Phosphorus can enter a wastewater system in a variety of forms. Organic and synthetic phosphorus are transformed by bacteria to the simple orthophosphate form. Because excess phosphorus can stimulate eutrophication, the excessive growth of algae and aquatic plants in streams, rivers, and lakes, it is important that phosphorus not enter water bodies in high concentration. Fortunately, orthophosphate usually is immobilized by a number of processes in the soil.
According to research by Bicki et al. (1985), if soil conditions below the onsite system treatment and disposal trenches are aerobic and unsaturated, phosphorus concentrations can be reduced by 85% to 95%.
Phosphate immobilization processes in the soil include adsorbtion to the soil particles or biomat, precipitation in the soil (Figure 4.1.3), or biological uptake.
Phosphorus fixation in the soil
From the North Carolina Onsite Guidance Manual