO 4 and oxalic acid have the advantages of promoting the formation of poorly soluble CaSO 4 and calcium oxalate, which improves the dissolution of Ca-P (see Eqs. ( 5) and ( 6)) [START_REF] Luyckx | Closing the phosphorus cycle: Multicriteria techno-economic optimization of phosphorus extraction from wastewater treatment sludge ash[END_REF][START_REF] Goss | Determination of calcium salt solubility with changes in pH and P CO2, simulating varying gastrointestinal environments[END_REF][START_REF] Shukla | Effect of pH on the solubility of CaSO4⋅2H2O in aqueous NaCl solutions and physicochemical solution properties at 35 • C[END_REF] (2.7 M) to extract P from ash at an L/S ratio of 4.3 mL/g for 0.5 h. The method was found to extract 62-92% of P from most ashes generated by various treatment plants [START_REF] Gorazda | Fertilisers production from ashes after sewage sludge combustion -A strategy towards sustainable development[END_REF]. However, a negative P extraction efficiency (-50%) was reported due to the high concentration of Al in ash and the caused precipitation of phosphate [START_REF] Gorazda | Fertilisers production from ashes after sewage sludge combustion -A strategy towards sustainable development[END_REF]. This does not mean that H 3 PO 4 would result in a low extraction but reminds the consideration of material compositions when selecting extractants. A comparison study showed that at similar acid/ash mass ratios, using H 3 PO 4 (0.49 g/g) and HNO 3 (0.45 g/g) achieved similar P extraction efficiencies (92% and 95%, respectively) [START_REF] Gorazda | From sewage sludge ash to calcium phosphate fertilizers[END_REF]. Using H 3 PO 4 also provides a complement and high phosphate concentration of the extract to produce P fertilizer with high purity. The use of acetic acid, formic acid, and gluconic acid only obtained low P extraction efficiencies in reported studies, probably due to the high pH (2. Many studies have demonstrated that the extractant concentrations, L/S ratios, and contact time can significantly affect the P extraction efficiency. Such influence is also correlated to the end pH of extraction solutions. It has been found that increasing extractant concentrations or L/S ratios could enhance end pH and the P extraction efficiency up to a certain level (at equilibrium) while other variables are fixed [START_REF] Luyckx | Closing the phosphorus cycle: Multicriteria techno-economic optimization of phosphorus extraction from wastewater treatment sludge ash[END_REF]. Most studies used an L/S ratio of 10 mg/L or higher for P extraction. However, considering the energy and capital costs, a low L/S ratio (as low as 2.3 mL/g) is preferred to achieve a concentrated P solution on the industrial scale [START_REF] Gorazda | Characteristic of wet method of phosphorus recovery from polish sewage sludge ash with nitric acid[END_REF]. Similarly, a low acid concentration (e.g., 0.01 N) may also not be practical as it requires a significant amount of solution (e.g., 1,000 mL/g) to achieve a high P extraction efficiency but results in a low P concentration in the extract. Decreasing contact time also reduces end pH, but it is the least influential factor compared to the other two and adequate extraction time should be allowed to obtain the maximum productivity [START_REF] Luyckx | Closing the phosphorus cycle: Multicriteria techno-economic optimization of phosphorus extraction from wastewater treatment sludge ash[END_REF]. Most studies have agreed that a contact time of 2 h is appropriate to reach the optimal P extraction from sludge-derived ash. Shorter extraction time (10 min) may be satisfactory on the industrial scale to obtain > 90% of extraction efficiency due to the intensive mixing process (1,200 rpm) [START_REF] Franz | Phosphate fertilizer from sewage sludge ash (SSA)[END_REF]. It should be noted that excessive contact time (e.g., 1 week) may be unfavorable due to P re-precipitation as well as the increase of leached heavy metals [START_REF] Ottosen | Extracting phosphorous from incinerated sewage sludge ash rich in iron or aluminum[END_REF]. From the economic perspective, minimizing extraction time (solids residence time) and L/S ratio without reducing P extraction efficiency would be desirable for the bargain of equipment size and associated capital costs.

Limited studies have considered the influence of the abovementioned factors on P extraction from sludge-derived hydrochar. To date, only H 2 SO 4 , HCl, citric acid, and oxalic acid have been used to extract P from hydrochar with satisfactory results, while few studies compared the differences among them [START_REF] Ovsyannikova | Feedstock-dependent phosphate recovery in a pilot-scale hydrothermal liquefaction bio-crude production[END_REF][START_REF] Acelas | Supercritical water gasification of sewage sludge: Gas production and phosphorus recovery[END_REF]. Ovsyannikova et al. found that the effects of acid (H 2 SO 4 ) concentration on P extraction from hydrochar were similar to those from ash [START_REF] Ovsyannikova | Feedstock-dependent phosphate recovery in a pilot-scale hydrothermal liquefaction bio-crude production[END_REF]. Acelas et al. found that a long extraction time (4-8 h) was required to reach a high P extraction efficiency (92-96%) from hydrochar using H 2 SO 4 and oxalic acid at pH = 2 and L/S = 1,000 mL/g [START_REF] Acelas | Supercritical water gasification of sewage sludge: Gas production and phosphorus recovery[END_REF]. Other studies reported that a 2-h contact time obtained nearly 100% of extracted P with sufficient H 2 SO 4 , HCl, and citric acid [START_REF] Ovsyannikova | Feedstock-dependent phosphate recovery in a pilot-scale hydrothermal liquefaction bio-crude production[END_REF][START_REF] Becker | Novel approach of phosphate-reclamation as struvite from sewage sludge by utilising hydrothermal carbonization[END_REF]. More studies are desired to comprehensively evaluate the impacts of processing variables on the P extraction from hydrochar, while current studies about ash can be used as guidance. Due to the complex compositions of ash and hydrochar derived from municipal sludge, it is important to optimize the extraction conditions for P recovery.

Concerning the optimization of extraction conditions for P recovery, response surface methodology (RSM) with second-order models has been widely used in the optimization of multi-parameter processes, which can efficiently optimize operational parameters with a minimum number of experimental runs [START_REF] Hu | Investigation of waste biomass co-pyrolysis with petroleum sludge using a response surface methodology[END_REF]. The RSM using a central composite design consists of 2 k (k = number of independent variables) of factorial points (coded as ± 1), 2 k of axial points (coded as ± α), and 2-6 of center points (coded as 0) [START_REF] Zin | Struvite production from food processing wastewater and incinerated sewage sludge ash as an alternative N and P source: Optimization of multiple resources recovery by response surface methodology[END_REF]. A quadratic poly-nominal model expressed by Eq. ( 8) can be performed to predict responses and examine the interactive relationships between the variables and the responses. Using RSM, Luyckx et al. optimized the cost-effective conditions for P extraction with only 16 runs for three variables (i.e., extractant concentration, L/S ratio, and contact time) at three levels [START_REF] Luyckx | Closing the phosphorus cycle: Multicriteria techno-economic optimization of phosphorus extraction from wastewater treatment sludge ash[END_REF]. By coupling a multi-criteria techno-economic optimization, they also identified the optimal extractants (H 2 SO 4 and oxalic acid) for the lowest cost and lowest heavy metal co-extraction, respectively.

Y = β 0 + ∑ 3 i=1 β i X i + ∑ 3 i=1 β ii X 2 i + ∑ 2 i=1 ∑ 3 j>i β ij X i X j ( 8
)
where Y is the response variable; X i and X j are the independent variables; β 0 , β i , β ii , and β ij are the intercept, linear, quadratic, and interaction coefficients, respectively.

Concerns of heavy metals

Along with P, dissolution of heavy metals is inevitable during acidic extraction, while the dissolved amount depends on the compositions of ash and hydrochar, as well as acid types. As shown in Table 2, higher P extraction efficiency is generally associated with more leachable heavy metals because of more free acids. Particularly, both inorganic and organic acids have been found effective in leaching As, Cu, and Zn from sludge-derived ash. Besides, Cd can be effectively extracted by inorganic acids. One study suggested that organic acids leached more heavy metals from ash (especially As, Cu, Pb, and Zn) than inorganic acids [START_REF] Fang | Phosphorus recovery and leaching of trace elements from incinerated sewage sludge ash (ISSA)[END_REF]. HCl was found to be an effective leaching agent for all tested metals (e.g., As, Cd, Cr, Cu, Ni, Pb, and Zn) [START_REF] Xu | Recovery of phosphorus as struvite from sewage sludge ash[END_REF]. H 3 PO 4 and citric acid were found to be efficient in extracting Ni [START_REF] Gorazda | Fertilisers production from ashes after sewage sludge combustion -A strategy towards sustainable development[END_REF][START_REF] Fang | Phosphorus recovery and leaching of trace elements from incinerated sewage sludge ash (ISSA)[END_REF]. Over 39% of Pb was also extracted by HNO 3 and oxalic acid along with most extracted P [START_REF] Fang | Phosphorus recovery and leaching of trace elements from incinerated sewage sludge ash (ISSA)[END_REF]. However, there is a lack of study showing the leachable heavy metals from sludgederived hydrochar. One study showed that citric acid (pH = 2) could achieve high P extraction efficiency from hydrochar but cause limited heavy metals in recovered struvite due to formed citrate complexes; however, the reagent cost may be too high [START_REF] Becker | Novel approach of phosphate-reclamation as struvite from sewage sludge by utilising hydrothermal carbonization[END_REF]. It was reported that dosing citric acid (at a stoichiometric ratio C 6 H 8 O 7 :Σ(Fe, Al, Ca, Mg) = ~1) to acidic leachate could form metal complexes staying in solution during struvite precipitation [START_REF] Meyer | Phosphorus Polluter Resour. Futur. -Remov. Recover. from Wastewater[END_REF]. Through a multi-criteria technoeconomic analysis, Luyckx et al. found that H 2 SO 4 (0.5 N, 10 mL/g, 2 h) and oxalic acid (0.5 N, 12.8 mL/g, 2 h) showed the best trade-off among high extraction efficiency of P (>80%), low leached amount of heavy metal and desirable operational costs [START_REF] Luyckx | Closing the phosphorus cycle: Multicriteria techno-economic optimization of phosphorus extraction from wastewater treatment sludge ash[END_REF]. Considering the high efficiency of low-cost H 2 SO 4 and the complexing effects of citrate acid, combining two acids might be an approach to find a balance among P extraction, costs, and concerning heavy metals.

As a result of leached heavy metals by acidic extraction, additional chemicals and/or energy inputs are necessary to separate P from heavy metals and ensure the quality of recovered products. Several separation techniques have been attempted, including cationic ion exchange, liquid-liquid extraction (PASCH process), nanofiltration, sequential precipitation, and sulfidic precipitation [START_REF] Petzet | On wet chemical phosphorus recovery from sewage sludge ash by acidic or alkaline leaching and an optimized combination of both[END_REF]. Nevertheless, none of them have been demonstrated to be applicable to a large scale due to their complexity and economic infeasibility [START_REF] Petzet | On wet chemical phosphorus recovery from sewage sludge ash by acidic or alkaline leaching and an optimized combination of both[END_REF][START_REF] Petzet | Recovery of phosphorus and aluminium from sewage sludge ash by a new wet chemical elution process (SESAL-Phos-recovery process)[END_REF]. Further efforts are needed to optimize the removal of metals from acidic extracts, such as developing cheap and regenerable resins for ion exchange and finding cost-effective adsorbents.

