Pilot-scale biogas production in a temperate climate using variable food waste

Abstract

Low-input, household-scale anaerobic digestion (AD) is practical for rural energy generation, but the heating and maintenance of such systems has prevented the use of this technology in temperate climates. This study quantified the temporal variation of fuel production from an unheated, pilot-scale (2 m3) anaerobic digester in southeastern Ohio, USA. The feedstock for the digester consisted of ground, mixed pre- and post-consumer food waste collected daily and diluted to 10% solids with rainwater collected on site. Fuel production varied from 7.96 × 10−6 to 8.45 × 10−2 m3 CH4 kgVSadded−1 following three separate inoculations over the course of two years. The positive relationship between ambient air temperature, biogas yield, and biomethane production rates for both years was a dominant driver affecting fuel quantity and quality. Biogas quality produced from variable feedstocks in these conditions was poor, with an average volumetric methane (CH4) content of 20% and an average CO2:CH4 ratio of 7.8. Methane yields did reach 50% during the warm seasons, but this yield was not consistently maintained. Despite low energy yields that resulted from the wide range of ambient temperatures (−18 °C–33 °C) and variable feedstocks, we achieved a moderate energy return on investment relative to previously published results describing AD system energy requirements. Pilot-scale, unheated AD systems using mixed food waste can be effective in temperate regions, but the systems should be managed to compensate for seasonal temperature changes and feedstock chemistry.