A comparative study of a fermentation process for total volatile fatty acids (TVFA) production using pilot-scale fixed-bed (FAS) and suspended biomass (FER) reactors in which similar operational conditions was carried out. The influence of the changes of ambient temperatures at fixed operational conditions was also studied. Oxidation-reduction potential (ORP) increased and effluent pH decreased as the hydraulic retention time (HRT) decreased, which was favourable for TVFA production. Equations describing the ORP and pH variations with the HRT were obtained. ORP variation with HRT for FAS and FER reactors followed a logarithmic function with a regression coefficient, R-2, equal to 0.98. The variations of pH with HRT followed polynomial functions with regression coefficients of 0.96 and 0.98 for FAS and FER reactors, respectively. Hydrolysis process increased with the experiment duration. At the beginning of the experiment, effluent soluble COD (SCOD) decreased with respect to the influent but further effluent SCOD increased showing higher values compared to the influent. Cold temperatures were more favourable than summer temperatures for the accumulation of TVFA at the liquid effluent. The FAS reactor was more effective in the production of TVFA than the FER reactor. The maximum yields of TVFA were obtained at an organic volumetric loading rate (B-V) of 1.9 g COD/l per day, corresponding to an HRT of 3.4 h, for both reactors. A maximum increase of ammonia and phosphorus was observed at the maximum value of HRT coinciding with an increase of pH and a decrease of ORP, as could be previously observed. The average P/SCOD ratio for the influent and effluent were 0.06 and 0.05, respectively, for FAS and FER reactors. The average Ammonia/SCOD ratio for the influent and effluent were 0.15 and 0.14, respectively. These results demonstrate that effluent quality was improved by the treatment employed in case a further process of nutrient removal is carried out. (C) 2003 Elsevier Ltd. All rights reserved.