Co-digestion of water lettuce with cow dung to produce biogas

Abstract

Increasing demand for energy has motivated studies on renewable energy originating from biomass. Water lettuce is an invasive weed on Lake Kyoga and there is need to utilize substantial volumes to benefit the communities around the lake. It hinders water transport and deprives aquatic fauna of oxygen. This study focuses on analyzing the co-digestion of water lettuce with cow dung under batch system for 30 days at ambient temperature of about 29°C in order to put the weed into use. The thermogravimetric analyser was used for proximate analysis where as a Soxhlet machine was used to determine the fat content. A completely randomized design was used together with a third order polynomial equation to simulate methane production. Triplicates of 750 ml vessels were used for the five co-digestion ratios in the experiment. The biogas was passed through sodium hydroxide at a 35% concentration to absorb carbon dioxide and collected by water displacement. Water lettuce contains 51.7% carbohydrates, 0.02% fat, 8.25% carbon and 15.8% protein. Basing on C/N ratio, water lettuce has ideal properties for biogas production when co-digested with cow dung. From the optimization model (using DataFit 9.1.32), the optimum methane yield of 101.1mL/g VS was obtained from the ratio 77:23 of water lettuce to cow dung (mass). The value of R2 (0.8153) showed that the optimization model can efficiently be applied to predict methane

production from the co-digestion of water lettuce with cow dung. These findings show that co- digestion of water lettuce with cow dung is a promising alternative source of renewable energy

although it is recommended that the experiment is carried out until no biogas is produced. Key words: Water lettuce, cow dung, co-digestion, ANOVA