Biogeochemical reactions in anaerobic digesters for biogas production yield enhanced ammonia emissions

The livestock industry in the United States is in the process of retrofitting existing animal waste management systems serving concentrated animal feeding operations (CAFOs) to collect biogas, primarily methane for renewable energy production. The shift from using animal waste solely for crop fertilizer to biofuels, including biogas, is often advocated as an environmentally friendly strategy, due to the possibility of reducing greenhouse gas emissions. Anaerobic digestion (AD) is currently one of the principal methods for converting waste biomass into bioenergy and distributed energy benefits. However, the implementation of closed and covered anaerobic digesters for the collection of biogases from animal feeding operations has potential negative environmental consequences including degrading air quality (e.g. enhanced reactive nitrogen emissions) and water quality (e.g. increased nitrate loading in ground water) relative to digestion of manure in systems open to the atmosphere. Here, ammonia (NH 3 ) flux from swine CAFOs was analyzed using field measurements and from outputs of a semiempirical mass-transfer model to compare the performance of the different systems. Regression analysis controlling for temperature demonstrates that NH 3 flux from biogas secondary lagoons (BSL) is statistically higher than from conventional lagoons (p < 0.001). Model simulations further confirm that increased total ammonium nitrogen and higher pH in digestate enhance volatilization potential during secondary storage and land application under typical management. Therefore, the flux of ammonia emission is significantly higher from secondary open lagoons containing digestate BSL received from a closed and covered AD compared to open-air lagoons using the conventional lagoon spray technology, showing the negative consequence of the changes which can lead to degraded air quality and water quality. Retrofitting AD without integrated nitrogen management may shift environmental impacts from methane to reactive nitrogen. Incorporation of mitigation technologies for the management of animal waste can substantially reduce this risk. Synopsis Anaerobic digesters for biogas production from animal manure processing have negative consequences to society and the environment and may not represent the cleaner energy future.