Performance Evaluation of a Floating Treatment Wetland in an Urban Catchment

Peter Nichols,Darren Drapper,Chris Walker,Terry Lucke

doi:10.3390/w8060244

Abstract

Floating Treatment Wetland (FTW) systems are purpose-built devices designed to replicate the water treatment processes that occur in and around naturally occurring floating vegetated islands. FTWs can be used to improve the water quality of water storage ponds by contributing to water treatment processes through adhesion, filtration, nutrient uptake (direct use by plants), and sequestration. This paper presents the results of a twelve-month investigation into the pollution removal performance of a FTW receiving stormwater runoff from a 7.46 ha urban residential catchment. As anticipated, there was a high degree of variation in FTW treatment performance between individual rainfall events. Overall pollution removal performance was calculated to be 80% for Total Suspended Solids (TSS), 53% for Total Phosphorous (TP), and 17% for Total Nitrogen (TN) for a FTW footprint of 0.14% of the contributing catchment. TSS and TP concentrations were found to be significantly reduced after FTW treatment. The minimum FTW footprint to catchment size ratio required to achieve regulated nutrient removal rates was calculated to be 0.37%. Sum of loads calculations based on flow resulted in pollution load reductions of TSS 76%, TP 55%, and TN 17%. Pollution treatment performance (particularly for TN) was found to be affected by low influent concentrations, and highly-variable inflow concentrations. The study demonstrated that FTWs are an effective treatment solution for the removal of pollution from urban stormwater runoff.

Highlights

Occurring floating vegetated islands are found in freshwater lakes and ponds, and are comprised of a matrix of floating organic material and plant associations growing at the water surface.The buoyancy of a naturally occurring floating island is a result of gasses (Nitrogen-N and Carbon dioxide-CO2 in aerobic conditions, and Methane-CH4 in anoxic conditions) trapped beneath the organic mat, and the air stored within the roots and leaves of vegetation growing on the island [1,2]
The improved removal rates are believed to be the result of the unique experimental design that excluded potential short-circuiting and focused on evaluating the pollution removal performance of the field-scale Floating Treatment Wetland (FTW) alone, rather than as part of a stormwater treatment train
The calculated pollution removal proportions from outflows were found to be less than the values specified by the local Queensland State Planning Policy and regulations (TSS 80%, total phosphorus (TP) 60%, and total nitrogen (TN) 45%) [21], and this is likely a reflection of the small FTW

Summary

Introduction

Occurring floating vegetated islands are found in freshwater lakes and ponds, and are comprised of a matrix of floating organic material and plant associations growing at the water surface. The UK Royal Society for the Protection of Birds constructed artificial islands for the conservation of threatened species as early as the 1960’s [4] Following these early successes, FTWs have since been used for a variety of purposes including treatment of pollution emanating from mine tailings [4,5,6,7], and pollution removal from stormwater [8,9,10,11,12,13,14]. TakenThe at the inlet and outlet points shown in modular FTW was constructed using four, 50 mm thick layers of recycled plastic fibre, with This matrix was thenplastic covered with a injected marine grade foam to provide buoyancy.

Sampling Protocol and Methodology

Results and Discussion

28 September 2015 1

Conclusions