Performance of a laboratory-scale two-phase anaerobic digestion system treating dairy wastewater was investigated using the pre-determined operating criteria for the anaerobic acidification reactor. The results, obtained from a 9 month operation, showed that overall, 90% COD and 95% BOD removal efficiencies at an organic loading rate (OLR) of 5 kg COD/m(3) d and a hydraulic retention time (HRT) of 2 days were achieved. The acidogenic phase completely stirred tank reactor was operated up to an OLR of 23 kg COD/m(3) d with a HRT of 0.5 days whereas the methanogenic-phase upflow anaerobic filter was loaded up to an OLR of 7 kg COD/m(3) d with a HRT of approximately 1.5 days. In the pre-acidification reactor, a maximum of 60% conversion rate of COD to volatile fatty acids (VFAs) was achieved at an OLR of 12 kg COD/m(3) d after which the conversion rate remained reasonably constant while a 60% removal of total fatty matter was obtained. Good separation of acid and methane phases was achieved as indicated by low and high methane yields in the first and second phases respectively. VFAs produced during the acidogenic phase were effectively consumed in the methanogenic phase. (C) 1998 Elsevier Science Ltd. All rights reserved.