Camilo Murillo Coba
email: camilo.murillo@emse.fr

Xavier Boucher
email: xavier.boucher@mines-stetienne.fr

François Vuillaume

Alexandre Gay

Jesus -Gonzalez-Feliu

Value Proposition in Smart PSS Engineering: Case Study in the Residential Heating Appliance Industry

Keywords: Smart PSS, PSS design, Value Proposition, Risk review

This paper explains the first steps of a smart PSS engineering approach, aimed at eliciting stakeholder needs, prototyping the value proposition, representing how the Original Equipment Manufacturer (OEM) will capture value and share it with a collaborative network of stakeholders while identifying and prioritizing risks from the value proposition. The approach addresses two gaps in the field of smart PSS design: (i) the need of visualizing solutions to support the transformation of value propositions for the stakeholders into a contract mechanism supporting value capture by the offering company and (ii) the importance of integrating risk management during the design of Smart PSS value proposition

Introduction and Scientific Positioning

Long-term strategies of manufacturing companies are dependent on their adoption of new Business Models (BM). The commoditization of products and digital development have posed an opportunity for manufacturing companies to transform their current business models into a digital service-oriented business model, in which value-in-use plays a major role. Since value proposition describes the benefits that customers can expect from the solution proposed by a firm [START_REF] Osterwalder | Value proposition design: How to create products and services customers want[END_REF], a clear design of this proposition seems vital to its market success. However, the formulation of a new value proposition implies several challenges. Among others, a large number of collaborative stakeholders, with distinct non-convergent needs and value expectations requires to be carefully considered during the whole process of solution design. Besides, a new value proposition may create value for several stakeholders (notably customers) but not necessarily generate value for the provider. In order to ensure the capture of value for the provider, a value proposition must be easily replicable in the form of a contract leading to a steady source of revenues. Therefore, design needs adapted mechanisms for economic value capture.

These challenges take special importance in the field of Product-Service Systems (PSS), particularly in 'Smart PSS' defined as "A digital-based ecosystem of value creation characterized by high complexity, dynamics and interconnectedness among stakeholders." [START_REF] Kuhlenkötter | New perspectives for generating smart PSS solutions-life cycle, methodologies and transformation[END_REF]. Value proposition representation and stakeholder mapping, seen as key elements required to build a pertinent usual PSS solution [START_REF] Ilg | Systematic prototyping of product-service systems[END_REF], play an essential role in Smart PSS engineering. A well-defined value proposition is crucial to develop long-lasting relationships with users and its relevance should be maintained over time [START_REF] Valencia Cardona | Challenges in the design of smart product-service systems (PSSs): Experiences from practitioners[END_REF]. Although value-based guidance and evidence of risk-based decisions are highly advisable in the design of a complex system [START_REF] Boehm | The incremental commitment spiral model: Principles and practices for successful systems and software[END_REF], scientific literature lacks guidelines on how to identify and evaluate risks during the phase of Smart PSS value proposition design. For a larger state-of-the-art in the field of value proposition design in PSS, the reader can refer to [START_REF] Fernandes | Towards product-service system oriented to circular economy: A systematic review of value proposition design approaches[END_REF].

Aiming at increasing the industrial applicability of Smart PSS design, this research is developed as a collaborative work with the company Elm Leblanc. Thus, the paper deals with the operationalization of methods for smart PSS value proposition building. More precisely, this paper presents a value proposition design approach complementing current frameworks with the following two specific added values: -The development of an iterative, multi-stakeholder value proposition building approach based on the mapping and visualization of the value creation for each involved stakeholder via a modelling and visualization tool (PS3M) to support the collaborative process [START_REF] Medini | Specifying a modelling language for PSS Engineering-A development method and an operational tool[END_REF].

-The inclusion of risk identification and evaluation in an early stage of the value proposition building processes, to mitigate and monitor them during the whole Smart PSS engineering process.

Thus, this paper presents a structured approach to assist Original Equipment Manufacturers (OEMs) in the definition of solution-oriented value propositions for smart PSS. This approach allows to (i) identify key stakeholders for the Smart PSS, (ii) collect key stakeholder needs, (iii) ideate functionalities to satisfy these needs, (iv) define alternative value propositions, (v) identify value capture mechanisms for the solution provider, and (vi) identify the key risks affecting the solution. A case study in the field of the thermo-technology industry (Elm Leblanc) illustrates the approach.

The structure of this paper is as follows. Section 2 describes the approach for designing and prototyping value propositions for Smart PSS. Section 3 illustrates the application to an industrial case. Finally, we present conclusions and perspectives.

Value Proposition Prototyping in Smart PSS Engineering

As collaborative research with Elm Leblanc, the authors propose to build the approach based on the scientific gaps already mentioned and the feedback from practitioners of Smart PSS engineering. This approach (Fig. 1) presented in this paper, provides an operational guide for the activities required for defining and prototyping the value proposition for a Smart PSS solution. This approach is part of an extended Smart PSS Engineering framework presented in [START_REF] Coba | Towards a riskoriented Smart PSS Engineering framework[END_REF]. This Smart PSS Engineering framework results from the study of PSS design literature, agile approaches for smart PSS, Value proposition design methods [START_REF] Osterwalder | Value proposition design: How to create products and services customers want[END_REF] [9], and Risk management [START_REF] Boucher | Integrated approach for risk management in servitization decision-making process[END_REF]. Thus, the approach described below follows an iterative logic inspired by [START_REF] Boehm | The incremental commitment spiral model: Principles and practices for successful systems and software[END_REF]. This logic facilitates to react to the different types of risks during the whole engineering process. This smart PSS value proposition design includes advances on the two gaps mentioned above, visualization and risks, within various engineering steps. The activities that contribute to risk identification along the whole approach are listed in Table . 1. These activities are associated with some of the framework stages presented in this paper. The identified risks are recorded in a design risk register, with their corresponding assessments of the probability of occurrence and potential impacts. The design team should come up with monitoring strategies to track the high-impact risks. A graphical decision-based computer tool, PS3M [START_REF] Medini | Specifying a modelling language for PSS Engineering-A development method and an operational tool[END_REF], allows capturing visually all useful elements and knowledge contributing to value proposition definition, supporting knowledge sharing among stakeholders along the design process. Hence, PS3M is used to represent graphically the components of the Smart PSS value proposition. It proposes a set of modelling views presenting complementary aspects of the ongoing value proposition, gathering a shared design knowledge base.

