Combined treatment of palm oil mill effluent (POME) by coagulation-flocculation and heterogeneous photocatalysis

Palm oil industry in Malaysia has grown rapidly over the years, putting the country as one of the world’s largest producers and exporters of palm oil and its product. However, this industry has also been identified as the single largest source of water pollution due to the discharge of palm oil mill effluent (POME) into the rivers. Hence, the main objective of this research is to investigate the possibility of applying a new combined process consisting of coagulation-flocculation and heterogeneous photocatalysis as a more sustainable way for POME treatment to significantly reduce its pollutants. In coagulation-flocculation pretreatment of POME, the present work focused towards demonstrating the performance of unmodified rice starch as an effective and environmentally friendly substitute for alum. By investigating the important operating parameters, it was found that the use of rice starch alone in room temperature enabled the removal of total suspended solids (TSS) up to 84.1% using the recommended dosage, initial pH, settling time and slow stirring speed at 2 g/L, pH 3, 5 min and 10 rpm, respectively. Using these pretreatment conditions, 17.4% of chemical oxygen demand (COD) from the POME could also be removed. Higher TSS (88.4%) and COD removal (27.0%) could be achieved when both rice starch and alum was used together during the pretreatment process. Through optimization using response surface methodology, the combined between both alum and rice starch not only enhanced TSS and COD removals; but it also significantly improved the process by reducing alum and settling time, while shifting the operating pH closer to the natural pH of POME. Final pretreatment conditions enabled the TSS and COD removals up to 86.7 and 49.2%, respectively under the optimum conditions of 0.38 g/L of alum, 0.28 g/L of unmodified rice starch, pH 4.45 and settling time of 5.54 min. Before applying photocatalytic treatment on the pretreated POME, a photocatalyst was firstly synthesized. By using sonochemical method, both nitrogen and chlorine were successfully doped into TiO2 (Stage 3). Detailed study showed that the recommended synthesis conditions were sonication amplitude of 40%, sonication duration of 3.62 h, N,Cl:Ti molar ratio of 2 and calcination temperature of 200 °C. Due to its reduced band gap, better visible light absorption properties, higher availability of reaction sites and higher surface acidity, N,Cl-codoped TiO2 exhibited superior photocatalytic activity by achieving soluble chemical oxygen demand (sCOD) and dissolved organic carbon (DOC) removals up to 89.5 and 86.0%, respectively after 12 h of irradiation by using the recommended treatment conditions of pH 5, 4 g/L of photocatalyst and air flowrate of 200 mL/min. The photocatalytic activity of N,Cl-codoped TiO2 also outperformed those by Aeroxide® P25 (about 50% removal of sCOD and DOC). In terms of reusability, the performance of N,Cl-codoped TiO2 was relatively stable up to two treatment cycles with sCOD and DOC removals higher than 80%. Completion of this study successfully demonstrated the effectiveness of using combined process, consisting of both coagulation-flocculation and heterogeneous photocatalysis to greatly reduce the pollutants from the POME.