Batch biogas prudction using palm oil mill effluent: effect of wastewater-sludge ratio

Abstract

This work was an attempt to understand the effect of wastewater-sludge ratio on the biogas production efficiency in batch digesters which mimic a similar industrial practice. The experiments were carried out at a controlled temperature of 35±0.5°C. Two series of the experiment were conducted and all treatments were replicated to ensure their consistency. In the first series, the wastewater-sludge ratios covered 1:1 (with sodium bi-carbonate addition), 1:1, 1:2 and 2:1. It was found that the ratios had a strong effect on the biogas production and the ratio of 1:2 gave the highest biogas producing efficiency followed by the ratio 1:1 (with sodium bi-carbonate addition). At 1:1 ratio, sodium bi-carbonate addition was required to start anaerobic digestion as a workale pH range whereas at 1:2 no chemical addition was required. However, at the ratio of 2:1 it was not economical to adjust the initial pH. The second series was to confine experiments to a narrower ratio range, namely: 1:1 (with sodium bi-carbonate addition), 1:1.5, 1:2, and 1:2.5. In both sets of experiments, the ratio 1:2 gave the best biogas production potential of 336 and 410 mL of biogas/ g COD removed, respectively. In all treatments, more than 80% of wastewater initial COD were removed. The modified Gompertz equation was used to estimate the maximum specific biogas production rate (MBPR). It was also found that the ratio of 1:2 gave the best MBPR in both experimental series (5050 and 24050 mL biogas/g COD day, respectively). A modified Monod-type model was also developed to describe the microbial growth, substrate consumption and biogas production satisfactorily. In general, sludge recycle provided active biomass which can use the substrate in the wastewater instantly without significant lag phase or delay. Suitable wastewater-sludge ratio also eliminated the need of any chemical addition to adjust the initial pH.