The 'Strategic contextualization' step provides the initial inputs with the identification of key stakeholders, key risks, and a representation of an Ecosystem map within PS3M. This step will not be detailed here, since it is considered as an input of subsequent steps explained below.

Collection of Stakeholder Needs

After the 'Strategic contextualisation' step, the challenges of design team are to create value in this existing ecosystem, then to estimate the potential market.

The Collection of Value Expectations of Key Stakeholders is made via Design Thinking [START_REF] Lewrick | The design thinking playbook: Mindful digital transformation of teams, products, services, businesses and ecosystems[END_REF], then integrated with both the visualization and risk approach proposed below. Contextual and in-depth interviews with the key stakeholders are necessary to elicit stakeholders' expectations and problems to be solved. The analysis of these interviews leads to the creation of customer profiles defined in terms of customer jobs, gains, and pains [START_REF] Osterwalder | Value proposition design: How to create products and services customers want[END_REF]. In addition, expectations are classified into five dimensions of value creation, providing a classification of value expectations to support value proposition building: economical, environmental, social, relational, and functional. These analyses help in understanding the different usage contexts linked to the expectations of each key stakeholder. Finally, current customer experiences are mapped as 'customer journeys' to highlight opportunities to improve the value-in-use experience. All these elements of information can be stored and shared within the design team through the PS3M Ecosystem map to ensure that the value proposition devised from expectations analysis will have high desirability.

The Estimation of Value Potential for the OEM is necessary to ensure the incorporation of the value proposition in a profitable business model. Overall estimation of the potential market becomes imperative before further developing the value proposition. This estimation is a qualitative characterization of the market into distinct customer classes together with a quantitative estimation of the market share in the short and middle term. PS3M proposes a specific 'Demand view' to visualize suitable markets along with potential customer classes with their associated use profiles in a shared project knowledge base. These potential market estimations are used as a source to identify risks affecting provider value capture: the risk register is updated.

Value Proposition Prototyping

Expectations identified in the previous step are a prior requirement for the ideation phase. Consequently, the design team must prioritize the jobs, pains, and gains of key stakeholders, considering their importance for each stakeholder [START_REF] Osterwalder | Value proposition design: How to create products and services customers want[END_REF]. Then, the ideation phase consists of a brainstorming session aimed at defining a general value proposition statement, and the functionalities that the solution will offer. These functionalities are captured and represented in the PS3M modeling tool via the combination of two views: the 'product view" depicts tangible and digital elements of the solution in terms of product options and the 'service view" contains intangible elements in terms of service packages. These representations ease risk identification based on of the capabilities required to deliver the services included in the value proposition. The risk register is updated accordingly.

The general value proposition statement has then to be refined to make explicit how physical, digital, and intangible elements generate value for the stakeholders. To support this activity, we propose the use of Value maps [START_REF] Osterwalder | Value proposition design: How to create products and services customers want[END_REF], which efficiency in Smart Services' design is proven: iteratively, a value map is developed for each key stakeholder separately, resulting in several alternative value propositions for each. These alternative value propositions are structured and formulated as 'service packages' targeted at different customer classes. Then, referring to all previous design knowledge already gathered, the design team defines decision criteria leading to choose the most appropriate value proposition for each key stakeholder, considering OEM's value expectations. At this point in the engineering process, validation of the chosen value propositions is conducted with OEM's internal actors that are in close contact with customers. This validation aims at examining the feasibility of the value proposition, from both a technical and market viability points of view.

Finally, the success conditions of key stakeholders are identified, then displayed in a diagram and registered in the PS3M tool. This representation allows identifying risks related to the potential incompatibilities amongst the success models of each key stakeholder [START_REF] Boehm | The incremental commitment spiral model: Principles and practices for successful systems and software[END_REF], since neglecting them may lead to the failure of the system delivering the PSS solution. These risks are recorded in the Smart PSS design risk register.

Value Capture Mechanism Design

Designing a value proposition requires presenting evidence of the profitability and scalability of the economic model associated with the value proposition [START_REF] Osterwalder | Value proposition design: How to create products and services customers want[END_REF]. Since previous research has little addressed this aspect, either for traditional [START_REF] Fernandes | Representing the value proposition of Product-Service Systems (PSS) in a value-based perspective[END_REF], or Smart PSS design [START_REF] Neuhüttler | Applying value proposition design for developing smart service business models in manufacturing firms[END_REF][START_REF] Liu | A framework integrating interval-valued hesitant fuzzy DEMATEL method to capture and evaluate co-creative value propositions for smart PSS[END_REF], it is important to define, at early stage design, the roles of the contract's owner, the revenue mechanism and the possible penalties when the expected functionalities and/or outcomes of the contract are badly delivered. A very clear definition of the contract helps to avoid any failure in value delivery [START_REF] Reim | Risks for functional products-Empirical insights from two Swedish manufacturing companies[END_REF].