Pretreatment with chelating agents

Chelating agents have been examined for the leachability of heavy metals from sludge-derived ash for the purpose of pretreatment (heavy metal removal) prior to P extraction. The pretreatment involves prewashing ash to remove heavy metals but retain P in the prewashed ash, which will be extracted with acids for P recovery. The studied chelating agents include ethylene diamine tetramethylene phosphonate (EDTMP), ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), Na 2 -EDTA, Na 2 -oxalate, and Na 4 -EDTA [START_REF] Luyckx | Closing the phosphorus cycle: Multicriteria techno-economic optimization of phosphorus extraction from wastewater treatment sludge ash[END_REF][START_REF] Fang | Phosphorus recovery and leaching of trace elements from incinerated sewage sludge ash (ISSA)[END_REF][START_REF] Li | Change in re-use value of incinerated sewage sludge ash due to chemical extraction of phosphorus[END_REF][START_REF] Fang | Recovery of phosphorus from incinerated sewage sludge ash by combined two-step extraction and selective precipitation[END_REF]. As shown in Table 2, EDTMP and EDTA could leach a considerable amount of P (9-37%) but with limited removal of most heavy metals/metalloids (≤33%) except for As (≥51%). Na 2 -EDTA, Na 2 -oxalate, Na 4 -EDTA, and NTA-NaOH showed limited leachability for both P (1-14%) and heavy metals (≤17%). It is also reported that increased P leaching and decreased heavy metal removal by chelating agents are due to the decreased pH of extracts after extraction. In terms of minimizing P loss (<15% of total P) and maximizing heavy metal removal (>25% of total metals), none of the above-mentioned chelating agents has been considered appropriate as the lost P outweighs the insufficient removal of heavy metals.

Alkalic extraction

Direct alkaline extraction is a promising alternative to avoid the codissolution of heavy metals, but it requires strong bases, e.g., NaOH and KOH. As expected, alkaline extractants (e.g., NaOH) could hardly leach heavy metals (<1%) due to high pH (Table 2). Under alkalic conditions (pH ≥ 12), only NAIP can be extracted while heavy metals remain in sludge-derived ash and hydrochar (Eqs. ( 9)-( 11)) [START_REF] Falayi | Alkaline recovery of phosphorous from sewage sludge and stabilisation of sewage sludge residue[END_REF][START_REF] Cao | Phosphorus recovery as vivianite from waste activated sludge via optimizing iron source and pH value during anaerobic fermentation[END_REF]. Consequently, this process is highly selective on the compositions of ash and hydrochar.

AlPO 4 + 4OH -⇌[Al(OH) 4 ] -+ PO 4 3- (9) Fe 3 (PO 4 ) 2 + 6OH -⇌3Fe(OH) 2 ↓ + 2PO 4 3- (10) FePO 4 + 3OH -⇌Fe(OH) 3 ↓+PO 4 3- (11)
Particularly, it is found that the P recovery rate is greatly dependent on the P/Ca molar ratio. Fig. 8 shows that P extraction efficiency reduces significantly with the decrease of P/Ca molar ratio in ash and hydrochar. It was also reported that alkalic extraction of P became infeasible when the CaO concentration in ash exceeded 20% [START_REF] Sonoda | Alkaline leaching of phosphate from sewage sludge ash[END_REF]. Therefore, Ca-rich ash or hydrochar would not be suitable for P extraction using alkaline leaching.

Similar to acidic leaching, P extraction efficiency of alkalic extraction improves with the increase of extractant concentrations (or higher extraction pH), L/S ratios, and contact time within certain ranges. Xu et al. found that P extraction efficiency enhanced from 1% to 25% of total P in ash when NaOH concentrations increased from 0.01 M to 0.8 M (L/S = 25 mL/g, 2 h), while it only improved 5% when L/S increased from 25 to 200 mL/g (0.01 N, 2 h) [START_REF] Xu | Fraction distributions of phosphorus in sewage sludge and sludge ash, Waste and Biomass Valorization[END_REF]. It was suggested that limited P could be extracted from hydrochar at low pH (e.g., < 10% at pH = 10.5), while the extraction efficiency did not increase significantly at pH > 12 (L/S = 100 mL/g, 16 h) [START_REF] Li | Phosphorus species transformation and recovery without apatite in FeCl3-assisted sewage sludge hydrothermal treatment[END_REF]. Sufficient contact time should be allowed to achieve maximum extraction efficiency. Luyckx the leached P from ash increased from 9% to 42% with increasing contact time (10 to 120 min) with 0.5 N NaOH at L/S = 50 mL/g [START_REF] Luyckx | Closing the phosphorus cycle: Multicriteria techno-economic optimization of phosphorus extraction from wastewater treatment sludge ash[END_REF].

Through a kinetics study, Li et al. observed that the P extraction equilibrium can be reached within approximately 1-4 h depending on the compositions of hydrochar. The P desorption results were found to fit Pseudo-second-order (Eq. ( 12)) better than Pseudo-first-order (Eq. ( 13)) [START_REF] Li | Phosphorus species transformation and recovery without apatite in FeCl3-assisted sewage sludge hydrothermal treatment[END_REF]. It indicates that the P desorption process is primarily controlled by chemisorption rather than diffusion, and the reaction rate is limited over time with the concentration of one or more substances [START_REF] Barca | Kinetics and capacity of phosphorus extraction from solid residues obtained from wet air oxidation of sewage sludge[END_REF].

q t = q e (1 -e -k1t ) (12)
q t = q e 2 k 2 t 1 + q e k 2 t ( 13
)
The above equations can be converted as linear forms (Eqs. ( 14) and ( 15)) to assist the calculation of model parameters: ln(q eq t ) = ln(q e )k 1 t (14)

t q t = 1 k 2 q e 2 + 1 q e t ( 15
)
where q t : Extraction capacity at time t (min), mg/g; q e : Extraction capacity at equilibrium, mg/g; k 1 : Pseudo-first-order rate constant, min -1 ; and, k 2 : Pseudo-second-order rate constant, g/(mg min).

The upstream treatment conditions can also affect the P extraction efficiency due to the transformation of P species (see Section 3.2). Higher incineration or hydrothermal temperatures generally reduces the relative percentages of NAIP and increases AP contents in sludgederived ash or hydrochar, thus reducing the extractable P by alkalic method [START_REF] Ovsyannikova | Experimental and thermodynamic studies of phosphate behavior during the hydrothermal carbonization of sewage sludge[END_REF]. Another study reached a very high extraction efficiency (>89% of total P) from hydrochar (200 • C for 4 h) using KOH solution (pH ≥ 12), as the sludge was produced from chemical P removal process with poly-aluminum sulfate during wastewater treatment. It was also suggested that adding FeCl 3 could promote the selective transformation of AP to NAIP under hydrothermal conditions and enhance alkalicextractable P [START_REF] Li | Phosphorus species transformation and recovery without apatite in FeCl3-assisted sewage sludge hydrothermal treatment[END_REF]. Many studies found that extracted P (<60% of total P) was limited from sludge-derived ash and hydrochar using alkalic method (Table 2), restricting the wide application of this approach.

A successful full-scale application of alkalic extraction technique in Gifu City, Japan, has been operating since 2010, with a reported P leaching rate of up to 75% of total P [START_REF] Petzet | On wet chemical phosphorus recovery from sewage sludge ash by acidic or alkaline leaching and an optimized combination of both[END_REF][START_REF] Sonoda | Alkaline leaching of phosphate from sewage sludge ash[END_REF][START_REF] Nakagawa | Phosphorus recovery from sewage sludge ash: A case study in Gifu, Japan[END_REF]. Briefly, P-rich leachate is extracted with 1 N NaOH at 50-70 • C for 1.5 h from locally produced ash from sewage sludge, and P is recovered as hydroxyapatite by subsequent precipitation using Ca(OH) 2 at 20-50 • C for 9 h. The remaining alkaline solution is recycled for P extraction to save chemical costs. The solid residue from extraction is washed by H 2 SO 4 solution (pH = 4.5-5.5) to remove hazardous metals and can be safely reused as construction materials. However, further reduction of the cost of chemicals is necessary and critical to make this process more economically practicable [START_REF] Sonoda | Alkaline leaching of phosphate from sewage sludge ash[END_REF][START_REF] Nakagawa | Phosphorus recovery from sewage sludge ash: A case study in Gifu, Japan[END_REF].

Sequential extraction

For acidic dissolution, metals/metalloids dissolved with P should be separated to improve the quality of recovered product, which requires additional energy or chemical input. Direct alkalic extraction can avoid the dissolution of interfering metals, but it does not apply to all sludgederived ash and hydrochar due to limited amount of NAIP. Therefore, several studies proposed the combined sequential extraction procedures to maximize extracted P and minimize leached metals/metalloids (Fig. 9).

These sequential processes in Fig. 9 can be categorized into three types based on the mechanisms: 1) Transforming Ca-P to Al-P and/or Fe-P under controlled pH around 3-4, e.g., Processes A-C; 2) eluting heavy metals by chelating agent (EDTA), e.g., Process D; and 3) adsorption-desorption, e.g., Process E. Table 3 evaluates the advantages and challenges of using these extraction methods.

In Process A, firstly an acidic elution (pH = ~3) was applied to dissolve Ca-P and reform P as AlPO 4 (K sp = 9.8 × 10 -21 ) precipitates simultaneously (0.4 N HCl, L/S = 10 mL/g, 24 h). Reactions are illustrated in Eqs. ( 16)-( 18) [START_REF] Petzet | On wet chemical phosphorus recovery from sewage sludge ash by acidic or alkaline leaching and an optimized combination of both[END_REF]. It was found that when the reaction pH was maintained in 3-4, most Ca along with heavy metals was removed as ions in the liquid stream by the subsequent dewatering process. On the contrary, most Al, Fe, and P stayed in the solid phase [START_REF] Petzet | On wet chemical phosphorus recovery from sewage sludge ash by acidic or alkaline leaching and an optimized combination of both[END_REF][START_REF] Petzet | Recovery of phosphorus and aluminium from sewage sludge ash by a new wet chemical elution process (SESAL-Phos-recovery process)[END_REF]. It is worth noting that the optimal pH for elution varies by the compositions of ash and hydrochar. P in the form of amorphous AlPO 4 can be easily extracted by the following alkaline solution without concerns of heavy metals leaching. However, it was noted that limited P (50%) could be extracted from Fe-rich ash using this method [START_REF] Petzet | On wet chemical phosphorus recovery from sewage sludge ash by acidic or alkaline leaching and an optimized combination of both[END_REF]. This may be because most Fe exists as acid-insoluble hematite (Fe 2 O 3 ) in sludge-derived ash, thus leached Fe cations are insufficient for forming Fe-P (nearly no dissolved Fe for pH > 2) [START_REF] Petzet | On wet chemical phosphorus recovery from sewage sludge ash by acidic or alkaline leaching and an optimized combination of both[END_REF][START_REF] Fang | Recovery of phosphorus from incinerated sewage sludge ash by combined two-step extraction and selective precipitation[END_REF].

