A Model of Ammonium Ion Dissociation in Liquid Dairy Manure

Abstract

Emission of Ammonia (NH3) from lagoons and other similar structures holding liquid dairy manures contributes to environmental pollution and also lowers the fertilizer-value of the liquid-effluent. In general, NH3 volatilization from these facilities depends on the concentration of free NH3 in the liquid, which is a function of the dissociation of ammonium ion (NH4+). The dissociation of NH4+, on the other hand, is dependent on the: liquid temperature, liquid pH, and concentration of total solids (TS). In this study the ammonium ion (NH4+) dissociation constant (Kd) was empirically modeled at a pH of 9; at four temperatures (5, 15, 25, and 35C), generally experienced in the US Pacific Northwest, and at five TS concentrations (0.5, 1.0, 1.5, 2.0, and 2.5%; w/w) common in flushed-dairy manure. The Kd of NH3 increased 1.61 times for every 10 oC rise in temperature. The data also indicated a linear decrease in the Kd of NH4+ with increase in the concentration of TS in the liquid. The resulting empirical model of the Kd of NH4+ as a function of temperature and TS had a coefficient of determination, R2, of 0.97; demonstrating a good fit to the experimental data. The Kds of NH4+ in the dairy manure liquid were 117%, 87%, 61%t, and 54% compared to the theoretical Kds of NH4+ in pure water at 5, 15, 25, and 35C, respectively. The results of this study emphasize the need for: (i) including both the liquid-TS and the liquid-temperature in models of Kd of NH4+ in livestock wastewaters, and (ii) covering the entire ranges, of both parameters, encountered in the region where the model will be used.