Nutrient removal from stormwater in floating treatment wetlands: prediction of kinetic rates, effect of initial concentration and plant performance assessment

Abstract

Nitrogen and phosphorus in stormwater cause an excessive influx of nutrients to stormwater-receiving waterbodies (e.g., rivers, lakes), inducing eutrophication and water quality deterioration. Floating treatment wetland (FTW) is a relatively recent, cost-effective and nature-based solution to this problem. However, predicting its treatment efficiency is still a challenge for the designers due to a lack of research. This study aimed to explore a new variable for predicting FTW nutrient removal kinetic rates for better accuracy in estimating treatment efficiency. Our study also aimed to assess the impact of initial concentration on nutrient removal performance in FTWs and evaluate the performance of four native Australian plant species. The experiments were conducted using Carex fascicularis, Juncus kraussii, Eleocharis acuta and Baumea preissii. The first experiment was performed with different plant dry biomass (D_T)-to-water volume (V_w) ratios (D_T/V_w) for each of the plant species over a 21 day period. The results showed that D_T/V_w follows a linear relationship with first-order nutrient removal kinetic rate. The second experiment was conducted over an 80 day period in 5 stages (16 days each) at high (12 mg L⁻¹ TN, 3.5 mg L⁻¹ TP) and low (2 mg L⁻¹ TN, 0.5 mg L⁻¹ TP) nutrient concentrations. It was found that nutrient removal kinetics were higher at low concentration and lower at high concentration for both TN and TP, which agrees with enzymatic response behaviour. C. fascicularis was able to remove 93.3% and 80.8% TN and TP, respectively, in just three days on average, being the top performer among the four plants. The outcomes of this study will facilitate precise estimation of treatment efficiency by FTWs under variable conditions.