Thermophilic anaerobic digestion of the screened solid fraction of dairy manure in a solid-phase percolating reactor system

Abstract

An increase in volatile solids (VS) content from solid fraction of dairy manure would reduce the size of biogas on-farm facilities and could be of greater interest to farmers than manure slurry biogas plants. This study examined the technical feasibility of a solid-phase batch thermophilic anaerobic digestion system for the screened solid fraction (SF) of dairy manure using the digested liquid fraction of dairy manure as inoculum. Inoculum to substrate ratio (I/S) and percolate recirculation strategy were the parameters studied. The manure slurry separation process resulted in a solid fraction that represented 16.8% of total manure mass; this aforementioned fraction showed an ultimate methane yield of 61.5 L CH4 kg−1 SF (265 L CH4 kg−1 VS) which represented 48% of methane potential from raw manure slurry. Five comparative experiments with 40 kg SF and different I/S ratios and percolate recirculation rates revealed that higher I/S ratios provide more security against inhibition states due to high VFA levels and low pH in percolate. The results of the present work have shown that increasing the percolate recirculation rate in intermittent and short recirculation operations improves the stability and speed of the process. Under this percolate recirculation strategy dry batch operation was possible with a I/S ratio of 0.6 obtaining methane yields of 145, 175, 204 and 220 L CH4 kg−1 VS after 15, 20, 30 and 60 days of operation.