The effect of biogas ebullition on ammonia emissions from animal manure-processing lagoons

Various models have been developed to determine ammonia (NH₃ ) emissions from animal manure-processing lagoons to enable relatively simple estimations of emissions. These models allow estimation of actual emissions without intensive field measurements or "one-size-fits-all" emission factors. Two mechanisms for lagoon NH₃ emissions exist: (a) diffusive gas exchange from the water surface and (b) mass-flow (bubble transport) from NH₃ contained within the ebullition gas bubble (as it rises to the surface) produced from anaerobic decomposition of organic matter. The purpose of this research is to determine whether gas ebullition appreciably affects NH₃ emissions and therefore should be considered in emissions models. Specifically, NH₃ mass-flow emissions were calculated and compared with calculated diffusive NH₃ emissions. Mass-flow NH₃ emissions were evaluated based on a two-film model, in connection with the acid dissociation constant of ammonium, to predict the degree of NH₃ gas saturation within the bubbles. Average daily ammoniacal nitrogen concentration, pH, and measured biological gas production (ebullition) in conjunction with literature values for Henry's law constant were used to calculate emissions from NH₃ saturation of ebullition gases. Ebullition enhancement of NH₃ surface emissions due to increased turbulence was estimated from average lagoon ebullition rates and literature values of turbulence enhancement. Ebullition enhancement of NH₃ surface emissions and ebullition mass-flow NH₃ emissions was determined to be <10% and <0.052%, respectively, of total NH₃ emissions. Therefore, because ebullition effects are small, they may be neglected when developing process models to estimate NH₃ emissions from water surfaces of swine manure processing lagoons.