A 30-1 anaerobic bioreactor with biomass recycling was used to provide a continuous reduction in sulphate and a continuous COD removal from wastewater, which consisted of the effluent from an industrial pig fattening farm, enriched with technical FeSO(4). 7H(2)O, a waste product from ferrous metallurgy. The concentrations of sulphate and COD in the wastewater amounted to 2.73 g l(-1) and 3.15 g l(-1), respectively. The HRT (hydraulic retention time) of 10-1.7 d produced an extent of sulphate and COD reduction which totalled 98% and 88%, respectively. When the HRT was further shortened, the efficiency of reduction in sulphate and COD decreased. The maximum removal rate constants for both the pollutants, calculated by means of a modified Stover-Kincannon model, were 80.9 g COD l(-1) d(-1) and 41.8 g SO(4)(2-) l(-1) d(-1), the values of the saturation constants being 91.582 g COD l(-1) d(-1) and 42.398 g SO(4)(2-) l(-1) d(-1). (c) 2008 Elsevier Ltd. All rights reserved.