Coupling Wetland Treatment to Land Treatment: an Innovative Method for Nitrogen Stripping?

Abstract

The ability of two small wetlands to remove added nitrate was studied as part of a wider investigation into the feasibility of using a combined forest irrigation-wetland treatment system to meet strict receiving water limits. In laboratory microcosm experiments, wetland sediments removed nitrate at rates between 0.019 - 0.609 g m−2 d−1, with rates being dependent upon nitrate loading and the vegetation that supplied the decaying organic matter (Typha orientalis > Carex spp. > Azollafiliculoides). Denitrification could account for between 32 - 100% of the observed nitrate loss, indicating that in some microcosms other nitrate removal processes were operating. Additions of bromide (a conservative tracer) and nitrate to the two wetlands demonstrated more rapid nitrate loss in a Typha stand (decay coefficient, ke = 4.44 d−1) compared to loss in an Azolla pond (ke = 1.1 d−1). The time course of bromide concentration at the wetland outlets, and its distribution within the wetlands, showed the presence of preferential flow paths and “dead” zones. This non-uniform flow is a common characteristic of wetlands and, in this case, may exert a major control on the scheme's overall nitrate removal efficiency.