In this step, the design team explores the different economic models associated with a Smart PSS value proposition, namely, product-oriented, use-oriented, and result-oriented. Aiming at addressing the gap mentioned in the previous paragraph, we propose to draft the Smart PSS contract via the PS3M's specific 'Offer view'. This representation aims at visualizing the alternative paths to deliver the Smart PSS solution, in commercial terms: (i) type of selling contracts, (ii) content of the contracts in terms of products, digital components, and services, (iii) demand forecasting, (iv) characteristics of the customers that are likely to sign the drafted contract. This representation leads to a first global definition of the cost and revenue structures associated with the Smart PSS value proposition.

Risk Review

The risk register contains the list of all risks identified and assessed through the process of design and prototyping of the value proposition. The classical stages of risk management, namely, identification, evaluation, and prioritization of risks are performed. If after having executed these stages, the OEM finds an acceptable level of risk, it will pursue the engineering process while monitoring and applying such mitigation strategies. On the contrary, if the OEM concludes that the risk level is too high, it will decide to reevaluate the value proposition, target another customer segment, or address other stakeholder jobs, pains, and gains, avoiding in this way to pursue efforts that might compromise the OEM's financial health.

Case Study

We illustrate the application of our value proposition prototyping framework via a deductive case study [START_REF] Bitektine | Prospective case study design: qualitative method for deductive theory testing[END_REF] starting from the evolution of a solution developed by a fabricant of gas boilers (the OEM) mainly addressed to social housing. The OEM also performs the maintenance of its products as part of its current service catalog. Converting the current product and service offering into a Smart PSS solution would enable the target customer and maintenance companies to monitor and operate a fleet of gas boilers. To assess the potential of this change, the proposed approach is applied by modelling suitable Smart PSS configurations using the PS3M toolkit: a focus group (design group) within the company was consulted and both primary and secondary data were collected and structured as follows.

Collection of Stakeholder Needs

Firstly, the design group elaborated a mapping of the current business ecosystem associated with the selling and maintenance of gas boilers, for the target customer. This mapping enabled the OEM to identify several key stakeholders, including endusers of the products, maintenance firms, and energy companies among others. Different risks were identified and recorded in a risk register. The design team identified the stages of the activity cycle of the target customer, namely configuration of the thermal system, installation, commissioning, operation, monitoring, maintenance, repair, and scraping. The OEM decided that the Smart PSS solution would be aimed at the operation and monitoring stage.

In-depth interviews were conducted with social landlords, tenants of the social housing units, and maintenance companies, with the objective to identify their needs. From the interviews' analysis, customer personas were created for end-users and maintenance staff personnel. Next, a list of customer jobs, pains, and gains are associated with each customer profile. These insights were categorized into value dimensions, e.g., unnecessary technical interventions impacting the monetary value dimension of the maintenance provider. Then, customer journeys were mapped based on the identified customer pains. These mappings were focused on depicting certain aspects of the user experience concerning the preventive and curative maintenance of the gas boilers, the gas boiler fleet management, the installation of the boiler, among others. The insights obtained with these mappings were used to formulate the potential functionalities of the Smart PSS solution. In parallel, an estimation of the potential market targeted at social landlords was conducted and then represented on the PS3M toolkit.

Value Proposition Prototyping

The needs of key stakeholders captured in the previous stage and expressed in user stories are used to carry out a creativity workshop. At the end of this workshop, the main functionality of the Smart PSS solution was defined, along with a catalog of services included in the solution. This catalog, modelled via PS3M, was used to identify risks concerning the deployment of the digital services. The most important risks associated with the deployment of these services concern data security, data storage, and data processing in real-time.

The design team linked gain creators and pain relievers to each of the envisioned services, by using PS3M (Fig. 2). Each of these services is related to the key stakeholders that will benefit from the service. Then, a discussion on the usage contexts of the solution helped to define service packages that addressed the different usage contexts of the customers, the social landlords, and the maintenance providers.

Value Capture Mechanism

In order to structure the content and terms of the contract, the design team used the PS3M tool to represent visually the components of the contract. The design team decided to sell the hardware and to offer a service contract to either the maintenance provider or the social landlord. This service contract offers a range of smart services, namely, breakdown prediction, remote control of the fleet of gas boilers, energy consumption monitoring, and remote optimization of the settings of the thermal system. This visual representation triggered the discussion within the design about the conditions of the contract, such as the billing plan deadlines and the terms of payment.

Risk Review

Throughout this value proposition design process, the most important identified risk concerned the identification of the key stakeholders to commercialize the Smart PSS solution. A previous project of the OEM, related to a similar remote-monitoring solution, showed that assuming the wrong stakeholder as a client led to the commercial failure of the Smart PSS solution. In that case, the end-user of the boiler was defined as the client of the solution. However, end-users were not interested in acquiring a solution to identify breakdowns of the product, as they expected the product to work properly all the time: a specific manner to control such a risk had to be devised. Other key risks detected during the next steps of the engineering process dealt with data theft and potential external sabotage of the system.

Conclusions

This paper proposed a Value Proposition Prototyping approach aimed at assisting OEMs in the definition of digital service-oriented value propositions. The main contributions of the research dealt with the development of a multi-stakeholder collaborative prototyping approach based on a graphical modelling tool (PS3M) together with the inclusion of risk detection and review linked to value creation. A case study illustrates the approach and underlines the complexity of transforming a usual product-based business model into a smart PSS solution.

The application of the proposed methodology remains at a preliminary level and needs more in-depth developments but already shows the potential of the proposed approach. Further work is required to include a method to evaluate the value proposition(s) that results from the ideation and selection stages. A quantitative risk evaluation may be appropriate in this context. The approach proposed in this paper is expected to be applied in a case study targeted at the private housing market. This new case study involves the participation of a larger number of stakeholders.

