Several models were developed to predict the efficiency of the intermittent sand filter (ISF) in removing algae from wastewater stabilization pond effluent; volatile suspended solids (VSS) was the analytical technique used to identify algal concentrations. The first (ISF model) and second (modified ISF model) models consisted of two distinct portions: a surface algal layer (SAL) component and a sand phase component. In the ISF model, the sand phase component was described in terms of 20 empirical sand filter efficiency terms (20 ^ coefficients); in the modified ISF model, a functional relationship between ^ and filter depth was developed. The modified ISF model was less accurate than the ISF model in predicting filter effluent quality. The third model (simplified ISF model) consisted of a single component (the sand phase). The mass of algae which was deposited to the SAL componenet in the first two models was, instead, forced into the top (2 inch) layer of sand. The functional relationship between the sand phase filter term and filter depth was recalculated and utilized to desribe the decrease in the concentration of algae during the filtration process. The simplified ISF model was comparable to the ISF model in predicting filter effluent quality. The simplified model predicted 85 percent VSS removal for 0.17 mm effective sand size (E') filters and 44 percent VSS removal for 0.40 and 0.68 mm E' filters. The application of the simplified ISF model is subject to limitations of maximum hydraulic loading rates of 0.7 million gallons per acre per day and miximum mass loadings of 49 grams of SS per m^2 per day for 0.17 mm effective size sand. Design curves, in which period of filter operation was described as a function of mass loading, were developed for ISF systems containing 0.17, 0.40, and 0.68 mm E' media. Wastewater stabilization pond effluents having calcium carbonate precipitation problems were included as a special case in this analysis.
Cowan, P. A. and Middlebrooks, E. J., "Modeling the Performance of the Intermittent Sand Filter" (1979). Reports. Paper 521.