Development of a biological electrochemical system for decentralised wastewater treatment and energy production in remote and under-served regions

Abstract

Wastewater treatment is one of many global challenges that we face, where currently 80% of wastewater discharges into our environment go untreated. The most widely used approach is a centralised model using sewage networks to collect the waste, often treating it linearly. The linear approach does not explore the value in the waste that can be extracted. The research explored opportunities to transition towards a circular economy where we extract the value from within the wastewater; to develop a new sustainable approach for decentralised wastewater treatment for remote and underserved regions that lack wastewater infrastructure. The research undertaken was to explore the ability of Biological Electrochemical Systems (BES) to transform our approach to wastewater treatment. BES can recover valuable resources such as energy in the form of electricity, hydrogen and methane as well as nutrients. The project explored the various BES technologies to understand which system will likely provide the initial route to commercialisation. Academic research on BES is spread mainly across Microbial Fuel Cells (MFCs) and Microbial Electrolysis Cells (MECs) with both technologies showing potential to revolutionise wastewater treatment. However, both MFCs and MECs face challenges with commercial deployment to meet the needs of underserved communities, from the complexity of design to cost. The research aimed to assess whether a less researched route of Electro-Methanogenesis is technologically and economically feasible. Electro-Methanogenic Reactors (EMR) can accelerate the performance of Anaerobic Digestion (AD); a widely commercially available technology. EMR provides multiple opportunities to improve our current approach to wastewater management and provide a compact decentralised treatment. The United Nations Sustainable Development Goals (SDG) have identified multiple challenges that could be tackled by EMR; including clean water and sanitation (SDG6), affordable and clean energy (SDG7) and sustainable cities and communities (SDG11). The research explored the scaleup of EMR from lab scale (2.3 L) to pilot (1300 L) , assessing both the technical challenges but also the economic viability. The lab experiments highlighted challenges, including, designing a continuous flow system and regulating pH through the organic loading rate. During batch testing, the EMR systems operating on brewery spent grain demonstrated a high substrate efficiency of 99.2 ± 15.3%, identifying that the system is effectively converting the breakdown of the organics into methane. The batch test results identified that EMR could provide better pH regulation and organic removal compared to AD systems. EMR was able to breakdown the Volatile Fatty Acids (VFA) once the pH had dropped below 5, which in the case of AD systems inhibited the microbial community. EMR is still susceptible to having a high Volatile Solids (VS) concentration as an increase in the concentration from 14.6 g/L to 21.1 g/L did not recover as quickly; with the EMR taking initially 7 days to increase the pH above 6 whilst the increased VS meant the pH reached 4.92 after 28 days. Scaling up the lab research to pilot scale proved to be a challenge with unforeseen operational barriers affecting performance. The pilot identified that defining the EMR operational parameters are crucial to ensure commercial applications where the technology will be deployed in less controlled environments. The results showed that EMR would likely require further post-treatment to reduce the COD and pathogens within the effluent. A Technological Economic Analysis (TEA) is used to compare the current solutions that are widely used in Kenya, including pit latrines, septic tanks and anaerobic digestions systems with EMR. The TEA was based on lab results and assumptions on future increases in EMR performance. Including COD removal increases through improved reactor design and updated costs obtained from manufacturers. The TEA identified that EMR had a lower lifetime cost compared to fixed dome AD systems, which are commonly used in Kenya. BES still have a long way to go before becoming widely commercially viable. EMR offers a route to commercialisation that could provide decentralised wastewater treatment. The research identifies the barriers and next steps to reach the goal of moving EMR towards implementation.