The fate of metals in wastewater treated by the activated sludge process and membrane bioreactors: a brief review.

The fate of metals in wastewater treatment by the conventional activated sludge process (ASP) and membrane bioreactors (MBRs) is reviewed. The review outlines the environmental and health impacts of metals, but focuses primarily on data reported for removal of toxic metals, and some other high-profile inorganic micropollutants such as aluminium and arsenic, by wastewater treatment processes. Information from pilot and full scale plants is included, with corroboratory reports from bench-scale tests. General trends in removal across different metals are considered, along with the impact of the key process operating determinant of solids retention time. It is concluded that the only consistent trend in metals removal is that it is most effectively achieved through efficient solids separation, and that this represents the primary advantage offered by the MBR. As such, MBRs achieve averaged metals removals which are consistently but not dramatically higher than the ranges reported by the ASP: 64-92% vs. 51-87%, with no more than a 55% decrease on average in effluent concentration. The slightly greater removal attained is attributable to the additional suspended solids retention attained by the membrane process. In either case, further removal of metals would demand a tertiary process for removal of the dissolved material.