Mechanisms of dinitrogen gas formation in anaerobic lagoons

Abstract

Anaerobic lagoons have been widely used to treat agricultural waste and waste from small municipalities for many years. Oxidation of ammonia is generally assumed to not occur in such lagoons because of their anaerobic environment. Nitrification, the most likely process that would lead to ammonia oxidation, has not been considered a significant process in anaerobic lagoons because of the negligible concentrations of dissolved oxygen measured in these systems. Therefore observed nitrogen losses are usually assumed to be due to ammonia volatilization. However, in field studies of primary swine waste lagoons in the south-eastern US Coastal Plain, rates of dinitrogen (N 2) gas production have been observed to be much greater than rates of NH 3 volatilization. This paper discusses possible mechanisms that could explain observations of N 2 gas generation in anaerobic waste lagoons. Chemical and microbial reactions have been documented that combine ammonia with nitrite, or nitrous acid, to form N 2 under anaerobic conditions. Nitrification and denitrification reactions have also been observed under microaerobic conditions. Each of these reactions requires low levels of oxygen for the initial nitrification of ammonia to nitrite. Diffusion rates of oxygen through the lagoon surface appear to be adequate to allow enough nitrite formation to explain observed N 2 fluxes.