Process Performance and Microbial Community Variation in High-Rate Anaerobic Continuous Stirred Tank Reactor Treating Palm Oil Mill Effluent at Temperatures Between 55 and 70 °C

Palm oil mill effluent (POME) is wastewater that is generated from the extraction of oil from palm fruit and palm kernels and is discharged at a relatively high temperature (80–90 °C). Anaerobic digestion (AD) of POME has been mostly investigated at a mesophilic range of temperatures (30–38 °C) and at a thermophilic temperature of 55 °C. The thermophilic AD of POME at higher temperatures could be advantageous as treatment at thermophilic temperatures may increase rates of biochemical reactions. Stepwise temperature increase is a methodology for adaptation of microorganisms to higher temperatures. In this study, the effects of stepwise temperature increments of 5 °C from 55 to 70 °C on the AD of POME were investigated in a continuous stirred tank reactor (CSTR) operated under high organic loading rates (OLRs). The process performance and microbial community structure at each temperature interval were evaluated. It was observed that the methane production rates of the CSTR increased with increasing OLRs up to values of 13.7 g/L days, 25.7 g/L days, and 26.5 g/L days at operating temperatures of 55 °C, 60 °C, and 65 °C, respectively. As a result of the increasing OLRs, the maximum rate of methane production increased from 3.8 L/L days at 55 °C to 4.4 L/L days at 60 °C. The microbial community structure analysis showed that there were notable reductions in the gene copy number of the bacterial domain and the Methanosarcinales order with increasing temperatures from 55 to 60 °C and to 65 °C, whereas hydrogenotrophic methanogens, especially the genus Methanobacterium, in the order Methanobacteriales became dominant at 60 °C and 65 °C. Thus, the methanogenesis pathway was suggested to be a combination of acetoclastic and hydrogenotrophic methanogenesis at 55 °C and 60 °C with an increased contribution of hydrogenotrophic methanogenesis at 60 °C, whereas methane was mainly generated via hydrogenotrophic methanogenesis at 65 °C. The thermophilic AD of POME at 60 °C was found to be promising because the methane content in the biogas and the methane production rates were optimal, with an average methane content of approximately 73%. Graphic Abstract