Olumide Wesley Awe

Yaqian Zhao

Ange Nzihou

Doan Pham

Nathalie Lyczko

Anaerobic co-digestion of food waste and FOG with sewage sludge -realising its potential in Ireland

Keywords: ABP, anaerobic by-products, AD, anaerobic digestion, AS, activated sludge, ASBR, anaerobic sequential batch reactor, BMW, biodegradable municipal solid waste, BMP, bio-methane potential, BNR, biological nutrient removal, BOD, biochemical oxygen demand, CGY, crude glycerine, CH 4 , methane, C/N, carbon nitrogen ratio, COD, chemical oxygen demand, CO 2 , carbon dioxide, CSTR, continuous stirred tank reactor, DM, dissolved matter, DS, dissolved solids, FFA, free fatty acid, FOG, fat, oil

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Introduction

Anaerobic digestion (AD) has been applied for over a century at wastewater treatment plants (WWTP) for stabilizing excess organic sludge and for biogas production. Growing interest in using this technology for treating other organic solids (organic waste and energy crops) comes with stricter regulations on organic waste disposal as well as the need to find alternative sources of energy [START_REF] Mata-Alvarez | Codigestion of solid wastes: A review of its uses and perspectives including modeling[END_REF][START_REF] Astals | Identification of synergistic impacts during anaerobic co-digestion of organic wastes[END_REF][START_REF] Silvestre | Optimisation of sewage sludge anaerobic digestion through co-digestion with OFMSW: Effect of collection system and particle size[END_REF][START_REF] Maragkaki | Pilot-scale anaerobic co-digestion of sewage sludge with agro-industrial by-products for increased biogas production of existing digesters at wastewater treatment plants[END_REF]. The urgent need for renewable energy generation and the need to divert biodegradable waste from landfill have pushed AD into the market for treatment of the organic fraction of municipal solid waste (OFMSW). Food waste (FW) is the most challenging biodegradable fraction of this because of its high moisture content and readily biodegradable nature [START_REF] Davidsson | Codigestion of grease trap sludge and sewage sludge[END_REF][START_REF] Palatsi | Anaerobic digestion of slaughterhouse waste: Main process limitations and microbial community interactions[END_REF][START_REF] Girotto | Food waste generation and industrial uses: A review[END_REF][START_REF] Xie | Synergistic effect from anaerobic co-digestion of sewage sludge and organic wastes[END_REF].

Generally, mono-digestion of single substrate has problems which include the seasonal nature of agricultural waste and variability of food waste. Sewage sludge has low organic loads. Municipal organic waste contains improper materials and is high in metal content. Animal manure is known to contain a high nitrogen concentration. Wasted oil is high in lipids. There can be long chain fatty acid (LCFA) production and inhibitions of methanogenic bacterial communities [START_REF] Astals | Identification of synergistic impacts during anaerobic co-digestion of organic wastes[END_REF][START_REF] Zonta | Modelling inhibitory effects of long chain fatty acids in the anaerobic digestion process[END_REF][START_REF] Edwards | Anaerobic co-digestion of municipal food waste and sewage sludge: A comparative life cycle assessment in the context of a waste service provision[END_REF]. Anaerobic co-digestion can solve most of these problems by co-digesting two or more organic waste streams for optimisation and increased biogas yield and kinetic performance. It is important to select the best co-substrate and blend ratio, to favour synergy, dilute harmful compounds, optimise methane production and maintain digestate quality [START_REF] Astals | Identification of synergistic impacts during anaerobic co-digestion of organic wastes[END_REF][START_REF] Xie | Synergistic effect from anaerobic co-digestion of sewage sludge and organic wastes[END_REF].

The key to successful anaerobic co-digestion is the substrate, especially its composition and the proportion of each substrate in the feed mixture. It is vital to obtain the best chemical composition in order to optimise the activity of the biomass in the anaerobic process such as carbon-nitrogen ratio, pH, alkalinity, etc. [START_REF] Silvestre | Biomass adaptation over anaerobic co-digestion of sewage sludge and trapped grease waste[END_REF][START_REF] Wang | Impacts of inoculum pretreatments on enzyme activity and biochemical methane potential[END_REF]. It is also important to avoid the inhibition of different components like ammonia, volatile fatty acids (VFAs), intermediate products, optimised biogas production and final effluent in the dewatering process [START_REF] Astals | Identification of synergistic impacts during anaerobic co-digestion of organic wastes[END_REF][START_REF] Xie | Synergistic effect from anaerobic co-digestion of sewage sludge and organic wastes[END_REF]. Nghiem et al. [START_REF] Nghiem | Full scale co-digestion of wastewater sludge and food waste: Bottlenecks and possibilities[END_REF] argue that technical and economic constraints may limit full-scale SS co-digestion operations. There are confusing laws in relation to SS and co-digestion with other waste, especially FW, meat and animal product, etc. in WWTP. These all create a major non-technical limitation coupled with waste availability and the lack of knowledge about adequate co-digestion strategies or their impact on the overall operation of a WWTP [START_REF] Silvestre | Optimisation of sewage sludge anaerobic digestion through co-digestion with OFMSW: Effect of collection system and particle size[END_REF][START_REF] Edwards | Anaerobic co-digestion of municipal food waste and sewage sludge: A comparative life cycle assessment in the context of a waste service provision[END_REF].

Ireland's major agro-industrial units such as wineries, breweries, distilleries, cheese factories, other food factories and livestock units produce wastes, with high organic load, that could be used for anaerobic digesters in functioning WWTPs that produce biogas in significant quantities. Thus, co-digestion could be an opportunity for the water industry in Ireland to increase its renewable energy generation, without any additional facility. Revenue can be generated through gate fees or service charges [START_REF] Edwards | A review of policy drivers and barriers for the use of anaerobic digestion in Europe, the United States and Australia[END_REF]. This could reduce the environmental footprint. At the same time, the application of co-digestion of FW with SS will help Ireland to meet the EU Landfill Directive [START_REF] Directive | Council Directive 1999/31/EC on the landfill[END_REF], which requires the reduction of biodegradable waste in landfill to 35% by 2020, based on the 1995 levels. Further, co-digestion could contribute to the achievement of targets set by the Climate Change Act and the EU Renewable Energy Directive [START_REF] Parliament | Directive 2009/28/EC of the European Parliament and of the Council of 23 April 2009[END_REF]. The Climate Change Act requires the reduction of carbon emissions by 34% by 2020, based on the 1990 levels. The EU Renewable Energy Directive requires 16% of Ireland's total energy to come from renewable sources by 2020 [START_REF] Parliament | Directive 2009/28/EC of the European Parliament and of the Council of 23 April 2009[END_REF][START_REF]Energy in Ireland 1990-2015[END_REF][20][START_REF]The Environment in your pocket[END_REF][START_REF]Guidelines to Defra / DECC's GHG Conversion Factors for Company Reporting: Methodology Paper for Emission Factors[END_REF].

Several studies have examined the benefits of FW co-digestion with SS, but only a few have examined the constraints. Optimal operational conditions in terms of percentages of co-substrates cannot be univocally defined but should be investigated for each specific case [START_REF] Ponsá | Anaerobic co-digestion of the organic fraction of municipal solid waste with several pure organic co-substrates[END_REF][START_REF] Esposito | Bio-methane potential tests to measure the biogas production from the digestion and co-digestion of complex organic substrates[END_REF][START_REF] Labatut | Biochemical methane potential and biodegradability of complex organic substrates[END_REF]. As for the physical and biochemical processes in a co-digestion system, only some have been investigated; other questions require to be further studied. These include the effect of the temperature on the process performance; the need for pre-treatments; the effect of solid mixtures' moisture content; the feeding systems of the solid mixture; the definition of the optimal stirring of the solid mixture in the digester. Thus, this paper aims to review the current knowledge on co-digestion of FW and fat, oil and grease (FOG) with wastewater treatment sludge. The benefits and constraints of the co-digestion technology will then be discussed with special reference to prospects for the application in Ireland.

