Contaminant distribution in intermittent sand filters

Intermittent sand filtration of septic tank effluent has shown promise as a viable method of protecting groundwater supplies from the public health hazard of viral contamination, However, Little is known about the effects of filter size or hydraulic loading rate upon the viral removal efficiency of intermittent sand filters. Six columns, ranging in size from 15 cm (6 inches) to 46 cm (18 inches) in diameter were filled with 0.66 m (2 ft.) of medium concrete sand having an effective diameter (d(10)) of 0.32 mm and a uniformity coefficient of 2.2. Dechlorinated tap water containing MS2 bacteriophage was applied to the tilters at 5.1, 8.1, 12.2, and 16.3 cm d(-1) (1.25, 2, 3, and 4 gal d(-1) ft(-2)), and effluent samples were collected from a grid of sampling cells located under each filter column, Using the gridded sampling capture, each 7 cm(2) area of the filter effluent was isolated for analysis. The samples were analyzed for phage titer using plaque assays in Escherichia coli cells. The ability of varying diameters of sand filter columns to remove viruses was demonstrated. Although increasing hydraulic loading rate caused treatment efficiency to deteriorate, average removal efficiencies ranged from 88.953 percent to 99.956 percent. Contour maps of effluent samples show that regions of filter media containing large interstitial pore spaces, such as along the filter wall, promote preferential flow and treatment degradation. In addition to hydraulic loading rate, the degree of preferential now occurring in a filter column is shown to be governed by column diameter. Large diameter filters proved to be more efficient virus adsorbers than small-diameter filters, suggesting that studies conducted using small-diameter columns may underestimate the treatment capacity of filters used in field conditions.