Barr developed a first-of-it-kind spent-lime treatment cell for the Ramsey-Washington Metro Watershed District to reduce phosphorus loading to Wakefield Lake, an impaired lake located in Maplewood. Spent lime, an abundant by-product of drinking-water treatment, provides several advantages when implemented as a filtration-type best management practice, including exceptional performance; rapid treatment (5-10 minutes of contact time) of large volumes of stormwater within a small footprint; easy regeneration by mixing; no toxicity for aquatic life; and ecological reuse of a “waste” material.
In operation for seven years, this treatment cell is primarily earthen, with wooden barriers to direct flow. Stormwater entering the cell infiltrates through the spent-lime material, with contact time averaging five minutes. The outlet consists of a riser with 1-inch-diameter holes. Peak outflow rates are often seen after the peak of a storm event, as the water level draws down around the riser and a head differential develops between the outlet and inlet of the cell. Monitoring data collected at the treatment cell shows that the spent lime removes phosphorus and metals to low levels (removal of 74.4 percent of ortho-phosphate and 66 percent of particulate phosphate) and, in some cases, to below typical detection.