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The complicated interactions that occur in mixed-species biotechnologies, including biosensors, hinder chemical detection specificity. This lack of specificity limits applications in which biosensors may be deployed, such as those where an unknown feed substrate must be determined. The application of genomic data and well-developed data mining technologies can overcome these limitations and advance engineering development. In the present study, 69 samples with three different substrate types (acetate, carbohydrates and wastewater) collected from various laboratory environments were evaluated to determine the ability to identify feed substrates from the resultant microbial communities. Six machine learning algorithms with four different input variables were trained and evaluated on their ability to predict feed substrate from genomic datasets. The highest accuracies of 93 ± 6% and 92 ± 5% were obtained using NNET trained on datasets classified at the phylum and family taxonomic level, respectively. These accuracies corresponded to kappa values of 0.87 ± 0.10, 0.86 ± 0.09, respectively. Four out of six of the algorithms used maintained accuracies above 80% and kappa values higher than 0.66. Different sequencing method (Roche 454 or Illumina sequencing) did not affect the accuracies of all algorithms, except SVM at the phylum level. All algorithms trained on NMDS-compressed datasets obtained accuracies over 80%, while models trained on PCoA-compressed datasets presented a 10–30% reduction in accuracy. These results suggest that incorporating microbial community data with machine learning algorithms can be used for the prediction of feed substrate and for the potential improvement of MFC-based biosensor signal specificity, providing a new use of machine learning techniques that has substantial practical applications in biotechnological fields.

We propose targets, based on real world data, necessary to design a financially viable microbial electrolysis cell (MEC) for the treatment of domestic wastewater. By reducing the cost of the anode and current collecting materials by 90%, a viable organic loading rate would be between 800 and 1,400g-COD/m 3 /d (2–3A/m 2 ). The anode and current collector materials account for 94% of the total material costs; consequently, cost savings in any other material are moot. If the bioanode can be reused after 20 years, further, significant savings could be achieved. To develop targets we used real world data, for the first time, to evaluate the financial viability of MECs against the current predominant method of wastewater treatment: activated sludge. We modelled net present values for eight potential scenarios and the performances required for MECs to break-even.

Mathematical modelling can reduce the cost and time required to design complex systems, and is being increasingly used in microbial electrochemical technologies (METs). To be of value such models must be complex enough to reproduce important behaviour of MET, yet simple enough to provide insight into underlying causes of this behaviour. Ideally, models must also be scalable to future industrial applications, rather than limited to describing existing laboratory experiments. We present a scalable model for simulating both fluid flow and bioelectrochemical processes in microbial fuel cells (MFCs), benchmarking against an experimental pilot-scale bioreactor. The model describes substrate transport through a two-dimensional fluid domain, and biofilm growth on anode surfaces. Electron transfer is achieved by an intracellular redox mediator. We find significant spatial variations in both substrate concentration and current density. Simple changes to the reactor layout can greatly improve the overall efficiency, measured in terms of substrate removal and total current generated.

The potential benefits of applying microbial electrolysis cell (MEC) technology to wastewater treatment are clear and profound. Previous pilot studies have demonstrated a ‘proof of concept' with domestic waste at ambient temperatures, but have not yet treated waste to required discharge standards, and have not reached energy neutrality. In addition, these reactors have been many orders of magnitude smaller than would be needed for full scale wastewater treatment plants. Scale-up affects many of the parameters that underpin performance; understanding its impact will be vital to further progress. Modifying a previously tested cassette-style design, we reduced the internal resistance, and increased the module size by a factor of 16, constructing an MEC with six 1 m2 anodes. This created an anodic surface area to volume ratio of 34 m2 m−3. The system was operated at a hydraulic retention time of 5 hours on settled domestic wastewater for 217 days, producing more current than a scaled-down reactor, which was run in parallel. The large MEC produced 0.8 L of 93% pure H2 d−1 at ambient winter temperatures (11.4 ± 2.5 °C). Chemical oxygen demand (COD) removal averaged 63.5% with an average effluent quality of 124.7 mgCOD L−1, achieving the European Urban Wastewater Treatment Directive (1991) consent.

Bioelectrochemical systems (BES) have long been viewed as a promising wastewater treatment technology. However, in reality, the performance of bioelectrochemical systems fed with real (and therefore complex) wastewaters is often disappointing. We have sought to investigate the combined impacts of complex substrates and presence of electron acceptors. In particular, this study illustrates and systematically evaluates the disparity in performance between a BES acclimatised with acetate and those acclimatised with more complex carbohydrates (glucose, sucrose or starch) and in the presence and absence of sulphate. Relative to acetate only, operating with complex carbohydrates reduced current by 73%-87% and coulombic efficiency by 4%-50%. Acclimation with complex carbohydrates seriously impeded the colonisation anode by Geobacteraceae, resulting in substantially reduced capacity to produce current (60.2% on average). Combined acclimation with sulphate further reduced current by 35% on average, and resulted in a total reduction of 83%-93% relative to the acetate control. However, the presence of an electrogenic sulphide-sulphur shuttle meant sulphate had little effect on the coulombic efficiency of the BES. The results indicate that a reduction in current and coulombic efficiency is, at present, an unavoidable consequence of operating a BES fed with complex wastewater. Researchers, designers and policy makers should incorporate such losses in both their plans and their prognostications.

