Optimising the Hydraulic Retention Time in a Pilot-Scale Microbial Electrolysis Cell to Achieve High Volumetric Treatment Rates Using Concentrated Domestic Wastewater

Scale-up and techno-economic analysis of microbial electrolysis cells for hydrogen production from wastewater

Microbial electrolysis cells (MECs) have demonstrated high-rate H₂ production while concurrently treating wastewater, but the transition in scale from laboratory research to systems that can be practically applied has encountered challenges. It has been more than a decade since the first pilot-scale MEC was reported, and in recent years, many attempts have been made to overcome the barriers and move the technology to the market. This study provided a detailed analysis of MEC scale-up efforts and summarized the key factors that should be considered to further develop the technology. We compared the major scale-up configurations and systematically evaluated their performance from both technical and economic perspectives. We characterized how system scale-up impacts the key performance metrics such as volumetric current density and H₂ production rate, and we proposed methods to evaluate and optimize system design and fabrication. In addition, preliminary techno-economic analysis indicates that MECs can be profitable in many different market scenarios with or without subsidies. We also provide perspectives on future development needed to transition MEC technology to the marketplace.

Investigating Variability in Microbial Fuel Cells

In scientific studies, replicas should replicate, and identical conditions should produce very similar results which enable parameters to be tested. However, in microbial experiments which use real world mixed inocula to generate a new "adapted" community, this replication is very hard to achieve. The diversity within real-world microbial systems is huge, and when a subsample of this diversity is placed into a reactor vessel or onto a surface to create a biofilm, stochastic processes occur, meaning there is heterogeneity within these new communities. The smaller the subsample, the greater this heterogeneity is likely to be. Microbial fuel cells are typically operated at a very small laboratory scale and rely on specific communities which must include electrogenic bacteria, known to be of low abundance in most natural inocula. Microbial fuel cells (MFCs) offer a unique opportunity to investigate and quantify variability as they produce current when they metabolize, which can be measured in real time as the community develops. In this research, we built and tested 28 replica MFCs and ran them under identical conditions. The results showed high variability in terms of the rate and amount of current production. This variability perpetuated into subsequent feeding rounds, both with and without the presence of new inoculate. In an attempt to control this variability, reactors were reseeded using established "good" and "bad" reactors. However, this did not result in replica biofilms, suggesting there is a spatial as well as a compositional control over biofilm formation. IMPORTANCE The research presented, although carried out in the area of microbial fuel cells, reaches an important and broadly impacting conclusion that when using mixed inoculate in replica reactors under replicated conditions, different communities emerge capable of different levels of metabolism. To date there has been very little research focusing on this, or even reporting it, with most studies using duplicate or triplicate reactors, in which this phenomenon is not fully observed. Publishing data in which replicas do not replicate will be an important and brave first step in the research into understanding this fundamental microbial process.

Membrane-based technologies for biohydrogen production: A review

Overcoming the existing environmental issues and the gradual depletion of energy sources is a priority at global level, biohydrogen can provide a sustainable and reliable energy reserve. However, the process instability and low biohydrogen yields are still hindering the adoption of biohydrogen production plants at industrial scale. In this context, membrane-based biohydrogen production technologies, and in particular fermentative membrane bioreactors (MBRs) and microbial electrolysis cells (MECs), as well as downstream membrane-based technologies such as electrodialysis (ED), are suitable options to achieve high-rate biohydrogen production. We have shed the light on the research efforts towards the development of membrane-based technologies for biohydrogen production from organic waste, with special emphasis to the reactor design and materials. Besides, techno-economic analyses have been traced to ensure the suitability of such technologies in bio-H₂ production. Operation parameters such as pH, temperature and organic loading rate affect the performance of MBRs. MEC and ED technologies also are highly affected by the chemistry of the membrane used and anode material as well as the operation parameters. The limitations and future directions for application of membrane-based biohydrogen production technologies have been individuated. At the end, this review helps in the critical understanding of deploying membrane-based technologies for biohydrogen production, thereby encouraging future outcomes for a sustainable biohydrogen economy.

A scalable model of fluid flow, substrate removal and current production in microbial fuel cells

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.

Scalability of microbial electrochemical technologies: Applications and challenges

During wastewater treatment, microbial electrochemical technologies (METs) are a promising means for in situ energy harvesting and resource recovery. The primary constraint for such systems is scaling them up from the laboratory to practical applications. Currently, most research (∼90%) has been limited to benchtop models because of bioelectrochemical, economic, and engineering design limitations. Field trials, i.e., 1.5 m³ bioelectric toilet, 1000 L microbial electrolysis cell and industrial applications of METs have been conducted, and their results serve as positive indicators of their readiness for practical applications. Multiple startup companies have invested in the pilot-scale demonstrations of METs for industrial effluent treatment. Recently, advances in membrane/electrode modification, understanding of microbe-electrode interaction, and feasibility of electrochemical redox reactions have provided new directions for realizing the practical application. This study reviews the scaling-up challenges, success stories for onsite use, and readiness level of METs for commercialization that is inexpensive and sustainable.

