Energy recovery from a high strength waste stream using pilot-scale bioelectrochemical systems

Abstract

Bioelectrochemical systems (BESs) have the potential to produce energy from wastewater. However, they are far from ready to be applied into industry. This study attempts to reduce the gap between pilot-scale and commercial application. First, we present a systematic review on the published semi-pilot and pilot-scale MECs, and benchmark their performance against existing wastewater treatment. 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 rates (VTRs) as the areas that require further research. It was hypothesised that the use of high strength return sludge liquor (RSL), rather than raw domestic wastewater, may boost BES performance. At a laboratory scale, it was seen that MFCs fed RSL performed as well as the pure substrate when comparing wastewater treatment performance, and COD saturation was reached with respect to VTRs. The use of RSL with a high soluble COD appears to reduce the effect of the slow breakdown of complex substrates in wastewater. Building on this success, a pilot-scale microbial electrolysis cell (MEC) was then operated in continuous flow for 6 months. The reactor was fed RSL, and successful optimisation of the hydraulic retention time (HRT) resulted in the highest VTR achieved by a pilot-scale MEC treating real wastewater. Peak HRT was 0.5-days, resulting in an average VTR of 3.82 kgCOD/m3 ·day and a 55% COD removal efficiency. Using the data obtained, a direct analysis of the potential savings from the reduced loading on AS was then made. Theoretical calculation of the required tank size with the estimated costs and savings indicate that the use of an MEC as a RSL pre-treatment technique could result in an industrially viable system. Throughout the thesis, there was a distinct variability between identical reactors when using wastewater. Variability reduces the overall performance and therefore increases costs, but more importantly, it highlights our lack of understanding of, and our inability to control and engineer these systems. Analysis in the previous chapters saw that variability was most obvious during inoculation with fresh wastewater, and appears to be caused by the total number of electrogens able to establish onto the anode. Artificially seeding the reactors enabled the creation of higher performing biofilms; however, this was only with sterile wastewater. Periods of stable current when running the pilot reactor in continuous flow give hope that modification to the design and operational conditions could reduce the impact of this variability. Assuming that other dimensions will be overcome by the use of modular electrodes, depth remains a major challenge, and even if this is accomplished, the natural variability from using wastewater may prevent this technology from ever being implemented into industry. However, if these issues can be solved, the switch to a more sustainable wastewater treatment method would be both economically and environmentally beneficial.