One of the ways by which the biosand filter removes bacteria and viruses is through biologically mediated mechanisms, such as predation and adsorption onto biofilms. In addition, some virus removal is thought to be due to attraction to positively charged ions which accumulate on the negatively charged sand particles. However, these processes take time to develop. A newly installed biosand filter typically removes only about 0.3 log, or about 50%, of E. coli. This increases over the following weeks as the biolayer develops and the filter “ripens”. However, the exact amount of time required to ripen a BSF is highly variable. Although it is possible for filters to be ripened in less than a month, field conditions do not to allow for the testing of each filter to determine whether or not that specific filter is ripened, so it is prudent to assume the larger time period.
For viruses, it is known that the longer the filter is in operation, the better the removal rates are. Experiments with residence times of six months to one year have shown much higher removal rates than experiments which are run for only a few months.
A good paper with information on this topic is the following:
Elliott MA, Stauber CE, Koksal F, DiGiano FA, Sobsey MD. Reductions of E. coli, echovirus type 12 and bacteriophages in an intermittently operated household-scale slow sand filter. Water Research. 2008 ;42(10-11):2662 - 2670.