Effect of inoculum to substrate ratio on the hydrolysis and acidification of food waste in leach bed reactor

► High inoculum:substrate ratio (ISR) significantly increased the hydrolysis rate of protein. ► High ISR exhibit only a modest increase in the decomposition of carbohydrate. ► Chemical oxygen demand of leachates increase with the increasing ISR from 20% to 80% (w/w), however not significant. ► High ISR reduce the waste throughput; thus lower ISR of 20% (w/w) during acidogenesis is recommended. The aim of present study was to determine an appropriate ISR (inoculum to substrate ratio) to enhance the hydrolysis rate and reduce the solid retention time of food waste in hydrolytic-acidogenesis leach bed reactor (LBR). LBR 1–4 were inoculated with 0%, 5%, 20% and 80% (w/w basis) of anaerobically digested sludge, respectively, using artificial food waste as substrate. Experiments were conducted in batch mode at mesophilic condition (35°C) for 17days. Higher ISR resulted in 4.3-fold increase in protein hydrolysis; whereas, only a modest increase in the decomposition of carbohydrate. Two kinetic models for carbohydrate and protein degradation were proposed and evaluated. The differences among four ISRs in volatile solids removal efficiencies were marginal, i.e. 52.4%, 62.8%, 63.2% and 71.7% for LBR 1–4, respectively; indicating that higher ISR was insignificant in enhancing the overall hydrolysis rate in LBR. Therefore, a lower ISR of 20% was recommended in the hydrolytic-acidogenic process.