CO‐DIGESTION OF GRASS SILAGE AND COW MANURE IN A CSTR BY RE‐CIRCULATION OF ALKALI TREATED SOLIDS OF THE DIGESTATE

Abstract

Three laboratory, continuously stirred tank reactors (CSTRs) co‐digesting grass silage and cow manure (forming 30% and 70% of substrate volatile solids (VS), respectively) were operated to evaluate the effects of re‐circulating an alkali‐treated and untreated solid fraction of the digestate back to the reactors. The CSTRs were operated at an organic loading rate (OLR) of 2 kg VS m−3 day−1 and hydraulic retention time (HRT) of 20 days with a semi‐continuous mode of feeding. The feasibility of co‐digestion with substrate VS containing 30% VS of crop was reinforced, resulting in average specific methane yield of about 180–185 l CH4 kg−1 VS. Re‐circulation of the solid fraction of digestate back to the reactors in both alkali‐treated and untreated forms decreased the methane yield by 11% and 21%, respectively, and resulted in operational problems such as scum formation and accumulation of the reactor materials. Batch studies were conducted to evaluate (i) the methane potentials of the solid fraction of digestate, and whole digestate with alkali treatments ranging from 20–60 g NaOH kg−1 VS of substrate, and (ii) methane potentials of the accumulated reactor materials as top, middle and bottom layers. The solid fraction of digestate treated with 20 g NaOH kg−1 VS showed higher specific methane yield (340 l CH4 kg−1 VS) than the higher range of alkali treatments. The bottom layers of the control reactor and the reactor fed with alkali‐treated solids gave a higher specific methane yield (93 and 85 l CH4 kg−1 VS, respectively), and all three layers of untreated solids gave similar methane potentials.