Fig. 1 .

 1 Fig. 1. Overall view of the prototyping approach.

Fig. 2 .

 2 Fig. 2. Value proposition representation on PS3M.

Table 1 .

 1 Activities contributing to risk identification during value proposition design

Stage of the approach Activities for risk identification

	Strategic contextualization	PEST and SWOT analyses, Ecosystem mapping
	Collection of stakeholders' needs	Potential market estimation
	Value proposition prototyping	Service catalog representation on PS3M, Value
		model proposition Model-Clash Spider Web [5]

Acknowledgments. Part of this work supported by EU DIGIFoF Erasmus+ Project.

Therefore, the complexity of the value proposition prototyping tasks increases, making it possible to test the capacity of this work to deal with higher complexity.


Xin Yang

Hongbo Liu

Yangyang Yao

Eric Lichtfouse
email: eric.lichtfouse@gmail.com

Improved electricity production and nitrogen enrichment during the treatment of black water using manganese ore-modified anodes

Keywords: Black water, Modified electrodes, Ammonium enrichment, Treatment performance, Resource recovery

The decline of fossil fuel availability is calling for alternative energy such as electricity produced by degradation of waste in microbial fuel cells. The performance of microbial fuel cells is controlled by the anode because microorganisms grow and attach on the anode. Here, we prepared carbon felt electrodes modified with manganese ore based on the hypothesis that manganese and silicon oxides would improve the cell performance to treat simulated black water. We compared properties of the manganese ore-modified electrodes with properties of classical carbon felt electrodes modified with magnetic ferroferric oxide. We also studied the removal of the chemical oxygen demand and the enrichment in ammonia nitrogen. Results show that the maximum voltage of the manganese ore-modified carbon felt electrode is 1.7 times higher than that of the magnetic ferroferric oxide-modified carbon felt electrode. The start-up time to reach the maximum voltage is reduced by 68%. The removal of chemical oxygen demand reached 91.2%. Up to 68.0% of ammonia nitrogen can be enriched at the cathode, imply-ing that ammonia nitrogen can be highly reduced in the effluent. Overall, the manganese ore-modified carbon felt electrode showed excellent redox performance, excellent corrosion resistance and low electron transfer resistance.

Introduction

Due to the rapid development of industrialization and urbanization, a large amount of industrial wastewater and domestic sewage is discharged [START_REF] Panagopoulos | Brine management (saline water & wastewater effluents): sustainable utilization and resource recovery strategy through minimal and zero liquid discharge (MLD & ZLD) desalination systems[END_REF][START_REF] Hasan | Health risk and water quality assessment of surface water in an urban river of Bangladesh[END_REF]. This has led to the deterioration of the water environment and caused shortage of water resources [START_REF] Panagopoulos | Environmental impacts of desalination and brine treatment -challenges and mitigation measures[END_REF][START_REF] Kurwadkar | Unregulated discharge of wastewater in the Mahanadi River Basin: risk evaluation due to occurrence of polycyclic aromatic hydrocarbon in surface water and sediments[END_REF][START_REF] Panagopoulos | Water-energy nexus: desalination technologies and renewable energy sources[END_REF]. Microbial fuel cells are developed devices that convert the chemical energy stored in organic matter into electricity by microbial metabolic processes [START_REF] Tahir | MnCo 2 O 4 coated carbon felt anode for enhanced microbial fuel cell performance[END_REF]. Since treatment of wastewater by microbial fuel cells can both degrade wastewater and produce electricity, microbial fuel cells have attracted attention as a promising green energy source. [START_REF] Zang | Nutrient removal and energy production in a urine treatment process using magnesium ammonium phosphate precipitation and a microbial fuel cell technique[END_REF]. Specifically, the biodegradation of wastewater at the anode generates electrons and protons which are recycled to the cathode to produce electricity [START_REF] Samsudeen | Bioelectricity production from microbial fuel cell using mixed bacterial culture isolated from distillery wastewater[END_REF]. Therefore, the anode is a key limiting step, whose efficiency controls the overall performance [START_REF] Fan | Quantification of the internal resistance distribution of microbial fuel cells[END_REF][START_REF] Ho | Cobalt oxides-sheathed cobalt nano flakes to improve surface properties of carbonaceous electrodes utilized in microbial fuel cells[END_REF].

Common anode materials such as graphite, carbon cloth, carbon felt and activated carbon are not efficient enough in terms of energy recovery [START_REF] Liu | High performance of microbial fuel cell afforded by metallic tungsten carbide decorated carbon cloth anode[END_REF][START_REF] You | Recent progress of carbonaceous materials in fuel cell applications: an overview[END_REF]. Alternatively, anodes made of precious metals such as platinum could improve the degradation of contaminants [START_REF] Xu | Effect of anodes decoration with metal and metal oxides nanoparticles on pharmaceutically active compounds removal and power generation in microbial fuel cells[END_REF]), yet such metals are expensive. Therefore, researchers have focused on developing cheaper metals such as magnetic ferric tetroxide, which presents unique physical and chemical properties, small particle size, narrow distribution, stable surface properties, and good biocompatibility [START_REF] Park | Enhanced electrical contact of microbes using Fe 3 O 4 /CNT nanocomposite anode in mediator-less microbial fuel cell[END_REF][START_REF] Peng | Enhanced performance and capacitance behavior of anode by rolling Fe3O4 into activated carbon in microbial fuel cells[END_REF][START_REF] Li | A renewable potentiometric immunosensor based on Fe3O4 nanoparticles immobilized anti-IgG[END_REF]. Manganese ore is attracting attention because the presence of Xin Yang and Hongbo Liu contribute equally to this work. manganese oxides improves catalysis and removal of organic matter [START_REF] Xie | Enhanced triclosan and nutrient removal performance in vertical up-flow constructed wetlands with manganese oxides[END_REF]. Indeed, MnO 2 is a common redox agent and is often used to decrease the chemical oxygen demand of wastewater [START_REF] Gao | Novel carbon fiber cathode membrane with Fe/Mn/C/F/O elements in bio-electrochemical system (BES) to enhance wastewater treatment[END_REF]. It is possible to improve corrosion resistance and redox properties of electrodes by modification of the electrodes with the manganese ore since it contains manganese oxides, silicon oxides and other metal oxides [START_REF] Li | Enhanced electricity generation and extracellular electron transfer by polydopamine-reduced graphene oxide (PDA-rGO) modification for high-performance anode in microbial fuel cell[END_REF].

