The lateral-flow sand filter (LFSF) is an alternative system for household septic-tank effluent treatment that utilizes periodic or continuous surface discharge. The purpose of this study was to document the LFSF system, evaluate its treatment capabilities in laboratory models, and optimize design parameters. Three 5.0-meter-long laboratory models, each with a different permeability sand fill, were dosed with septic-tank effluent for six months. After an initial development period, percentage removals for biochemical oxygen demand (BOD), total organic carbon (TOC), suspended solids, and coliform bacteria from all the models were excellent. Orthophosphate initially was attenuated but showed increasing values after a few months. Recommendations are made as to a suitable permeability range of sand fills to use in the LFSF system, adequate depth of sand fill and overall system dimensions. Laboratory modeling shows that the LFSF system is capable of providing a high level of treatment to household wastewater and that it should be suitable as a remedial or alternative system for household wastewater disposal. Water Environ. Res., 66,919 (1994).