Performance evaluation of an anaerobic hybrid digester treating palm oil mill effluent

A high-strength palm oil mill effluent was treated using a hybrid anaerobic digester, in which the bottom two-thirds were occupied by a sludge blanket and the upper one-third by submerged PVC rings, under mesophilic conditions. The influent COD concentration varied considerably (11.6-65.0 g/l). The digester was subjected to quantitative increases in organic loading rate (OLR) from 3.3 to 18.6 kg COD/m(3) day at a set hydraulic retention time (HRT) of 3.5 days. The data showed that the anaerobic digestion process is feasible for the treatment of, and methane generation from, palm oil mill effluent without pretreatment. A COD removal efficiency and methane yield of 92.3% and 0.335 m(3)/kg CODremoved, respectively, could be achieved at an OLR of 16.2 kg COD/m(3) day. The data also showed that the hybrid digester could be maintained at a HRT of 3.5 days with an OLR of 18.6 kg COD/m(3) day. This rate is significantly faster than the rates achieved in previously reported studies. Decreasing digester efficiency was basically characterized by accumulation of propionic acid.