Optimization of Biogas Production from Abattoir Waste by Bioaugmentation

Abstract

Biogas production from abattoir waste using conventional technology is a very slow and inefficient process. The process can be improved by use of bioaugmentation and optimisation of operating parameters. The general objective of this study was to optimize biogas production from abattoir waste through bioaugmentation. The specific objectives were to: evaluate composition of abattoir waste, characterize physiochemical properties, analyze effects of using rumen inoculum and bioaugmentation utilizing Bacillus subtilis and Escherichia coli on biogas yield and composition and establish optimum temperature, percentage of rumen inoculum and hydraulic retention time for biogas production from mixed abattoir waste. Biogas was produced in 250ml flask reactors from mixed abattoir waste using 0, 20 and 50% rumen inoculum based on volume ratio at 35 °C for 30 days. The most effective percentage was applied in the second experiment that produced biogas at 37 °C in 25 days in 500ml flask reactors using Bacillus subtilis, Escherichia coli, Bacillus subtilis + Escherichia coli mixture for bioaugmentation and control experiment without any bioaugmentation. Box-Behnken design was used to optimize temperature, percent rumen inoculum and hydraulic retention time for biogas production from mixed abattoir waste in the last experiment. Analysis of the mean biogas volume, biogas potential, methane content and depletion of total solids and chemical oxygen demand were used to determine the most effective set up while response surface methodology was applied in determining optimum conditions for the Box-Behnken designed experiment. Rumen inoculum of 20 and 50% (v/v) achieve a significant increase in biogas potential and methane content over use of 0% rumen inoculum. Rumen inoculum of 20 and 50% achieve production potentials of 0.068 and 0.069 ml/mgTS withvii methane content being 54.13 and 60.12% respectively compared to 0% rumen inoculum which attains a potential and methane content of 0.052 ml/mgTS and 48.91% respectively. Similarly, combining Bacillus subtilis and Escherichia coli significantly improves biogas yield and methane content compared to when each microbe is used separately during digestion of abattoir waste. The combined power of the microbes achieves a potential of 0.083 ml/mgTS and a methane content of 66.92%. Separately E.coli and Bacillus subtilis achieve a potential of 0.077 and 0.076 ml/mgTS and methane content of 62.71 and 62.24% respectively. Optimum levels of temperature, percent rumen inoculum and hydraulic retention are 37.93 °C, 70.45% and 16.28 days respectively. This study recommends optimisation of biogas production from abattoir waste using a combination of E.coli, Bacillus subtilis and 70.45% rumen inoculum at temperature of 37.93°C.