Phosphorus Removal

Phosphorus adsorption takes place in the soil media below the biozone. The concentration of P in the biozone is often in the linear range of reported nonlinear isotherms (McCray et al., 2005). A linear isotherm is represented by the equation:

$S=K_DC$ (15)

where $S$ is the mass of solute sorbed per unit dry weight of solid (mg/kg), $C$ is the concentration of the solute in solution in equilibrium with the mass of solute sorbed onto the solid (mg/L), and $K_D$ is a linear distribution coefficient (L/kg). McCray et al. (2005) recommends $K_D$= 15.1 L/kg, the linear sorption isotherm constant as median value, but the value may vary from the 10th percentile ($K_D$= 5 L/kg) to 90th percentile ($K_D$= 128 L/kg) for modeling purpose. Similarly, a median value of $S_{max}$= 237 mg/kg is recommended for the maximum $P$ sorption capacity. This value may underestimate the P sorption capacity of the soil in some cases. A larger value (~800mg/kg) can be used (Zanini et al., 1998) when the $P$ sorption capacity is underestimated. The concentration of P in the biozone is often reported low; thus, only the linear portion of a nonlinear isotherm is enough for estimation.

Phosphorus sorption isotherm described in Equation (15) gives an estimate of $P$ sorption capacity given the $P$ concentration and the distribution coefficient. According to this equation, effluent $P$ concentration leaching to sub-soil layer should be zero until the soil is saturated with$P$; however, small amount of soluble $P$ leaches to sub-soil layer with daily inflow of $P$ to the biozone. The effluent $P$ concentration is estimated by a linear relationship suggested by Bond et al. (2006) in which the outflow P concentration is proportional to the total amount P in the soil layer based on soil type as depicted in Figure 6:4-2.

Figure 6:4-2 Water-soluble P as a function of Mehlich-3 P for native Autryville loamy sand, Wasda muck, Georgeville silt loam, and Pacolet sandy clay loam soils (Bond et al., 2006).