Ca 9 Al(PO 4 ) 7 + 21HCl→9Ca 2+ + Al 3+ + H 3 PO 4 + 21Cl - (16
)

Al(OH) 3 + 3H + →Al 3+ + 3H 2 O ( 17
)
Al 3+ + H 3 PO 4 →AlPO 4 ↓ + 3H + ( 18
)
Process B is similar to Process A. However, instead of using weak acid for elution, Process B initially dosed a strong acid solution (1 N HCl, L/S = 10 mL/g, 24 h) to dissolve almost all P (leachate pH = ~1) [START_REF] Lim | Selective acidic elution of Ca from sewage sludge ash for phosphorus recovery under pH control[END_REF]. Without separation, the reaction pH was then raised to 4 for Al-P precipitation, whereas Ca and most heavy metals remaining in the liquid stream can be washed away. A higher P extraction efficiency may be achieved than Process A due to the lower elution pH, but this has not been demonstrated by comparison studies. Higher amounts of chemicals are required than Process A for the extra dosage of acid and base. Both Processes A and B did not use H 2 SO 4 for pre-elution to avoid the formation of insoluble CaSO 4 .

Process C also used an excessive amount of acid to dissolve most P in the first step (0.2 N H 2 SO 4 , L/S = 20 mL/g, 2 h). After filtration, P was precipitated as Al-P/Fe-P from the leachate by adjusting pH from 1.5 to 3 [START_REF] Wang | Sustainable reclamation of phosphorus from incinerated sewage sludge ash as value-added struvite by chemical extraction, purification and crystallization[END_REF]. Different from Processes A and B, this procedure used acid to redissolve P precipitates, and the extract was further washed with cation exchange resin (CER) for a high-purity product. Although it could be applicable for both Al-rich and Fe-rich feedstocks, it involves additional chemicals and equipment. The use of CER did not improve purity (76%) of recovered fertilizer [START_REF] Wang | Sustainable reclamation of phosphorus from incinerated sewage sludge ash as value-added struvite by chemical extraction, purification and crystallization[END_REF]. Further optimization is necessary for Process C. Fig. 8. Alkaline-extractable P versus the corresponding P/Ca molar ratio in sludge-derived ash and hydrochar. Data sourced from [START_REF] Shi | Hydrothermal conversion of dewatered sewage sludge: Focusing on the transformation mechanism and recovery of phosphorus[END_REF][START_REF] Ovsyannikova | Feedstock-dependent phosphate recovery in a pilot-scale hydrothermal liquefaction bio-crude production[END_REF][START_REF] Ovsyannikova | Experimental and thermodynamic studies of phosphate behavior during the hydrothermal carbonization of sewage sludge[END_REF][START_REF] Petzet | On wet chemical phosphorus recovery from sewage sludge ash by acidic or alkaline leaching and an optimized combination of both[END_REF].

Unlike Processes A-C, process D utilized chelating agent -EDTA to remove heavy metals by forming metal-EDTA complexes. The eluted ash can be extracted by a strong acid solution to obtain the most P and limited heavy metals. However, due to the significant loss of P (21%) during elution, the P extraction efficiency by this process was relatively low [START_REF] Fang | Recovery of phosphorus from incinerated sewage sludge ash by combined two-step extraction and selective precipitation[END_REF]. Meanwhile, not all heavy metals (especially for Cu, Ni, and Zn) can be diminished from the extracts. Moreover, the cost of EDTA is very high ($2000/tonne). The use of H 2 SO 4 for the 2nd extraction caused a high content of CaSO 4 (68% by weight) in the recovered material, further limiting the recovery value. Therefore, alternative chelating agents may be explored to minimize P loss, improve heavy metals removal, and reduce the cost.

Process E used activated alumina (Al 2 O 3 ) to adsorb P from the acidic extract and desorb P using subsequent alkalic extraction. The involved reactions are shown by Eqs. ( 19) and ( 20) [START_REF] Zhai | Recovery of phosphorus from sewage sludge in combination with the supercritical water process[END_REF]. Based on the equations, there is no need to adjust pH during the acidic adsorption and alkalic desorption steps, and the alumina only works as an adsorbent that can be reused. However, due to the co-adsorption of heavy metals (e.g., Cr and Zn) from the acidic extract, regeneration of alumina is necessary, and its reusability requires examination. Furthermore, a large amount of powdered alumina is needed for its low adsorption capacity (e.g., 6.2 and 15.4 mg P/g alumina for solutions with 15 and 50 mg P/L, respectively).

Al 2 O 3 + 3H 2 O→Al(OH) 3 (19) 2Al(OH) 3 + H 2 PO 4 -⇌AlPO 4 + OH -+ 2H 2 O (20)
The above sequential extraction methods have noticeable advantages of depolluting heavy metals from extracts using relatively simple procedures. They also provide alternative options for recovering P from Fig. 9. Schematic diagrams of sequential P extraction processes for sludge-derived ash and hydrochar. A) SESAL-Phos [START_REF] Petzet | On wet chemical phosphorus recovery from sewage sludge ash by acidic or alkaline leaching and an optimized combination of both[END_REF][START_REF] Petzet | Recovery of phosphorus and aluminium from sewage sludge ash by a new wet chemical elution process (SESAL-Phos-recovery process)[END_REF]; B) Acid-base Leaching [START_REF] Lim | Selective acidic elution of Ca from sewage sludge ash for phosphorus recovery under pH control[END_REF][START_REF] Zin | Struvite production from food processing wastewater and incinerated sewage sludge ash as an alternative N and P source: Optimization of multiple resources recovery by response surface methodology[END_REF][START_REF] Lee | Identification of phosphorus forms in sewage sludge ash during acid pre-treatment for phosphorus recovery by chemical fractionation and spectroscopy[END_REF][START_REF] Zin | Maximizing ammonium and phosphate recovery from food wastewater and incinerated sewage sludge ash by optimal Mg dose with RSM[END_REF]; C) Three-step Extraction [START_REF] Wang | Sustainable reclamation of phosphorus from incinerated sewage sludge ash as value-added struvite by chemical extraction, purification and crystallization[END_REF]; D) Two-step Extraction [START_REF] Fang | Recovery of phosphorus from incinerated sewage sludge ash by combined two-step extraction and selective precipitation[END_REF]; E) Alumina Adsorption [START_REF] Zhai | Recovery of phosphorus from sewage sludge in combination with the supercritical water process[END_REF]. HMs = heavy metals; CER = cation exchange resin; EDTA = ethylenediaminetetraacetic acid. ash and hydrochar with complex compositions. It is worthy to further develop the sequential P extraction approaches by addressing the process challenges and investigating their technical and economic feasibility for large-scale applications. Detailed economic analysis and more comprehensive examinations are needed to verify their applicability to various ash and hydrochar. Process optimizations, such as finding cheaper extractants, recycling process water, lowering L/S ratio and extraction time, and improving extraction efficiency, are demanded for cost-effectiveness.

P recovery as fertilizers by precipitation

After liquid extraction, P is often recovered as solid fertilizers by the subsequent chemical precipitation for its high recovery rate and economic efficiency, allowing easy transportation and storage. Three major crystallization processes are commonly considered to produce P fertilizers from wastewater streams: Struvite (MgNH 4 PO 4 ⋅6H 2 O), Ca-P, and vivianite [Fe 3 (PO 4 ) 2 ⋅8H 2 O)]. The mechanisms of crystallization can be briefly described as: 1) Crystal formation through primary nucleation by foreign particles or supersaturation conditions, 2) crystal growth via secondary nucleation by mass transfer and surface integration, and 3) aggregation of crystals [START_REF] Daneshgar | Economic and energetic assessment of different phosphorus recovery options from aerobic sludge[END_REF][START_REF] Abel-Denee | Using mass struvite precipitation to remove recalcitrant nutrients and micropollutants from anaerobic digestion dewatering centrate[END_REF]. Table 4 compares the three crystallization processes and Table 5 summarizes the common precipitants used for each process. The review about effects of operational factors (e. g., pH, temperature, mixing, precipitants, molar ratio, seeding, foreign ions, organic matter, and reactor types) on the crystallization processes can be found elsewhere [START_REF] Wu | Potentials and challenges of phosphorus recovery as vivianite from wastewater: A review[END_REF][START_REF] Peng | A comprehensive review of phosphorus recovery from wastewater by crystallization processes[END_REF][START_REF] Li | Phosphorus recovery through struvite crystallisation: Recent developments in the understanding of operational factors[END_REF][START_REF] Kumar | Assessing the feasibility of N and P recovery by struvite precipitation from nutrient-rich wastewater: a review[END_REF].

Struvite

Struvite crystallization has been widely developed to recover P from both ash and hydrochar derived from municipal sludge (Table 6). Theoretically, struvite starts to form at a Mg 2+ :NH 4 + :PO 4 3-molar ratio of 1:1:1 under saturation conditions as shown in Eq. ( 21) (Table 4). The solution saturation condition can be indicated by the saturation index (SI), which is defined as the ion activity product (IAP) of constituent ions over the equilibrium solubility product (K sp ) of the precipitate, as described in Eqs. ( 27) and ( 28) [START_REF] Daneshgar | Optimization of P compounds recovery from aerobic sludge by chemical modeling and response surface methodology combination[END_REF]. Precipitation does not occur when the system is undersaturated (SI < 0), while it happens under supersaturated conditions (SI > 0). Due to the complex contents of P extracts, two chemical equilibrium modeling programs, Visual MINTEQ and PHREEQC, have been widely used to estimate SI values based on the inputs of ion concentrations, pH, and temperature of the solution [START_REF] Li | Phosphorus recovery through struvite crystallisation: Recent developments in the understanding of operational factors[END_REF].

The saturation point of struvite is highly affected by the solution pH, with a favorable range of 8-10. Increasing ions concentrations and pH can enhance the solution saturation, but a pH higher than 10.5 leads to the sharp decrease of NH 4 + and inhibit the formation of struvite [START_REF] Kumar | Assessing the feasibility of N and P recovery by struvite precipitation from nutrient-rich wastewater: a review[END_REF].

IAP = {Mg 2+ }{NH 4 + }{PO 4 3-} (27)
SI = log(IAP/K sp ) (28)
where {} donates the effective concentration or activity of ions.

