Impacts of Anaerobic Digestion and Solids Separation on Ammonia Emissions from Stored Dairy Manure

Abstract

<abstract> <bold>Abstract. </bold>Ammonia is one of the major air pollutants from concentrated animal feed operations (CAFOs), which have potential to negatively impact the environment and ecosystem. Anaerobic digestion (AD), on the other hand, is steadily being adopted, more and more, for extracting bioenergy from dairy wastewater in CAFOs prior to storage or land application. The AD process is associated with changes in manure physical-chemical properties, which may affect ammonia emission from AD effluents. Field and laboratory studies were conducted to evaluate the effects of AD and solids-liquid separation on ammonia emissions during subsequent storage of the AD. The lab manure storage tests were conducted for 21 d in plastic containers with open surface areas of 386 cm2 filled with 2.5 L manure samples obtained at the following four points in a full-scale AD system: raw manure (RM) feed substrate, raw manure supplemented with other substrates (AD influent), AD effluent, and AD effluent after solids-liquid separation (AD liquid effluent). Higher ammonia fluxes were observed on day 1 from the AD effluent (5.58 g m-2 d-1) and RM (2.12 g m-2 d-1) storages but both decreased to zero after 21 d. Lower ammonia fluxes were observed from AD liquid effluent (0.53 g m-2 d-1) and AD influent (0.12 g m-2 d-1), which remained constant during the entire 21 d of the study. The cumulative ammonia emissions during the 21 d test were: 1608 ± 34 mg from AD effluent; 660 ± 19 mg from RM; 429 ± 44 mg from AD liquid effluent; and 99 ± 11 mg from AD influent. Field measurements of ammonia emissions from both AD liquid effluent and RM storages were conducted using a floating chamber and a photoacoustic gas analyzer. Ammonia emissions from full-scale manure storages were similar to those obtained in the lab. Ammonia emissions from storages of AD (AD liquid effluent) and non-AD manure (RM) from winter to summer ranged from 0.12 to 1.4 g m-2 d-1 and 0.09 to 0.79 g m-2 d-1, respectively. In general, anaerobic digestion of dairy manure, therefore, resulted in significant increases in ammonia emissions from stored effluents. However, solids-liquid separation further mitigated ammonia emissions from AD liquid effluent.