Production of aerobic granular sludge in sequencing batch reactor for the treatment of palm oil mill effluent

Abstract

Malaysia is the world’s second largest producer of palm oil. However, production of palm oil results in the generation of large quantities of polluted wastewater known as palm oil mill effluent (POME). It was estimated that for one tonne of crude palm oil (CPO) production, about five to eight tonnes of water are required for processing purposes and around 50% (three to four tonnes) will end up as POME. Direct discharge of raw POME without any treatment will deteriorate the surrounding environment as the organic compounds in the wastewater such as tannins and humic acids, tend to inhibit growth and reduce the rate of photosynthesis of aquatic biota. The unpleasant odour and blackish colour of POME also causes aesthetic problems and raise public concerns. Conventional ponding systems which rely solely on indigenous bacteria often failed to withstand the excessive pollution load and resulted in poor treatment efficiency. Aerobic granular sludge (AGS) produced in sequencing batch reactor (SBR) is a promising alternative to ponding system as it tends to withstand higher organic loadings and greater biomass retention. In this study, AGS was produced in laboratory scale SBR with raw final discharged POME as feed. Three bacterial strains, Escherichia coli (AL1), Bacillus cereus (AL2) and Lysinibacillus fusiformis (ZB2) were added as inoculum into the SBR. These strains were screened in this study based on their abilities to reduce colour intensity and COD level of final discharged POME, while maintaining high viable cell counts when cultivated in POME for five days. The SBR system was operated at the volume exchange ratio (VER) of 50%, hydraulic retention time (HRT) of six hours and organic loading rate (OLR) of 5.0 kg/COD/m3/d for 200 days. The SBR system entered steady state during day-80 of operation period indicated by mixed liquor volatile suspended solids (MLVSS) concentration of above 3,000 mg/L and sludge volume index (SVI) of below 80 mL/g. The SBR system achieved outstanding biomass concentration of 19,200 mg/L, six times higher than normal operating SBR. SVL index as low as 12 mL/g was considered one of the best among similar studies. The microbial communities of AGS were examined at different stages of granulation using Miseq amplicon sequencing system. Results showed the microbial communities of AGS of the age of 20 days, 80 days and 180 days were dominated by phylum Firmicutes and Proteobacteria, whereas the relative abundance of Phyla Actinobacteria and Bacteroidetes reduced as the AGS aged.