Media for Enhanced Phosphorus Removal from Secondary Wastewater Effluent

Abstract

Constructed subsurface flow wetlands have the potential for high phosphorus removal through chemical adsorption onto the media. Over the past decade researchers at Griffith University, Australia have been using 240 L mesocosms to investigate the long term sustainability and performance efficiency of using different soil media and plant species to maximise nutrient removal from secondary wastewater effluent. The performance efficiency of 15 different media mixes including amendments for enhanced phosphorus adsorption, and 10 different plant species have been investigated. Total phosphorus removal was highest (94–99%) in sand media amended with water treatment residuals (WTR), followed by ‘brickies loam’ (92%), then sand amended with red mud or krasnozems (86–89%) and lowest in gravel (44%). Phosphorus removal using sand with WTR had the greatest efficiency when treating influent with P concentrations ranging from 0.5 to 8 mg/L (90–99%). The addition of biochar to sand did not enhance P removal during a two year trial. Leaching and export of P occurred from the media either after heavy rain or loading with stormwater having low P concentrations (<0.06 mg/L). However, following such ‘flushing events’ P sorption and hence P removal efficiency were enhanced. Mature plant assemblages could uptake up to 16gP/m2/year. Of the herbaceous plants the perennial grasses Pennisetum alopecuroides and Vetiver-Chrysopogon zizanioides, and the sedge Carex appressa had the highest biomass. The woody species Callistemon pachyphyllus and Melaleucaquinquenervia had the highest carbon sequestration (1000 g C/m2/year). Thus various sandy loam media can be used to enhance long term effective phosphorus removal, and plants can be used for carbon credits.