Substrates for co-digestion

Previous studies have investigated well the anaerobic co-digestion of SS with several kinds of other materials. These include: source-sorted OFMSW [START_REF] Silvestre | Optimisation of sewage sludge anaerobic digestion through co-digestion with OFMSW: Effect of collection system and particle size[END_REF][START_REF] Cavinato | Mesophilic and thermophilic anaerobic co-digestion of waste activated sludge and source sorted biowaste in pilot-and full-scale reactors[END_REF], confectionery waste [START_REF] Lafitte-Trouqué | Dual anaerobic co-digestion of sewage sludge and confectionery waste[END_REF], municipal solid waste [START_REF] Stroot | Anaerobic codigestion of municipal solid waste and biosolids under various mixing conditions -I. digester performance[END_REF], FW [START_REF] Kim | Increased anaerobic production of methane by co-digestion of sludge with microalgal biomass and food waste leachate[END_REF][START_REF] Kim | A comparison study on the high-rate co-digestion of sewage sludge and food waste using a temperature-phased anaerobic sequencing batch reactor system[END_REF][START_REF] Giuliano | Co-digestion of livestock effluents, energy crops and agro-waste : Feeding and process optimization in mesophilic and thermophilic conditions[END_REF][START_REF] Koch | Co-digestion of food waste in a municipal wastewater treatment plant: Comparison of batch tests and full-scale experiences[END_REF], fruit and vegetable waste [START_REF] Liu | Pilot-scale anaerobic codigestion of municipal biomass waste: Focusing on biogas production and GHG reduction[END_REF], industrial waste [START_REF] Maragkaki | Pilot-scale anaerobic co-digestion of sewage sludge with agro-industrial by-products for increased biogas production of existing digesters at wastewater treatment plants[END_REF][START_REF] Murto | Impact of food industrial waste on anaerobic co-digestion of sewage sludge and pig manure[END_REF], grease trap sludge [START_REF] Davidsson | Strategies for enzyme dosing to enhance anaerobic digestion of sewage sludge[END_REF][START_REF] Luste | Anaerobic co-digestion of meat-processing by-products and sewage sludge -Effect of hygienization and organic loading rate[END_REF][START_REF] Long | Anaerobic co-digestion of fat, oil, and grease (FOG): A review of gas production and process limitations[END_REF][START_REF] Martínez | Anaerobic co-digestion of FOG and sewage sludge: Study of the process by Fourier transform infrared spectroscopy[END_REF], meat processing waste [START_REF] Palatsi | Anaerobic digestion of slaughterhouse waste: Main process limitations and microbial community interactions[END_REF][START_REF] Luste | Anaerobic co-digestion of meat-processing by-products and sewage sludge -Effect of hygienization and organic loading rate[END_REF], crude glycerol [START_REF] Silvestre | Optimisation of sewage sludge anaerobic digestion through co-digestion with OFMSW: Effect of collection system and particle size[END_REF][START_REF] Nghiem | Co-digestion of sewage sludge and crude glycerol for on-demand biogas production[END_REF], and sludges from pulp and paper industry and enzyme production [START_REF] Wang | Impacts of inoculum pretreatments on enzyme activity and biochemical methane potential[END_REF][START_REF] Wang | Determination of methane yield of cellulose using different experimental setups[END_REF][START_REF] Einola | Screening for an optimal combination of municipal and industrial wastes and sludges for anaerobic co-digestion[END_REF].

The gas yield varies with the amounts of carbohydrate, proteins and lipids that are present in the mixes. Lipids provide the highest biogas yield, but require longer retention time owing to their slow kinetic of biodegradability; whereas, carbohydrates and proteins show fast conversion rates but low biogas yield. Thus, in the co-digestion process protein-rich wastes can provide the buffering capacity and a wide range of nutrients, and wastes with high carbon content can balance the C/N ratio for all substrates characterised by a low C/N ratio, thereby decreasing the risk of ammonia inhibition [START_REF] Wang | Effects of substrate concentration on methane potential and degradation kinetics in batch anaerobic digestion[END_REF]. To overcome this limitation, researchers have concentrated on co-digestion of FW with SS to augment nutrient imbalance [START_REF] Mata-Alvarez | Codigestion of solid wastes: A review of its uses and perspectives including modeling[END_REF][START_REF] Astals | Identification of synergistic impacts during anaerobic co-digestion of organic wastes[END_REF][START_REF] Maragkaki | Pilot-scale anaerobic co-digestion of sewage sludge with agro-industrial by-products for increased biogas production of existing digesters at wastewater treatment plants[END_REF][START_REF] Silvestre | Biomass adaptation over anaerobic co-digestion of sewage sludge and trapped grease waste[END_REF][START_REF] Giuliano | Co-digestion of livestock effluents, energy crops and agro-waste : Feeding and process optimization in mesophilic and thermophilic conditions[END_REF][START_REF] Liu | Pilot-scale anaerobic codigestion of municipal biomass waste: Focusing on biogas production and GHG reduction[END_REF][START_REF] Wang | Determination of methane yield of cellulose using different experimental setups[END_REF][START_REF] Shin | Variations in methanogenic population structure under overloading of pre-acidified high-strength organic wastewaters[END_REF][START_REF] Sun | Effects of lipid concentration on anaerobic co-digestion of municipal biomass wastes[END_REF].

Different waste combinations are possible for the co-digestion with SS based on the availability of substrates through different seasons. It is desirable that variation in the characteristics of the waste be minimum to avoid disturbances in the reactor [START_REF] Fonoll | Anaerobic co-digestion of sewage sludge and fruit wastes: Evaluation of the transitory states when the co-substrate is changed[END_REF]. The next sections address composition and characteristics of FW and FOG before discussing their co-digestion with SS.

FW and FOG

Among the materials for anaerobic co-digestion, FW and FOG are commonly used because of their high organic carbon and protein content in FW, as well as high lipid content of FOG waste. It has been reported that more than 1.3 billion tonnes of FW are discarded every year, with about 120 million tonnes coming from Europe [START_REF]Food Losses and Waste in the Context of Sustainable Food Systems. A Report by the High Level Panel of Experts on Food Security and Nutrition of the Committee on World Food Security (Rome: UN Committee on World Food Security (CFS))[END_REF]. According to the Food and Agriculture Organisation of the United Nation [START_REF] Gustavsson | Global Food Losses and Food Waste -Extent, Causes and Prevention (Rome: Food and Agriculture Organization of the United Nations (FAO))[END_REF], the food waste and losses amount to roughly US$ 680 billion in developed countries and over US$ 310 billion in developing countries. This also amounts to wasting of resources used in producing the loss and wasted food, which includes land, water, labour and capital, energy, and the unnecessary production and emission of GHGs, thereby contributing to global warming and climate change [START_REF] Girotto | Food waste generation and industrial uses: A review[END_REF]. According to EPA's 'Stop Food Waste' program, Ireland generates over 1 million tonnes of food waste annually (from the MSW) and this includes; households, commercial firms and food producers. This is alarming because, about 15% of the population experience some type of food poverty and if those on low incomes are added, the figures rise to 35%. Most of this wasted food goes to landfill. It is estimated that each kilogram (kg) of food waste costs €2 (purchasing ingredients, energy to cook, labour to prepare, and disposal to landfill). This amounts to an average of €1000 per household in Ireland (about €2000/tonne). To reduce the volume may create financial savings by the avoidance of landfill levy charges. These have been rising from the initial €30/tonne in 2010, €50/tonne in 2011 and €75/tonne in 2012, to meet European Union Council Directive 1999/31/EC on the landfill of waste [START_REF] Directive | Council Directive 1999/31/EC on the landfill[END_REF][START_REF] Epa | Licensed Sites 2014 Report on Air Emissions[END_REF].

FW refers to raw or cooked food materials, including food loss before, during or after meal preparation in the household, as well as food discarded in the manufacturing/production, distribution, wholesale/retail and food service sectors (including restaurants, schools and hospitals, etc.). FW is a major vermin attractant, and source of odour emission, toxic gas emission and groundwater contamination in collection, transportation and landfill, owing to high organic concentration and moisture content [START_REF] Lee | Effect of volatile fatty acid concentration on anaerobic degradation rate from field anaerobic digestion facilities treating food waste leachate in South Korea[END_REF]. FW can be highly variable depending on its source. Its composition differs significantly from one place to another. This can hinder its successful digestion on its own. It can also contain plastics, metals and other impurities [START_REF] Edwards | Anaerobic co-digestion of municipal food waste and sewage sludge: A comparative life cycle assessment in the context of a waste service provision[END_REF][START_REF] Edwards | A review of policy drivers and barriers for the use of anaerobic digestion in Europe, the United States and Australia[END_REF].

FOG comprises greasy by-products separated from wastewater from restaurants, hotels, institutional kitchens, meat processing plants, food industry, and dissolved air floatation of the WWTPs etc. Municipalities consider the release of FOG into the collection system/sewer lines to be illegal. Within sewer lines, it forms hardened solids (fatbergs) as a result of both chemical and physical changes, resulting in sewer overflow, reducing conveyance capacity, road diversions, and sometimes flooding of properties [START_REF] He | Mechanisms of fat, oil and grease (FOG) deposit formation in sewer lines[END_REF][START_REF] Curran | Sustainable management of fat, oil and grease (FOG) waste -A global challenge[END_REF]. These cost millions of euros to municipalities every year in repairing, cleaning, and maintenance fees. According to Curran [START_REF] Curran | Sustainable management of fat, oil and grease (FOG) waste -A global challenge[END_REF], fatbergs (FOG) are rampant in the UK with about 15 tonnes found in Kingston in 2013, which took six weeks to remove, costing €20 million to repair. It was estimated that FOG caused 50-75% sewerage pipeline blockages. Accordingly, municipalities implement pre-treatment steps to aid the removal of grease from the kitchen waste streams. This is usually accomplished by using grease abatement devices called grease traps (typically 190 L in size and installed directly below the sink inside the food production facility), or grease interceptors (typically 3800-7500 L in size and installed below the ground and outside buildings [START_REF] He | Physico-chemical characterization of grease interceptors with and without biological product addition[END_REF]. These devices are pumped out at regular intervals to prevent the FOG from entering the sewerage system. The operational challenges with FOG include sludge floatation, inhibition of methanogenic and acetoclastic bacteria, substrate and product transport limitation, digester foaming, clogging of gas collection and handling systems, blockage of pipes and pumps [START_REF] Davidsson | Codigestion of grease trap sludge and sewage sludge[END_REF][START_REF] Luostarinen | Increased biogas production at wastewater treatment plants through co-digestion of sewage sludge with grease trap sludge from a meat processing plant[END_REF][START_REF] Long | Anaerobic co-digestion of fat, oil, and grease (FOG): A review of gas production and process limitations[END_REF][START_REF] Maragkaki | Improving biogas production from anaerobic co-digestion of sewage sludge with a thermal dried mixture of food waste, cheese whey and olive mill wastewater[END_REF].

