Filtration occurs as the water flows through the sand. BioSand Filters (BSF) remove almost all solid particles and pathogens from water. They have also been shown to remove some viruses, turbidity, and metals. This happens through mechanical trapping, predation by other microbes, absorption by the sand, and natural death. 

However, the most important part of a BSF is the biolayer, where microbes live in the top of the sand. The biolayer takes about 30 days to grow, and adapts to the particular water source that is being used. Every time water is poured into the filter, the biolayer microbes consume the pathogens in the water.