NUMERICALLY SIMULATED UNSATURATED FLOW FOR SAND FILTER DESIGN.

Sand filter systems have been developed to treat sewage from individual residences for situations where the standard septic tank-seepage field system is inappropriate. Basic design of sand filter systems has been by rule-of-thumb based on previous experience, frequently resulting in inefficient or inadequate systems. A procedure that uses a two-dimensional Alternating-Direction-Implicit finite difference approximation of the partial difference equation describing unsaturated water flow through porous media was developed to improve upon the hydraulic design of sand filter systems. The influences of selected design factors on sand filter flow parameters were evaluated using numerical simulation, with surface application distribution, application rate, application cycle length, inflow period length, and filter medium depth found as being significant factors for predicting filter flow characteristics and for making design recommendations.