Optimizing mass flow and extracellular electron transfer to promote energy recovery during treating municipal wastewater in a biochar enhanced anaerobic membrane bioreactor (AnMBR)

Abstract

The anaerobic membrane bioreactor (AnMBR) is an alternative technology with energy neutrality potential when treating municipal wastewater (MWW). However, the poor bio-reaction kinetic conditions caused by low COD concentrations in MWW greatly limits the CH yield. In this study, the average methanogenesis rate was significantly increased from 164.8 to 275.5 mL/g COD with the two times biochar addition of 5 g/L. And following the second time addition of 5 g/L biochar, the CH component significantly increased to a high level of 89.0 ± 6.3 %. In addition, the rate for acetate converted into CH increased from 3.3 to 10.5 mmol/(g ·d) with biochar addition of 10 g/L, indicating that acetotrophic methanogenesis route was significantly promoted by biochar. Bio-electrochemistry analysis showed that biochar accelerated 0.74 mol e/g /min of electron transfer velocity in AD system. The addition of biochar enriched organic metabolism bacteria such as , and , and upregulated the expression of functional genes related to organics degradation such as , and facilitates the direct transfer of electrons and acetates produced by and r to convert CO into CH through microbial mediation. Thus, the COD flow shifted significantly towards more CH production. The COD mass flow showed that biochar facilitated more bio-catabolism (up about 52.7 %) and less bio-anabolism (down about 92 %), and more COD in wastewater was converted to CH, resulting a significant promoting of energy production. The energy balance analysis indicated that the net energy consumption was reduced by 15 % after two times addition of 5 g/L biochar, and the key to achieve the net energy output during treating MWW is the efficient operation of AnMBR at ambient temperature.