Co-digestion of brewers' spent grain with dairy cattle manure

ABSTRACT Anaerobic digestion (AD) of dairy manure presents many potential environmental, economic and social benefits for farmers and communities. However, digestion of dairy manure alone results in relatively low output of methane, whereas co-digestion of manure with other substrates can result in increased output of methane as well as income for farmers from tipping fees for organic waste recycling. Brewers’ spent grain (BSG), the main solid waste product of beer brewing, has been examined elsewhere as a feedstock for AD systems but is not well studied as a co-substrate with dairy manure in semi-continuous farm digesters. This study compared co-digestion of dairy manure with BSG against a manure only control in 7.5L mixed, daily-fed lab digesters for 95 days. Addition of 2% TS as BSG to a base of 5% TS as dairy manure resulted in a 136% increase in methane production, despite organic loading rate only increasing by 49%. The protein and carbohydrate rich BSG is more degradable than dairy manure, also verified by a 56% increase in volatile solids reduction for the co-digestion treatment compared to control. Assuming no synergistic effect, BSG in this study resulted in 0.264 L CH4/g VS added (0.064 L CH4/ g wet BSG added). Co-digestion of BSG with dairy manure at an organic loading rate of 2.35 gVS/L-d at 35oC for 95 days resulted in no apparent inhibition of methanogens while digestate pH remained steady in the neutral range. BSG addition did result in a slight increase in H2S content of biogas, likely due to the sulfur released as proteins in the BSG were degraded. Overall we conclude that BSG is a suitable substrate for co-digestion with dairy manure in mixed agricultural digesters.