The addition of sulphate (4 g/l) to the influent of a mesophilic (35 +/- 2 degrees C) laboratory-scale anaerobic hybrid reactor (volumetric loading rate 6 kg/m(3)/day) treating a propionate-, butyrate- and ethanol-containing (1:1:1 on a COD basis; total COD of 12 g/l) wastewater resulted in severe process disturbance, with a complete inhibition of the propionate-degrading ability of the sludge. Severe inhibition of acetate removal was also observed, with concentrations of propionate and acetate in the reactor effluent of 4000 and 1000 mg/l, respectively. An oscillating pattern of sulphate and sulphide concentrations was observed in the reactor effluent. A control reactor, which had no influent sulphate, maintained a COD removal efficiency of > 95% throughout the trial period. A number of remediation strategies were undertaken with the sulphate-fed reactor. Initially, the levels of total sulphide in the digester were reduced to 1000 mg/l by inclusion of a nitrogen gas sparging system at 4.12 l/l/day (5.36 l/m(2)/day). Although the levels of total sulphide were reduced in the reactor, little improvement in COD removal was observed. Re-inoculation of 21 of non-sulphate-adapted seed sludge (25 g VSS/l;6.0% inoculum) into the digester did not improve the reactor performance either, suggesting that no improvement could take place in the absence of appropriate sulphate-reducing bacteria or sulphate-adapted syntrophic and methanogenic bacteria. Successful bioaugmentation of the reactor was achieved by the inoculation of 21 sulphate-adapted sludge (25 g VSS/1;6.0% inoculum) from a full-scale digester. Improvement in reactor performance was observed after one retention time (48 h) and the COD removal efficiency exceeded 95% by the conclusion of the trial. (C) 1998 Elsevier Science Ltd. All rights reserved.