Bacterization of Biostimulant (Brewers Spent Grains) on Hydrocarbon Degradation of Crude Oil Contaminated Garden Soil

Abstract

The influence of bio-stimulants (Brewers Spent Grains - BSG) on hydrocarbon degradation of bacteria used in the remediation of oil-contaminated soil was evaluated using standard microbiological techniques. The result revealed that the test soil contained 2.0 x107cfu/g total heterotrophic bacteria counts (THBC); of which 4.0 x102 cfu/g were oil degrading bacteria (ODB). The values of ODB/THBC ratio recorded for soil sample was less than one; and indication with a system with low level of hydrocarbon pollution. However, results obtained when microbially (B. subtilis) -augmented bio-stimulants were used in a Bacterization-Biostimulation (BB) remediation protocol revealed enhanced degradation of crude oil and it components. Viable cell measurement showed that the higher the biostimulants/contaminant (BC) - ratio employed the more the heterotrophic activity but less hydrocarbonoclastic activity. However, results have shown that soil contamination with crude oil drastically reduced the population of denitrifying bacteria but increased the population of oil degrading bacteria in soil but has concentration-dependent effects on the densities of heterotrophic bacteria. For soils remedied for 8 weeks with bacterized - BSG, the degradation rates were remarkably high and near 100% as against 44.02% recorded for the control (treatment with spent grains alone). This shows that biostimulation was better when "cropped" with oil degrading bacterium. The best degradation (99.09%) was achieved when 1% of BSG was applied (2.08%) at a BC ratio of 0.48: 1 which induced oil degraders’ growth rate and generation time of 0.00077 and 898.83 h-1 in 8 weeks respectively.  Beyond these ratios the treatments created “diauxic influence”, retarding the growth and activities of hydrocarbonoclastic bacteria while heterotrophic bacteria proliferate. The study revealed that augmenting biostimulants with strong strains of hydrocarbons degrading bacteria would stimulate the activities of indigenous degraders and ensure a hasten natural attenuation process in contaminated ecosystems. Longer incubation would certainly have led to complete or higher hydrocarbon degradation when hydrocarbonoclastic degradation enters the second log phase. Enhanced remediation with brewer's spent grains using BB protocol is strongly recommended but will be sustainable if the organic amendment stabilized with a fibre-rich carrier.