The performance of electrode materials can be improved by modification with both metal and non-metal materials [START_REF] Qiao | Carbon nanotube/polyaniline composite as anode material for microbial fuel cells[END_REF][START_REF] Zhu | Improved performance of membrane free single-chamber air-cathode microbial fuel cells with nitric acid and ethylenediamine surface modified activated carbon fiber felt anodes[END_REF][START_REF] Liu | Anode modification with formic acid: a simple and effective method to improve the power generation of microbial fuel cells[END_REF]. For instance, due to the lower resistance of charge transfer, modifying a carbon paper anode with iron oxide increased the output power density by 31% [START_REF] Harshiny | Biosynthesized FeO nanoparticles coated carbon anode for improving the performance of microbial fuel cell[END_REF]). Similarly, a microbial fuel cell carbon felt anode modified with MnO 2 improved the output power to 3580 mW/m 2 , which is 24.7% higher than the unmodified anode [START_REF] Zhang | Enhanced power generation of microbial fuel cell using manganese dioxide-coated anode in flow-through mode[END_REF].

Many researchers focus on the removal of pollutants in water with microbial fuel cells. For example, by using air-cathode microbial fuel cells for wastewater treatment, ammonia nitrogen can be removed up to 99.0% (Yang et al. 2020a, b). Nitrification/denitrification can be enhanced by constructing a multi-anode microbial fuel cell [START_REF] Huang | The relationship between energy production and simultaneous nitrification and denitrification via bioelectric derivation of microbial fuel cells at different anode numbers[END_REF]. The removal of organic matter and nutrients from microbial fuel cells can be enhanced by pre-treatment modifications, such as chemical and thermal treatment of carbon brushes [START_REF] Shahid | Effect of modified anode on bioenergy harvesting and nutrients removal in a microbial nutrient recovery cell[END_REF]. However, under the context of circular economy, ammonia nitrogen as a resource should be recovered as much as possible. In this study, we explored the enrichment and recovery of ammonia nitrogen from black water. By doing this, we used the inexpensive and widely available manganese ore for anode modification and prepared carbon felt anodes modified with various amounts of manganese ore and magnetic ferroferric oxide. Besides the enrichment effects on ammonium nitrogen, we also investigated their performance on treating simulated black water and producing electricity in a wastewater microbial fuel cell.

Experimental

Materials

We constructed a dual-chamber and a single-chamber microbial fuel cell; the sizes and start-up method are shown in Supplementary Text S1. The microbial fuel cell was started using the open-circuit constant potential start-up method that we have previously used [START_REF] Ren | Accelerated start-up and improved performance of wastewater microbial fuel cells in four circuit modes: role of anodic potential[END_REF]. The main composition of the experimentally simulated black water is 1200-1500 mg/L chemical oxygen demand, 90-120 mg/L ammonia nitrogen (NH 4 + -N) and 8-15 mg/L phosphate (PO 4 3--P). Detailed composition of the simulated black water and composition of trace elements in the stock solution are given in Supplementary Tables S1 andS2, respectively.

Preparation of modified anode materials

Manganese ore tailings were obtained from a manganese ore production site in Guangxi, China, which were crushed and passed through a 200-mesh sieve to obtain the powdered manganese ore. Main constituents of the crushed manganese ore powder were determined using the X-ray diffraction analysis (Fig. S1a). The magnetic ferroferric oxide was produced using the co-precipitation method [START_REF] Chen | Facile synthesis and magnetic properties of monodisperse Fe 3 O 4 /silica nanocomposite microspheres with embedded structures via a direct solution-based route[END_REF]). X-ray diffraction analysis of the composition of the produced magnetic ferroferric oxide was assessed (Fig. S1b).

The mixture of manganese ore powder (0.8 g), acetylene black (0.1 g) and polyvinylidene fluoride (0.1 g) was mashed to powder in an agate mortar; 1.0 g of this powder was added to 10 mL of N-methylpyrrolidone in a beaker and stirred magnetically for 4 h to form a slurry. This slurry was poured evenly on a 5 × 4 cm carbon felt and coated with a film applicator; then, the coated electrode was dried for 10 h in a vacuum oven at 102 °C to obtain manganese ore-modified carbon felt electrodes with a loading of 1.0 g. According to the above method, 0.5 g, 1.0 g and 1.5 g loaded manganese ore-modified carbon felt electrodes and magnetic ferroferric oxide-modified carbon felt electrodes were prepared similarly. Detailed preparation processes of the manganese ore-modified carbon felt electrodes and magnetic ferroferric oxide-modified carbon felt electrodes could refer to Supplementary Tables S3.

