Media, Microbes and Macrophytes – their role in improving the effectiveness of bioretention systems. Getting the right mix

Abstract

Bioretention systems are becoming the most widely used stormwater treatment technologies for many local authorities. Their popularity is based primarily on existing models that demonstrate very high pollutant removal efficiencies and their small physical footprint compared to wetland systems. Structurally bioretention systems consist of a basin or trench filled with media and planted with vegetation. Physical, chemical and biological treatment processes facilitated by the media, microbes and macrophytes improve stormwater quality as it percolates through the media. Limited research has been conducted on the suitability of media and yet media specifications abound! There is little published research on the long term performance of media or plants on nutrient removal; however laboratory studies have shown that media can become rapidly saturated with phosphorus. Media with organic matter can also leach nutrients. Our research at Griffith University using experimental bioretention mesocosms has been focussing on the effectiveness of different media types for nutrient removal. Our 5 year old trials have shown that sand, loam and gravel media have now become P saturated, however vegetation improves PO4 removal. Vegetation also improves N removal by uptake of NOx. Microbes play an important role in N transformation involving ammonification of organic N, nitrification and denitrification. By applying accelerated loading rates of N and P in our “stormwater” we have a good indication of the P saturation capacity and hence long term removal efficiency. We have also been quantifying plant biomass to assess the role of different macrophytes in nutrient uptake and bioaccumulation.