Influence of ultrasound modes on sonoelectrochemical degradation of Congo red and palm oil mill effluent

Sonoelectrochemical oxidation (SEO) treatment is a competitive technology for the degradation of organic pollutants in water. The operation modes of ultrasound (continuous and pulse) not only affect the pollutant degradation efficiency but also the energy consumption of SEO. In this study, the influence of ultrasound modes on SEO degradation of Congo red (CR) and real wastewater – palm oil mill effluent (POME) were investigated. The SEO was operated in two modes: continuous ultrasound throughout the whole process and intermittent ultrasound at a cycle of 15 min of ultrasound and 5 min without ultrasound. It was found that the intermittent SEO gave a higher degree of degradation of CR at 95% as compared to continuous SEO and electrochemical oxidation (EO) at 85% and 64%, respectively. The improved performance could be attributed to the supply of ultrasound that minimized electrode passivation and promoted mass transfer and production of hydroxyl radicals. The intermittent supply of ultrasound diminished the interference of ultrasound on the electrochemical process. A similar observation was also observed where the highest COD reduction of POME was recorded by intermittent SEO at about 65%. In terms of energy consumption, degradation of CR by SEO only required half of the energy consumed by the analogous purely electrochemical process at 250–265 kWh/kg dye removed. Surprisingly, the energy consumption for SEO degradation of POME only reduced from 32.82 (electrochemical) to 25.91 and 30.78 kWh/kg COD removed for continuous and intermittent SEO, respectively. Such contrast in energy saving highlights the need for further study as the complex constituents of real effluents will result in different SEO treatment performance. Nonetheless, the capability of SEO in reducing the COD indicates its potential to be used as pre- or post-treatment for other advanced processes in POME handling.