A Process-based Model for Ammonia Emission from Storages of Flushed Dairy Manure

Abstract

Ammonia (NH3) is one of the major gaseous pollutants emitted from livestock facilities. Estimates indicate that the largest portion (about 80%) of the total nitrogen entering a dairy facility is lost as NH3 from manure storages; such as anaerobic lagoons. Direct measurements of NH3 emissions from these storage structures are not only tedious but also quite complex and expensive exercises. Process-based models offer an alternative cost-effective approach of making emissions estimations. This research coupled theoretical and empirical analyses of NH3 emissions mechanisms to increase the reliability of process-based NH3 emission models. A process-based model was developed to predict NH3 emission from dilute dairy manure via incorporation of two newly developed empirical sub-models of: the overall mass transfer coefficient (KoL) of NH3 from liquid dairy manure; and the dissociation constant (Kd) of ammonium (NH4+) in liquid dairy manure. The KoL was modeled based on lagoon liquid temperature (TL), air velocity, air temperature, and total solids (TS) concentrations. The Kd was modeled based on TL, and TS concentrations. The model predictions were validated with directly measured NH3 emissions using an open-path ultra-violet differential optical absorption spectroscopy (UV-DOAS) technique. Directly measured NH3 emission fluxes from our study lagoon ranged from 16.1 to 41.2 g/m2/s, which compared well against our model predicted fluxes with a normalized mean error (NME) of 15%. Sensitivity analyses showed NH3 emission is most sensitive to the lagoon-liquid temperature compared to the other factors (air temperature, air velocity, and total solids concentrations) examined in this study.