Modeling Phosphorus Treatment Capacities of On-Site Wastewater Lateral Flow Sand Filters

Abstract

The lateral flow sand filter (LFSF) is an on-site wastewater technology widely used to treat septic tank effluent (STE) in Nova Scotia, Canada. In this study, linear and nonlinear isotherm models (accounting for both sorption and precipitation processes) were developed and evaluated to simulate cumulative phosphorus (P) treatment in eight LFSFs that operated from September 2004 to October 2011. The LFSFs varied in design based on slope (5, 30%), sand grain size (fine, medium, coarse), and filter length (5.5, 8 m). The HYDRUS two-dimensional (2D) model was used to estimate the mass of sand involved in active P treatment to normalize the isotherm influent P loads for each LFSF. The isotherm equations tested were linear, Langmuir, Freundlich, and 2-part piecewise linear. Linear, Freundlich, and 2-part piecewise linear functions were also combined with batch sorption experiment derived Langmuir equations and subsequently tested. For the six 8-m long LFSFs, the individual 2-part piecewise linear isotherms had the best model performance. The P removal processes in the lower STE-loaded medium sand, 5.5-m-long LFSFs 7 and 8 were adequately modeled by both Langmuir and Freundlich isotherms, but all seven isotherms had satisfactory model performance. Phosphorus treatment isotherms were also developed by averaging input parameters from the individual filter isotherms on the basis of filter length and sand grain size. The average P treatment isotherms had poor model performance results for the 8-m-long LFSFs, possibly caused by different P removal rates related to filter slope and residence time. All of the eight LFSF designs were not effective at long-term P treatment with effluent concentrations exceeding