Analysis methods and data acquisition

Electrochemical analysis

Electrochemical properties of the manganese ore-modified carbon felt electrode and the magnetic ferroferric oxidemodified carbon felt electrode were determined using the electrochemical workstation CHI660E (Shanghai Chenhua). A three-electrode system was used for electrochemical analysis, with a platinum electrode as the counter electrode, the modified anode as the working electrode, the saturated calomel electrode as the reference electrode and a 1 mM potassium ferricyanide solution as the electrolyte.

Measurements and calculations

Chemical oxygen demand was measured with the potassium dichromate method [START_REF] Dhanjai | Water analysis|determination of chemical oxygen demand[END_REF]. Ammonia nitrogen was measured with the spectrophotometric method [START_REF] Lin | Determination of ammonia nitrogen in natural waters: Recent advances and applications[END_REF]) and total phosphorus with the ammonium molybdate spectrophotometric method [START_REF] Labry | Addressing the measurement of particulate organic and inorganic phosphorus in estuarine and coastal waters[END_REF]). The voltage was recorded every 1 min using the PicoLog online data collector. This closed loop current was calculated according to Ohm's law, and the microbial fuel cell produces a current density I (mA/m 2 ) that can be obtained by calculating with the equation I = U/(RS). The power density (mW/ m 2 ) was calculated as P = IU, where U is the voltage (mV), R is the resistance (Ω), and S is the surface area of the anode (m 2 ) [START_REF] Du | Nitrification and denitrification in two-chamber microbial fuel cells for treatment of wastewater containing high concentrations of ammonia nitrogen[END_REF].

Results and discussion

Carbon felts modified with different loadings of manganese ore and magnetic ferroferric oxide were used as anodes for microbial fuel cells to explore their performance in treating black water.

Electrochemical characterization

Cyclic voltammetry curves of our manganese ore-modified carbon felt electrodes display redox peaks with higher symmetry than the magnetic ferroferric oxide-modified carbon felt electrodes, and the distance between the redox peaks is smaller (Fig. 1a,b). This indicates that the manganese ore-modified carbon felt electrodes has better reversibility after the redox reaction and excellent catalytic oxidation. This observation implies that the manganese ore-modified carbon felt electrode has lower redox resistance and high reversibility favoring the redox reaction, according to the literature [START_REF] Qiu | Enhanced degradation of diclofenac with Ru/Fe modified anode microbial fuel cell: kinetics, pathways and mechanisms[END_REF].

The curves of the manganese ore-modified carbon felt electrode and the magnetic ferroferric oxide-modified carbon felt electrode are similar in the high-frequency region, showing a rounded semicircle (Fig. 1c,d). However, as shown in Table 1, the minimum polarization resistances are 1.0 Ω/cm 2 for the 0.5 g manganese ore-modified carbon felt electrode, 2.0 Ω/cm 2 for the 1.0 g magnetic ferroferric oxide-modified carbon felt electrode and 2.0 Ω/cm 2 for the carbon felt electrode (the polarization resistance values of the other electrodes are shown in Supplementary Table S4). The lowest polarization resistance of the manganese oremodified carbon felt electrode is likely due to the fact that the manganese oxide has a lower charge transfer resistance compared with the magnetic ferroferric oxide [START_REF] Zhang | Binder-free graphene and manganese oxide coated carbon felt anode for high-performance microbial fuel cell[END_REF]. A lower polarization resistance reduces the transfer resistance of electrons and increases the electron mobility [START_REF] Fan | Different modified multi-walled carbon nanotube-based anodes to improve the performance of microbial fuel cells[END_REF]. This is in agreement with the literature verifying that a small amount of manganese ore can improve the electron transfer rate [START_REF] Aswin | The internal resistance of a microbial fuel cell and its dependence on cell design and operating conditions[END_REF]. Another explanation is that manganese oxide is known to favor microbial attachment and improve the electron transfer rate [START_REF] Zhou | Enhanced copper-containing wastewater treatment with MnO 2 /CNTs modified anode microbial fuel cell[END_REF]Yang et al. 2020a, b). In addition, for the same mass of manganese dioxide or iron oxide, the amount of electrons absorbed by manganese is higher than that of iron [START_REF] Jiménez-Cedillo | Adsorption capacity of iron-or iron-manganese-modified zeolite-rich tuffs for As(III) and As(V) water pollutants[END_REF], which could be one of the reasons for the lower resistance of manganese ore-modified carbon felt electrode. Therefore, the electron transfer resistance of the manganese ore-modified carbon felt electrode is lower. Manganese ore-modified carbon felt electrode rich in manganese dioxide makes the electron absorption transfer easier.

Tafel polarization curves demonstrate that the corrosion voltage is -0.10 V for the 0.5 g manganese ore-modified carbon felt electrode versus -0.18 V for the unmodified carbon felt electrode (Fig. 1e,f). In contrast, negative voltages for magnetic ferroferric oxide-modified carbon felt electrodes are maximizing at -0.22 V. The higher corrosion resistance of manganese ore-modified carbon felt electrodes is explained by the higher resistance of manganese oxides and SiO 2 to corrosion [START_REF] Anzini | The effect of manganese and silicon additions on the corrosion resistance of a polycrystalline nickel-based superalloy[END_REF]. The loading of ferroferric oxide leads to more corrosion of the electrode because the ferroferric oxide is an unstable intermediate that tends to rust when contacting air and water. Therefore, manganese ore carbon felt electrodes have better corrosion resistance.

Overall, we found that the manganese ore-modified carbon felt electrode has better redox performance, lower polarization resistance and higher corrosion resistance than magnetic ferroferric oxide-modified carbon felt and carbon felt electrodes, which is more suitable for practical use.

