Bioretention Media Mixtures

The research conducted on bioretention media was aimed to address phosphorus removal in bioretention cells. The study reviewed potential amendments to bioretention media mixtures that could remove particulate and dissolved phosphorus from stormwater runoff. The study reviewed the use of water treatment residuals (WTRs), biochar, coconut coir, other industrial byproducts (e.g. Fly Ash), and general sand/compost/gravel mixes.  It was found that certain industrial byproducts are capable of efficient phosphorus removal and have a high sorption capacity for phosphorus.

However, there has not been enough research on the practical use of the industry byproducts of interest (e.g. blast furnace slag). When using an amendment for bioretention media, there needs to be an adequate amount of research that displays the ability of the material to remove phosphorus while not remobilizing any harmful pollutants (like Al3+) in order to ensure public and environmental safety. The use of WTR’s to remove phosphorus from wastewater or stormwater has been a popular research topic for past few decades and some of that research has been focused on the potential negative human and environmental effects of WTR application. The research focused on the human and environmental impacts of WTRs found that there can be an increase in exported aluminium and natural organic matter, but the levels exported were within surface body water standards.

The use of coconut coir as a bioretention media amendment has been shown to remove phosphorus from stormwater, but not as efficiently as WTRs. The use of biochar to remove phosphorus from stormwater has produced mixed results. Some studies actually show a net export of phosphorus from biochar amended sands, and some studies show a reduction in phosphorus. However, the particular biochar mixes vary greatly based on the components the biochar. This study concluded that biochar would not be a useful amendment for phosphorus removal in bioretention cells until further research can show an effective use of biochar for phosphorus removal. The study found that certain types of general sand mixes can be more effective at removing phosphorus from stormwater. The general mixes of interest include: zeolite and silica sand, activated bauxite, maerl, peat, wollostonite, laterite, vegitated sand with loam, and gravel and cobbles with cattails. The issue with most of the general mixes is that the phosphorus sorption capacity is variable and can be difficult to quantify. It was concluded that using one or more of the general mixes along with WTRs would promote long term phosphorus removal in bioretention cells.

The research conducted also found that the use of compost in bioretention cells can cause an export of phosphorus and nitrogen. However, compost is effective at reducing heavy metals in stormwater runoff. It is suggested that if compost is to be used in a bioretention cell that it should be placed above a layer that is amended with phosphorus removing materials.The Colorado Stormwater Center concluded that the use of WTRs as an amendment for bioretention cells could reduce the levels of phosphorus in stormwater runoff for many years (depending on how much WTR is applied) while maintaining an adequate hydraulic retention time and not causing a human or environmental health issue.