In real precipitation process, the optimal pH and Mg:N:P molar ratio vary due to the complex contents of sludge-derived ash and hydrochar and their extracts. Supplemental Mg 2+ and NH 4

+ chemicals are typically needed due to their low concentrations in extracts. Liang et al. reported that at pH 9 and Mg:N:P of 1:1:1, over 98% of P can be precipitated from acidic extract within 40 min [START_REF] Liang | A comparison between sulfuric acid and oxalic acid leaching with subsequent purification and precipitation for phosphorus recovery from sewage sludge incineration ash[END_REF]. Similarly, another study also obtained 99% of P precipitation efficiency at Mg:N:P = 1:1:1 and pH 9.5 in 2 h [START_REF] Wang | Sustainable reclamation of phosphorus from incinerated sewage sludge ash as value-added struvite by chemical extraction, purification and crystallization[END_REF]. However, Xu et al. found that more MgCl 2 ⋅6H 2 O and NH 4 Cl and higher pH were added for struvite formation from acidic extracts and the optimal conditions were Mg:N:P = 1.6:1.6:1 at pH 10, with the influence ranked as pH > N:P > Mg:P [START_REF] Xu | Recovery of phosphorus as struvite from sewage sludge ash[END_REF]. Some studies indicated that the optimal P (>97%) and N recovery (>70%) from alkaline extracts were reached at higher Mg:P (>1.5) and lower N:P (0.6) molar ratios at pH > 9.3 [START_REF] Zin | Struvite production from food processing wastewater and incinerated sewage sludge ash as an alternative N and P source: Optimization of multiple resources recovery by response surface methodology[END_REF][START_REF] Zin | Maximizing ammonium and phosphate recovery from food wastewater and incinerated sewage sludge ash by optimal Mg dose with RSM[END_REF]. It is noticed that using Mg salts (e.g., MgCl 2 ) can achieve a high P precipitation efficiency (≥87%) within a short period (≤2h), while using cheaper but water-insoluble MgO could cause many issues, such as lower efficiency, longer retention time, larger reactor volume, or even blockage [START_REF] Desmidt | Global phosphorus scarcity and full-scale P-recovery techniques: A review[END_REF]. K-struvite (MgKPO 4 ⋅6H 2 O) is another form of struvite, which replaces NH 4 + with K + under higher pH at 11. One study found that sludge-derived hydrochar showed a high potential of P recovery (≥88% overall P recovery) as K-struvite (K sp = ~ 10 -22 ) through KOH extraction and subsequent crystallization with additional MgCl 2 [START_REF] Li | Phosphorus species transformation and recovery without apatite in FeCl3-assisted sewage sludge hydrothermal treatment[END_REF]. However, both KOH and MgCl 2 are expensive chemicals, which would increase the overall costs. Abbona et al. found that a small amount of Ca 2+ (e.g., Ca:P = 0.1) could largely reduce the purity of struvite, especially at low pH due to the substitution of brushite or amorphous Ca-P for their low induction time, while high Mg content and high pH are needed to overcome this challenge [START_REF] Abbona | The initial phases of calcium and magnesium phosphates precipitated from solutions of high to medium concentrations[END_REF]. The use of Ca (OH) 2 for pH adjustment for struvite precipitation is also discouraged [START_REF] Daneshgar | Impact of pH and ionic molar ratios on phosphorous forms precipitation and recovery from different wastewater sludges[END_REF]. The tradeoffs among chemical consumption, purity, and market values of recovered products should be considered in the cost optimization.

Although additional ammonium source is needed to promote the formation of struvite, it also provides the opportunities of recovering N from waste streams, such as ammonium-rich wastewater. Adding food processing wastewater (27,096.5 mg/L NH 4

+ -N) to alkaline extracts,
+ NH4 + + HnPO4 n-3 + 6H2O→MgNH4PO4•6H2O + nH + (21)
DCP:

Ca 2+ + HPO4 2-→CaHPO4 (22)
; ACP/TCP:

3Ca 2+ + 2PO4 3-→Ca3(PO4) 2 (23)
; OCP: 8

Ca 2+ + 2HPO4 2-+ 4PO4 3-→Ca8(HPO4) 2 (PO4) 4 (24)
; HAP:

5Ca 2+ + 3PO4 3-+ OH -→Ca5(PO4) 3 OH (25)
3Fe 2+ + 2PO4 3-+ 8H2O→Fe3(PO4) 2 •8H2O (26)
Solubility product (pK sp , 25 -N were achieved from sludgederived ash and wastewater, respectively [START_REF] Zin | Struvite production from food processing wastewater and incinerated sewage sludge ash as an alternative N and P source: Optimization of multiple resources recovery by response surface methodology[END_REF]. HTL treatment of municipal sludge generates two major waste streams: P-rich hydrochar and ammonium-rich aqueous phase, which can be exactly utilized as struvite production sources, achieving sustainable nutrients recovery along with the development of continuous-flow HTL system [START_REF] Ovsyannikova | Feedstock-dependent phosphate recovery in a pilot-scale hydrothermal liquefaction bio-crude production[END_REF]. By combining HTL aqueous and hydrochar acidic extracts, a P precipitation efficiency of over 87% was achieved at pH 9 [START_REF] Ovsyannikova | Feedstock-dependent phosphate recovery in a pilot-scale hydrothermal liquefaction bio-crude production[END_REF][START_REF] Becker | Novel approach of phosphate-reclamation as struvite from sewage sludge by utilising hydrothermal carbonization[END_REF]. Noticeably, the overall P recovery efficiency from ash and hydrochar tended to be low (as low as 50%), although the precipitation rates were high. Particularly, in a pilot-scale continuous-flow HTL treatment (350 • C, 22 MPa), it was reported that total recovery for P based on the sludge feedstock was only 23.7%. The low overall efficiency was caused by the dispersion of Pcontaining particles in biocrude (33% P of sludge) during HTL phase separation and limited extraction efficiency (49.9% P of hydrochar) during acidic leaching [START_REF] Ovsyannikova | Feedstock-dependent phosphate recovery in a pilot-scale hydrothermal liquefaction bio-crude production[END_REF]. Therefore, future studies should minimize P loss to other phases and maximize the P extraction efficiency from hydrochar to enhance the overall nutrient recovery efficiency and economic benefits, accomplished with the optimization of the continuousflow HTL operations.

A preliminary economic analysis based on chemical costs and struvite income indicated that struvite precipitation coupled with b Prices obtained from Alibaba [START_REF] Group | Manufacturers, Exporters & Importers from the worlds largest online marketplace[END_REF].

hydrothermal treatment of municipal sludge can be marginally profitable, primarily due to the addition of costly Mg salt (MgCl 2 ) [START_REF] Munir | Phosphate recovery from hydrothermally treated sewage sludge using struvite precipitation[END_REF]. Thus, alternative chemicals and methods should be applied to optimize nutrient recovery. Many research gaps also require attention, such as the reusability and disposal of post-recovery processing water, the utilization of solid residue (i.e., hydrochar after P extraction), comprehensive cost analysis of each recovery process, etc.

Calcium phosphates

Ca-P as commercially available products, their precipitation is a common P recovery pathway for sludge-derived ash and hydrochar. According to the inventory by the European Sustainable Phosphorus Platform, Ca-P are the only fertilizer products that have been recovered under full-scale applications of wet-chemical extraction and precipitation from sludge-derived ash (Fig. 10) [START_REF]European Sustainable Phosphorus Platform, ESPP -DPP -NNP phosphorus recovery technology catalogue[END_REF]. Depending on the solution pH and kinetics, various forms of Ca-P can exist in precipitates: Dicalcium phosphate (CaHPO 4 ), octacalcium phosphate [Ca 8 H 2 (PO 4 ) 6 ⋅5H 2 O] and hydroxyapatite [Ca 5 (PO 4 ) 3 OH] are stable at pH around 5, 6 and above 7, respectively; however, the precipitated phases will eventually recrystallize as hydroxyapatite that is thermodynamically more stable [START_REF] Desmidt | Global phosphorus scarcity and full-scale P-recovery techniques: A review[END_REF]. It was found that with a phosphate concentration of 0.01-0.5 M, the precipitates occurred in the order of brushite (CaHPO 4 ⋅2H 2 O), monetite (CaHPO 4 ), and amorphous Ca-P [Ca 3 (PO 4 ) 2 ⋅xH 2 O], which were controlled by solution pH [START_REF] Abbona | The initial phases of calcium and magnesium phosphates precipitated from solutions of high to medium concentrations[END_REF]. Unlike struvite, Ca-P can reach supersaturation and precipitate at a very low pH (>4). It is found that adding Ca chemicals with a solution pH above 5 could result in a high P precipitation efficiency of > 89% from leachates of sludge-derived ash and hydrochar (Table 6). However, due to the co-dissolved Al and Fe cations during acidic extraction, a large portion of phosphate could exist in the form of undesirable AlPO 4 (pK sp = 20) and/or FePO 4 (pK sp = 15) combined with their hydroxides when precipitation at moderate pH (4.4-6.5), which reduces the plant availability of recovered products [START_REF] Liang | A comparison between sulfuric acid and oxalic acid leaching with subsequent purification and precipitation for phosphorus recovery from sewage sludge incineration ash[END_REF][START_REF] Fang | Recovery of phosphorus from incinerated sewage sludge ash by combined two-step extraction and selective precipitation[END_REF]. Therefore, pre-treatment or purification might be needed to complexes, and therefore P was recovered as low-value AlPO 4 -based product during precipitation [START_REF] Liang | A comparison between sulfuric acid and oxalic acid leaching with subsequent purification and precipitation for phosphorus recovery from sewage sludge incineration ash[END_REF]. Enhancing precipitation pH could also overcome the co-precipitation of Al as it predominates as a soluble anion, Al(OH) 4 -, in alkalic solution (pH > 10) [START_REF] Fang | Recovery of phosphorus from incinerated sewage sludge ash by combined two-step extraction and selective precipitation[END_REF]. Therefore, the compatibility of extractants and the subsequent target fertilizers from precipitation should be carefully considered to avoid the generation of low-value Al-P or Fe(III)-P. Typically, lime (CaO), Ca(OH) 2 , and CaCl 2 are supplied to P extracts to achieve specific stoichiometric molar ratios (Ca:P = 1-1.67) and precipitate Ca-P, whereas a higher amount of Ca demand might be needed due to the co-precipitation of gypsum (CaSO 4 ) and carbonate (CaCO 3 ) [START_REF] Peng | A comprehensive review of phosphorus recovery from wastewater by crystallization processes[END_REF]. This not only causes extra chemical costs but also reduces P contents in recovery products. Such impacts are also related to reaction pH [START_REF] Desmidt | Global phosphorus scarcity and full-scale P-recovery techniques: A review[END_REF]. It has been suggested that SI values (>0) of hydroxyapatite can be effectively improved by increasing phosphate concentration (>1.6 mg P/L), pH (7)[START_REF]Mineral Commodity Summaries: Phosphate Rock[END_REF][START_REF] Huang | Transformation of phosphorus during (hydro)thermal treatments of solid Biowastes: Reaction mechanisms and implications for P reclamation and recycling[END_REF][START_REF] Wei | Development, current state and future trends of sludge management in China: Based on exploratory data and CO2-equivaient emissions analysis[END_REF][START_REF] Eurostat | Sewage sludge production and disposal from urban wastewater (in dry substance (d.s))[END_REF], or Ca:P ratio (1.67-16.7), and it could also be slightly enhanced by temperature from 5 to 30 • C [START_REF] Song | Effects of solution conditions on the precipitation of phosphate for recovery: A thermodynamic evaluation[END_REF]. However, there seems a lack of optimization study for Ca-P precipitation from extracts of sludge-derived ash and hydrochar, although high precipitation rates have been achieved in various studies. The retention time for precipitation is generally maintained within 15-120 min depending on the mixing intensity, but the detailed optimizing process is lacking [START_REF] Gorazda | Fertilisers production from ashes after sewage sludge combustion -A strategy towards sustainable development[END_REF][START_REF] Franz | Phosphate fertilizer from sewage sludge ash (SSA)[END_REF][START_REF] Gorazda | From sewage sludge ash to calcium phosphate fertilizers[END_REF].