Anode potential can affect the degradation pathway of complex substrates in bioelectrochemical systems (BESs), thereby influencing current production and coulombic efficiency. However, the intricacies behind this interplay are poorly understood. This study used glucose as a model substrate to comprehensively investigate the effect of different anode potentials (− 150 mV, 0 mV and + 200 mV) on the relationship between current production, the electrogenic pathway and the abundance of the electrogenic microorganisms involved in batch mode fed BESs. Current production in glucose-acclimatized reactors was a function of the abundance of Geobacteraceae and of the availability of acetate and formate produced by glucose degradation. Current production was increased by high anode potentials during acclimation (0 mV and + 200 mV), likely due to more Geobacteraceae developing. However, this effect was much weaker than a stimulus from an artificial high acetate supply: acetate was the rate-limiting intermediate in these systems. The supply of acetate could not be influenced by anode potential; altering the flow regime, batch time and management of the upstream fermentation processes may be a greater engineering tool in BES. However, these findings suggest that if high current production is the focus, it will be extremely difficult to achieve success with complex waste streams such as domestic wastewater. Electronic supplementary material The online version of this article (10.1007/s00253-020-10547-6) contains supplementary material, which is available to authorized users.

© 2016 The Author(s). The objective of this work was to quantify the number of exoelectrogens in wastewater capable of producing current in a microbial fuel cell by adapting the classical most probable number (MPN) methodology using current production as end point. Inoculating a series of microbial fuel cells with various dilutions of domestic wastewater and with acetate as test substrate yielded an apparent number of exoelectrogens of 17 per ml. Using current as a proxy for activity the apparent exoelectrogen growth rate was 0.03 h-1. With starch or wastewater as more complex test substrates similar apparent growth rates were obtained, but the apparent MPN based numbers of exoelectrogens in wastewater were significantly lower, probably because in contrast to acetate, complex substrates require complex food chains to deliver the electrons to the electrodes. Consequently, the apparent MPN is a function of the combined probabilities of members of the food chain being present.

Bioelectrochemical systems (BESs) have the potential to produce energy from wastewater. However, they are far from ready to be applied into industry. The development of large and pilot-scale systems to harness energy and value-added chemicals is widely regarded as one of the greatest research challenges in this field. There are several reasons for this: i) they are expensive, ii) they are difficult to engineer, iii) the data that can be derived from them is often limited, rarely in duplicate, and is disproportionate to the time commitment. Given these restrictions, systematic reviews of large and pilot-scale systems can be helpful in determining the direction of future research. These reviews need to standardise very different reactor set-ups, operational conditions, and methods of reporting data. Here we present an analysis of the energy production from semi-pilot and pilot-scale BESs, and benchmark their performance against existing wastewater treatment. The parameters used include complexity of wastewater; chemical oxygen demand (COD) loading rate; conductivity; reactor depth; volumetric treatment rate; effluent quality; energetic treatment balance; and temperature. We find that factors which are perceived to be problematic, such as low conductivities and temperatures, have been overcome by BESs at pilot-scale, and that these systems have met the regulatory requirements for discharge standards. We identify reactor depth and volumetric treatment rate as the areas for future research to focus on. The first of these issues will need an engineering solution, while the second is likely to come from improved understanding of the complex microbial digestion pathways. Material science may help both. Importantly, these pilot studies have shown that renewable energy production from wastewater is possible, and with targeted future research, could become a reality.

The formation of an electrochemically active biofilm is critical to the function of a Microbial Electrolysis Cell (MEC). We used Illumina 16S rDNA sequencing to analyse the formation and composition of anodic biofilms of two pilot-scale MECs, operated in continuous flow mode on domestic wastewater for over six months, and inoculated with that same wastewater. We observe: (i) a clear correlation between the frequency of detection of taxa in the MECs and their abundance in the metacommunity, (ii) the existence of a “core community” that was present across sites and (iii) the percentage of Geobacter tended to increase with longevity of retention time of the wastewater in the reactor. This suggests that: (i) community composition was largely governed by stochastic processes, (ii) that the technology should work on most if not all domestic wastewaters, as long as the anodes are seeded with the target wastewater and (iii) that deterministic factors may also play a role in establishing the anodic community. Geobacter, the archetypical electrogen in bioelectrochemical systems, comprised only 1.0 ± 0.7 % of the sequences recovered from a functioning pilot-scale MEC anode. Our results imply that influent flow rate may need to be optimised separately for start-up and for operating conditions for maximal performance.