According to Davidsson et al. [START_REF] Davidsson | Codigestion of grease trap sludge and sewage sludge[END_REF] and Nghiem et al. [START_REF] Nghiem | Co-digestion of sewage sludge and crude glycerol for on-demand biogas production[END_REF], the high lipids content of FOG results in process instability because of inhibition of LCFAs. Therefore, co-digestion of FOG with sewage sludge or animal manure is a better choice. Because of its high methane potential of 0.7-1.1 m 3 CH 4 /kg VS [START_REF] Astals | Identification of synergistic impacts during anaerobic co-digestion of organic wastes[END_REF], and the high theoretical methane potential of 0.9-1.4 L/g at 65-70% CH 4 [START_REF] Davidsson | Codigestion of grease trap sludge and sewage sludge[END_REF][START_REF] Long | Anaerobic co-digestion of fat, oil, and grease (FOG): A review of gas production and process limitations[END_REF], the co-digestion of FOG with SS is more beneficial, especially when the waste is collected in-house. It is important to control the amount of FOG added to the mix to avoid high concentration of LCFAs production in the digester, as they can inhibit methanogenic activities [START_REF] Mata-Alvarez | Codigestion of solid wastes: A review of its uses and perspectives including modeling[END_REF][START_REF] Astals | Identification of synergistic impacts during anaerobic co-digestion of organic wastes[END_REF][START_REF] Palatsi | Anaerobic digestion of slaughterhouse waste: Main process limitations and microbial community interactions[END_REF][START_REF] Zonta | Modelling inhibitory effects of long chain fatty acids in the anaerobic digestion process[END_REF][START_REF] Silvestre | Biomass adaptation over anaerobic co-digestion of sewage sludge and trapped grease waste[END_REF][START_REF] Palatsi | Strategies for recovering inhibition caused by long chain fatty acids on anaerobic thermophilic biogas reactors[END_REF][START_REF] Wang | Determining the limits of anaerobic codigestion of thickened waste activated sludge with grease interceptor waste[END_REF]. High concentration of LCFA will cause operational problem during co-digestion [START_REF] Silvestre | Biomass adaptation over anaerobic co-digestion of sewage sludge and trapped grease waste[END_REF][START_REF] Cho | Effect of long chain fatty acids removal as a pretreatment on the anaerobic digestion of food waste[END_REF][START_REF] Wang | Simultaneous removal of hydrogen sulfide and mercury from simulated syngas by iron-based sorbents[END_REF][START_REF] Razaviarani | Pilot-scale anaerobic co-digestion of municipal wastewater sludge with restaurant grease trap waste[END_REF][START_REF] Borowski | Co-digestion of pig slaughterhouse waste with sewage sludge[END_REF]. There are operational challenges associated with FOG digestion such as foaming and biomass flotation related to adsorption of lipids onto biomass, and clogging in the liquid or gas systems [START_REF] Fonoll | Anaerobic co-digestion of sewage sludge and fruit wastes: Evaluation of the transitory states when the co-substrate is changed[END_REF][START_REF] Neczaj | Anaerobic treatment of sewage sludge and grease trap sludge in continuous co-digestion[END_REF].

Compositions and characteristics of FW

The composition of FW varies with eating habits. Its characteristics in terms of carbohydrates, lipids and proteins differ considerably based on its components [START_REF] Silvestre | Optimisation of sewage sludge anaerobic digestion through co-digestion with OFMSW: Effect of collection system and particle size[END_REF][START_REF] Silvestre | Biomass adaptation over anaerobic co-digestion of sewage sludge and trapped grease waste[END_REF][START_REF] Giuliano | Co-digestion of livestock effluents, energy crops and agro-waste : Feeding and process optimization in mesophilic and thermophilic conditions[END_REF][START_REF] Liu | Pilot-scale anaerobic codigestion of municipal biomass waste: Focusing on biogas production and GHG reduction[END_REF][START_REF] Iacovidou | The household use of food waste disposal units as a waste management option: A review[END_REF]. For example, bread, potatoes and flour are rich in carbohydrates; meat, fish and eggs and cheese are rich in protein. Fresh vegetables present an even composition of carbohydrate and protein content, as shown in Table 1. FW elemental compositions are commonly expressed in terms of available carbon (C), hydrogen (H), nitrogen (N), oxygen (O) and Sulphur (S). The elemental composition of carbohydrates, proteins and lipids indicates that an important quantity of carbon is available in FW, which makes it highly degradable.

Compositions and characteristics of FOG sludge

The origins of the FOG determine the total solids and fat compositions. Long et al. [START_REF] Long | Anaerobic co-digestion of fat, oil, and grease (FOG): A review of gas production and process limitations[END_REF] note that the composition and chemical characteristic of FOG can vary greatly depending on its origin, the abatement device configuration (size, inlet/outlet piping, number of baffles), and the device pumping frequency. The frequency of pump-out can influence its biochemical oxygen demand (BOD), and total solids (TS) variations [START_REF] Martínez | Anaerobic co-digestion of FOG and sewage sludge: Study of the process by Fourier transform infrared spectroscopy[END_REF][START_REF] He | Physico-chemical characterization of grease interceptors with and without biological product addition[END_REF]. Suto et al. [START_REF] Suto | Innovative anaerobic digestion investigation of fats, oils, and grease[END_REF] observe that waste cooking oil typically forms three layers whether in pipes, containers or abatement devices, which are primary (top floatable layer), organic-rich wastewater (middle aqueous layer and food particles) and other settled solids (bottom sludge layer). The volume of each layer will depend on the frequency of evacuation and the types of abatement devices or container used. Waste cooking oil collected from restaurants, hotels and other food outlet grease collection devices has a high fat content. Some studies [START_REF] Luste | Anaerobic co-digestion of meat-processing by-products and sewage sludge -Effect of hygienization and organic loading rate[END_REF][START_REF] Long | Anaerobic co-digestion of fat, oil, and grease (FOG): A review of gas production and process limitations[END_REF] have concentrated on unsaturated fat rather than saturated fat. Oleic acid was reported as the most common fatty acid found in grease trap abatement devices and WWTPs. The TS of FOG ranges from 2 to 22% before dewatering [START_REF] Luste | Anaerobic co-digestion of meat-processing by-products and sewage sludge -Effect of hygienization and organic loading rate[END_REF].

Table 2 shows the main characteristics of different FOG waste samples as reported in the literature on sewage sludge co-digestion with FOG at lab scale.

Co-digestion of FW with SS

Apart from the feedstock compositions, a wide range of successful results concerning the co-digestion of SS with FW demonstrate factors such as the organic loading rate (OLR), sludge composition (primary, secondary or mixed), digester configuration, mixing conditions and temperature range [START_REF] Mata-Alvarez | Codigestion of solid wastes: A review of its uses and perspectives including modeling[END_REF]. Many researchers have reported the benefits of co-digestion of SS with FW for improved methane production and dilution of toxic compounds [START_REF] Cavinato | Mesophilic and thermophilic anaerobic co-digestion of waste activated sludge and source sorted biowaste in pilot-and full-scale reactors[END_REF][START_REF] Koch | Co-digestion of food waste in a municipal wastewater treatment plant: Comparison of batch tests and full-scale experiences[END_REF][START_REF] Liu | Pilot-scale anaerobic codigestion of municipal biomass waste: Focusing on biogas production and GHG reduction[END_REF][START_REF] Sun | Effects of lipid concentration on anaerobic co-digestion of municipal biomass wastes[END_REF]. Table 3 shows a summary of studies of methane yield during co-digestion of SS, WAS with FW. Most researchers adopted continuous stirred tank reactors, but a few adopted two-phased anaerobic digestion, and anaerobic sequence batch reactors. The parameters for these benefits are of equal importance, especially when it comes to increasing the co-digestion performance. Table 3 shows that the higher the OLR, the higher the methane production. The volatile solids (VS) removal rate (efficiency) seems to be a function of the percentage of the FW added to the mixture on VS basis [START_REF] Wang | Determination of methane yield of cellulose using different experimental setups[END_REF][START_REF] Sun | Effects of lipid concentration on anaerobic co-digestion of municipal biomass wastes[END_REF].