Besides the conventional recovery of Ca-P, Liu and Qu developed a novel method of recovering P as monocalcium chlorophosphate (CaClH 2 PO 4 ⋅H 2 O) crystals under strongly acidic conditions (pH < 0) from sludge-derived ash [START_REF] Liu | Design and optimization of a reactive crystallization process for high purity phosphorus recovery from sewage sludge ash[END_REF]. The mechanism is that CaClH 2 PO 4 ⋅H 2 O is the least soluble P form at a pH range of -3-0 compared to other compounds, except for CaSO 4 . The advantages were the high product purity and the elimination of undesired impurities (e.g., AlPO 4 and FePO 4 ) and heavy metals with a simple setup. However, the overall P recovery Fig. 10. Full-scale plants operating or under planning using wet-chemical techniques for P recovery from sludge-derived ash [START_REF]European Sustainable Phosphorus Platform, ESPP -DPP -NNP phosphorus recovery technology catalogue[END_REF]. t/y = tonnes per year; t/d = tonnes per day. efficiency from ash was relatively low (≤62%), due to the low P extraction efficiency (≤73%) and precipitation efficiency (≤85%), which can be further improved. The process also requires a high temperature (95 • C) to evaporate water to induce the crystallization of CaClH 2 PO 4 ⋅H 2 O, with estimated chemical and energy costs of US$ 5.8/ kg P. The recovered product can be further calcinated to produce CaHPO 4 for market interests [START_REF] Liu | Design and optimization of a reactive crystallization process for high purity phosphorus recovery from sewage sludge ash[END_REF]. However, this approach can hardly be applied unless the following drawbacks are addressed: 1) The total P recovery needs to be improved by process optimization; 2) the overall cost is far beyond the market value of Ca-P (up to US$ 2/kg P) in Table 4; and, 3) the requirements of equipment quality and capital costs are much higher than typical recovery processes due to more corrosive conditions (high temperature and extremely low pH).

Vivianite

P recovery as vivianite has become popular particularly from ironrich municipal sludge, where vivianite accounts for a major P fraction (up to 90%) [START_REF] Wilfert | Vivianite as the main phosphate mineral in digested sewage sludge and its role for phosphate recovery[END_REF]. One significant advantage of vivianite is its foreseeable economic value with a much higher market price than other recovered products as shown in Table 4. Vivianite has also exhibited a great potential to serve as Fe and P fertilizers simultaneously to prevent Fe chlorosis and enhance crop yield as demonstrated by several field studies [START_REF] Yaya | Assessment of the Fertilizing effect of Vivianite on the Growth and yield of the Bean '" phaseolus vulgaris "' on Oxisoils from Ngaoundere (Central North)[END_REF][START_REF] Díaz | Testing the ability of vivianite to prevent iron deficiency in pot-grown grapevine[END_REF]. Lab-scale studies indicated that vivianite crystallization for P recovery was feasible under complex environment, with enhanced precipitation efficiency (nearly 100%) from pH 5 to 8 or Fe:P ratio from 1 to 2 and high purity (e.g., 82% vivianite and 10% silica), whereas the effect of temperature was found insignificant (25-55 • C) [START_REF] Liu | Recovery of phosphate from aqueous solutions via vivianite crystallization: Thermodynamics and influence of pH[END_REF][START_REF] Priambodo | Phosphorus recovery as ferrous phosphate (vivianite) from wastewater produced in manufacture of thin film transistor-liquid crystal displays (TFT-LCD) by a fluidized bed crystallizer (FBC)[END_REF][START_REF] Zhang | Effect of sodium alginate on phosphorus recovery by vivianite precipitation[END_REF]. To the best of knowledge, there is a lack of studies about recovering P as vivianite from sludge-derived ash and hydrochar. This is possibly due to several challenges: 1) Addition of expensive iron(II) salts; 2) prerequisite of reductive and anoxic conditions for recovery; 3) purity demand of product: Vivianite is only metastable at ambient conditions, while impurities may cause full oxidization within 2 days [START_REF] Wu | Potentials and challenges of phosphorus recovery as vivianite from wastewater: A review[END_REF]. To further develop this novel and profitable precipitation approach, future research should focus on optimizing operational parameters, reasonable purification procedures, potential risks of contaminants in recovered products, as well as comprehensive life cycle and economic assessment.

Contaminants in recovered fertilizers

Studies have shown that P in recovered Ca-P and struvite has a high plant availability (>89% of total P) as shown in Table 7. However, along with P recovery from sludge-derived ash and hydrochar (particularly by acidic extraction), concerning contaminants (mainly heavy metals) could also be captured in the final products, which can limit their application to agriculture. Without any pre-treatment or purification, the recovered fertilizer may contain concerning amounts of heavy metals (e.g., Cd, Cu, and Zn) [START_REF] Weigand | RecoPhos: Fullscale fertilizer production from sewage sludge ash[END_REF][START_REF] Gorazda | Fertilisers production from ashes after sewage sludge combustion -A strategy towards sustainable development[END_REF]. Even purified by a significant amount of CER (2 kg/kg ash) prior to precipitation, a considerable quantity of Cu and Zn can still be present [START_REF] Xu | Recovery of phosphorus as struvite from sewage sludge ash[END_REF]. A life cycle assessment of various recovery technologies (e.g., LEACHPHOS®, PASCH, Reco-Phos®, Fertilizer Industry, and EcoPhos®) suggested that the decontamination of heavy metals caused higher environmental impacts (e.g., emissions and energy demand) [START_REF] Amann | Environmental impacts of phosphorus recovery from municipal wastewater[END_REF]. A techno-economic and environmental assessment revealed that P recovery costs were highly dependent on the purity requirements of recovered products, with an estimated additional cost of US$ 1.2-2.4 per capita per year associated with significant depollution of heavy metals [START_REF] Egle | Phosphorus recovery from municipal wastewater: An integrated comparative technological, environmental and economic assessment of P recovery technologies[END_REF]. Several recently developed sequential extraction methods (see Section 4.3) provide promising alternatives for P recovery and depollution with simple procedures [START_REF] Zin | Struvite production from food processing wastewater and incinerated sewage sludge ash as an alternative N and P source: Optimization of multiple resources recovery by response surface methodology[END_REF][START_REF] Liang | A comparison between sulfuric acid and oxalic acid leaching with subsequent purification and precipitation for phosphorus recovery from sewage sludge incineration ash[END_REF][START_REF] Petzet | Recovery of phosphorus and aluminium from sewage sludge ash by a new wet chemical elution process (SESAL-Phos-recovery process)[END_REF][START_REF] Zin | Maximizing ammonium and phosphate recovery from food wastewater and incinerated sewage sludge ash by optimal Mg dose with RSM[END_REF]. Their recovered products showed limited heavy metals contents, all below most regulatory fertilizer standards (Table 7).

Regarding sludge-derived hydrochar, there is a lack of information about heavy metals contents in its recovered fertilizers. Thus, the necessity of purification during recovery remains unknown. Unlike fully combusted ash, hydrochar derived from municipal sludge has been found to contain various persistent organic pollutants, such as polycyclic aromatic hydrocarbons, polychlorinated biphenyls, polychlorinated dibenzo-dioxins/-furans, pharmaceuticals, personal care products [START_REF] Silva Thomsen | Hydrothermal liquefaction of sewage sludge; energy considerations and fate of micropollutants during pilot scale processing[END_REF][START_REF] Gong | Polycyclic aromatic hydrocarbon formation during the gasification of sewage sludge in sub-and supercritical water: Effect of reaction parameters and reaction pathways[END_REF][START_REF] Wiedner | Chemical evaluation of chars produced by thermochemical conversion (gasification, pyrolysis and hydrothermal carbonization) of agro-industrial biomass on a commercial scale[END_REF][START_REF] Heiner Brookman | Influence of hydrothermal carbonization on composition, formation and elimination of biphenyls, dioxins and furans in sewage sludge[END_REF][START_REF] Mitroshkov | Analysis of Perfluorinated, Pharmaceutical, Personal Care Compounds and Heavy Metals in Waste Water Sludge using GC-MS/MS and Multicollector ICP-MS[END_REF]. Organic micropollutants (e.g., benzotriazole and carbamazepine) have been identified in struvite directly recovered from municipal sludge [START_REF] Rastetter | Ecotoxicological assessment of phosphate recyclates from sewage sludges, Water, Air[END_REF]. Therefore, the transformation and risks of organic contaminants should also be investigated during the P recovery from hydrochar. Studies reporting on optimum HTL temperatures/ pressures to destroy trace contaminants of emerging concern in HTL aqueous and hydrochar from municipal sludge are lacking. Due to limited studies, the impacts of organic pollutants are not discussed in this review. Potential contamination from pathogens is not a concern as all microorganisms shall be eliminated because of the thermochemical processes.

Conclusions and prospects

P recovery and recycling are necessary to mitigate environmental challenges (eutrophication), improve water quality, address P scarcity, and sustain local food production in a circular economy approach. Ash and hydrochar from municipal sludge thermal treatment are promising sources for P recovery. Wet chemical extraction and precipitation processes are technically feasible, cost-effective, and reliable for P recovery from ash and hydrochar. However, high concentrations of heavy metals (e.g., up to 10,000 mg/kg of Cu) in ash and hydrochar could complicate the recovery processes. The contents and species of P and co-existing metals (Al, Ca, Fe, and Mg especially) also affect the extraction step and quality of recovered products. This review highlighted the key features, challenges, and recent advances of P recovery from sludgederived ash and hydrochar, and advantages and limitations of different processes are discussed. The major findings of this review summarized below will contribute to the improvement of P recovery from waste streams.

