Food processing wastewaters and slurries typically contain high concentrations of biodegradable organic matter. Before the wastewater can be discharged the pollutant concentration must be reduced. One way to achieve this is through the use of a biological species ('biomass') that consumes the organic matter ('substrate'). We investigate biological reactor models in which the growth rate is given by a Contois expression with a variable yield coefficient. We investigate the reduction in pollutant concentration when wastewaters are passed through one of two reactor configurations: a single reactor and a two-reactor cascade. In the latter scenario the total residence time is fixed and the residence time in the first reactor is taken to be a design parameter. It is found that in many cases an optimal reactor cascade may outperform the single reactor by two orders of magnitude. (c) 2006 Elsevier B.V. All rights reserved.