MATHEMATICAL MODELING OF UNSATURATED FLOW AND TRANSPORT IN SOIL–BASED WASTEWATER TREATMENT SYSTEMS

A numerical model is used to investigate the impact of the infiltrative-surface crust on the hydraulictreatment volume and on unsaturated transport and transformation of orthophosphate and ammonium in soil-based wastewater treatment systems (SWTS). The simulated SWTS is a subsurface trench underlain by a natural soil. Crusts at the base and on the sidewall of the trench are incorporated in the model. Unsaturated water-flow and contaminant-transport parameters are selected from the ranges of values measured in field and laboratory experiments that have been reported in the literature. The process of contaminant sorption to soil is included for both contaminants. Biochemical ammonium transformation and phosphate precipitation are simulated assuming first-order kinetics. The simulations illustrate that the presence of an infiltrative-surface crust greatly improves treatment of orthophosphate and ammonium. The infiltrative crust causes reduced infiltration velocities and a somewhat larger hydraulic-treatment volume. The slower velocities result in longer hydraulic residence times and thus allow more time for biochemical removal. Increased hydraulic volumes are due mainly to infiltration through the sidewall crust in mature systems. Slower contaminant velocities due to sorption also improve biochemical treatment. The impact of two septic-tank effluent (STE) -application methods on treatment is evaluated for an uncrusted system. Uniform application across the infiltration trench resulted in improved treatment due to a larger overall hydraulic residence time compared to a focused application in the center area of the trench. This research illustrates that numerical models are useful for gaining a better understanding of crust development and the associated impact on contaminant treatment.