• Considering the largely varied contents of sludge-derived ash (13.5-25 ), establishing regional-specific P recovery process is necessary to ensure stable P recovery performance. • For wet-chemical processes, extraction is the critical first step to maximize P recovery and minimize potential pollution by optimizing operational parameters. Minimal process inputs (e.g., water, chemicals, and energy) should be targeted to improve process sustainability. • In terms of process applicability, acidic extraction (pH < 2) is suitable for all types of ash and hydrochar and could achieve nearly 100% P extraction, while depollution step is often necessary due to the significant heavy metals contamination (e.g., As, Cd, Cu, Pb, and Zn). Pretreatment with chelating agents is not cost-effective for P recovery and heavy metals removal. Alkaline extraction (pH ≥ 12) is only appropriate to feedstocks with high contents of NAIP, but it avoids the contamination of heavy metals (<1% leachable). The recent advances of sequential extraction approaches allow them to be applicable to all feedstocks, with desirable P recovery (up to 91%) and limited heavy metals. Ideally, after leaching heavy metals from ash and hydrochar, the remaining solid residues could be landapplied, and heavy metals in leachates could be precipitated and recovered, thus achieving zero waste. However, the new processes are lacking economic and pilot/full-scale assessments. • Struvite and Ca-P are mostly precipitated from P-rich extracts to produce fertilizers for their high plant-availability (>89% of total P), while optimization is needed for the highest cost-efficiency. Vivianite represents a highly profitable recoverable product, which demands detailed studies to demonstrate its applicability to P recovery from sludge-derived ash and hydrochar. Metal salts (e.g., MgCl 2 , CaCl 2 , and FeCl 2 ) are efficient for precipitation, but they are much more expensive than oxygen-bearing minerals (e.g., MgO and CaO). The combined use of mineral salts and oxides could balance the chemical cost and recovery efficiency. Possible interference by Al 3+ and Fe 3+ cations should be avoided during precipitation to assure the product quality. • Without proper purification steps, recovered P fertilizers from sludge-derived ash could contain concerning amounts of heavy metals (e.g., Cd, Cu, and Zn), while methods using sequential extraction can fulfill the safe metals concentrations of most fertilizer ordinances. However, there is a lack of studies about the potential risks associated with heavy metals and organic contaminants of recovered products from hydrochar.

HTL has been found the most promising technology in handling municipal sludge owing to the potential for commercialization and netpositive energy recovery. It represents a justifiable operation for sludge treatment, as carbon is maximized to biocrude and P is enriched in hydrochar to allow its recovery. P recovery from sludge-derived hydrochar is a critical and necessary step for promoting the implementation of full-scale HTL at wastewater treatment plants. This review underscores the following challenges and prospects of building towards a sustainable waste management system: Integrated wastewater treatment, HTL (energy recovery), and nutrient recovery.

• Feedstock-specific processes using low-cost (e.g., H 2 SO 4 ) and effective (e.g., oxalic acid) extractants should be developed for P extraction. The optimal ratios of combined acids for extraction can be investigated to explore any possible synergistic effects. An alternative technically and economically feasible purification process (e.g., sequential extraction and cost-effective adsorption) is encouraged to address the contamination issue in acidic extraction. • Sequential extraction processes (A-E) are promising to extract most P with limited heavy metals, preventing additional costs for purification. However, the extraction efficiency and applicability can be further improved to accommodate hydrochar with various compositions. For Processes A and B, the requirements on hydrochar compositions (e.g., Al contents) should be identified for the maximum P extraction efficiency. For Process C, raising the pH from 3 to 5 during the pre-precipitation step could enhance the overall P extraction efficiency, and removing the step of cation exchange without lowering the product purity is necessary to reduce the cost and complexity. For Process D, cost-effective chelating agents (e.g., optimal combination of different extractants) should be discovered to reduce P loss, enhance heavy metals removal, and minimize the cost. For Process E, improving the adsorption capacity and examining the reusability of activated alumina are needed for better costeffectiveness. • The plant availability of P in recovered fertilizers through agronomic evaluation by pot or field trials is beneficial to their agricultural application and public acceptance. The fate and transport modeling of heavy metals and organic pollutants in recovered products should be comprehensively examined for the determination of ecological risk assessment. Detailed techno-economic and environmental assessments focusing on various wet-chemical approaches for P recovery are essential to identify the advantages and weaknesses of each process and guide and facilitate their improvement. • Discovering the ultimate disposal/utilization routes of hydrochar residue after recovery of P and heavy metals, recycling, and disposal of post-recovery process water can facilitate the complete cycle of sustainable waste management. To determine the usability of postrecovery hydrochar, the following examinations can be considered: Agricultural application as soil amendment, adsorption as a direct adsorbent or activated hydrochar, and heterogeneous catalysis as magnetic hydrochar. • The optimization of process design and phase separation of HTL is necessary to allow most energy in biocrude and P in hydrochar. P recovery methods should be developed specifically based on the optimized HTL treatment for maximum energy recovery. The life cycle assessment and cost analysis of HTL combined with nutrient recovery can illustrate the environmental and economic benefits of upgrading wastewater treatment plants into wastewater biorefineries. • Legislation is a key driver in promoting P recovery and recycling. Environmental regulations of stringent discharge limit and mandatory P removal could concentrate P into the sludge stream and its derivative hydrochar. Policies of facilitating the use of recovered fertilizers, such as tax-free and renewable resources credits, could stimulate the development of sustainable and innovative P recovery technologies and their full-scale applications.

Fig. 5 .

 5 Fig. 5. Distribution plot of heavy metals in (a) sludge-derived ash [6,31,48,50-55,57-67,82,83] and (b) sludge-derived hydrochar[START_REF] Marrone | Bench-scale evaluation of hydrothermal processing technology for conversion of wastewater solids to fuels[END_REF][START_REF] Shi | Hydrothermal conversion of dewatered sewage sludge: Focusing on the transformation mechanism and recovery of phosphorus[END_REF][START_REF] Shah | Hydrothermal liquefaction of high ash containing sewage sludge at sub and supercritical conditions[END_REF][START_REF] Conti | Valorization of animal and human wastes through hydrothermal liquefaction for biocrude production and simultaneous recovery of nutrients[END_REF][START_REF] Wang | Hydrothermal treatment coupled with mechanical expression at increased temperature for excess sludge dewatering: Heavy metals, volatile organic compounds and combustion characteristics of hydrochar[END_REF][START_REF] Ekpo | A comparison of product yields and inorganic content in process streams following thermal hydrolysis and hydrothermal processing of microalgae, manure and digestate[END_REF][START_REF] Zhang | Efficient phosphorus recycling and heavy metal removal from wastewater sludge by a novel hydrothermal humification-technique[END_REF][START_REF] Zhang | The hydrochar characters of municipal sewage sludge under different hydrothermal temperatures and durations[END_REF][START_REF] Zhang | A two-step process for sewage sludge treatment: Hydrothermal treatment of sludge and catalytic hydrothermal gasification of its derived liquid[END_REF][START_REF] Breulmann | Process conditions of pyrolysis and hydrothermal carbonization affect the potential of sewage sludge for soil carbon sequestration and amelioration[END_REF][START_REF] Wang | Pyrolysis study of hydrothermal carbonization-treated digested sewage sludge using a Py-GC/MS and a benchscale pyrolyzer[END_REF][START_REF] Leng | Bio-char derived from sewage sludge by liquefaction: Characterization and application for dye adsorption[END_REF][START_REF] Escala | Hydrothermal carbonization as an energy-efficient alternative to established drying technologies for sewage sludge: A feasibility study on a laboratory scale[END_REF][START_REF] Liu | Effect of hydrothermal carbonization on migration and environmental risk of heavy metals in sewage sludge during pyrolysis[END_REF][START_REF] Wang | Influence of pyrolysis temperature on characteristics and environmental risk of heavy metals in pyrolyzed biochar made from hydrothermally treated sewage sludge[END_REF][START_REF] Xu | The fate of heavy metal during subcritical and supercritical water gasification of sewage sludge[END_REF].

FeC 6 H

 6 5 O 7 -) and therefore improve the solubility of Ca-P and Fe-P and extracted P[START_REF] Iwama | Extraction of phosphorus and recovery of phosphate from steelmaking slag by selective leaching[END_REF][START_REF] Du | Effects of cooling rate and acid on extracting soluble phosphorus from slag with high P2O5 content by selective leaching[END_REF]. It should be noted that a much larger amount of citric acid is required to achieve the low pH due to its weak strength (K a1 = 8.4 × 10 -3 ) compared to oxalic acid (K a1 = 5.9 × 10 -2 ) and other strong acids. Regardless of other conditions, P extraction efficiency decreases with the increase of pH and remains minimum within the pH of 4-10[START_REF] Petzet | On wet chemical phosphorus recovery from sewage sludge ash by acidic or alkaline leaching and an optimized combination of both[END_REF][START_REF] Fang | Phosphorus recovery and leaching of trace elements from incinerated sewage sludge ash (ISSA)[END_REF][START_REF] Lee | Identification of phosphorus forms in sewage sludge ash during acid pre-treatment for phosphorus recovery by chemical fractionation and spectroscopy[END_REF]. The Cracow University of Technology developed a patented method (PolFerAsh) of using a high concentration of H 3PO 4

  7-4.1) during extraction. Despite the variance among different acids, controlling pH of extraction (or post extracts) is more effective in assuring P extraction efficiency.

Fig. 7 .

 7 Fig. 7. Linear fit of actual P extraction efficiency in terms of H + /P molar ratio (a) and predicted extraction efficiency (b) from ash[START_REF] Takahashi | Technology for recovering phosphorus from incinerated wastewater treatment sludge[END_REF][START_REF] Donatello | Production of technical grade phosphoric acid from incinerator sewage sludge ash (ISSA)[END_REF][START_REF] Fang | Recovery of phosphorus from incinerated sewage sludge ash by combined two-step extraction and selective precipitation[END_REF] and hydrochar[START_REF] Ovsyannikova | Feedstock-dependent phosphate recovery in a pilot-scale hydrothermal liquefaction bio-crude production[END_REF] using H 2 SO 4 . Predicted extraction efficiency (%) = used amount of H 2 SO 4 (kg)/required amount of H 2 SO 4 (kg) × 100. Required amount of H 2 SO 4 is estimated by Eq. (7).

  remove interference ions before precipitation. It was suggested that oxalic acid extraction could lead to unremovable Al and Fe ions by CER because they formed Al(C 2 O 4 ) 2 -

Table 1

 1 Concentrations (mg/kg) of heavy metals in phosphate rock from various regions[START_REF] Weigand | RecoPhos: Fullscale fertilizer production from sewage sludge ash[END_REF][START_REF] Gorazda | Fertilisers production from ashes after sewage sludge combustion -A strategy towards sustainable development[END_REF].

	Country	As	Cd	Cr	Cu	Hg	Ni	Pb	Zn	U
	China	9-26	0-2.5	18-33	-	0.005-0.211	-	1.5-6	-	23-31
	Morocco	1.3-13	15-38	75-291	1-22	0.04-0.855	26	7-30.1	172-345	75-155
	Middle East	2.4-35	1.5-35	25-230	5-31	0.002-0.02	20-80	1-33	29-630	40-170
	Russia	1-10	0.1-1.3	13-23	15-30	0.004-0.01	2-15	1.8-33	19-23	10-85
	US	7-14	6.1-92	60-637	9.6-23	0.05-0.233	17-37	4.6-17	204-403	65-180

  . It was also reported that citric acid could form metal complexes (e.g., CaC 6 H 5 O 7 -, FeC 6 H 5 O 7 , FeHC 6 H 5 O 7 , and

Table 2

 2 Summary of recent studies about P extraction from ash and hydrochar derived from municipal sludge (extractants in the order of decreasing acidity or increasing pH) a .