A 100-L microbial electrolysis cell (MEC) was operated for a 12-month period fed on raw domestic wastewater at temperatures ranging from 1°C to 22°C, producing an average of 0.6L/day of hydrogen. Gas production was continuous though decreased with time. An average 48.7% of the electrical energy input was recovered, with a Coulombic efficiency of 41.2%. COD removal was inconsistent and below the standards required. Limitations to the cell design, in particular the poor pumping system and large overpotential account for many of the problems. However these are surmountable hurdles that can be addressed in future cycles of pilot scale research. This research has established that the biological process of an MEC will to work at low temperatures with real wastewater for prolonged periods. Testing and demonstrating the robustness and durability of bioelectrochemical systems far beyond that in any previous study, the prospects for developing MEC at full scale are enhanced.

The factors that affect microbial community assembly and its effects on the performance of bioelectrochemical systems are poorly understood. Sixteen microbial fuel cell (MFC) reactors were set up to test the importance of inoculum, temperature and substrate: Arctic soil versus wastewater as inoculum; warm (26.5 °C) versus cold (7.5 °C) temperature; and acetate versus wastewater as substrate. Substrate was the dominant factor in determining performance and diversity: unexpectedly the simple electrogenic substrate delivered a higher diversity than a complex wastewater. Furthermore, in acetate fed reactors, diversity did not correlate with performance, yet in wastewater fed ones it did, with greater diversity sustaining higher power densities and coulombic efficiencies. Temperature had only a minor effect on power density, (Q10: 2 and 1.2 for acetate and wastewater respectively): this is surprising given the well-known temperature sensitivity of anaerobic bioreactors. Reactors were able to operate at low temperature with real wastewater without the need for specialised inocula; it is speculated that MFC biofilms may have a self-heating effect. Importantly, the warm acetate fed reactors in this study did not act as direct model for cold wastewater fed systems. Application of this technology will encompass use of real wastewater at ambient temperatures.

Addressing the need to recover energy from the treatment of domestic wastewater, a 120-L microbial electrolysis cell was operated on site in Northern England, using raw domestic wastewater to produce virtually pure hydrogen gas (100 ± 6.4 %) for a period of over 3 months. The volumetric loading rate was 0.14 kg of chemical oxygen demand (COD) per cubic metre per day, just below the typical loading rates for activated sludge of 0.2-2 kg COD m(-3) day(-1), at an energetic cost of 2.3 kJ/g COD, which is below the values for activated sludge 2.5-7.2 kJ/g COD. The reactor produced an equivalent of 0.015 LH(2)L(-1) day(-1), and recovered around 70 % of the electrical energy input with a coulombic efficiency of 55 %. Although the reactor did not reach the breakeven point of 100 % electrical energy recovery and COD removal was limited, improved hydrogen capture and reactor design could increase the performance levels substantially. Importantly, for the first time, a 'proof of concept' has been made, showing that this technology is capable of energy capture as hydrogen gas from low strength domestic wastewaters at ambient temperatures.

This chapter reviews the production of hydrogen in microbial electrolysis cells (MEC) used to treat wastewater. The technology has huge potential for application within the wastewater industry, reducing energetic and economic costs of operation. As both electrodes are anaerobic, the engineering of these systems is simpler than their microbial fuel cell counterparts, meaning they could be relatively easily retrofitted into the existing wastewater treatment infrastructure. The key parameters for assessing MEC performance are described, and a critical review of the literature from the past decade of MEC research assesses whether the rhetoric matches the reality. Finally, there is a nod to the future of the technology, where consideration is given to the prospects for deployment as a wastewater treatment technology and what may be the most valuable product to make beyond hydrogen. Ultimately, if the technology is to be credible, it requires further testing at a plausible scale and under realistic conditions.

The wastewater industry is facing a paradigm shift, learning to view domestic wastewater not as a waste stream which needs to be disposed of but as a resource from which to generate energy. The extent of that resource is a strategically important question. The only previous published measurement of the internal chemical energy of wastewater measured 6.3 kJ/L. It has long been assumed that the energy content in wastewater relates directly to chemical oxygen demand (COD). However there is no standard relationship between COD and energy content. In this study a new methodology of preparing samples for measuring the internal chemical energy in wastewater is developed, and an analysis is made between this and the COD measurements taken. The mixed wastewater examined, using freeze-drying of samples to minimize loss of volatiles, had 16.8 kJ/L, while the domestic wastewater tested had 7.6 kJ/L nearly 20% higher than previously estimated. The size of the resource that wastewater presents is clearly both complex and variable but is likely to be significantly greater than previously thought. A systematic evaluation of the energy contained in wastewaters is warranted.