Biological removal of ammonia and nitrate from simulated mine and mill effluents

Abstract

Although nitrification/denitrification processes have been used to remove ammonia and nitrate from municipal and industrial processes, there are few reported results on the removal of these ions from mine effluents. Ammonia and nitrate are present in the effluent due to the widespread use of ammonium-nitrate-fuel oil (ANFO) as a blasting agent, and the use of other nitrogen-containing reagents in the mill. Unlike other process effluents, there is very little nutrient content in mine effluents. The objective of this work was to develop a microbial process for the removal of ammonia from simulated mine effluents. Biologically, this is accomplished in two steps. In the first step, ammonia is oxidized to nitrate, and in the second step, nitrate is reduced to dinitrogen gas. Ammonia oxidation (nitrification) was tested using both continuously stirred tank reactors and trickling filters. The stirred tank reactor was chosen for the combined system because it produced final effluents with lower ammonia concentrations. For nitrate reduction (denitrification), a packed bed reactor operated in the upflow mode was tested with methanol being used as a carbon and energy source. The nitrate profiles within the denitrification reactor showed an exponential decay, a characteristic of plug flow conditions. A simulation model was developed for the denitrification reactor which described performance as a function of feed nitrate (NO 3) and methanol concentrations (MeOH) and retention time (RT): Productivity (mg NO 3/l per hour)=0.722(NO 3)+(MeOH)[1.416−0.024 (MeOH)+0.0011(NO 3)−0.776(RT)]. The methanol to nitrate ratio required for denitrification was 0.86:1. When the two processes were set up in series, it was successful in removing ∼90% of the nitrogen.