Sequential batch thermophilic solid-state anaerobic digestion of lignocellulosic biomass via recirculating digestate as inoculum – Part I: Reactor performance

Abstract

Sequential batch thermophilic (55°C) solid-state anaerobic digestion (SS-AD) of yard trimmings was evaluated at a total solids (TS) content of 22% via recirculating digestate as the inoculum. The substrate-to-inoculum (S/I) ratio of 1 (TS basis) was favored over 2 and 3 due to significantly higher methane yield and volumetric productivity. At an S/I ratio of 1, sequential batch SS-AD gradually reached steady state by 3 runs (30days/run) with increases in both methane yields (up to 11.5%) and cellulose degradation (up to 55%), indicating that recirculated digestate could be a feasible inoculum to establish long term stable SS-AD of lignocellulosic biomass. The initial sharp increases of volatile fatty acids during runs 2–4 indicated faster hydrolysis of organic matter than during run 1, suggesting that microbes were probably more acclimated due to digestate recirculation. At steady state, 51% (w/w) of the digestate was recirculated as the inoculum.