The experiments were performed at 30 °C. The VFA-mixtures used were enriched with sufficient N, P and S and also 0, 1 ml/l of a trace element solution according to Zehnder (1980) and 100 mg/l yeast extract was added. In all experiments digested sewage sludge from the municipal sewage treatment plant at Ede was used as a basic inoculum at an initial sludge concentration of 12-15 g VSS/l.