Sand filters are an amazingly simple yet extremely effective water filtration technique. Once a sand filter has been running for a short time, natural bacterial colonies form in the surface layers of sand. This layer of life is called a bio-film or hypogeal layer. It consumes any biological material; meaning:
The water produced from an exemplary slow sand filter is of excellent quality with 90-99% bacterial cell count reduction.– NDWC.
“Although they are often the preferred technology in many developing countries because of their low energy requirements and robust performance, they are also used to treat water in some of the most developed countries, such as the UK, where they are used to treat water supplied to London.” – Wikipedia
< A simple sand filter. Note that the outlet is above topmost layer of sand, keeping the filer media wet. This is important to keep the hypogeal layer alive.