Optimization of soil mixtures in bioretention cells to reduce nutrient loading to the environment from storm water
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Bioretention cells are used as a way to remove some pollutants and particulate matter from stormwater. Typically a bioretention cell is placed at the edge of a parking lot or effluent stormwater structure to capture stormwater runoff. The bioretention cell has limited treatment of phosphorous which can lead to eutrophication in receiving waters. To help with the uptake of phosphorous, drinking water treatment plant residuals (DWTRs) from Missouri were added to bioretention cell soil mixes. The DWTR is created when coagulant is added to the influent drinking water at the treatment plant. Flocs form and fall to the bottom of the clarifier. The sludge which is created from this process (or DWTR) is then pumped to a lagoon for holding and drying. All of the DWTRs in this study were taken from the lagoons at the respective drinking water treatment plants. The DWTRs tested included lime, ferric chloride, polyaluminum chloride, and aluminum sulfate. First, an equilibrium study was conducted to test the sorption capacity of each DWTR. It was found that the DWTRs can sequester 15 to 40 milligram phosphorous per gram of DWTR when tested alone of the bioretention mix. When added to the lab-scale bioretention cell the removal efficiency was approximately 54% (lime), 87% (ferric chloride), and 99% (aluminum sulfate) at 5% DWTR addition (by volume) to the bioretention mix with 6.2 mg/L of phosphorous in the influent water for 3 hours.