Chromium removal mechanisms and bacterial community in an integrated membrane bioreactor system

Abstract

A wastewater with high salinity (35 g NaCl/L) and chromium (0-200 mg Cr/L) was treated with an integrated system of a submerged anaerobic membrane bioreactor (MBR), followed by an aerobic MBR and a powdered activated carbon (PAC) column. The final effluent from the system was generally below 10 mg Cr/L (95% removal), although the chemical oxygen demand removal was not high (70%), possibly because of the inhibition of anaerobic and aerobic biomass by Cr and high salinity. This is the first study that investigated the performance of an integrated system with a synthetic tannery wastewater under variations in both chromium and salinity over 100 days. Molecular techniques detected the presence of certain bacteria in the submerged anaerobic MBR that could survive under both high salinity and chromium concentrations; these findings could be valuable as these bacteria could be isolated and then bioaugmented into an MBR for the treatment of tannery wastewater. The study also examined the distribution of chromium in anaerobic biomass and determined the main chromium removal mechanisms under high salinity. After 24 h of addition, the largest quantity of chromium was removed as a precipitate and the second largest was found in the residual ash of the biomass. High salinity positively affected chromium adsorption by the biomass, but only during the first 5 min. This work is important because it has implications not only for treatment plant design but also for studies on chromium removal by biomass.