A two-stage anaerobic digestion system for treating mixed abattoir wastes was compared with a conventional single-pass reactor (SPR). The aim of the work was to investigate a means of overcoming the problems caused by accumulation of volatile fatty acids (VFA) and ammonia in conventional completely mixed SPRs, which are thought to arise as a result of the low carbon: nitrogen ratio of the feedstock. The two-stage digester consisted of a first stage 30 1 working volume completely mixed reactor in which the hydraulic retention time (HRT) was significantly shorter than the solid retention time (SRT). This mode of operation was referred to as a hydraulic flush, and was designed to retain the fibrous components of the feedstock within the reactor whilst rapidly washing out hydrolysis and fermentation intermediates. The hydraulic flush reactor (HFR) operated at a 10-day SRT and 2-day HRT. The performance of the HFR was compared with that of an SPR at total solids (TS) loading rates of 3, 4, 5, 6, 7 and 8 kg TS m(-3) per day. The SPR reactor failed at a solids loading rate of 5 kg TS m(-3) per day and only showed a maximum of 41% solids reduction whereas the HFR showed up to 66% solids reductions and performed satisfactorily up to 7 kg TS m(-3) per day. An anaerobic filter (AF) was employed as second stage to the HFR to provide a means of treating the liquid effluents derived from first stage. The AF was run at a HRT of I day with loading rates in the range 4.0-13.1 kg COD m(-3) per day. A COD removal of around 95% and a methane production rate of up to 0.34 m(3) CH4 per kg COD removed was achieved. The best performance of the two-stage system m(3) was 6.6% solids reduction, 81.7% COD removal and an overall methane yield of 0.21 m(3) CH4 per kg TS added, at a solids loading. rate of 7.02 kg TS m(-3) per day. (C) 2002 Elsevier Science Ltd. All rights reserved.