Performance of the single-chamber wastewater microbial fuel cell

The anode materials were 0.5 g manganese ore-modified carbon felt electrode and 1.5 g magnetic ferroferric oxide-modified carbon felt electrode, respectively. Results show that the manganese ore-modified carbon felt electrode increased power production to a maximum voltage of 215 mV in 1100 min, whereas it took 3500 min for the magnetic ferroferric oxide-modified carbon felt electrode to increase power production to a maximum of 124 mV (Fig. 2a). We calculated the maximum power density of the manganese oremodified carbon felt electrode (23.11 mW/m 2 ) as 3.0 times higher than that of the magnetic ferroferric oxide-modified carbon felt electrode (7.68 mW/m 2 ). This might be explained by the fact that despite the excellent electrical conductivity of the magnetic ferroferric oxide-modified carbon felt electrode [START_REF] Song | Bacteria-affinity 3D Macroporous graphene/MWCNTs/ Fe 3 O 4 foams for high-performance microbial fuel cells[END_REF], there is more biofilm formation on this electrode, and in turn its electron transfer is limited. This hypothesis is strengthened by a study showing that the grafting of Fe 2+ onto a graphite polyester composite electrode leads to higher specific surface area and pore space, more biofilm formation and lower power density than the unmodified electrode [START_REF] Narayanasamy | Improved performance of Pseudomonas aeruginosa catalyzed MFCs with graphite/polyester composite electrodes doped with metal ions for azo dye degradation[END_REF]. On the other hand, a study showed that the electrode performance of iron oxide modified electrode was unstable, and the magnetic ferroferric oxide itself was also an unstable iron intermediate oxide which is easily oxidized to generate iron oxide rust and seriously affects the electron transfer effect [START_REF] Fu | Low electrical potential anode modified with Fe/ferric oxide and its application in marine benthic microbial fuel cell with higher voltage and power output[END_REF]. Moreover, manganese dioxide increases the electrode wettability and the attachment of microorganisms that allows electronic transfer [START_REF] Gohari | Improving performance and antifouling capability of PES UF membranes via blending with highly hydrophilic hydrous manganese dioxide nanoparticles[END_REF]. In contrast, the electrode resistance of manganese ore-modified carbon felt electrode is lower, so the manganese ore-modified carbon felt electrode is easier for electron transfer. And its time to reach the maximum voltage is faster. Overall, we find that the manganese ore-modified carbon felt electrode produces more electricity and has a shorter start-up time and a greater power density.

As shown in Fig. 2b andc, the chemical oxygen demand removal effect of both electrodes is similar. The removal rate reaches 91.2% for the manganese ore-modified carbon and(e, f) Tafel polarization curves. Note manganese ore-modified carbon felt (Mn-CF) electrode has better redox peak and higher corrosion voltage felt electrode and 89.5% for the magnetic ferroferric oxidemodified carbon felt electrodes. The removal of ammonia nitrogen by the manganese ore-modified carbon felt electrode (84.5%) is slightly higher than that of the magnetic ferroferric oxide-modified carbon felt electrode (72.2%). Some scholars have improved the activity of microbial fuel cells by electrodepositing manganese dioxide on the anode, with chemical oxygen demand removal rates reaching close to 100% and generating more electricity [START_REF] Phonsa | Electrodeposition of MnO 2 on polypyrrole-coated stainless steel to enhance electrochemical activities in microbial fuel cells[END_REF]. By using activated carbon combined with iron and nitrogen carbon nanospheres as cathodes, the chemical oxygen demand removal rate of 80.2% could be achieved [START_REF] Zhang | Highly efficient improvement of power generation and waste removal in microbial fuel cell by associating activated carbon with a novel porous iron-nitrogen-doped carbon nanosphere[END_REF]. It has been proposed that MnO 2 can act as a redox mediator to accelerate the electron transfer between microbial cells and pollutants, thus facilitating the removal of pollutants [START_REF] Linx | Activated carbon modified with nano manganese dioxide triggered electron transport pathway changes for boosted anaerobic treatment of dyeing wastewater[END_REF]. Other metal oxides in manganese ore increase the surface roughness and specific surface area of the electrode; the manganese ore-modified carbon felt electrode can be loaded with more microorganisms [START_REF] Zhang | Carnation-like MnO 2 modified activated carbon air cathode improve power generation in microbial fuel cells[END_REF]. For the magnetic ferroferric oxide-modified carbon felt electrode, the magnetic ferroferric oxide itself is superparamagnetic and the trivalent iron can be used as an electron acceptor for microbial metabolism. Microorganisms can use Fe 2+ /Fe 3+ as a redox mediator to accelerate the decomposition of organic matter in microbial fuel cells [START_REF] Lovley | Dissimilatory Fe(III) and Mn(IV) reduction[END_REF]). Therefore, although the manganese oremodified carbon felt electrode is slightly more effective than the magnetic ferroferric oxide-modified carbon felt electrode for pollutant removal, the removal rates are relatively close.

Treatment effects of dual-chamber wastewater microbial fuel cells and optimization of ammonia recovery

The manganese ore-modified carbon felt electrode was used as the anode of the dual-chamber microbial fuel Fig. 2 Comparison of (a) power generation, b chemical oxygen demand (COD) removal and c ammonia removal for manganese oremodified carbon felt (Mn-CF) and magnetic ferroferric oxide-modi-fied carbon felt (Fe-CF) electrodes. Note that the manganese oremodified carbon felt (Mn-CF) electrode has the highest voltage and the fastest rise, but the removal effect is similar for both electrodes cell. The effects of different influent ammonia concentrations on its performance were investigated. The results are shown in Fig. 3. Under low concentration of influent ammonia, the ammonia enrichment rate of the cathode chamber reached 68% which was higher than that of the medium concentration condition (41%) and the high concentration condition (34%). The chemical oxygen demand removal rate reached 86.7% at low concentrations, 90.5% at medium concentrations and 87.1% at high concentrations, respectively. The low concentration of influent ammonia is more favorable for the enrichment of ammonia nitrogen in the cathode chamber.

