Phosphorus removal in SAT is accomplished by adsorption and chemical precipitation. The adsorption occurs quickly, and the slower occurring chemical precipitation replenishes the adsorption capacity of the soil. Typical phosphorus removals for SAT are presented in Table 8.17, including travel distances through the soil. If phosphorus removal is critical, a phosphorus adsorption test using the specific site soil can be conducted (Reed and Crites, 1984). To conduct an adsorption test, about 10 g of soil is placed in containers containing known concentrations of phosphorus in solution. After periodic shaking (for up to 5 d), the solution is decanted and analyzed for phosphorus. The difference in concentrations is attributed to adsorption onto the soil particles. The detailed procedure is presented by Enfield and Bledsoe (1975). Actual (long-term) phosphorus retention at an SAT site will be 2 to 5 times greater than the values obtained in the 5-d phosphorus adsorption test (USEPA, 1981). If the travel distance to the critical point for phosphorus removal is known, the "worst case" phosphorus concentration can be calculated using Equation 8.13:
Px = Total phosphorus at a distance x on the flow path (mg/L). P = Total phosphorus in the applied wastewater (mg/L). k = 0.048 (d).
t = Detention time (d-1) = x(0.40)/KxG, where x is distance along the flow path (ft), Kx is the hydraulic conductivity in soil in direction x (ft/day), and G is the hydraulic gradient (G = 1 for vertical flow; H/L for horizontal flow).
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