The invention relates generally to a system for treating effluent containing particulate and dissolved pollutants. More specifically, the invention relates to a system and method for removing excess phosphorous (P) and nitrogen (N), such as dissolved P and N in the form of P or N compounds and ions, from stormwater and other polluted wastewater effluent. The invention also relates to improved media for long-term phosphorus retention in bioretention systems and an improved outlet for promoting nitrogen retention.
Bioretention systems often include a basin to contain the effluent to be treated. The bottom of the basin is commonly composed of a porous media that is planted with vegetation. As the effluent passes through the bioretention system, particulate pollutants are removed by filtration. Dissolved phosphorus can be removed from the effluent by biological processes of the system such as vegetative and microbial biomass uptake, as well as chemical adsorption/precipitation processes (herein referred to under the category of sorption), which are effected by properties of the media. Dissolved nitrogen can also be removed from the effluent by vegetative and microbial biomass uptake, as well as biological transformations, including denitrification, that eventually convert nitrogen into nitrogen gas.
Bioretention systems have been documented as being cost-effective effluent management facilities for stormwater runoff in terms of removing sediments and sediment bound phosphorus and nitrogen. This can be particularly relevant in watersheds that have been impacted by urban and/or agricultural runoff to such an extent that they are the subject of what are referred to as Total Maximum Daily Load (TMDL) criteria for nutrients. However, typical bioretention systems with sandy media and free discharge outlets are less effective for dissolved nitrogen and long term dissolved phosphorus removal. Methods to increase retention of nutrient pollutants to meet TMDL criteria can be important in the design of effluent management facilities, so they can more effectively process effluent in a more acceptable and efficient manner.
Accordingly, there is a need for improved bioretention systems and methods and for improved media to be used in bioretention systems.