Evaluation of Co-Digestion at a Commercial Dairy Anaerobic Digester

Co-digestion of dairy manure with off-farm waste has become a common practice on US farms, however, little data at a commercial-scale is present within the literature. In response, a mesophilic, mixed plug-flow reactor co-digesting 16.36% v/v food processing substrates with dairy manure, was monitored for its performance and substrate effects. Co-digestion, as compared to substrate or manure-only digestion, allowed for more preferred levels of key micronutrients, neutral pH, and additional alkalinity while also producing C/N and C/N/P ratios of 28:1 and 112:4:0.5, respectively. Reduction percentages were 45.36, 55.28, 67.72, and 99.87% for TS, VS, COD, and VFA, respectively, while fecal coliform bacteria as an indicator organism showed a 2 log 10 reduction. A manure-only modeled baseline was developed for comparison with the experimental co-digestion data with co-digestion resulting in a 110% increase in biogas production and a tripling of gross receipts with 72% of all receipts being directly due to substrate supplementation. Specific methane productivities for the manure-only and co-digestion scenarios were 0.23 and 0.37 m 3 CH 4 /kg VS load , respectively. Addition of substrates tripled project gross revenues and accounted for 72% of all receipts, however, inclusion of substrates led to significant increases in total nitrogen and phosphorous loading to the farm.