Palm oil mill effluent treatments and biomethane production using rice husk ash and palm kernel ash

Abstract

Palm oil mill effluent (POME) is one of the major wastes in palm oil industry. Prior to the disposal, POME should be pretreated as it can reduce soil fertility and contribute to human health hazards. Until now, many approaches have been investigated to improve waste management of POME. Traditionally, sedimentation process is used for removal of suspended solids trapped in the water. However, one-step treatment only is unable to remove high amount of solids in shorter time. The use of coagulation-flocculation (CF) prior to sedimentation process helps to form proper flocs, thickened the volatile solid (VS) in the sludge and facilitate the sedimentation process. Based on previous work, numerous natural coagulants had been used in POME treatment such as chitosan, Moringa oleifera seed, rice starch and other novel biocoagulants. In this study, the potential of rice husk ash (RHA) and palm kernel ash (PKA) were assessed as natural coagulant. A comparison study between natural coagulants and aluminium sulphate (alum) were conducted using jar test. The optimum dosage, pH and settling time of each coagulant were investigated based on responses of chemical oxygen demand (COD) and total solid (TS). Response surface methodology was used to predict the removal efficiencies and optimum conditions for each sample. Results showed that, alum was capable to remove COD up to 52.36 ± 1.49 % and 84.94 ± 0.12 % of TS using 6.50 g of alum, pH 6.0 and 41.69 min of settling time. At optimum condition, 52.38 ± 0.85 % of COD and 83.88 ± 1.03 % of TS were removed using 6.00 g RHA, pH 3.6 and 57.00 min of settling time. Meanwhile, PKA was capable to remove 50.50 ± 0.97 % of COD and 80.64 ± 0.64 % of TS by using 7.14 g of PKA, pH 5.97 and 40.65 min of sedimentation time. The results showed that RHA and PKA had attained similar removal efficiencies of COD and TS as alum. The flocs produced from CF process were further tested for biogas production in 150 mL serum bottle. The test was done by using biomethane potential (BMP) assay to determine the effect of each coagulants used towards amount of biogas produced. BMP assay for RHA produced maximum amount of biogas (480.25 mL/gVS) within 36 days. The methane potential was 244.03 mL/gVS and the methane content in the serum from overall biogas produced was 66.71%. To predict the performance of biogas produced, first order kinetic model was selected as it is prevalently used to evaluate anaerobic digestion process from various substrates. The results obtained from kinetic study revealed that RHA possesed the highest kinetic constant, k = 0.0403. From this work, it is proven that RHA was better than PKA and it is capable to replace alum in POME treatment as well as good enhancer for biogas productions.