In addition, thermophilic anaerobic co-digestion showed higher biogas production than mesophilic co-digestion in the past studies. Cavinato et al. [START_REF] Cavinato | Mesophilic and thermophilic anaerobic co-digestion of waste activated sludge and source sorted biowaste in pilot-and full-scale reactors[END_REF] found an increase in methane production of 160% when operation the digester under thermophilic temperature condition followed by a mesophilic digester with OLR of 2.21 kg/m 3 d, using a 75:25 SS:FW mixture ratio on (on VS basis). The same trend was observed in other studies investigated as shown in Table 3.

The methane yields were found to increase significantly with an increased percentage of FW in mixtures with SS, illustrating the value of the co-digestion process [START_REF] Xie | Synergistic effect from anaerobic co-digestion of sewage sludge and organic wastes[END_REF][START_REF] Wang | Determination of methane yield of cellulose using different experimental setups[END_REF]. FW is a highly biodegradable co-substrate, which, until a certain threshold limit, improves the biogas production of the sewage sludge digesters just by increasing the OLR. Any addition of FW whether above or below the optimum mixing ratio can limit the digestion and reduction in biogas yields, or even result in a complete failure through system instability [START_REF] Silvestre | Optimisation of sewage sludge anaerobic digestion through co-digestion with OFMSW: Effect of collection system and particle size[END_REF][START_REF] Edwards | Anaerobic co-digestion of municipal food waste and sewage sludge: A comparative life cycle assessment in the context of a waste service provision[END_REF][START_REF] Kim | Status of the phylogenetic diversity census of ruminal microbiomes[END_REF]. Evaluating the economic and environmental suitability of using OFMSW as a substrate in two German WWTPs, Krupp et al. [START_REF] Krupp | Feasibility study for co-digestion of sewage sludge with OFMSW on two wastewater treatment plants in Germany[END_REF] argued that using OFMSW as sewage sludge co-substrate was the more advantageous solution when compared to OFMSW composting and mono-digestion. It was also reported that the improved carbon to nitrogen (C/N) ratio and kinetic reactions achieved by the addition of FW to SS were prerequisites for improving methane yield [START_REF] Mottet | Estimating anaerobic biodegradability indicators for waste activated sludge[END_REF]. Some researchers have suggested that the optimum C/N for digestion performance is in the range of 20-30; others have equally demonstrated that digestion can be successfully performed under a wider range of C/N ratios [START_REF] Xie | Synergistic effect from anaerobic co-digestion of sewage sludge and organic wastes[END_REF][START_REF] Wang | Determination of methane yield of cellulose using different experimental setups[END_REF][START_REF] Kim | The optimisation of food waste addition as a cosubstrate in anaerobic digestion of sewage sludge[END_REF][START_REF] Heo | Single-stage anaerobic codigestion for mixture wastes of simulated korean food waste and waste activated sludge[END_REF][START_REF] Sosnowski | Anaerobic co-digestion of sewage sludge and organic fraction of municipal solid wastes[END_REF][START_REF] Zhang | Anaerobic co-digestion of biosolids and organic fraction of municipal solid waste by sequencing batch process[END_REF]. Palatsi et al. [START_REF] Palatsi | Anaerobic digestion of slaughterhouse waste: Main process limitations and microbial community interactions[END_REF] and Wang et al. [START_REF] Wang | Determination of methane yield of cellulose using different experimental setups[END_REF] argued that any C/N ratio more than 30 will result in instability, mainly through nutrient deficiency. This will severely affect microorganisms' activity, thereby leading to lower substrate removal and methane production. But if the C/N ratio is less than 6, it will negatively affect the digestion process, chiefly as a result of low carbon availability in combination with high ammonia (NH 3 ) concentration, which can cause toxicity to anaerobic microorganisms [START_REF] Koch | Co-digestion of food waste in a municipal wastewater treatment plant: Comparison of batch tests and full-scale experiences[END_REF][START_REF] Wang | Ammonia effect on hydrogenotrophic methanogens and syntrophic acetate-oxidizing bacteria[END_REF].

Generally, SS is characterised by a low C/N in the range of 6-9, whereas, the addition of FW having high C/N ratio [START_REF] Parliament | Directive 2009/28/EC of the European Parliament and of the Council of 23 April 2009[END_REF][START_REF]Energy in Ireland 1990-2015[END_REF](20)[START_REF]The Environment in your pocket[END_REF][START_REF]Guidelines to Defra / DECC's GHG Conversion Factors for Company Reporting: Methodology Paper for Emission Factors[END_REF] can improve the ensuing mixtures [START_REF] Kim | Increased anaerobic production of methane by co-digestion of sludge with microalgal biomass and food waste leachate[END_REF][START_REF] Heo | Single-stage anaerobic codigestion for mixture wastes of simulated korean food waste and waste activated sludge[END_REF][START_REF] Zhang | Batch and semi-continuous anaerobic digestion of food waste in a dual solid-liquid system[END_REF]. They reported an improvement of C/N in the range of 6-15. Wang et al. [START_REF] Wang | Determination of methane yield of cellulose using different experimental setups[END_REF] submitted that in addition, FW provides essential carbon to sewage digestion performance, mainly because of its influence on the process kinetics. In anaerobic digestion microbiology, hydrolysis has been described as rate-limiting steps, because SS is rich in proteins (30% DM) by composition, and also has a lower hydrolytic potential than carbohydrates and lipids [START_REF] Liu | Pilot-scale anaerobic codigestion of municipal biomass waste: Focusing on biogas production and GHG reduction[END_REF][START_REF] Zhang | Anaerobic co-digestion of biosolids and organic fraction of municipal solid waste by sequencing batch process[END_REF]. Therefore, the addition of easily degradable FW, has been reported to improve the degradation efficiency and speed up the hydrolysis of SS, because of the faster growth of anaerobic microorganisms [START_REF] Koch | Co-digestion of food waste in a municipal wastewater treatment plant: Comparison of batch tests and full-scale experiences[END_REF][START_REF] Wang | Determination of methane yield of cellulose using different experimental setups[END_REF]. A faster grow rate of the microorganisms will result in a higher hydrolytic rate and, thus, a higher acidification and methanogenesis potential [START_REF] Edwards | Anaerobic co-digestion of municipal food waste and sewage sludge: A comparative life cycle assessment in the context of a waste service provision[END_REF][START_REF] Ponsá | Anaerobic co-digestion of the organic fraction of municipal solid waste with several pure organic co-substrates[END_REF]. Therefore, the co-digestion of FW with SS will improve not only the kinetic coefficients of hydrolysis, but also the overall performance. Further, temperature increase can result in even better kinetics. Angelidaki and Sanders [START_REF] Angelidaki | Assessment of the anaerobic biodegradability of macropollutants[END_REF] concluded that the effect of temperature on the hydrolysis kinetic originates from the combined effect of hydrolytic enzymes, bacteria growth and solubility of the substrate on the improved activity.

Co-digestion of SS with FW allows the WWTPs to increase their revenue streams from the increase renewable energy generation (biogas). According to Defra [START_REF]The Environment in your pocket[END_REF], by generating additional renewable energy, the water industry would be able to increase its revenue (gate fee), by reducing their electricity bills and also by selling the surplus energy to the grid. FW addition will also dilute some potential toxic compounds such as heavy and light metal ions and other organic compounds [START_REF] Nghiem | Full scale co-digestion of wastewater sludge and food waste: Bottlenecks and possibilities[END_REF][START_REF] Edwards | A review of policy drivers and barriers for the use of anaerobic digestion in Europe, the United States and Australia[END_REF][START_REF] Maragkaki | Improving biogas production from anaerobic co-digestion of sewage sludge with a thermal dried mixture of food waste, cheese whey and olive mill wastewater[END_REF][START_REF] Iacovidou | Food waste co-digestion with sewage sludge -Realising its potential in the UK[END_REF]. SS contains a high amount of metals and this may affect the digestion process. The co-digestion of SS with FW can reduce the inhibitory effect of these toxic compounds [START_REF] Angelidaki | Defining the biomethane potential (BMP) of solid organic wastes and energy crops: A proposed protocol for batch assays[END_REF]. The resulting efficient dilution would not only increase the degradation efficiency of SS, but also the biogas yields.

