Performance of a lab-scale bio-electrochemical assisted septic tank for the anaerobic treatment of black water

Septic tanks are used for the removal of organic particulates in wastewaters by physical accumulation instead of through the biological production of biogas. Improved biogas production in septic tanks is crucial to increase the potential of this system for both energy generation and organic matter removal. In this study, the effect on the biogas production and biogas quality of coupling a 20 L lab-scale septic tank with a microbial electrolysis cell (MEC) was investigated and compared with a standard septic tank. Both reactors were operated at a volumetric organic loading rate of 0.5 gCOD/L d and a hydraulic retention time between 20 and 40 days using black water as an input under mesophilic conditions for a period of 3 months. The MEC-septic tank was operated at an applied voltage of 2.0 ±0.1V and the current experienced ranged from 40 mA (0.9 A/m2 projected electrode area) to 180 mA (5 A/m2 projected electrode area). The COD removal was of the order of 85% and the concentration of residual COD was not different between both reactors. Yet, the total phosphorous in the output was on average 39% lower in the MEC-septic tank. Moreover, the biogas production rate in the MEC-septic tank was a factor of 5 higher than in the control reactor and the H2S concentration in the biogas was a factor of 2.5 lower. The extra electricity supplied to the MEC-septic tank was recovered as extra biogas produced. Overall, it appears that the combination of MEC and a septic tank offers perspectives in terms of lower discharge of phosphorus and H2S, nutrient recuperation and a more reliable supply of biogas.