The anaerobic biological treatment of volatile fatty acid (VFA) - and sucrose - based wastewaters was investigated in two anaerobic bioreactors. R1 and R2, over a 300-day trial period. During the trial, the operating temperature of both reactors was lowered, in a stepwise fashion.. from 37 to 16 degreesC. The VFA-fed reactor maintained an excellent level of performance, regardless of operating temperature, reaching COD removal efficiencies of 95% at IS degreesC, and a biogas methane content in excess of 70% at 16 degreesC, at an imposed OLR of 20 kg COD m(-3) d(-1). However, an increase in the applied liquid upflow velocity to the bottom chamber of the reactor from 5 to 7.5 m h(-1) on day 236 resulted in a considerable decline in reactor performance. COD removal efficiencies in excess of 80% were achieved by the sucrose-fed reactor at 18 degreesC, at an imposed OLR of 20 kg COD m(-3) d(-1). An increase in the liquid upflow velocity applied to the sucrose-fed reactor resulted in enhanced reactor performance and stability, with respect to decreasing temperature. The different responses of both reactors to increased upflow velocity was associated with variations in the microbial population structure of the sludges. as determined by culture-independant molecular approaches, specifically the presence of high levels of delta-Proteobacteria and hydrogenotrophic methanogens in the VFA-fed biomass. High levels of Methanomicrobiales sp., in particular Methallocorpusculum parvum sp.. were observed in both RI and R2 during the trial. There was a distinct shift from acetoclastic methanogenic dominance to hydrogenotrophic dominance in both reactors in response to a decrease in the operating temperature. (C) 2004 Federation of European Microbiological Societies. Published by Elsevier B.V. All rights reserved.