Simulated effects of flow rate on phosphate retention in soils

Phosphate in waste water disposed in the field can be partially or wholly retained in soil, eliminating ground water pollution. The retention ability of soil is affected by many factors, such as soil pH, minerals, structure, water content, solute concentration, solution pH, etc. The flow rate of waste water within a soil profile may be an important factor as well. It is the purpose of this paper to simulate different flow patterns in soils and their effect on the renovation of waste water by soils. A system of partial differential equations, including water flow and phosphate transport equations with a P-soil reaction model, were solved simultaneously to simulate the saturated or unsaturated flow of phosphate solution and its reactions in soils. Euler implicit and general explicit finite difference methods were applied to solve these equations. The results indicate that phosphate retention by soils from a finite quantity of waste effluent could be increased by decreasing the flow rate. An on-and-off intermittent disposal of waste water would also increase the retention capacity when compared to continuous flooding of the same amount of effluent. The main cause of these effects is the difference of reaction time of phosphate with soils.