Assessing the feasibility of using permeable reactive barriers for phosphorus removal from stormwater

Engineers and scientists are currently faced with the challenge of finding innovative, effective, and environmentally friendly approaches that can meet lower phosphorus standards being promulgated in Minnesota and throughout the U.S. Traditional phosphorus removal approaches such as settling ponds and wetlands may not be capable of removing phosphorus from stormwater to the degree needed to meet phosphorus standards. These traditional settling approaches may not be able to remove small particles upon which the phosphorus is often concentrated in stormwater (Pilgrim, 2002). Other approaches have limitations—sand filters have the potential to clog (Barrett et. al., 2003); biological filter performance may be limited by season, temperature, and light availability (Dodds, 2003); and chemical treatment can generate substantial hydrated floc that eventually must be disposed (Pilgrim, 2002). To effectively and permanently remove phosphorus, phosphate must be incorporated into recalcitrant organic material or be bound to cations such as calcium, magnesium, or unreducible trace metals such as aluminum. Spent lime is an abundant waste byproduct of drinking water treatment and the primary component of spent lime is calcium carbonate. Fortunately, calcium chemically prefers to be bound to phosphate over carbonate, and phosphate is readily converted into calcium phosphate in the presence of high concentrations of calcium carbonate (Stumm and Morgan, 1996). Spent lime has an advantage over limestone in that it consists of recently precipitated and hence more available calcium carbonate. The use of spent lime for stormwater treatment is a new concept. A treatment cell with spent lime is not precipitating or flocculating phosphate (e.g., like alum), and it is not necessarily intended to filter as do sand filters. Rather, it is a chemical substitution reaction whereby the newly formed calcium phosphate simply resides in the cell where the calcium carbonate once resided.""