Particle size, inoculum-to-substrate ratio and nutrient media effects on biomethane yield from food waste

This study investigates the effects of particle size reduction at different inoculum-to-substrate ratios and nutrient media supplementation on the assessment of biomethane production from food waste, under batch mesophilic conditions. Two different food waste samples were used and the best method for testing biomethane potential was chosen based on their characterisation and methane yields. Results obtained indicate that Inoculum-to-substrate ratios of 3:1 and 4:1 helped to stabilise test reactors with smaller particle sizes of 1 mm and 2 mm, respectively. Consequently, an overall biomethane yield increase of 38% was reported (i.e., from 393 NmLCH4 gVS−1added to 543 NmLCH4 gVS−1added). This could potentially imply a better assessment of energy outputs from anaerobic digestion of food waste (i.e., 43.5% higher energy output as electricity from biogas, using commercial scale Combined Heat and Power (CHP) units). Although nutrient media supplementation did not enhance methane yield from optimum inoculum-to-substrate ratio (3:1) and particle size (1 mm), it was found that its application helped to stabilise food waste digestion by avoiding volatile fatty acids accumulation and high propionic-to-acetic acid ratio, consequently, improving the overall test kinetics with 91% lag time reduction from 5.6 to 0.5 days. This work supports the importance of key variables to consider during biomethane potential tests used for assessing methane yields from food waste samples, which in return can potentially increase the throughput of anaerobic digestion system processing food waste, to further increase the overall energy output.