This research is aimed at improving the thermophilic aerobic treatment system of anaerobically digested
palm oil mill effluent (POME) through optimization of mixed liquor suspended solids (MLSS) at high
concentration range for the first time besides other parameters of organic loading rate (OLR), dissolved
oxygen (DO) concentrations and settling time. Chemical oxygen demand (COD), biochemical oxygen
demand (BOD) and total suspended solids (TSS) removal ratios were measured as indicators of the system
performance. Experimental results clearly indicate that MLSS and OLR are the crucial parameters in
thermophilic aerobic treatment. Optimal COD, BOD and TSS removals up to 86%, 87% and 89%, respectively,
were achieved with MLSS concentration and OLR of 27,000 mg/L and 2.5 g COD/(L day), respectively. More
significantly, high MLSS concentration range which is yet to be reported was successfully applied and this
has resulted in superior performance as compared to other thermophilic treatment studies. No significant
improvement of the treatment efficiency was attained with DO concentration exceeding 5 mg/L and
settling time up to 24 h. Hence, operation at DO concentration of 2 mg/L and settling time of 2 h gives a
good compromise between effluent quality and system operating costs.