Sunday, June 01, 2008 8:00 PM
Developing Stormwater Treatment Systems to Remove Nitrogen
As property is converted from natural to developed land use, impervious area is added and remaining pervious areas may be compacted by construction equipment traffic. These changes reduce the potential for rainfall to infiltrate into the ground and increase the post-development volume of stormwater runoff discharged from the site. In addition to an increase in runoff volume, human activities associated with development routinely increase the concentration of nutrients (nitrogen and phosphorus), suspended solids, heavy metals, and many other pollutants in stormwater runoff. The combination of increased runoff volume and pollutant concentrations results in significant increases in stormwater pollutant loads to receiving surface waters.
Eutrification of lakes, rivers, and estuaries is a common result of increased nutrient loadings from stormwater runoff. Eutrification can degrade water quality, negatively impact fish and wildlife, complicate treatment for water supplies, and limit human recreation. The primary productivity of the receiving water is normally limited by phosphorus, nitrogen, or both. Significant nitrogen and/or phosphorus load reductions (50%+) are sometimes needed to satisfy Total Maximum Daily Load (TMDL) program stormwater load reduction requirements or to improve surface water quality.
Our first choice for reducing stormwater pollutant loads should be stormwater volume reduction at or near the source. Nitrogen and phosphorus load reductions greater than 80% can be accomplished using infiltration practices and storage and reuse. In many areas it is not possible to use infiltration practices, due to the presence of low-permeability soils, high groundwater conditions, or regulatory restrictions. Stormwater storage and reuse may be cost prohibitive due to the required storage and associated infrastructure or may also be limited by regulatory issues. Chemical coagulation and settling of stormwater can consistently achieve a 90%+ load reduction for phosphorus but only 30% to 40% for nitrogen. Other traditional stormwater treatment methods, primarily wet detention, can remove up to about 70% of the phosphorus, but only 30% to 40% of the nitrogen. Wetland treatment, which requires a significant land area, can be very effective in removing nitrogen when the influent concentration is much higher than typical stormwater (well above 1 mg/L).
Similar to chemical treatment for phosphorus removal, innovative stormwater treatment technologies are needed which can consistently remove 50% to 90% of stormwater nitrogen loads. This is complicated by the relatively low concentrations of nitrogen in stormwater as compared to wastewater and highly variable stormwater flow rates.
What are your ideas for removing nitrogen from stormwater runoff?