Co-digestion of Dairy Cattle Waste in a Pilot-Scale Thermophilic Digester Adapted to Poultry Litter Feedstock: Stress, Recovery, and Microbiome Response

Abstract

The adaptability of a thermophilic anaerobic digester stabilized long-term with poultry litter as sole feedstock was measured with regard to co-digestion with dairy cattle manure. Both feedstocks have low C/N ratios but differ in carbohydrates, protein, and fiber content. The digester was a pilot-scale (40 m3) sCSTR (semi-continuously stirred tank reactor) with a 20-day hydraulic retention time (HRT). Dairy manure was added as three step-wise increases in concentration: 20, 40, and 80%. At 20% dairy manure, methane production was unchanged (17–19 m3 day−1). But as the fraction of dairy manure increased, methane, ammonia, sCOD, acetate, and propionate decreased. At 80% dairy manure, methane volume dropped by 60%. Acetate concentration decreased from an initial 878 ± 209 mg L−1 during poultry litter mono-digestion to 285 ± 169 mg L−1 with 80% dairy manure. Upon returning to mono-digestion, methane production was on a trajectory of recovery although acetate remained low. Bacterial community structure shifted during co-digestion, but community resilience was also underway following return to the original feedstock for one HRT. The dominant bacterial phylotypes belonged to the groups Thermotogales, Synergistales, Clostridiales, and Halanaerobiales, and three uncharacterized orders (MBA08, SHA-98, OPB54) in class Clostridia. Co-digestion led to a shift in dominance with reductions in Thermotogales and OPB54, and an increase in MBA08. This experiment provides guidance for digester operators who wish to mix livestock and poultry manures for thermophilic co-digestion.