The semi-continuous stirred tank reactors (SCSTR) were applied under anaerobic mesophilic conditions at 37 +/- 3 degrees C to treat the supernatant layer of the raw palm oil mill effluent (POME). In this study, the anaerobically digested sludge POME (ADS-POME) was mixed with rumen fluid (RF) taken from the first compartment of cow's stomach (5% and 10% by volume) and used for co-digestion and then compared with (ADS-POME) for mono-digestion. The treatment was done at different hydraulic retention times (HRT): 20, 10 and 6 days and organic loading rates (OLR): 1.65, 2.85 and 4.06 gm COD/L d. The results showed that co-digestion (10% RF + 90% ADS-POME) has high COD removal of 96.48% at 20 days of HRT with lowest OLR of 1.65 gm COD/L d and the higher methane content was 61.8% at 6 days HRT with 4.06 gm COD/L d of OLR. All digesters had a good stability in terms of pH. The concentration of MLVSS was decreased with long HRT and low OLR within all digesters. The co-digestion showed an efficient ability for COD removal efficiency and maintains methane content within acceptable levels. (C) 2010 Elsevier B.V. All rights reserved.