Coagulants are very effective for treating stormwater runoff and commonly achieve removal efficiencies of 80-90 percent for total phosphorus (TP), 35-65 percent for total nitrogen, 90+ percent for total suspended solids (TSS) and 90+ percent for pathogens. These efficiencies are higher than traditional stormwater treatment approaches. Enhanced treatment also requires much less land per watershed area treated than traditional systems. For example, in Largo, Florida, an existing 3-acre wet pond effectively treats runoff from a 1,200-acre watershed. A typical retrofit project includes the diversion of runoff for common rain events (1- to 2-inch depth) from an existing storm sewer/channel to a floc settling pond. Runoff inflow rate is continuously monitored, and coagulant is added on a flow proportionate basis to maintain the desired dose. Precipitates capture dissolved and particulate pollutants. Flow metering equipment, coagulant feed pump and programmable controller, and coagulant storage are placed in a small attractive building. Because the systems require less land and have high mass pollutant removal efficiencies, coagulant treatment typically has the lowest life cycle cost (construction plus 20-years annual O&M cost) per mass pollutant removed, $200-600/lb. TP and $100-400/lb. TN.

The first enhanced coagulant treatment project was completed in Tallahassee, Florida in the late 1980’s. This in-line system was constructed to reduce stormwater pollutant loads to a small hypereutrophic lake in a highly urbanized area when there was no available land to construct traditional stormwater BMPs. Coagulant is automatically injected into each major storm sewer based on the measured stormwater flow rate in each pipe. An extensive 2-year post construction monitoring program was conducted to evaluate the effects on lake water, sediment chemistry, and aquatic organisms with positive results.  

Systems evolved in the 1990s to include off-line floc settling ponds and combined with other treatment train components including wetland treatment to reduce coagulant use. This presentation includes: 1) an introduction to coagulant treatment and pollutant removal mechanisms; 2) history and effectiveness of use; 3) initial in-line project case studies, costs, results; and 4) off-line project case studies, costs, results. This presentation is important to all stormwater managers looking for a cost-effective approach to reduce TP, TN, TSS or pathogen loads in stormwater.