Microbial fuel cells for bioelectricity production from waste as sustainable prospect of future energy sector

Microbial fuel cell (MFC) is lauded for its potentials to solve both energy crisis and environmental pollution. Technologically, it offers the capability to harness electricity from the chemical energy stored in the organic substrate with no intermediate steps, thereby minimizes the entropic loss due to the inter-conversion of energy. The sciences underneath such MFCs include the electron and proton generation from the metabolic decomposition of the substrate by microbes at the anode, followed by the shuttling of these charges to cathode for electricity generation. While its promising prospects were mutually evinced in the past investigations, the upscaling of MFC in sustaining global energy demands and waste treatments is yet to be put into practice. In this context, the current review summarizes the important knowledge and applications of MFCs, concurrently identifies the technological bottlenecks that restricted its vast implementation. In addition, economic analysis was also performed to provide multiangle perspectives to readers. Succinctly, MFCs are mainly hindered by the slow metabolic kinetics, sluggish transfer of charged particles, and low economic competitiveness when compared to conventional technologies. From these hindering factors, insightful strategies for improved practicality of MFCs were formulated, with potential future research direction being identified too. With proper planning, we are delighted to see the industrialization of MFCs in the near future, which would benefit the entire human race with cleaner energy and the environment.

Alkaline thermal pretreatment of waste activated sludge for enhanced hydrogen production in microbial electrolysis cells

Resource utilization of waste activated sludge (WAS) has become a mainstream development direction. Alkaline thermal pretreatment (TPT) was found to greatly promote the bioaccessibility and biodegradability of the sludge. The organic matter including soluble chemical oxygen demand (SCOD), soluble carbohydrate, soluble protein and volatile fatty acids (VFAs) after low temperature (90 °C) pretreatment was 4.8%-65.9% higher than that after high temperature (180 °C) pretreatment. These increasements could be contributed by the alkaline treatment condition and the longer treatment time. The alkaline condition reduced the resistance of cell wall to the temperature. The pretreatment time at 90 °C was two times of that at 180 °C, allowing more organic matter to be released. But the total energy consumption of low temperature pretreatment (2580.7 kJ/L) was 30.5% lower than that of high temperature pretreatment (3711.8 kJ/L). The sludge fermentation liquid (SFL) was then employed as the substrate in microbial electrolysis cells (MECs), and the utilization efficiency of acetic acid was the highest (74.9%-83.2%). The hydrogen yield using low temperature pretreated sludge was 0.44 m³/(m³·d), which was higher than that of using high temperature pretreated sludge (0.31 m³/(m³·d)). These results suggested that alkaline TPT at 90 °C was an effective way to hydrolyze sludge and further enhance hydrogen production in MECs.

Scaling-up up-flow microbial electrolysis cells with a compact electrode configuration for continuous hydrogen production

Maintaining high current densities is a key challenge in scaling-up microbial electrolysis cell (MEC) reactors. In this study, a novel 10 L MEC reactor with a total electrode surface area greater than 1 m² was designed and evaluated to maximize the current density and H₂ recovery. Performances of the reactor suggest that the longitudinal structure with parallel vertical orientation of the electrodes encouraged high fluid mixing and the sheet metal electrode frames provided distributed electrical connection. Results also demonstrated that the electrode pairs located next to reactor walls decreased current density, as did separating the electrodes with separators. High volumetric H₂ production rate of 5.9 L/L/d was achieved at a volumetric current density of 970 A/m³ (34 A/m²). Moreover, the observed current densities of the large reactor were accurately predicted based on the internal resistance analysis of small scale MECs (0.15 L), demonstrating the scalability of the single chamber MEC design.

Hydrogen production in single-chamber microbial electrolysis cells using Ponceau S dye

In this study, Ponceau S dye, which is one of the hazardous dyes found in industrial wastewater, was examined for hydrogen production in single chamber-free membrane-free microbial electrolysis cells at different concentrations (10-40 mg L^-1). A gas content analysis (hydrogen, carbon dioxide, and methane) was measured daily using gas chromatography to determine the effects of the Ponceau S on hydrogen production levels. Hydrogen was successfully produced in the presence of Ponceau S dye, but the gas production levels were affected by the concentrations of Ponceau S. The maximum hydrogen production was measured as 18 mL at a concentration level of 20 mg L^-1. Decolorization ratios of Ponceau S were in the range of 85-100%. Hydrogen production rates increased in the presence of Ponceau S (20 mg L^-1); however, yield (%) of the production decreased when compared to the control group. The percentage of COD removal was 94.78% in the presence of 40 mg L^-1 of Ponceau S. In conclusion, hydrogen can be generated using wastewaters contaminated with azo dyes such as Ponceau S, and decolorization of the dye can be achieved, simultaneously.