The influence of the hydraulic retention time on the microbial activity has been investigated, using a single methanization stage and a cascade of two upflow anaerobic sludge bed (UASB) reactors. The latter process scheme was clearly superior treating a synthetic fruit canning wastewater. For a COD reduction of 90%, a retention time of around 9-10 h was required in a single-stage UASB, while 4-5 h were sufficient in the two-stage mode of operation. In this case, the total volume of the methanization cascade was reduced by 50% compared to the single-stage one. (C) 2007 Elsevier Inc. All rights reserved.