What’s a Profitable Herd Size for RNG Production? Farm Size Impacts Economic Feasibility of Covered Manure Storage to RNG in Iowa

Highlights 1,000 cows is the break-even herd size for a ten-year life RNG project when the manure storage is emptied once annually. 1,400 cows is the break-even herd size for a ten-year life RNG project when the manure storage is emptied twice annually. 11,000 wean-finish swine is the break-even herd size for a ten-year life RNG project when manure is stored in a lagoon. Deep pit swine farm projects are less viable due to reduced carbon credit revenue and required infrastructure. ABSTRACT. Impermeable covers have been one of the most popular choices for implementing manure anaerobic digestion to renewable natural gas projects; however, limited studies have evaluated how economies of scale affect the profitability of impermeable cover projects. We assessed dairy farms ranging from 100 to 5,000 head and swine farms ranging from 1,200 to 35,000 head to determine the herd size necessary for economically viable projects. System construction costs were estimated using literature values, with potential revenue streams estimated using average market prices. Estimated break-even farm sizes for ten-year project lives were 1,000 dairy cows if the manure storage was emptied once annually, 1,400 dairy cows if the manure storage was emptied twice annually, and 11,000 wean-finish pigs using lagoon manure storage. Wean-finish pigs initially using a deep pit manure storage required more than 27,000 pigs to financially break-even within ten years. While there are roughly 7,600 swine farms and 170 dairy farms in Iowa, only 79 individual farms in Iowa qualify as economically feasible renewable natural gas projects due to typical farm sizes and manure management systems prevalent in the Midwestern United States. RNG projects on these 79 farms could reduce manure management CO 2 e emissions by 1,138,000 MG annually. Future work must explore and develop strategies for renewable natural gas systems on farms with less than 1,000 cows or 10,000 pigs. Keywords: Anaerobic digestion, Biogas, Biogas upgrading, Impermeable covers, LCFS credits, Manure management, Manure storage, Renewable natural gas, RIN credits.