Enhancing desalination and wastewater treatment by coupling microbial desalination cells with forward osmosis

Abstract

Both microbial desalination cells (MDCs) and forwards osmosis (FO) are emerging technologies for energy-efficient water/wastewater treatment. However, incomplete treatment/desalination in both technologies creates challenges for further development and also synergy between the two. In this study, an integrated system was developed by coupling MDCs with a FO cell to enhance COD (chemical oxygen demand) removal in synthetic organic solution and desalination of saline water. The organic solution was treated in the MDC anode and its effluent was sent to the FO for water recovery. The diluted draw solution from the FO was desalinated in the MDC. Compared to a standalone MDC, the MDC-FO system reduced the wastewater volume by 64% and improved the conductivity reduction in saline water by two times. The coupled system also achieved higher COD removal than a standalone FO cell. The MDC-FO system was further investigated for the effects of COD, salt concentration and hydraulic retention time (HRT). It was observed that varying COD concentration had little impact on desalination. Lowering the initial NaCl concentration enhanced the COD removal to 93.0% and the conductivity reduction to 99.4%. In addition, the COD removal rate and the conductivity decrease rate were proportionally facilitated with a shortened HRT. These results have collectively demonstrated that the MDC-FO system has improved performance, compared with the standalone process. It holds great promise to either treat brackish water, or serve as pre-desalination of high-salinity water, with simultaneous wastewater treatment.