Development of an enzymatic hydrolysis pre-treatment strategy to improve batch anaerobic digestion of wastewater generated in desiccated coconut processing plants

Abstract

Enzymes are biological catalysts that accelerate biochemical reactions involved in biological treatment of wastewater. Commercially processed enzymes such as lipases improve the biodegradation by accelerating the hydrolysis rate of lipids. In this study, wastewater produced in desiccated coconut processing plants was pre-treated with a commercial lipase extracted from porcine pancreas followed by anaerobic digestion. Anaerobic digestion was conducted using twenty four identical 50 ml anaerobic batch reactors for 60 days at ambient temperature of 31±1ᵒC and pressure of 0.997±0.002atm without any mixing in reactor bulk liquid. The wastewater generated in desiccated coconut processing plants consists of high concentrations of medium chain saturated triglycerides. These triglycerides hydrolyzed quickly as the enzyme was added during enzymatic pre-treatment, resulting higher initial biogas production rate in the beginning and a higher daily biogas production rate during first 10 days. The initial and daily biogas production rate throughout the first 10 days of the pH adjusted samples showed higher biogas production rate than pH not adjusted samples as it was the most favorable pH value for the optimal growth of the methanogens and it also performed alkaline chemical hydrolysis of lipids in the beginning. After 60 days of complete degradation, the cumulative biogas production and percentage of VS reduction were almost similar in every reactor showing that the inhibition caused by lipids is a temporary inhibition but the rate of the reaction can be accelerated by enzyme addition, initial pH adjustment and maintaining a proper substrate to inoculum ratio. Bio-methane yield increased when the amount of substrate was increased as the quantity of hydrolyzed triglycerides available for the anaerobic microorganisms to convert into methane gas was higher. The bio-methane yield of enzyme added reactors were higher than enzyme not added samples because enzymes accelerated the hydrolysis of lipids. Among all 24 anaerobic reactors, the highest initial biogas production rate of 25.43 ml/day was observed in the reactor with highest lipase addition of 0.1% at inoculum to substrate ratio (v/v) of 2:3 under pH adjusted into 7.0 in wastewater in the beginning. The highest average biogas production during the first 10 days of 7.16 ml/day was also observed in the similar reactor. After 60 days of complete degradation, for the same reactor, cumulative biogas production of 95 ml, cumulative methane production of 81.55 ml, percentage of volatile solid reduction of 67.68% and experimental bio-methane yield of 42.75 ml CH4/g VS substrate added were also achieved.