FW degradability through AD has been widely studied [START_REF] Silvestre | Optimisation of sewage sludge anaerobic digestion through co-digestion with OFMSW: Effect of collection system and particle size[END_REF][START_REF] Zhang | Batch and semi-continuous anaerobic digestion of food waste in a dual solid-liquid system[END_REF][START_REF] Kafle | Anaerobic digestion treatment for the mixture of Chinese cabbage waste juice and swine manure[END_REF][START_REF] Banks | Anaerobic digestion of sourcesegregated domestic food waste: Performance assessment by mass and energy balance[END_REF][START_REF] Mata-Alvarez | A critical review on anaerobic co-digestion achievements between 2010 and 2013[END_REF]. In many studies, it was reported that the digestion of FW was limited either owing to the lack of sufficient nutrients such as nitrogen, phosphorous and to a smaller extent metals, or to VFAs accumulation resulting from its extremely high degradability [START_REF] Astals | Identification of synergistic impacts during anaerobic co-digestion of organic wastes[END_REF][START_REF] Banks | Anaerobic digestion of sourcesegregated domestic food waste: Performance assessment by mass and energy balance[END_REF]. FW was successfully digested in other cases, but only after modifications of the digester: chemicals, the introduction of pre-treatment steps, the recirculation of nutrients through leachate or increases in temperature [START_REF] Wang | Impacts of inoculum pretreatments on enzyme activity and biochemical methane potential[END_REF][START_REF] Kafle | Anaerobic digestion treatment for the mixture of Chinese cabbage waste juice and swine manure[END_REF][START_REF] Banks | Anaerobic digestion of sourcesegregated domestic food waste: Performance assessment by mass and energy balance[END_REF][START_REF] Zhang | The anaerobic co-digestion of food waste and cattle manure[END_REF]. Most of these studies were experimental, with the implementation of AD of FW at full scale reported to be limited and problematic [START_REF] Astals | Identification of synergistic impacts during anaerobic co-digestion of organic wastes[END_REF][START_REF] Maragkaki | Improving biogas production from anaerobic co-digestion of sewage sludge with a thermal dried mixture of food waste, cheese whey and olive mill wastewater[END_REF][START_REF] Chen | Anaerobic digestion of food wastes for biogas production[END_REF]. de Baere [START_REF] De Baere | Anaerobic digestion of MSW in Europe[END_REF] provides a survey of AD's effect on MSW across Europe, with emphasis on FW.

Co-digestion of FOG with SS

FOG is desirable for co-digestion. It has a high methane potential (1 m 3 CH 4 /kg VS). Much work has suggested the increase in methane through co-digestion with SS. Most studies were centred on FOG waste produced in the dissolved air flotation unit of the WWTP [START_REF] Silvestre | Biomass adaptation over anaerobic co-digestion of sewage sludge and trapped grease waste[END_REF][START_REF] Luste | Anaerobic co-digestion of meat-processing by-products and sewage sludge -Effect of hygienization and organic loading rate[END_REF][START_REF] Martínez | Anaerobic co-digestion of FOG and sewage sludge: Study of the process by Fourier transform infrared spectroscopy[END_REF][START_REF] Noutsopoulos | Anaerobic co-digestion of grease sludge and sewage sludge: The effect of organic loading and grease sludge content[END_REF]. Several researchers have investigated the FOG waste coming from meat processing factories co-digested with SS [START_REF] Neczaj | Anaerobic treatment of sewage sludge and grease trap sludge in continuous co-digestion[END_REF][START_REF] Buendía | Feasibility of anaerobic co-digestion as a treatment option of meat industry wastes[END_REF][START_REF] Pitk | Co-digestion of sewage sludge and sterilized solid slaughterhouse waste: Methane production efficiency and process limitations[END_REF]. Some have also investigated the restaurants' grease-trap waste under mesophilic anaerobic co-digestion with SS [START_REF] Martínez | Anaerobic co-digestion of FOG and sewage sludge: Study of the process by Fourier transform infrared spectroscopy[END_REF][START_REF] Wang | Determining the limits of anaerobic codigestion of thickened waste activated sludge with grease interceptor waste[END_REF]. Carrere et al. [START_REF] Carrere | Improving methane production during the codigestion of waste-activated sludge and fatty wastewater: Impact of thermo-alkaline pretreatment on batch and semi-continuous processes[END_REF] and Pastor et al. [START_REF] Pastor | Co-digestion of used oils and urban landfill leachates with sewage sludge and the effect on the biogas production[END_REF] investigated restaurant fatty wastewater. These studies concern used oils from the restaurants co-digested in a mesophilic condition with SS (Table 4).

Luostarinen et al. [START_REF] Luostarinen | Increased biogas production at wastewater treatment plants through co-digestion of sewage sludge with grease trap sludge from a meat processing plant[END_REF] reported a 60% increase in methane yield when grease from a meat plant was co-digested with SS. Wan et al. [START_REF] Wan | Semi-continuous anaerobic co-digestion of thickened waste activated sludge and fat, oil and grease[END_REF] reported an increase of 137% in methane production, when a grease content of 64% on a VS basis was added. Astals et al. [START_REF] Astals | Identification of synergistic impacts during anaerobic co-digestion of organic wastes[END_REF], using FOG as a co-substrate at a full-scale digester, reported a 50% increase in biogas production. Kabouris et al. [START_REF] Kabouris | Methane recovery from the anaerobic codigestion of municipal sludge and FOG[END_REF] investigated FOG as a co-substrate and achieved a significant result in biogas production. Silvestre et al. [START_REF] Silvestre | Biomass adaptation over anaerobic co-digestion of sewage sludge and trapped grease waste[END_REF] observed a methane yield of 138% when a grease trap with 23% grease content on a VS basis was added with SS.

As shown in Table 4, FOG can come from two sources: from industrial processes (including other treatment plants) and from the same WWTP. In the treatment plants, FOG represents 25-40% of the wastewater total chemical oxygen demand and it is usually removed to about 50-90%, prior to biological treatments [START_REF] Hosseini Koupaie | Mesophilic batch anaerobic co-digestion of fruit-juice industrial waste and municipal waste sludge: Process and cost-benefit analysis[END_REF]. It saves money to treat the FOG sludge by using it as a co-substrate with sewage sludge. Noutsopoulos et al. [START_REF] Noutsopoulos | Anaerobic co-digestion of grease sludge and sewage sludge: The effect of organic loading and grease sludge content[END_REF] reported an increase of 285% in biogas production by comparing the digestion of 40% SS and 60% FOG mixture on a VS basis (2.1 kg VS fog m 3 /d) with a mono-digester of SS, which had an OLR of 1.9 kg VS m 3 /d. The co-digestion failed when the OLR of the FOG was further increased to 2.5 kg VS fog m 3 /d. Alanya et al. [START_REF] Alanya | Anaerobic co-digestion of sewage sludge and primary clarifier skimmings for increased biogas production[END_REF] using FOG from a primary settler and the scum from a concentration tank, co-digested with SS, obtained an increase in biogas production of 410%, with similar FOG (OLR of 2.4 kg VS fog m 3 /d). Davidsson et al. [START_REF] Davidsson | Codigestion of grease trap sludge and sewage sludge[END_REF], using a lower OLR (0.7 kg VS fog m 3 /d) and Silvestre et al. [START_REF] Silvestre | Biomass adaptation over anaerobic co-digestion of sewage sludge and trapped grease waste[END_REF] similarly (0.6 kg VS fog m 3 /d) also recorded a significant increase in biogas production (150%) and (210%), when compared to the SS mono-digester. The differences in these results can be attributed to the digesters' operational conditions, the adaptation periods, the characteristics of the feedstock (input materials) and the anaerobic biomass. Comparable results were also obtained with FOG coming from other origins.

According to Mata-Alvarez et al. [START_REF] Mata-Alvarez | A critical review on anaerobic co-digestion achievements between 2010 and 2013[END_REF], there is no clear threshold for FOG OLR. This is because several safe co-digestion performances with high biogas yields have been reported for OLR fog up to 0.8 kg VS fog m 3 /d. The degradation products, LCFAs, may severely inhibit methanogenesis. Contrary to early studies, the inhibition (LCFA), which was long assumed irreversible, has lately been proved reversible [START_REF] Long | Anaerobic co-digestion of fat, oil, and grease (FOG): A review of gas production and process limitations[END_REF][START_REF] He | Mechanisms of fat, oil and grease (FOG) deposit formation in sewer lines[END_REF][START_REF] Palatsi | Strategies for recovering inhibition caused by long chain fatty acids on anaerobic thermophilic biogas reactors[END_REF], because of the increasing consumption of acetate and butyrate, an indication of recovery [START_REF] Cirne | Anaerobic digestion of lipid-rich waste -Effects of lipid concentration[END_REF]. In addition, when digesting lipidrich materials, a lag phase is usually observed between LCFA and methane production [START_REF] Girault | Anaerobic co-digestion of waste activated sludge and greasy sludge from flotation process: Batch versus CSTR experiments to investigate optimal design[END_REF].

