This paper reviews the operation of a full-scale, fixed-bed digester treating a citric acid production wastewater with a GOD: sulphate ratio of 3-4:1. Support matrix pieces were removed from the digester at intervals during the first 5 years of operation in order to quantify the vertical distribution of biomass within the digester. Detailed analysis of the digester biomass after 5 years of operation indicated that Hz and propionate-utilising SRB had outcompeted hydrogenophilic methanogens and propionate syntrophs. Acetoclastic methanogens were shown to play the dominant role in acetate conversion. Butyrate and ethanol-degrading syntrophs also remained active in the digester after 5 years of operation.Laboratory-scale hybrid reactor treatment at 55 degrees C of a diluted molasses influent, with and without sulphate supplementation, showed that the reactors could be operated with high stability at volumetric loading rates of 24 kgCOD.m(-3).d(-1) (12 h HRT). In the presence of sulphate (2 g/l(-1); COD/sulphate ratio of 6:1), acetate conversion was severely inhibited, resulting in effluent acetate concentrations of up to 4000 mg.l(-1).