Abstract: Palm Oil Mill Effluent (POME) cause serious pollutions of soil, water and environment but could be exploited as substrate for microbial citric acid production because of its rich mineral and high carbohydrate contents. Therefore in this study, Aspergillus niger ATCC 9642 was grown in the different concentrations (25, 50, 75 and 100%) of cellulase-digested POME for 7 days. Result showed that citric acid was highest (0.78±0.02 g L-1) in the 75% POME but lower (0.32±0.04 g L-1) in the undiluted POME. When supplemented with methanol (3%), citric acid production increased (1.02±0.03 g L-1) only in the 100% POME. Citric acid was not detected in the 25% and the non-digested POME. Biomass measurement showed that the 100% POME gave the highest (10.2±0.6 g L-1) mycelial weight among the digested treatments. As the dilution increased, the biomass concentration decreased proportionally. Biosolids from the non-digested POME supplemented with glucose (1%) weighed 13.4±0.15 g L-1. The Chemical Oxygen Demand (COD) of the digested POME (100%) dropped by 47.4% but the presence of methanol did not significantly affect the COD kinetics. However, at higher POME dilutions, COD reduction rate was greater than 63% but in the non-digested batches, COD reduction was negligible. The sugar utilization patterns showed that glucose was completely used up in the different POME dilutions except the undigested and glucose (1%) supplemented batches which had 75 and 40% sugar uptake, respectively. The use of POME hydrolysates as substrate could lower the present cost of citric acid as well as save treatment costs and help solve environmental problems.