Mechanistic modelling of ammonia emission from laying hen manure at laboratory scale

Abstract

A mechanistic model was developed for estimating ammonia (NH3) emission from laying hen manure based on measurements from a laboratory-scale wind-tunnel experiment. Ammonia emissions were measured continuously for 3–6 d from manure samples under 17 different combinations of air temperatures (20.0–33.5°C) and air velocities (0.25–1.17 m s−1). The convective mass transfer coefficient, KG, which was estimated using chamber air velocity and temperature, averaged 6.81 ± 4.04 m h−1 and ranged from 0.47 to 16.76 m h−1 (R2 = 0.91). The average dissociation constant ratio α, estimated as a function of manure pH and moisture content, was 0.175 ± 0.103 (R2 = 0.81). Total ammonia nitrogen (TAN) has been reported in the literature as a key factor influencing NH3 emission from manure, however, its influence observed in these laboratory studies was statistically insignificant (p = 0.16). The model was developed using two independent groups of experimental data, one for model training, and the other for model validation. The model had adequate accuracy in estimating NH3 emission from layer manure with normalised mean squared error (NMSE) of 0.08 and R2 of 0.79. Sensitivity analyses suggested that NH3 emission was most sensitive to manure pH, followed by air temperature and air velocity. The results can serve as a basis for developing a reliable, practical and simple NH3 emission estimation model for determining compliance with NH3 emission regulations and developing effective mitigation strategies.