Economic feasibility and evaluation of a novel manure collection and anaerobic digestion system at a commercial swine finisher enterprise

A case study conducted at a commercial swine-finishing farm demonstrated that a novel manure management system increased economic feasibility of an anaerobic digester by eliminating the need for post-digestion manure storage construction at the farm. Uniquely designed underfloor manure storage pits collected manure for delivery to the digester, and then stored post-digested manure (digestate) in underfloor storage within the same swine houses. It was unknown if the introduction of biologically active digestate into these pits would produce pig living space air quality that was adverse to pig health, growth or survival, or if explosive methane levels would be generated within the buildings. Monitoring of air quality indicators both before and after digestate introduction to underfloor manure storage pits resulted in no observations of hydrogen sulfide (H2S) or methane (CH4) concentrations above critical safety levels in swine housing. Hourly mean ammonia (NH3) concentrations at pig level (0.15 m above the floor) before digestate was present in the buildings were higher (P < 0.05) compared to when digestate was present (24 ± 2.8 ppm vs. 17 ± 1.0 ppm). Air quality measures did not indicate that digestate introduction into underfloor manure pits caused degradations of air quality at pig level. No obvious etiologic effects on swine were observed. Evaluation of the electric cogeneration system showed that cost-savings of electricity produced from biogas combustion was approximately equal to the producer's debt service for capital investment. External funding and low interest financing were necessary for electric cost-savings to offset finance payments.