Effect of lipid inhibition in anaerobic wastewater treatment : a case study using desiccated coconut wastewater

Abstract

Anaerobic treatment processes have been extensively developed for the treatment of various wastewaters containing high concentrations of organic substrates including proteins, carbohydrates and lipids. The desiccated coconut industry in Sri Lanka is one of the industries where wastewater with high concentrations of lipids containing medium chain saturated fatty acids (~55%), long chain saturated fatty acids (~35%) and long chain unsaturated fatty acids (~10%) is generated. The long chain organic molecules of lipids undergo a series of complex biochemical reactions such as hydrolysis, acidogenesis, acetogenesis and methanogenesis during anaerobic digestion. According to previous studies, high concentrations of lipids have an inhibitory effect on anaerobic treatment processes, due to the mass transfer limitations and toxicity caused by long chain fatty acids (LCFA) towards syntrophic substrate-degrading bacteria. Undesirable effects of accumulation of LCFA in anaerobic reactors comprise of gradual drop down of rate of biogas production and derogatory effects on quality characteristics of effluents such as chemical oxygen demand (COD), biochemical oxygen demand (BOD) and oil and grease removal efficiencies after anaerobic treatment, causing the treatment process inefficient. According to the case study, desiccated coconut wastewater (DCWW) has pH 4.0 - 5.5, COD 4,000 mg/l - 8,000 mg/l, BOD 1,000 mg/l - 5,000 mg/l and total lipids 4,000 mg/l. In anaerobic treatment of DCWW, the COD removal efficiency rapidly changes from 90% - 30% due to the lipid inhibition caused by medium chain and LCFA in DCWW such as lauric acid and myristic acid. The treatment efficiency of anaerobic digestion can be enhanced via different physicochemical strategies.