Evaluation of microbial electrolysis cells in the treatment of domestic wastewate

Abstract

Wastewater can be an energy source and not a problem. This study investigates whether rapidly emerging bioelectrochemical technologies can go beyond working in a laboratory under controlled temperatures with simple substrates and actually become a realistic option for a new generation of sustainable wastewater treatment plants. The actual amount of energy available in the wastewater is established using a new methodology. The energy is found to be considerably higher than the previous measurement, or estimates based on the chemical oxygen demand with a domestic wastewater sample containing 17.8 kJ/gCOD and a mixed wastewater containing 28.7 kJ/gCOD. With the energy content established the use of bioelectrochemical systems is examined comparing real wastewater to the ‘model’ substrate of acetate. The abundance of exoelectrogenic bacteria within the sample, and the acclimation of these systems is examined through the use of most probable number experiments. It is found that there may be as few as 10-20 exoelectrogens per 100 mL. The impact of temperature, substrate and inoculum source on performance and community structure is analysed using pyrosequencing. Substrate is found to have a critical role, with greater diversity in acetate fed systems than the wastewater fed ones, indicating that something other than complexity is driving diversity. Laboratory scale microbial electrolysis cells are operated in batch mode fail when fed wastewater, whilst acetate fed reactors continue working, the reasons for this are examined. However a pilot scale, continuous flow microbial electrolysis cell is built and tested at a domestic wastewater treatment facility. Contrary to the laboratory reactors, this continues to operate after 3 months, and has achieved 70% electrical energy recovery, and an average 30% COD removal. This study concludes that wastewater is a very complex but valuable resource, and that the biological systems required to extract this resource are equally complex. Through the work conducted here a greater understanding and confidence in the ability of these systems to treat wastewater sustainably has been gained.