It seems that FOG with an original total solids (TS) of 1-2% does not favour incineration, even though the energy content in the waste is very high, with VS content of over 95% [START_REF] Luostarinen | Increased biogas production at wastewater treatment plants through co-digestion of sewage sludge with grease trap sludge from a meat processing plant[END_REF]. But, if thickened to a TS > 5%, it can be suitable for co-digestion. The single-substrate digestion of SS from FOG (grease trap) is unwise since long-chain fatty acids are known to inhibit anaerobic microorganism [START_REF] Long | Anaerobic co-digestion of fat, oil, and grease (FOG): A review of gas production and process limitations[END_REF][START_REF] Cho | Effect of long chain fatty acids removal as a pretreatment on the anaerobic digestion of food waste[END_REF]. It was reported that thermophilic conditions in the AD led to inhibition at much lower concentrations of oleate than in mesophilic conditions [START_REF] Kim | Increased anaerobic production of methane by co-digestion of sludge with microalgal biomass and food waste leachate[END_REF].

Limitations and constraints of SS co-digestion with FW and FOG

The key technical challenges limiting anaerobic co-digestion of sewage sludge with organic wastes will now be discussed briefly. The slow degradation process (FOG) will result in inefficiency of the system. The substrates are typically made of complex structures that are difficult for bacteria to access and some can contain non-degradable substances (lignin). This leads to a long retention time inside the digester; thus, a slow and inefficient process. Although there is a range of pre-treatment options, the wrong selection can lead to high investment costs and high operational costs. Therefore, the right pre-treatment option should be an AD specific process. Poor management of nutrients and the presence of toxic substances in the substrates is another technical challenge. The variability of FW and FOG and their composition limits their co-digestion with SS to some extent, because of the imbalance they can cause the anaerobic digestion systems. The dynamics of AD processes are complex. There is need for a stable digestion system if optimum biogas is to be produced, but the microorganisms acclimatised with some mixtures, and any changes in the mixtures may result in changes in process reactions and digestion instability. The specific methane production potential h may also vary with the seasonal variation of FW [START_REF] Wang | Impacts of inoculum pretreatments on enzyme activity and biochemical methane potential[END_REF][START_REF] Cavinato | Mesophilic and thermophilic anaerobic co-digestion of waste activated sludge and source sorted biowaste in pilot-and full-scale reactors[END_REF][START_REF] Liu | Pilot-scale anaerobic codigestion of municipal biomass waste: Focusing on biogas production and GHG reduction[END_REF][START_REF] He | Mechanisms of fat, oil and grease (FOG) deposit formation in sewer lines[END_REF][START_REF] Pagés-Díaz | Anaerobic co-digestion of solid slaughterhouse wastes with agro-residues: Synergistic and antagonistic interactions determined in batch digestion assays[END_REF]. Some researchers reported an accumulation of the light and heavy metals (Zn, Pb and Ni) and other improper materials in the digester, especially when mechanical-sorted OFMSW was used as co-substrate [START_REF] Xie | Synergistic effect from anaerobic co-digestion of sewage sludge and organic wastes[END_REF][START_REF] Pahl | Anaerobic co-digestion of mechanically biologically treated municipal waste with primary sewage sludge -A feasibility study[END_REF]. According to Chen et al. [START_REF] Chen | Anaerobic digestion of food wastes for biogas production[END_REF], light metal ions and biodegradation intermediates play important roles in process performance. They can also cause toxicity in anaerobic digestion. There may be adverse changes by reducing the bacteria growth and changes in the microbial community. Therefore, a slight addition of FW to the mixes (SS/FW) might increase the concentration of these light metal ions, making them toxic to the anaerobic microbial community.

Light metal ions (cations) in FW are essential for the growth of anaerobic microorganisms, which at high concentrations can be toxic in the digestion process, through osmosis [START_REF] Liu | Pilot-scale anaerobic codigestion of municipal biomass waste: Focusing on biogas production and GHG reduction[END_REF]. Any FW rich in vegetable co-digested with sewage sludge will likely show an increase in potassium (K + ), which may inhibit digestion process; and likewise, FW containing sodium (Na + ) may be toxic to the bacteria [START_REF] Cavinato | Mesophilic and thermophilic anaerobic co-digestion of waste activated sludge and source sorted biowaste in pilot-and full-scale reactors[END_REF]. The main biodegradation intermediates of anaerobic digestion are VFAs and ammonia (NH 3 ). Therefore, accumulation of the intermediates beyond certain limits in the digester can cause toxicity disruptions of the co-digestion process. There is an increase in VFA concentration initially when FW is co-digested with SS because of rapid acidification of soluble organic compounds that are present in FW [START_REF] Wang | Impacts of inoculum pretreatments on enzyme activity and biochemical methane potential[END_REF][START_REF] Sun | Effects of lipid concentration on anaerobic co-digestion of municipal biomass wastes[END_REF][START_REF] Sosnowski | Kinetic investigations of methane co-fermentation of sewage sludge and organic fraction of municipal solid wastes[END_REF]. There is no definite level for acceptable VFAs in the digester, but VFAs of 6000 mg/l can be present in the digester without being toxic provided the pH is kept within optimal range [START_REF] Zhang | Influence of initial pH on thermophilic anaerobic co-digestion of swine manure and maize stalk[END_REF].

The lipids content in FOG and FW will affect the digestion process owing to excessive production of LCFAs; which are proven to be toxic to anaerobic bacteria [START_REF] Xie | Synergistic effect from anaerobic co-digestion of sewage sludge and organic wastes[END_REF][START_REF] Chen | Inhibition of anaerobic digestion process: A review[END_REF]. The levels at which LCFAs become toxic vary widely, depending on which acids are predominant in the digester. The main cause of NH 3 production and accumulation in the digester is the degradation of FW as they are rich in protein [START_REF] Wang | Ammonia effect on hydrogenotrophic methanogens and syntrophic acetate-oxidizing bacteria[END_REF]. In the digester, NH 3 and ammonium ion (NH + 4 ) are always present. A large value of NH + 4 is associated with greater buffering capacity of the digestate, increasing the digester's stability. Appels et al. [START_REF] Appels | Principles and potential of the anaerobic digestion of waste-activated sludge[END_REF] shows that NH + 4 can inhibit the activity of methanogens and hence reduce the biogas production when the NH + 4 concentration is above 3.0 g L -1 . In the literature, it seems that NH 3 was reported to be more inhibitory than NH + 4 because it can penetrate through the cell membranes [START_REF] Wang | Impacts of inoculum pretreatments on enzyme activity and biochemical methane potential[END_REF][START_REF] Appels | Principles and potential of the anaerobic digestion of waste-activated sludge[END_REF][START_REF] Yenigün | Ammonia inhibition in anaerobic digestion: A review[END_REF][START_REF] Fotidis | Effect of ammonium and acetate on methanogenic pathway and methanogenic community composition[END_REF]. The level at which NH 3 concentration becomes inhibitory in the digesters is uncertain. Interestingly, Heo et al. [START_REF] Heo | Single-stage anaerobic codigestion for mixture wastes of simulated korean food waste and waste activated sludge[END_REF] and Arhoun et al. [START_REF] Arhoun | Biogas production from pear residues using sludge from a wastewater treatment plant digester. Influence of the feed delivery procedure[END_REF] reported that a mixture with a higher percentage of food waste than sewage sludge may not likely have ammonia inhibition, because of the availability of higher carbon in the mix.

There are operational constraints associated with handling and sorting of FW. FW must be free of impurities and unwanted materials prior to co-digestion with SS. Impurities in FW such as plastics, metals, glass and other packaging parts are likely to cause serious operational problems and affect co-digestion performance and the WWTP. Plastics can cause serious clogging in the mechanical system and wear out pumps and form the top layer inside the reactors. The presence of plastics (phthalates) can affect the quality of the digestate and make it unacceptable as soil conditioner in agricultural land [START_REF] Silvestre | Optimisation of sewage sludge anaerobic digestion through co-digestion with OFMSW: Effect of collection system and particle size[END_REF][START_REF] Xie | Synergistic effect from anaerobic co-digestion of sewage sludge and organic wastes[END_REF]. The presence of metals can also cause clogging in the conveyor line. All these can lead to higher operational costs, loss in the important fraction of FW during pre-treatment process, and reduction in biogas yield.

Animal by-products regulation (ABPR) is another impediment to co-digesting sewage sludge with FW. The water industry is required to have this permit to process FW. Any FW processing that has been contaminated by meat and other animal materials falls within ABPR. The regulation defines three categories of animal by-products (ABP) which includes FW originating from households, restaurants and catering facilities, with category three being the least harmful [START_REF] Parliament | Directive 2009/28/EC of the European Parliament and of the Council of 23 April 2009[END_REF][START_REF] Van Foreest | Perspectives for biogas in Europe[END_REF][110][111]. This was transposed in Ireland under European Union (ABP) Regulation S.I No 187 of 2014 [START_REF] Epa | Licensed Sites 2014 Report on Air Emissions[END_REF]112]. The ABPR permit requirements are that FW must firstly be pre-treated to ensure the removal of packaging material and other impurities and secondly, be pasteurised before being added to the digester [112].

The main aim of the ABPR is to sanitise food production, FW and to prevent transfers of pathogens. According to Defra [START_REF]The Environment in your pocket[END_REF], it is likely that the ABPR covers the use of co-digestion. In Ireland, the personnel responsible for ABPR in the Department of Agriculture, Food and Marine (DAFM) need to be informed and application must be made in line with the conditions. The process is done in three stages; before construction, before operation and validation of operation (when the plant is commissioned).

