Stormwater nitrogen removal performance of a floating treatment wetland

Abstract

The nitrogen (N) removal efficiency and effluent quality of two parallel stormwater retention ponds, one retrofitted with a floating treatment wetland (FTW) and one without any vegetation, was compared in a field trial. This study shows that inclusion of FTWs in stormwater retention ponds has potential to moderately improve N removal. Median FTW outlet event mean concentrations (EMCs) were lower than median inlet and control pond outlet EMCs for all species of N, except for NH(4)-N. Performance was statistically better from late spring to end autumn due to higher organic nitrogen (ON) removal and denitrification in presence of the FTW. Low dissolved oxygen (DO), higher temperature and increased organic matter (OM) and microbial activity below the FTW, likely facilitated the higher denitrification rates observed over this period. Greater sediment N accumulation in the FTW pond also contributed to its higher overall N removal. Higher OM availability in the FTW pond due to release of root exudates and supply of detritus from plant die-back may have contributed to floc formation in the water column, increasing particulate ON settlement. Enhanced ON mineralisation may also be responsible but was probably limited in summer due to the low DO induced by the FTW. Direct uptake by the plants appears to be of less importance.

Stormwater Treatment Evaluation of a Constructed Floating Wetland after Two Years Operation in an Urban Catchment

“…This difficulty was also discussed by Headley & Tanner [21]. Figure 7 depicts the differences between the treatment system design of the current study (Figure 7a) and those used in other studies [6][7][8] (Figure 7b) and [5] (Figure 7c). …”

“…This difficulty was also discussed by Headley & Tanner [21]. Figure 7 depicts the differences between the treatment system design of the current study (Figure 7a) and those used in other studies [6][7][8] (Figure 7b) and [5] (Figure 7c). While the experimental design for the current study only considered the differences between the inflow and outflow to the CFW itself (note sampler locations in Figure 7a), the experimental design of the other studies (Figures 7b and 7c) included the inflow and outflows from both the stormwater pond and the CFW.…”

“…A comparison between the removal efficiencies presented in other studies on large-scale CFWs [5][6][7][8][9][10] and those revealed in this study is difficult due to the different treatment system designs, sampling procedures, and evaluation procedures used in the studies. This difficulty was also discussed by Headley & Tanner [21].…”

“…A literature review identified six previous studies [5][6][7][8][9][10] which investigated the removal of total suspended solids (TSS) and nutrients from stormwater ponds by CFWs at seven different locations. All of the stormwater ponds investigated had a surface area of 100 m 2 or larger.…”

“…Since biofilm forms on the root mass below the water surface, additional water treatment processes occur through adhesion, filtration, nutrient uptake (direct use by plants), and sequestration [4,5]. The oxygen depletion under the mats can provide ideal conditions for denitrification, due to low oxygen levels and a carbon-rich environment from plant debris [6]. An increasing amount of research has been conducted on CFWs over the last 10 years.…”