Low-Temperature Anaerobic Membrane Bioreactor for Energy Recovery from Domestic Wastewater.

Abstract

Anaerobic membrane bioreactor (AnMBR) treatment, which combines the anaerobic microbial conversion of organic compounds into methane-rich biogas with membrane separation of treated wastewater and microbial biomass, has been proposed for direct energy recovery from domestic wastewater. We demonstrated in a bench-scale investigation that AnMBR can achieve 92 ± 5% chemical oxygen demand (COD) removal at 15°C, but that dissolved methane in the permeate represents 40-50% of the total methane produced. If unrecovered, this methane is a lost energy source and results in substantial greenhouse gas emissions. This work motivated an evaluation of the trade-offs between the membrane biofilm’s role in treatment and its contribution to fouling. We demonstrated that the development of a biofilm enriched in active syntrophic bacteria and methanogens significantly improved effluent quality, while maintaining acceptable fluxes. However, methanogenesis in the biofilm resulted in substantial levels of dissolved methane in the permeate. The lower temperature limit of AnMBR treatment was explored by sequentially lowering the operating temperature of the system from 15, 12, 9, 6, to 3°C under conditions supporting biofilm treatment. COD removal > 95% was achieved at temperatures as low as 6°C. COD removal fell to 86 ± 4.0% at 3°C and, at this temperature, essentially all COD removal occurred in the biofilm, suggesting that the biofilm was less inhibited by temperature decreases than the suspended biomass. Finally, we evaluated the life cycle environmental and economic impacts of AnMBR technology compared to aerobic treatment systems. AnMBR will not be net energy positive in the foreseeable future without reduction in fouling control energy demands. Currently, AnMBR is better suited for higher strength domestic wastewater treatment. Further, global warming impacts were over an order of magnitude higher than aerobic systems arising from the direct emission of effluent dissolved methane. Future research is necessary to (1) promote increased biological activity in suspended biomass at low temperatures such that membrane biofilm treatment is reduced and dissolved methane oversaturation avoided, (2) develop low-energy dissolved methane recovery technologies to limit global warming impacts, and (3) establish fouling control strategies that reduce energy demands thereby improving the net energy balance.