Future research suggestion and prospect of application in Ireland

The limitations associated with these three substrates (FOG, FW and SS), mainly the technical challenges and increased costs from chemical additions, increases in temperature and proper digestion management, and increasing concerns about the reliability of mono-digestion of FW have led to the investigation of co-digestion with SS as a better alternative. Further, wastewater treatment processes are very energy intensive, and could only benefit from initiatives to reduce their costs [START_REF] Adam | Upscaling of an electronic nose for completely stirred tank reactor stability monitoring from pilot-scale to real-scale agricultural co-digestion biogas plant[END_REF]. Sustainability requires adopting a systems approach with integrated solutions, rather than addressing single issues in isolation. Thus, a WWTP can serve as an energy producer, rather than end-of-pipe solution to clean wastewater.

There is growing interest in biogas, and there is large untapped potential to produce it both as a fuel in transport applications and for heating. According to Iacovidou [START_REF] Iacovidou | The household use of food waste disposal units as a waste management option: A review[END_REF], co-digestion is a breakthrough for the wastewater sector and its implementation needs an interdisciplinary approach. Care must be taken to avoid overloading the digesters to allow smooth process without problems. Overloading can cause failure of the whole process or reduction in biogas production, pipe blockages, foaming, and insufficient mixing of the substrates in the digester [START_REF] Nghiem | Co-digestion of sewage sludge and crude glycerol for on-demand biogas production[END_REF][START_REF] He | Mechanisms of fat, oil and grease (FOG) deposit formation in sewer lines[END_REF][START_REF] Cho | Effect of long chain fatty acids removal as a pretreatment on the anaerobic digestion of food waste[END_REF][START_REF] Ganidi | Anaerobic digestion foaming causes -A review[END_REF].

In Ireland, the co-digestion of FW with SS is almost non-existent and the few farm-based plants co-digest animal manure with municipal solid wastes, energy crops and glycerin from biodiesel plants. There are only three small-scale anaerobic digestion facilities in Ireland; McDonnel Farm Biogas Limited, Ballyshannon Recycling Limited and Bio-Energy and Organic Fertilizer (BEOFS). The current operational and regulatory framework makes anaerobic digestion complicated, because both FW and SS are covered by different regulatory regimes. The regulation becomes even more complicated when SS is co-digested with FW with the process falling between two sets of regulations [START_REF] Epa | Licensed Sites 2014 Report on Air Emissions[END_REF]. There are issues with getting waste management license requirements, quality of the co-digestate and getting the renewable energy generation credits. All these problems impede the adaptation of co-digestion of SS with FW by the water industry in Ireland.

In Ireland, anaerobic digestion fell within the EU Pollution Prevention and Control provisions, known in Ireland as the Integrated Pollution Control system which came into effect in 1994. It was replaced by the Integrated Pollution Prevention and Control (IPPC) system, introduced in Ireland in 2004, and implemented through the Environmental Protection Agency (EPA) (IPPC) Regulation 2013 S.I 283 of 2013 [115]. The IPPC Directive itself regulates the quantity and nature of pollutants emitted into the environment by the licensee, ensures that energy is used more efficiently, applies the precautionary principle and the polluter pays principle, prevents accidents, and makes improvements in all processes [112].

As a result, a Waste Management License was required under the EPA Act 1992, in order to operate an anaerobic digestion plant, where sewage sludge will be digested with household waste, industrial and commercial wastes EPA [START_REF] Epa | Licensed Sites 2014 Report on Air Emissions[END_REF]. A Waste Management License is required for the delivery, treatment and disposal of these wastes. Even in the UK, where regulations are similar to Ireland, these regulations (IPPC permit and Waste Management License) were merged to simplify regulation under Environmental Permit Regulations, to control the water industry. The control affects not only the size of the combined heat and power (CHP) plants, but also co-digestion of SS and FW, and the use of the biogas produced, and the storage and spreading of the digestate for a safer and cleaner environment [START_REF] Edwards | Anaerobic co-digestion of municipal food waste and sewage sludge: A comparative life cycle assessment in the context of a waste service provision[END_REF][START_REF]Guidelines to Defra / DECC's GHG Conversion Factors for Company Reporting: Methodology Paper for Emission Factors[END_REF][START_REF] Epa | Licensed Sites 2014 Report on Air Emissions[END_REF][START_REF] Ohandja | Improving anaerobic digester performance: lessons learnt from digester clean-out operations[END_REF]. The EPA waste permit in Ireland takes several months to process depending on the size of the plant. It is renewable annually, by a fresh application, which increases the water industry's costs.

In Ireland, an application for grid connection must be submitted to the Commission for Energy Regulation (CER). This is a complicated procedure and has considerable costs attached. CER provides licensing for construction and operation of CHP generator, but units under 1 MW are automatically licensed by order. The final hurdle lies with the Department of Communication, Energy and Natural Resources, which grants Renewable Energy Feed in Tarif (REFIT), for which AD-CHP application is up to 15 ct/kWh. This can only be done after planning permission is granted and grid connection offer has been issued. David McDonnell, of McDonnell Farm Biogas Limited, states in a REFIT report by the Sustainable Energy Authority of Ireland (SEAI) [START_REF]Energy in Ireland 1990-2015[END_REF], that it takes a minimum of two years to get the project from inception to completion. This long delay would deter investors in such a project.

Furthermore, the digestate from the co-digestion of SS with FW and FOG may require a potentially very expensive permit for disposal via land application. This is because it was not considered a waste digestate in the revised Waste Framework Directive and therefore, it is not acceptable for application to land directly [110]. According to the Chartered Institution of Water and Environmental Management [START_REF] Ciwem | Co-digestion of sewage sludge and waste[END_REF], the regulation that governs the production and use of such co-digestate is currently not clear and needs to be defined. This raises the question of what to do with the ensuing huge amount of co-digestate if not otherwise managed. All these problems of definition and procedure can cause impede developments in co-digestion in the water industry.

Waste generally should be considered as a resource and an economic opportunity. Co-digestion can help Ireland to reduce the amount waste going to landfills and a means to reduce greenhouse gas (GHG) emissions, prevent climate change, reduce electricity generation from fossil fuels, and thus create a sustainable environment. The relevant stakeholders and the regulators should work together, to support changes in regulatory frameworks necessary for the application of co-digestion of SS with FW and FOG.

There should be encouragement of all the WWTPs in Ireland to accept FW and FOG as co-substrates with their SS to increase biogas production, earn supplementary revenue, become energy self-sufficient, and improve their process performance. At present, WWTPs in Ireland do not accept FW and FOG and their excess capacity is underused. Change could begin with Ringsend WWTP, as discussed below.

Ringsend WWTP is modelled on the design, build, and operate contract, undertaken to provide up to tertiary treatment for a 1.7 million population equivalent (P.E.) (Figure 1). Operation began in 2003 following construction which started in 1999. Ringsend is home to the largest double decker sequencing batching reactors design in the world and is also essential to Dublin Bay's blue flag status. The plant used several innovative technologies to deal with the large pollution load on a constrained site. Sewage sludge is treated using the Cambi Thermal Hydrolysis Process and anaerobic digestion (AD) before being thermally dried [START_REF]Ringsend Wastewater Treatment Works[END_REF][START_REF] Awe | Analysis of energy consumption and saving in wastewater treatment plant : Case study from Ireland[END_REF]. The installed capacity of the plant is 120,000 tDS/yr of sewage sludge, with biogas yield of average of 410 m 3 /tDS. Currently, the anaerobic digestion produces 45,000 m³/day of biogas. This is used to fuel boilers and to generate electricity and recover heat through the CHP system, which can generate more than 2 MW of electricity, more than 50% of the heat and electricity required at the plant. The CHP system also generates electricity from natural gas for the plant's total energy demand. A total of 4 MW of capacity is installed at the plant [START_REF]Ringsend Wastewater Treatment Works[END_REF][START_REF] Awe | Analysis of energy consumption and saving in wastewater treatment plant : Case study from Ireland[END_REF]. More details of the plant can be found from Celtic Anglian Water website (CAW) [START_REF]Ringsend Wastewater Treatment Works[END_REF].

Conclusions

The review found that co-digestion of SS with FW and FOG provides an opportunity to increase biogas production especially from the existing biogas plant at the WWTPs. There is need for further research on the specific OLR which can be used to determine accurately the right quantity of organic wastes which can be added to a digester before there is potential for digester failure. This is because there is no agreed OLR specified for optimum results from the literature, probably because of different operating conditions. The incentive for renewable energy production in Ireland is the lowest amongst the EU countries which is just 15 ct/kWh as compared with Germany (28 ct/kWh), United Kingdom (20 ct/kWh) and France (25.78 ct/kWh). There should therefore be an urgent review in order to encourage the needed investment in the area of renewable energy generation, and open doors to a competitive market for digestate and energy supply within the country. The regulations and permit system should be simplified. Discrepancies among the bodies in charge should be eliminated, for the benefit of the environment.

