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.