Influence of medium specific retention on nitrification in unsaturated treatment wetlands

Abstract

Hydraulic loading rate (HLR) and the size distribution of the treatment medium have been shown to influence treatment efficacy in vertical flow treatment wetlands (VFTW). The effect of operational parameters like dose volume (DV) has received comparatively little attention. Using conversion of ammonium to nitrate as a measure of performance, this study defines the ratio of DV to the retention volume (RV), controlled primarily by the specific retention (Sr) of the medium, Ψ = DV/RV, as a parameter unifying the effects of HLR, DV and Sr on VFTW performance. Experiments were conducted in unplanted mesocosms using two media with unique size distributions and Sr (sand, d50 = 0.4 mm, Sr = 0.19 and gravel, d50 = 5 mm, Sr = 0.09), two DV values and four HLR values. More nitrate was produced in sand than gravel at any HLR. In both sand and gravel, nitrification efficiency increased as HLR decreased and, for a given HLR, lower DV. Use of the parameter Ψ minimized the interacting effects of media size, HLR and DV. When Ψ was identical (Ψ = 0.17), both sand and gravel achieved nearly identical nitrate concentrations despite the sand receiving twice the HLR. Similarly, nitrate concentration increased as the value of Ψ decreased for a given HLR regardless of medium size. Thus Ψ might be used to optimize HLR to the size of the medium and optimize DV for a given HLR. The parameter has potential to be applied in other porous medium based treatment systems.