	Extractants	Feedstock	Extraction conditions	Pre-pH	Post-pH	PEE (%)	Leached heavy metals (%)					Ref.
							As	Cd	Cr	Cu	Ni	Pb	Zn
	HCl	Ash	0.1 N, 10 mL/g, 2 h	1.1	3.1	16	-	11	0.5	15	8	ND	16	[31]
			0.5 N, 10 mL/g, 2 h	0.4	1.5	91	-	29	5	41	11	35	33
			0.8 N, 10 mL/g, 0.16 h	-	2	88	-	-	-	-	-	-	-	[51]
			5.2 N, 3 mL/g, 0.16 h	-	-1.22	72	-	-	-	-	-	-	-	[82]
			3.9 N, 2.5 mL/g, 0.16 h	-	-0.63	73	-	-	-	-	-	-	-
			0.5 N, 25 mL/g, 2 h	-	-	80	-	-	-	-	-	-	-	[123]
			0.8 N, 25 mL/g, 2 h	-	-	83	-	-	-	-	-	-	-
			0.4 N, 10 mL/g, 24 h	-	3.22	0.7	-	65	0.5	42	16	0.7	26	[106]
			0.4 N, 75 mL/g, 2 h	-	-	95	-	-	-	-	-	-	-	[114]
			0.2 N, 50 mL/g, 2 h	-	-	87-98	-	-	4-24	60-63	0-63	42-57	15-86	[55]
		Hydrochar	1 N, 10 mL/g, 2 h	-	-	98	-	-	-	-	-	-	-	[22]
	H 2 SO 4	Ash	0.1 N, 10 mL/g, 2 h	1.2	3.1	16	-	12	0.5	16	8	ND	17	[31]
			0.5 N, 10 mL/g, 2 h	0.6	1.8	90	-	30	5	42	11	3	36
			0.2 N, 20 mL/g, 2 h	1.3	3.2	19	53	-	-	35	6	1	27	[64]
			0.4 N, 20 mL/g, 2 h	1.26	2.1	100	100	-	-	43	7	10	32
			1.0 N, 20 mL/g, 2 h	0.9	1.1	107	100	-	-	43	9	15	32
			0.38 N, 20 mL/g, 2 h	-	-	72-91	-	-	-	-	-	-	0-56	[63]
			0.5 N, 30 mL/g, 2 h	-	-	82	-	-	-	-	-	-	-	[62]
			0.3 N, 50 mL/g, 2 h	-	-	65-85	-	-	-	-	-	-	-	[124]
			0.4 N, 20 mL/g, 2 h	-	-	94	10	-	1	36	15	10	33	[116]
			0.38 N, 20 mL/g, 2 h	-	-	100	-	-	-	78	-	-	-	[83]
			0.38 N, 20 mL/g, 2 h	-	-	99	-	-	-	60	-	-	-
			0.2 N, 20 mL/g, 2 h	-	-	88	-	-	-	12	-	-	7	[125]
			0.6 N, 20 mL/g, 0.5 h	-	-	89-90	-	-	-	60-66			31-50	[66]
		Hydrochar	1,000 mL/g, 8 h	-	2	80-92	-	-	-	-	-	-	-	[110]
			0.2 N, 10 mL/g, 2 h	-	-	23	-	-	-	-	-	-	-	[22]
			1 N, 10 mL/g, 2 h	-	-	100	-	-	-	-	-	-	-
	HNO 3	Ash	0.1 N, 10 mL/g, 2 h	1.0	2.9	17	-	11	0.5	15	7	ND	17	[31]
			0.5 N, 10 mL/g, 2 h	0.3	1.4	88	-	27	4.5	37	10	21	33
			0.1 N, 20 mL/g, 2 h	1.2	3.4	3	20	-	-	20	4	1	23	[64]
			0.2 N, 20 mL/g, 2 h	0.88	2.2	83	28	-	-	40	6	18	14
			0.5 N, 20 mL/g, 2 h	0.85	1.0	101	43	-	-	42	7	49	32
			2.7 N, 2.3-3.7 mL/g, 2 h	-	0.13-1.1	80-96	-	-	-	-	-	-	-	[126]
	H 3 PO 4	Ash	8.1 N, 4.3 mL/g, 0.5 h	-	-	-50-92	-	54-71	22-37	37-68	19-45	-	-	[48]
	Oxalic acid	Ash	0.1 N, 10 mL/g, 2 h	1.5	4.1	18	-	1	0.5	17	6	ND	14	[31]
			0.5 N, 10 mL/g, 2 h	1.0	2.2	95	-	6	6	35	9	ND	33
			0.2 N, 20 mL/g, 2 h	1.5	4.6	32	88	-	-	33	5	12	23	[64]
			0.4 N, 20 mL/g, 2 h	1.3	2.4	96	91	-	-	35	6	21	23
			1.0 N, 20 mL/g, 2 h	1.2	1.2	104	93	-	-	37	6	39	24
			0.5 N, 30 mL/g, 2 h	-	-	93	-	-	-	-	-	-	-	[62]
			0.34 N, 50 mL/g, 2 h	-	-	92-98	-	-	-	-	-	-	-	[124]
		Hydrochar	1,000 mL/g, 8 h	-	2	84-95	-	-	-	-	-	-	-	[110]
	Citric acid	Ash	0.1 N, 10 mL/g, 2 h	2.3	3.9	13	-	8	0.5	13	7	1	15	[31]
			0.5 N, 10 mL/g, 2 h	1.9	2.7	21	-	12	1	17	8	1	20
			0.3 N, 20 mL/g, 2 h	2.0	3.6	52	74	-	-	30	4	1	18	[64]
			0.6 N, 20 mL/g, 2 h	1.8	3.1	75	99	-	-	43	9	4	32
			1.5 N, 20 mL/g, 2 h	1.7	2.6	105	100	-	-	73	56	12	73
		Hydrochar	10 mL/g, 2 h	-	2	59-95	-	-	-	-	-	-	-	[127]
			1 N, 10 mL/g, 2 h	-	-	60	-	-	-	-	-	-	-	[22]
	Gluconic acid	Ash	0.1 N, 10 mL/g, 2 h	2.4	4.0	10	-	7	0.5	12	6	6	13	[31]
			0.5 N, 10 mL/g, 2 h	2.0	2.9	21	-	11	1	17	7	13	21
	Formic acid	Ash	0.1 N, 10 mL/g, 2 h	2.4	3.5	17	-	7	ND	10	7	ND	12	[31]
			0.5 N, 10 mL/g, 2 h	1.9	2.7	11	-	9	ND	14	8	ND	16
	Acetic acid	Ash	0.1 N, 10 mL/g, 2 h	2.9	4.1	5	-	3	ND	5	3	ND	4	[31]
			0.5 N, 10 mL/g, 2 h	2.5	3.6	9	-	7	ND	10	7	ND	11
	EDTMP	Ash	0.04 N, 20 mL/g, 2 h	2.1	4.1	19	51	-	-	10	1	14	11	[64]
			0.08 N, 20 mL/g, 2 h	1.9	3.5	20	77	-	-	14	3	23	18
			0.2 N, 20 mL/g, 2 h	1.7	2.6	26	95	-	-	22	5	33	23
			0.2 N, 10 mL/g, 2 h	1.3	2.5	15	-	-	-	10	-	-	10	[107]
	EDTA	Ash	0.04 N, 20 mL/g, 2 h	4.8	6.5	9	70	-	-	7	3	14	14	[64]
			0.08 N, 20 mL/g, 2 h	4.7	6.2	18	86	-	-	12	4	19	17
			0.2 N, 20 mL/g, 2 h	4.7	5.5	37	81	-	-	20	5	24	18
			0.2 N, 10 mL/g, 2 h	4.7	5.5	25	-	-	-	5	-	-	15	[107]
	Na2-EDTA	Ash	0.1 N, 10 mL/g, 2 h	4.5	5.4	8	-	6	ND	12	5	13	11	[31]
			0.5 N, 10 mL/g, 2 h	4.5	5.0	12	-	8	ND	14	7	17	16
	Na 2 -oxalate	Ash	0.1 N, 10 mL/g, 2 h	8.6	8.5	1	-	ND	ND	5	ND	ND	1
			0.5 N, 10 mL/g, 2 h	10.2	9.1	2	-	ND	ND	5	ND	ND	1
	Na 4 -EDTA	Ash	0.1 N, 10 mL/g, 2 h	10.4	9.8	6	-	ND	ND	8	ND	10	4
			0.5 N, 10 mL/g, 2 h	9.9	10.1	14	-	3	ND	9	ND	10	5
	EDTA + NaOH	Ash	0.1 N, 10 mL/g, 2 h	10.9	10.0	6	-	ND	ND	8	ND	0.5	4
			0.5 N, 10 mL/g, 2 h	10.7	10.4	13	-	ND	ND	8	ND	0.5	5
	NTA + NaOH	Ash	0.1 N, 10 mL/g, 2 h	10.9	10.2	3	-	ND	0.5	7	ND	7	3
			0.5 N, 10 mL/g, 2 h	10.8	9.9	5	-	ND	0.1	7	ND	8	4

(continued on next page)

Table 2 (

 2 et al. reported that continued )

	Extractants	Feedstock	Extraction conditions	Pre-pH	Post-pH	PEE (%)	Leached heavy metals (%)					Ref.
							As	Cd	Cr	Cu	Ni	Pb	Zn
	KOH	Hydrochar	100 mL/g, 16 h	-	11	32	-	-	-	-	-	-	-	[97]
				-	12	90-96	-	-	-	-	-	-	-
	NaOH	Ash	0.1 N, 10 mL/g, 2 h	12.6	12.5	1	-	ND	ND	ND	ND	0.5	0.5	[31]
			0.5 N, 10 mL/g, 2 h	13.0	12.7	34	-	ND	ND	0.1	ND	ND	1
			0.5 N, 25 mL/g, 2 h	-	-

Table 3

 3 Evaluations of the state of various sequential P extraction processes.

	Processes	Advantages	Challenges
	A) SESAL-	o Limited extraction of	• Feedstock selective (efficient
	Phos	heavy metals	only for Al-rich material)
		o No required separation of	• Low P extraction efficiency (up
		interfering metals from	to 78%)
		extracts	• Lack of economic analysis
		o Reduced chemical	• Unknown disposal of ash residue
		demand
		o Reusable alkalic extracts
		as Al-precipitant
	B) Acid-base	o Modified from SESAL-	• Feedstock selective
	Leaching	Phos	• Additional chemical demand for
		o Improved P extraction	pH control
		efficiency (up to 86%)	• Unknown disposal of ash residue
		o Heavy metals in	• Detailed cost analysis needed
		recovered fertilizer
		below regulatory limits
		o Possible financial return
		(limited study)
	C) Three-step	o Applicable for Al/Fe-rich	• Complex process
	Extraction	material	• Extra chemical, equipment, and
		o Desirable P extraction	energy demand
		efficiency (80%)	• Additional cost for resin
		o Further removal of Al/Fe	regeneration (optional if no
		ions from extract	requirement on product purity)
		o Heavy metals in	• Lack of economic analysis
		recovered fertilizer	• Unknown disposal of byproducts
		below regulatory	(ash residue and process water)
		standards
	D) Two-step	o No requirement on	• Low P extraction efficiency
	Extraction	feedstock	(73%) due to elution loss
		o Less extracted heavy	• Ordinary removal of heavy
		metals than acidic	metals
		extraction	• Low purity of recovered material
		o Possible financial return	• Expensive chemical (EDTA)
		(limited study)	• Detailed economic analysis
		o Desirable chemical	needed
		demand	• Unknown disposal of byproducts
			(ash residue and process water)
	E) Alumina	o No requirement on	• High demand on adsorbent
	Adsorption	feedstock	• Lack of comprehensive study
		o Desirable P extraction	• Unknown reusability of
		efficiency (>91%)	adsorbent
		o Low heavy metals	• Lack of economic analysis
		contents in extracts	• Unknown disposal of byproducts
		o Reusable sorbent and	(ash residue and leachate)
		process water