The transfer of ammonia from the anode chamber through the proton membrane to the cathode chamber is mainly driven by the potential difference and concentration difference [START_REF] Kim | Analysis of ammonia loss mechanisms in microbial fuel cells treating animal wastewater[END_REF]. Under medium concentration conditions, the excellent redox performance and low electron transfer resistance of the manganese ore-modified carbon felt electrode enabled the microorganisms to degrade the organic carbon source in the early stage rapidly. In this case, the chemical oxygen demand removal reached 76.0% on the first day. These two properties lead to a low organic carbon source during the reaction, which generates low electric field forces in the late stage and therefore does not promote ammonia transfer effectively. Under high-concentration conditions, the organic carbon source is sufficient. But the ammonia nitrogen transfer rate is still relatively low. This is probably because the high concentration of ammonia nitrogen has a negative effect on microorganisms, thus leading to the insufficient electric field driving force [START_REF] Puig | Microbial fuel cell application in landfill leachate treatment[END_REF]. Therefore, the manganese ore-modified carbon felt electrode is more favorable for the enrichment and potential recovery of ammonia nitrogen in the cathode chamber. Meanwhile, its removal rate of chemical oxygen demand remains high under low concentration of influent ammonia.

We conducted further investigations to enrich ammonia at the cathode using the driving effect of electric field force. In particular, we added a phosphate buffer to the cathode to enrich the cathodic medium in ammonia and phosphate ions. Then, to ensure the removal of ammonia nitrogen and to avoid wasting buffer solution, magnesium ions were added to the cathode chamber for the recovery of struvite (Fig. 3e). The reaction is: According to Eq. 1, a 1:1:1 molar ratio of magnesium ion, phosphate ion and ammonia ion allows optimal precipitation of struvite. Therefore, after complete reaction of ammonia nitrogen at low concentrations, we transferred 50 mL of cathode solution into a beaker, adding magnesium sulfate heptahydrate, stirred magnetically, settled for 5 min and recovered struvite by filtration (Fig. 3f). Ammonia was found undetectable in the filtrate. Therefore, our method allow complete recovery of ammonia nitrogen as precipitated struvite from the cathodic medium of the dual-chamber microbial fuel cell.

Ammonia nitrogen was successfully recovered at the cathode, which is beneficial to the recycling of resources. This work presents a method of using modified electrodes to recover resources in microbial fuel cell treatment of black water. This can generate electricity and recover ammonia nitrogen while removing chemical oxygen demand in black water. If this approach can be improved (1)

Mg 2+ + PO 3- 4 + NH + 4 + 6H 2 O → MgNH 4 PO 4 ⋅ 6H 2 O ↓
and expanded, then microbial fuel cells can recover more resources and will relieve resource pressure.

Conclusion

A manganese ore-modified carbon felt electrode was successfully prepared. This electrode has excellent redox performance and lower polarization resistance that is more favorable for electron transfer, which brings about better power generation. The maximum voltage of the manganese oremodified carbon felt electrode is 1.7 times higher than that of the magnetic ferroferric oxide-modified carbon felt electrode. Meanwhile, the start-up time to reach the maximum voltage is 68% shorter than that of the magnetic ferroferric oxide-modified carbon felt electrode, which can achieve faster start-up of the system. The application of manganese ore-modified carbon felt electrode in microbial fuel cells can improve the removal of chemical oxygen demand and enrichment of ammonia nitrogen when treating black water. Ammonia nitrogen was successfully recovered at the cathode, which is beneficial to the recycling of resources.

Fig. 1

 1 Fig. 1 Comparison of electrochemical characterization of carbon felt (CF), manganese ore-modified carbon felt (Mn-CF) and magnetic ferroferric oxide-modified carbon felt (Fe-CF) electrodes with different loadings: a, b cyclic voltammetric curves (reference electrode:

Fig. 3

 3 Fig. 3 Comparison of ammonia recovery and ammonia nitrogen transfer effect and chemical oxygen demand (COD) removal effect under different influent ammonia concentrations: a low concentration, b medium concentration, c high concentration, d removal effect,

Acknowledgements

The authors would like to acknowledge the cofunding of this work by the National Natural Science Foundation of China (No. 52070130) and the Natural Science Foundation of Shanghai (No. 22ZR1443200).

Supplementary Information

The online version contains supplementary material available at https:// doi. org/ 10. 1007/ s10311-022-01490-1.

Declarations

Conflict of 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.

Supplementary Material

Improved electricity production and nitrogen enrichment during the treatment of black water using manganese ore-modified anodes Text S1 Microbial fuel cell device size and start-up method

We constructed a dual-chamber and a single-chamber microbial fuel cell. The dual-chamber microbial fuel cell system is made of 0.5 cm thick plexiglas, and the cathode and anode chambers each consist of a square plexiglas containers, and is separated from the anode chamber by a Nafion117 proton exchange membrane. The dimensions of both cathode and anode chambers are 6 x 6 x 6 cm; the anode is sealed and the cathode is aerated with an aeration head. The anode is a 5 x 4 cm modified electrode, and the cathode is a 5 x 4 cm stainless steel wire mesh connected to a 1000Ω resistor through a titanium wire to form a circuit. The single-chamber microbial fuel cell has the same configuration and dimensions as the dual-chamber device, except without a proton exchange membrane between the cathode and anode chambers.

The single-chamber microbial fuel cell and dual-chamber microbial fuel cell systems were started simultaneously using