> For the complete documentation index, see [llms.txt](https://swatplus.gitbook.io/io-docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://swatplus.gitbook.io/io-docs/theoretical-documentation/section-6-management-practices/septic-systems/biozone-algorithm/phosphorus-removal.md).

# 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).](/files/upLHZPcWcVHB13JZ4Jt1)