Effect of the Inoculum-to-Substrate Ratio on Putative Pathogens and Microbial Kinetics during the Batch Anaerobic Digestion of Simulated Food Waste

Abstract

The effects of the inoculum (anaerobic digestion effluent) to substrate (simulated food waste) ratio (ISR) 4.00 to 0.25 on putative pathogens and microbial kinetics during batch mesophilic anaerobic digestion were investigated. Red fluorescent protein labelled (RFPAKN132) <i>Escherichia coli</i> JM105 was introduced as a marker species, and together with the indigenous <i>Clostridium</i> sp., <i>Enterococcus</i> sp., <i>Escherichia coli</i>, and total coliforms were used to monitor pathogen death kinetics. Quantitative polymerase chain reaction was also used to estimate the bacterial, fungal, and methanogenic gene copies. All the ISRs eliminated <i>E. coli</i> and other coliforms (4 log₁₀ CFU/mL), but ISR 0.25 achieved this within the shortest time (≤2 days), while ISR 1.00 initially supported pathogen proliferation. Up to 1.5 log₁₀ CFU/mL of <i>Clostridium</i> was reduced by acidogenic conditions (ISR 0.25 and 0.50), while <i>Enterococcus</i> species were resistant to the digestion conditions. Fungal DNA was reduced (≥5 log₁₀ copies/mL) and was undetectable in ISRs 4.00, 2.00, and 0.50 at the end of the incubation period. This study has demonstrated that ISR influenced the pH of the digesters during batch mesophilic anaerobic digestion, and that acidic and alkaline conditions achieved by the lower (0.50 and 0.25) and higher (4.00 and 2.00) ISRs, respectively, were critical to the sanitisation of waste.