Virus Removal by Sand Filtration of Septic Tank Effluent

Septic tank effluent (STE) is applied to stratified sand filter columns at a rate of 6.1 cm/day (1.5 gal/day/sq ft) and the ability of the sand filter to remove enteric viruses is demonstrated. The sites of virus retention are determined. Seven 10‐cm (4‐in.) diameter sand columns are constructed of 25 cm (10 in.) of dry, coarse sand, 10 cm (4 in.) of dry, fine sand, and 25 cm (10 in.) of glass sand, with the sand layers separated by 5 cm (2 in.) of 1.9‐cm (3/8‐in.) rounded gravel. Household septic tank effluent containing poliovirus vaccine (Type I, strain LSc) is applied to the filters, and influent and effluent samples are collected. Enterovirus analyses are performed using plaque assays in buffalo green monkey (BGM) kidney cell cultures. Following the filter runs, the columns are dissected into 2.5‐cm (1‐in.) discs and the discs were assayed for enteroviruses. The filter columns retain all viruses when the virus loading remains less than 33,000,000 PFU/1 (plaque‐forming units per liter). Analyses of the filter discs show most viruses are retained in the top few centimeters of biologically active sand, and a mass balance reveals virus inactivation in the sand filter.