Nitrate Removal in Shallow, Open-Water Treatment Wetlands
journal contributionposted on 07.10.2014, 00:00 by Justin T. Jasper, Zackary L. Jones, Jonathan O. Sharp, David L. Sedlak
The diffuse biomat formed on the bottom of shallow, open-water unit process wetland cells contains suboxic zones that provide conditions conducive to NO3– removal via microbial denitrification, as well as anaerobic ammonium oxidation (anammox). To assess these processes, nitrogen cycling was evaluated over a 3-year period in a pilot-scale wetland cell receiving nitrified municipal wastewater effluent. NO3– removal varied seasonally, with approximately two-thirds of the NO3– entering the cell removed on an annual basis. Microcosm studies indicated that NO3– removal was mainly attributable to denitrification within the diffuse biomat (i.e., 80 ± 20%), with accretion of assimilated nitrogen accounting for less than 3% of the NO3– removed. The importance of denitrification to NO3– removal was supported by the presence of denitrifying genes (nirS and nirK) within the biomat. While modest when compared to the presence of denitrifying genes, a higher abundance of the anammox-specific gene hydrazine synthase (hzs) at the biomat bottom than at the biomat surface, the simultaneous presence of NH4+ and NO3– within the biomat, and NH4+ removal coupled to NO2– and NO3– removal in microcosm studies, suggested that anammox may have been responsible for some NO3– removal, following reduction of NO3– to NO2– within the biomat. The annual temperature-corrected areal first-order NO3– removal rate (k20 = 59.4 ± 6.2 m yr–1) was higher than values reported for more than 75% of vegetated wetlands that treated water in which NO3– was the primary nitrogen species (e.g., nitrified secondary wastewater effluent and agricultural runoff). The inclusion of open-water cells, originally designed for the removal of trace organic contaminants and pathogens, in unit-process wetlands may enhance NO3– removal as compared to existing vegetated wetland systems.