In this work, both models for batch and continuous anaerobic digestion of palm oil mill effluent were developed based on Monod’s kinetics. Then the authors attempt to understand the effect of wastewater-sludge (WW:S) ratio on the biogas production efficiency in batch digesters. The experiments were carried out at a controlled temperature of 35±0.5 °C. Two series of the experiment were conducted. In the first series, the wastewater-sludge ratios covered 1:1 (add sodium bi-carbonate), 1:1, 1:2 and 2:1. It was found that the ratio of 1:2 gave the highest biogas producing efficiency followed by the ratio 1:1 (add sodium bi-carbonate). At 1:1 ratio, sodium bi-carbonate addition was required to start anaerobic digestion at a workable pH range whereas at 1:2 ratio the initial pH is in the workable range without the need of its addition. However, at the ratio of 2:1 the starting pH was too low to adjust pH economically by adding sodium bi-carbonate. The second series was to confine experiments to a narrower ratio range, namely: 1:1 (add sodium bi-carbonate), 1:1.5, 1:2, 1:2.5. In both sets of experiment, the ratio 1:2 gave the best biogas production potential of 76.62 and 78.52 ml of biogas/g COD removed respectively. In all treatments, the process was able to remove more than 80% of wastewater initial COD. The modified Gompertz equation was used to estimate the maximum specific biogas production rate (MBPR or Rm/S0). It was also found that the ratio of 1:2 gave the best MBPR in both experimental series (26.87 ml biogas/g COD-day). A modified Monod-type Model was also developed to describe the microbial growth, substrate consumption and biogas production in continuous operation. In general, sludge recycle provided active biomass which can use the substrate in the wastewater instantly without significant lag phase or delay. Furthermore, continuous-flow model developed, with parameters estimated from batch experiments, predicted the experimental kinetics of the actual continuous experiments satisfactory.