Palm oil mill effluent processing via hybrid plasma and acoustic treatment

The large quantities of palm oil mill effluent (POME) that need to be processed daily to meet global palm oil demand creates a significant pollution problem. The two-stage anaerobic–aerobic process conventionally used to treat POME before its discharge to minimize its environmental impact, however, involves long retention times. We demonstrate in this work that it is possible to significantly reduce such treatment times through the introduction of plasma and MHz-order sound waves in the form of surface acoustic waves (SAWs) to the second stage aerobic treatment process, which, considerably improve the reduction of the POMEs' total suspended solids (TSS) concentration, chemical oxygen demand (COD) and biological oxygen demand (BOD); the plasma exposure enhances oxygen dissolution and the dissolution of the organic compounds, whereas the SAW promotes convective mass transfer and decomposition of the bacteria. With plasma pretreatment alone before aerobic treatment, it is possible to achieve a twofold reduction in the total treatment time to achieve target TSS, COD and BOD values, whereas irradiating the POME with the SAWs during aerobic treatment was found to reduce treatment times by up to fourfold. Combining both methods into a serial treatment process was then observed to lead to overall reduction in the total treatment duration by up to fivefold, therefore highlighting the promise of the hybrid technology for adoption in industry-scale treatment systems, particularly given the simplicity and cost effectiveness of plasma generation and the low cost and chip-scale dimensions of the SAW devices that facilitate ease of scale-up through massive parallelization.