This work reports a novel system for the treatment of acidic metal-containing wastewaters, the Extractive Membrane Bioreactor-Sulfate-Reducing Bacteria (EMB-SRB) system. In this system, hydrogen sulfide is produced in the bioreactor by the sulfate-reducing bacteria, transfers through a dense phase membrane, and precipitates metal ions in the wastewater. The non-porous membrane prevents the SRB from having direct contact with the toxic metals, extremes of pH, or high salinity in the wastewater. Silicone rubber, which is permeable to H2S but virtually impermeable to ionic species in the system, was used as a membrane. The rate of mass transfer of H2S across the membrane was studied and found to be well described by a resistances-in-series model. k(ov) values vary in the range 5 x 10(-6)-10 x 10(-6) ms(-1) 1 depending on the membrane thickness. A continuous EMB-SRB system was operated and more than 90% (w/v) of the Zn2+ present in a wastewater was removed. A film of metal precipitate was found to build up on the inside (wastewater) side of the membrane, and became the dominant resistance contributing to the overall mass transfer coefficient during operation.