How it Works
Slow sand filters work through the formation of a gelatinous layer (or biofilm) called the Schmutzdecke in the top few millimeters of the fine sand layer. The Schmutzdecke is formed in the first 10–20 days of operation and consists of bacteria, fungi, protozoa, rotifera and a range of aquatic insect larvae. As a Schmutzdecke ages, more algae tend to develop and larger aquatic organisms may be present including some bryozoa, snails and Annelid worms. The Schmutzdecke is the layer that provides the effective purification in potable water treatment, the underlying sand providing the support medium for this biological treatment layer. As water passes through the Schmutzdecke, particles of foreign matter are trapped in the mucilaginous matrix and dissolved organic material is adsorbed and metabolized by the bacteria, fungi and protozoa. The water produced from a well-managed slow sand filter can be of exceptionally good quality with 90-99% bacterial reduction. (source: Wikipedia)
In fact , the P3 BioSand Bag Filter achieved laboratory certified results that showed 100% removal of fecal coliform and 99.999% removal of bacteria and virus.