Table 4

 4 Comparison of P recovery crystallization processes[START_REF] Wu | Potentials and challenges of phosphorus recovery as vivianite from wastewater: A review[END_REF][START_REF] Peng | A comprehensive review of phosphorus recovery from wastewater by crystallization processes[END_REF][START_REF] Li | Phosphorus recovery through struvite crystallisation: Recent developments in the understanding of operational factors[END_REF][START_REF] Desmidt | Global phosphorus scarcity and full-scale P-recovery techniques: A review[END_REF][START_REF] Lc | Next generation calcium phosphate-based biomaterials[END_REF][START_REF] Liu | Recovery of phosphate from aqueous solutions via vivianite crystallization: Thermodynamics and influence of pH[END_REF][START_REF] Pham | Hydroxyapatite gel for the improved removal of Pb2+ ions from aqueous solution[END_REF].

	Recovery	Struvite	Calcium phosphates a	Vivianite
	considerations
	Precipitation
	equations	Mg 2+

Table 6

 6 P recovery through wet-chemical extraction and precipitation from sludge-derived ash and hydrochar a .

	Feedstock	Extractants	Precipitants	Precipitation conditions	Recovered products	PPE	PRE	Ref.
						(%)	(%)
	Ash	0.4 N H 2 SO 4	NaOH	pH = 4.4-6.5	AlPO 4 , Ca 3 (PO 4 ) 2 , FePO 4	> 95	-	[116]
		0.08 N EDTA pre-eluted,		pH = 4.4-6.3		> 93	-
		followed by 0.4 N H 2 SO 4
		0.4 N H 2 SO 4	Ca(OH) 2	pH > 5.1		> 99	-
		0.08 N EDTA pre-eluted,		pH > 5.0		100	-
		followed by 0.4 N H 2 SO 4
		2.7 N HNO 3	6% lime milk	pH = 7.3	Dicalcium phosphate	> 90	-	[122]
		8.1 N H 3 PO 4		pH = 7.0		> 90	-
		8.1 N H 3 PO 4	Hydrated lime	pH = 6	Dicalcium phosphate	100	-	[48]
		5.2 N HCl	CaCl 2	pH = -1.5	Monocalcium	85	62	[82]
		5.2 N HCl	Ca(OH) 2	pH = -1.2	chlorophosphate	51	37
		3.9 N HCl	CaCl 2	pH = -1.1		70	51
		HCl pre-eluted (pH = 3),	CaCl 2	Ca:P = 1:1.5	Hydroxyapatite	89.6	60	[106]
		followed by NaOH (pH = 13)
		0.5 N H 2 SO 4	MgCl 2 ⋅6H 2 O and NH 4 Cl	Mg:N:P = 1:1:1, pH = 9.0 (purified by	Struvite	98.3	69.1	[62]
				CER)
		0.5 N oxalic acid	NaOH	pH = 9.0	Aluminum and iron	78.8	65.5
		HCl pre-eluted (pH = 4),	MgCl 2 ⋅6H 2 O and NH 4 + -	Mg:N:P = 2:0.6:1, pH = 9.28	hydroxyphosphates Struvite	99.9	72.7	[53]
		followed by 1 N NaOH HCl pre-eluted (pH = 4),	rich food wastewater MgCl 2 ⋅6H 2 O and NH 4 + -	Mg:N:P = 1.59:0.6:1, pH = 9.63	Struvite	97.9	70.9	[135]
		followed by 1 N NaOH	rich food wastewater
		0.2 N H 2 SO 4	MgCl 2 ⋅6H 2 O and NH 4 Cl	Mg:N:P = 1:1:1, pH = 9.5 (purified by	Struvite	99.2	79.7	[125]
				0.5 g/mL CER)
		0.2 N HCl	MgCl 2 ⋅6H 2 O and NH 4 Cl	Mg:N:P = 1.6:1.6:1, pH = 10 (purified	Struvite	97.2	85-95	[55]
				by 0.04 g/mL CER)
	Hydrochar	1 N HCl + activated alumina,	Ca(OH) 2	pH = 9	Calcium phosphate	> 97	> 85	[96]
		followed by 0.1 N NaOH Citric acid (pH = 2)	MgCl 2 and NH 4 + -rich HTC	Mg:N:P = 1.3:1:1, pH = 9	Struvite	87	-	[127]
			aqueous
		KOH (pH = 12)	MgCl 2 ⋅6H 2 O	Mg:P = 2:1, pH = 11 (EDTA chelated to	K-struvite	92	88	[97]
				prevent interferential precipitation)
		H 2 SO 4 (pH = 2)	MgCl 2 ⋅6H 2 O and NH 4 + -	Mg:P = 2.5:1, pH = 11 (EDTA chelated) Mg:N:P = 2.6:1.1:1, pH = 9	Struvite	99 99	95 50.7	[22]
			rich HTL aqueous

a PPE = phosphorus precipitation efficiency (from solution); PRE = phosphorus recovery efficiency (from feedstock); Ref. = references; CER = cation exchange resin; EDTA = ethylenediaminetetraacetic acid; HTC = hydrothermal carbonization; HTL = hydrothermal liquefaction; "-" donates "not available".

Table 5

 5 Commercial precipitants used for P crystallization[START_REF] Wu | Potentials and challenges of phosphorus recovery as vivianite from wastewater: A review[END_REF][START_REF] Peng | A comprehensive review of phosphorus recovery from wastewater by crystallization processes[END_REF][START_REF] Li | Phosphorus recovery through struvite crystallisation: Recent developments in the understanding of operational factors[END_REF].

	Type	Precipitants	Contents	Solubility in water (g/ L) a	Costs (US $/mol) b	Remarks
	Oxygen-bearing	Magnesia	94% MgO	0.09 (30 • C)	0.006-0.008	• Low-cost but less efficient
	minerals	Brucite	90% Mg(OH) 2	0.009 (18 • C)	0.009-0.013	• More alkaline, reducing alkalinity consumption
		Quicklime	90% CaO	1.19 (25 • C)	0.004-0.006	• Poorly soluble, applicable for acidic extracts
		Hydrated lime	90% Ca(OH) 2	1.59 (25 • C)	0.007-0.008
	Salts	Magnesium chloride	99%	543 (20 • C)	0.027-0.041	• Efficient but expensive
		hexahydrate	MgCl 2 ⋅6H 2 O			• More acidic, requiring additional alkalinity for pH
		Magnesium sulphate	99%	360 (20 • C)	0.020-0.037	adjusting
		heptahydrate	MgSO 4 ⋅7H 2 O			• Highly soluble, appropriate for both acidic and
		Calcium chloride	94% CaCl 2	813 (25 • C)	0.014-0.019	alkalic extracts
		Ferrous sulphate	98%	256 (20 • C)	0.017-0.023
		heptahydrate	FeSO 4 ⋅7H 2 O
		Ferrous chloride tetrahydrate	98% FeCl 2 ⋅4H 2 O	625 (20 • C)	0.020-0.041
	a Data obtained from PubChem [142].

Table 7

 7 Contents of P and heavy metals in recovered fertilizers from sludge-derived ash compared to commercial fertilizer (triple superphosphate) a . Or 50 mg/kg P 2 O 5 when P 2 O 5 > 5%.

	Recovered products	Country	Total P (%	Plant-available P (%	Heavy metals (mg/kg)						Ref.
			P 2 O 5 )	P 2 O 5 )	As	Cd	Cr	Cu	Hg	Ni	Pb	Zn
	Dicalcium phosphate	Germany Poland	40.3 42.85 44.77	38.0 b 38.22 b 39.48 b	9.1 --	2.16 52.17 16.49	118 --	663 --	0.7 --	47.3 --	51.4 11.97 0.008	1,580 --	[6] [48]
	Hydroxyapatite Struvite	Germany Korea Korea	35 24.2 28.2	-23.8 c 27.9 c	27.93 2.7 28	0.07 ND 1	ND 5.0 ND	4.5 5.5 3	---	0.20 3.2 ND	3.37 ND ND	20.73 9.0 44	[106]* [53]* [135]*
		China	32.6	-	-	-	-	10.6	-	-	-	5.4	[62]*
		China	-	-	-	-	-	150-800	-	-	-	260-350	[55]
		China	-	-	16	-	-	22	-	-	12	28	[125]
	Monocalcium	Denmark	34	-	1	<0.013	<0.2	2.8	0.084	0.94	1.1	5.4	[82]
	chlorophosphate Commercial fertilizer Fertilizer ordinance by	-Germany	51.0 -	46.9 b -	7.5 40	4.2 1.5 d	121.2 300	13 70	-1	41.2 80	3.1 150	182.9 1,000	[155] [6,19]
	country	Poland	-	-	50	50	-	-	2	-	140	-	[48]
		Austria	-	-	-	15	667	778	1	-	100	3,333	[55]
		Switzerland	-	-	-	3	200	400	3	50	200	1,300	[19,55]
		Korea	-	-	45	5	250	400	-	45	130	1,000	[53]
		China	-	-	-	10	500	-	5	-	150	-	[55]

a Ref. = references; ND = non-detected; "-" donates "not available"; "*" donates "studies using sequential extraction". b Ammonium citrate soluble. c Citrate acid soluble. d

  .7% P 2 O 5 , 5.6-21.1% Al 2 O 3 , 8.4-20.8% CaO, and 4.3-20% Fe 2 O 3 ) and hydrochar (5.2-13.6% P 2 O 5 , 0.6-17% Al 2 O 3 , 1.4-9.2% CaO, and 1.8-13.7% Fe 2 O 3

Acknowledgments

This research was funded by the Natural Sciences and Engineering Research Council of Canada (NSERC) and Metro Vancouver Industrial Research Chair Program in Advanced Resource Recovery from Wastewater (IRCPJ 548816-18). The authors would also like to thank the anonymous reviewers for their insightful comments and suggestions, which greatly helped the improvement of the manuscript.

a Extraction conditions include extractant concentration (N), liquid to solid ratio (mL/g), and contact time (h).

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.