An Empirical Model of Ammonium Ion Dissociation in Liquid Dairy Manure

Abstract

Emission of ammonia (NH3) from lagoons or other similar animal manure holding structures not only degrades environmental quality but also lowers the fertilizer value of the liquid effluent. In general, NH3 emissions from these facilities depend on the concentration of free NH3 in the liquid (NH3(aq)), which is a function of the dissociation of ammonium ions (NH4+). The dissociation of NH4+ is dependent on manure properties, including temperature, pH, ionic strength, and total suspended solids (TSS) concentration. The dissociation constant of NH4+ (Kd) in flushed dairy manure was determined at a pH of 9, temperatures of 5C, 15C, 25C, and 35C, and total solids (TS) contents of 0.5%, 1.0%, 1.5%, 2.0%, and 2.5% w/w, and a regression model was obtained. The Kd value increased by approximately 1.6 for every 10C rise in temperature. The data also indicated a logarithmic decay in Kd with an increase in the concentration of TS in the liquid. The coefficient of determination (R2) of 0.97 for the nonlinear empirical model of Kd as a function of both temperature and TS concentration demonstrated good fit to the experimental data. The Kd values in the dairy manure with 1.5% TS were 117%, 87%, 61%, and 54% compared to the theoretical Kd values in pure water-ammonium solutions at 5C, 15C, 25C, and 35C, respectively. Similar trends were observed at other TS concentrations, suggesting that the effects of solids were more significant at elevated temperatures. The results of this study emphasize the need for including both the TS concentration and the temperature in models of Kd for liquid dairy manure, in addition to ensuring that the entire ranges of TS and temperature encountered in the region where the model will be used are covered.