The discharge of effluents rich in sulphate and nitrate is of increasing concern, as the former leads to mineralisation of surface water, and the latter is a plant nutrient. A process is described whereby sulphate is reduced to sulphide under anaerobic conditions with producer gas as substrate. Sulphide stripped off is then converted to elemental sulphur by reacting it with biologically produced iron(III). The study consisted of laboratory batch and continuous experiments, as well as pilot scale studies. Sulphate was reduced to sulphide at a rate of 1.2 g SO4 1−1 d−1 with H2/CO as substrate, and 2.4 g SO4 1−1 d−1 with only CO as substrate. Nitrate was converted to ammonia in the anaerobic reactor. Sulphide was removed from 2 000 mg 1−1 (as SO4) to less than 90 mg 1−1. The iron(II), produced after reduction of iron(III) with H2S, was oxidised biologically to iron(III). The volumetric reaction rate was 5.5 g Fe 1−1 d−1.