The Effect of Idle Time on Reduction of Viruses in an Intermittently Operated, Householdscale Slow Sand Filter

Virus challenge experiments were conducted at full-scale and bench-scale on an intermittently operated household scale slow sand filter popularly known as the biosand filter. Results reported here detail the effect of idle time on the concentration of coliphages PRD-1 and MS2. Benchscale experiments incorporated sampling within the sand bed during the idle time at two depths, 10- and 30-cm. Our results indicate that waterborne virus concentrations decrease during the idle time of intermittently operated slow sand filters and that silica sand undergoes an “aging” process over days-to-weeks of daily filtration. This aging process alters the sand in a way that it acquires the ability to reduce the concentration of viruses during the idle time. Rates of reduction in aged sands were comparable for PRD-1 and MS2 and varied between 0.030-log10 and 0.062-log10 per hour of idle time. Rates of reduction in survival controls averaged less than 0.003-log10 per hour of idle time.