Phosphorus removal from wastewater applied to land

Low concentrations of phosphorus (P) are desired in surface waters to minimize production of organic materials which cause problems in water use. The low P concentrations characteristic of the soil solution suggest that soils represent an appreciable sink for P in wastewaters applied to land. The mechanisms responsible for maintaining low (usually less than 0.5 mg P 1-1 soil-solution P concentrations and for control of the chemical mobility of P in the soil profile appear in most cases, to involve a sorption reaction at soil surfaces with a hydrous oxide structure. Recent work has led to an interpretation of P sorption by soils which has direct relevance to an understanding of P removal from wastewater applied to land. Crops harvested at wastewater treatment sites, also represent a sink for added P. Phosphorus removal of 10% of that applied is typical, but 30 to 40% of the P applied at a rate of 200 to 300 kg P ha-1 yr-1 may be removed by harvested forage crops. Schemes of wastewater application to land involve either infiltration and percolation of the wastewater through the soil profile or overland flow. Most complete P removal is generally observed in infiltration-percolation schemes which permit maximum contact between wastewater and soil, and particularly if wastewater is applied to give a P input comparable to that under normal agricultural practice. Several mathematical models have been developed to predict P movement through a soil profile, but the P sorption data required are dificult to obtain. A simple mass-balance model is proposed, the P sorption parameters of which are easily measured, are independent of soil type, and can be adjusted to account for the kinetics of P sorption, thereby providing an estimate of the longevity of a treatment site for P removal.