Recycling separated liquid-effluent to dilute feedstock in anaerobic digestion of dairy manure

The major concern of recycling anaerobic digestion (AD) effluent in the digester centers on accumulation of total ammonia nitrogen (TAN) and salinity, both of which can potentially inhibit methane production. In the current study, 30%, 50%, and 80% of separated-liquid AD effluent, were recycled in a series of batch AD experiments. The inhibitions to specific methane potential (SMP) caused by TAN and salinity were evaluated. Recycling up to 80% of un-treated effluent resulted in the best SMP averaging 0.265 ± 0.005 m3 [CH4] Kg−1 [volatile solids], which averaged 10% more compared to recycling 80% treated effluent and 5% more compared to no recycling (the control). After acclimation, up to 6.39 g N L−1 increase in TAN resulted in SMP averaging 0.112 ± 0.002 m3 [CH4] Kg−1 [volatile solids] and up to 12 parts per thousand increase in salinity resulted in SMP averaging 0.163 ± 0.005 m3 [CH4] Kg−1 [volatile solids]. A mass balance for a hypothetical 5000 cows dairy farm showed effluent recycle of up to 66% for maintaining 8% solids in anaerobic digester. Moreover, in the proposed system, the effluent going off-farm was on w/w basis 64% less water, 66% less solids, and 52% less nitrogen compared to the effluent produced with no recycle facility. •Study evaluated effects of recycling separated liquid-effluent on anaerobic digestion (AD).•Up to 80% separated liquid-effluent is recyclable resulting in ∼5% more methane yield.•Methane production improved when using acclimated seed in the AD.•66% liquid-effluent recycle can maintain 8% total solids in AD.