If carefully applied, co-digestion can deliver beneficial synergies for the water industry and authorities responsible for food waste management. It is proposed that all relevant stakeholders and regulators should support changes to current regulatory frameworks to open a way forward for co-digestion in Ireland.

Figure 1 .

 1 Figure 1. General view of Ireland and the Ringsend WWTP [118,119].

Table 1 .

 1 Composition of different components of food waste as reported in literature (on % dry weight/ matter.

	Waste	Carbohydrates	Proteins	Lipids	References
	Grain products	74	12	0.7-2.0	[120]
	MFC (meat-fish-cheese)	12	55	33	[121]
	BP (bread-pasta)	82	13	4	[121]
	Vegetables Fruits	72 94	11 3	4 ˂1	[122] [122]
	Animal-derived food waste	2-6	70-75	20-30	[123]
	Cakes and pastries (Starbucks)	30-60	5-10	10-40	[78]
	Mixed food waste	63.87	21.34	12.4	[124]
	Whole milk (powder)	37.7	26.5	27.4	[125]
	Skim milk	53	36	-	[125]
	Cheese	-	25-35	20-45	[125]
	Legumes a	46-74	23-29	1.3-5	[126]
	Fresh vegetables b Meat and bones	27.1 ˂1	26.9 70-75	1.4 23-30	[126] [127]
	Egg (including shell)	2	35	32	[125]
	Fish meat	-	75.6	20.2	[128]
	Fruit	83	4	2	[129]
	Bread Potato	84	14 1	2 ˂1	[127] [127]
	Rice Flour	76.3 70	7.4 12	0.8-2.4 ˂1	[125] [127]

a Excluding soybeans which are composed by 41% protein, 19.6% lipids and 31.6% carbohydrates. b Mainly spinach.

Table 2 .

 2 FOG waste characteristics coming from different origins as reported in literature (DAF (dissolved air flotation), FL (floatable layer), AL (aqueous layer), SL (sludge layer).

	Parame-	COD								Refer-
	ters	(g/L)	TS (g/L)	VS (g/L) VS (%TS)	PH	C/N TKN (g/L) Fat (%)	ences
	27 different	FL: 478	-	-	96.2	-	-	-	48.6
	restau-	AL: 66.2
	rant
	grease
	samples	SL: 107.1								[66]
	Samples	FL: 303.4	-	-	100	3.9-6.2	-	-	-
	from	AL: 66.2
	two food
	service
	establish-
	ments in
	NC	SL: 51.9								[51]
	Grease trap	-	159	158	99.4	5.6	-	-	-
	sludge									[37]
	DAF sludge	-	78	68	87.2	6.8				[37]
	Post	1207	219	185	84.4	-	8.8	2	6
	flotation
	sludge									[63]
	FOG (res-	2698	777	777	100	5	-	-	-
	taurants)									[61,130]
	FOG (res-	1510	258	257	99.6	-	-	-	-
	taurants)									[62]
	FOG (res-	1500	968	955	98.7	-	-	-	-
	taurants)									[58]
	FOG (meat	-	25	25	100	5.1	-	-	-
	pro-
	cessing
	plants)									[54]
	FOG DAF-	-	173	170	98.2	4.4	-	-	-
	treatment
	plant									[5]
	FOG DAF-	298	146	123	84.2	-	20	4287	47
	treatment
	plant									[12]
	FOG DAF-	321	160	143	89.4	-	39	3556	100
	treatment
	plant									[12]
	FOG DAF-	258	126	101	80.1	-	23	3166	38
	treatment
	plant									[12]
	FOG DAF-	177	75	63	84	-	10	3428	15
	treatment
	plant									[12]
	FOG DAF-	196	96	86	89.6	5.5	42	2200	-
	treatment
	plant									[61]
	FOG	-	32	30	93.8	4.2	22	-	-	[68]

Table 3 .

 3 Comparative studies of methane yield during co-digestion of sewage sledge (SS), WAS with food waste (FW).

	Sludge/sub-strate	Mixing ratio (%)	T (°C)	Digester type	OLR (kg/ m 3 d)	SMP (m 3 CH 4 / kg)	Improve-ment	Refer-ences
	SS + FW	50:50	35	ASBR	2.40 (VS)	0.12-0.22 (VS)	×1.85	[73]
	SS + FW	60:40	35	ASBR	3.50 (VS)	0.18 (VS)	-	[131]
	SS + FW	0.9:1	35	CSTR	7.20 (VS)	0.33 (VS)	×1.72	[50]
	*WAS + Biowaste	1:1	35	CSTR	4.80 (VS)	0.42 (VS)	-	[132]
	FW (mono)	75:25	35	TPAD		0.23-0.44 (VS)	×1.87	[100]
	SS + FW	80:20	35	CSTR	n.d	0.27-0.33 (VS)	×1.2	[133]
	WAS + FW + FVW	50:25:25	35	CSTR	1.20	0.56-2.04 (VS)	-	[34]
	WAS + FW	100	37	CSTR	1.22	0.11-077 (VS)		[26]
	PS + WAS + FW	29:71	35	CSTR	10.3	0.95-3.66 (VS)	×3.85	[132]
	WAS + FW	10:90	35	CSTR	n.d	0.19-0.35 (VS)	-	[134]
	WAS + OFMSW	n.d	37	CSTR	n.d	0.21 (VS)	×2.28	[26]

Notes: ORL organic basis units in brackets, SMP organic basis units in brackets, n.d: non-detailed, ×: multiplication factor to the SMP of mono-digestion considering the same units shown for each study. *WAS: Waste Activated Sludge.

Table 4 .

 4 Comparative studies of methane yield during co-digestion of SS and FOG.

	Sludge/substrate	Mixing ratio (%)	T (°C)	Digester type	OLR (kg/ m 3 d)	SMP (m 3 CH 4 / kg)	Improve-ment/inhibi-tion	References
	PS:WAS + FOG	100:0	35	CSRT	1.90 (VS)	0.56 (VS)	-
	(74:26)							[89]
	PS:WAS + FOG	80:20	35	CSRT	2.90 (VS)	0.98 (VS)	×1.75
	(74:26)							[89]
	PS:WAS + FOG	40:60	35	CSRT	3.10 (VS)	1.72 (VS)	×3.07	[89]
	PS:WAS + FOG	60:40	35	CSRT	3.50 (VS)	1.43 (VS) ×2.55/LCFA,
	(74:26)						foaming	[89]
	PS:WAS + FOG	70:30	35	CSRT	1.60 (VS)	0.35 (VS)	Reduced
	(74:26)							[89]
	SS + FOG	77:23	35	CSRT	1.60 (VS)	0.37 (VS)	×1.48	[12]
	SS + grease trap	77:23	36	CSRT	1.58 (VS)	0.63 (VS) ×1.27/LCFA
							@30%	[62]
	PS:WAS + FOG	100:0	35	CSRT	3.0 (VS)	0.83	-
	(50:50)							[5]
	PS:WAS + FOG	90:10	35	CSRT	3.0 (VS)	1.08	×1.3
	(50:50)							[5]
	PS:WAS + FOG	100:0	52	CSRT	2.5 (VS)	0.68	Reduced by
	(50:50)						×0.82	[5]
	PS:WAS + FOG	90:10	52	CSRT	2.4 (VS)	0.83	Same
	(50:50)							[5]
	PS:WAS + FOG	70:30	52	CSRT	4.45 (VS)	2.23	×2.70
	(50:50)							[5]
	SS + FOG	36.6:63.6	35	CSRT	11 COD	0.31	-
						(COD)		[15]
	WAS + FOG	64:36	37	CSRT	2.34 (VS)	0.60 (VS)	×2.37	[95]
	WAS + FOG	48:52	36	CSRT	1.2 (VS)	0.55 (VS) ×2.07/@74%	[61]
	SS + used oil	80:19.4	38	CSRT	0.91 (VS)	0.47 (VS) Inhibition at
							53%	[94]
	SS + FOG	99.8:0.2	34	CSRT	0.77 (VS)	0.30 (VS) Adsorption	[39]
	SS + FOG	34.5:65.5	37	CSRT	2.16 (VS)	0.75 (VS) ×4.18/inhibi-	[58]
							tion @ 83.5%

Notes: ORL: organic basis units in brackets, SMP: organic basis units in brackets, n.d: non-detailed, ×: multiplication factor to the SMP of mono-digestion considering the same units shown for each study.

Acknowledgement

The first author acknowledges the support from the Civil Engineering Department of University College Dublin, scholarship support from Student Universal Support Ireland (SUSI) and Université de Toulouse, Mines Albi, CNRS UMR 5302, Centre RAPSODEE, Campus Jarlard, Albi, F-81013 cedex 09, France.

Disclosure statement

No potential conflict of interest was reported by the authors.