Dissolved zinc is present in natural waters and process streams generated by the mining and metallurgical industry. These streams usually have a low pH. By using sulfate reducing bacteria, sulfide can be produced that precipitates with zinc as zinc sulfide (sphalerite), which can be easily separated from the wastewater and even reused as zinc concentrate. In this study, a sulfate reducing gas-lift bioreactor was operated at pH 5.5 using hydrogen as electron donor for sulfate reduction. We demonstrate effective zinc removal (7.2 mmol L-1 d(-1)) with low zinc effluent concentrations (0.65-8.8 mu M) in a system combining sulfide generation by sulfate reducing bacteria (7.2-10.6 mmol SO(4)(2-)L(-1)d(-1)) at low pH (5.5) with the bio-precipitation of crystalline sphalerite. To investigate the effect of the sulfide excess on the settling properties of the sphalerite precipitates, the sulfide excess concentration was varied about two orders of magnitude (0.008-2.2 mM). The results show that crystalline sphalerite was formed in all cases, but larger particles with better settling properties were formed at lower sulfide concentrations. (C) 2009 Elsevier B.V. All rights reserved.