Bio (Slow) sand filters are used in water purification for treating raw water to produce a potable product. They are typically 1 to 2 meters deep, can be rectangular or cylindrical in cross section and are used primarily to treat surface water. The filter can be produced anywhere in the world using readily available local materials such as concrete container, layers of sand and gravel (to eliminate sediments, pathogens and other impurities from the water). Slow sand filters have a number of unique qualities: unlike other filtration methods, slow sand filters use biological processes to clean the water, and are non-pressurized systems. Slow sand filters do not require chemicals or electricity to operate. When water is poured into the top of the filter, the organic material it is carrying is trapped at the surface of the fine sand, forming a biological layer or ‘schmutzdecke’. Over a period of one to three weeks, micro-organisms colonize the schmutzdecke, where organic food and oxygen derived from the water abounds and consumes bacteria and other pathogens found in the water.
- Cleaning is traditionally by use of a mechanical scraper, which is usually driven into the filter bed once it has been dried out. However, some slow sand filter operators use a method called "wet harrowing", where the sand is scraped while still under water, and the water used for cleaning is drained to waste;
- Unlike other water filtration technologies that produce water on demand, slow sand filters produce water at a slow, constant flow rate and are usually used in conjunction with a storage tank for peak usage. This slow rate is necessary for healthy development of the biological processes in the filter.
In the base of each bed is a series of herringbone drains that are covered with a layer of pebbles which in turn is covered with coarse gravel. Further layers of sand are placed on top followed by a thick layer of fine sand. The whole depth of filter material may be more than 1 metre in depth, the majority of which will be fine sand material. On top of the sand bed sits a supernatant layer of raw, unfiltered water.
Water is poured into the top of the filter as needed, where a diffuser plate placed above the sand bed dissipates the initial force of the water. Traveling slowly through the sand bed, the water then passes through several layers of gravel and collects in a pipe at the base of the filter. At this point, the water is propelled through plastic piping encased in the concrete exterior, and out of the filter for the user to collect.
- As they require little or no mechanical power, chemicals or replaceable parts, and they require minimal operator training and only periodic maintenance, they are often an appropriate technology for poor and isolated areas.
- Slow sand filters, due to their simple design become a ‘do-it-yourself’ project.
- Slow sand filters are recognized by the World Health Organization , Oxfam, United Nations  and the United States Environmental Protection Agency  as being superior technology for the treatment of surface water sources. According to the World Health Organization, "Under suitable circumstances, slow sand filtration may be not only the cheapest and simplest but also the most efficient method of water treatment."
Studies conclude that the Biosand filter removes:
· More than 90% of fecal coliform
· 100% of protozoa and helminthes
· 50-90% of organic and inorganic toxicants
· 95-99% of zinc, copper, cadmium and lead
· <67% of iron and manganese
· <47% of arsenic
· All suspended sediments
Using a 50 gallon plastic drum filled with rocks, gravel, charcoal, and sand to purify water for drinking, the filter can be made inexpensively with locally available materials. The sand filter cleans out 99% of organic contaminants, including giardia and cryptosporidium, and is inexpensive to build from locally available materials.
1. 50 gal. Plastic drum: washed
2. 5-6 buckets fine sand 1/16-1/8” diameter; washed and free from organic matter
3. 2 buckets charcoal: crushed and compacted with no sticks
4. 2 buckets gravel: ¼-1/2” diameter
5. Enough rocks (2-3 buckets) to cover outlet pipe: 2-4” diameter
6. Outlet pipe: PVC or HDPE pipe with 1/8” holes drilled every ½”
7. Pipe Cap
8. Bulkhead fitting with feminine threads
9. Hose valve with male threads
Information taken from the websites at http://www.wasrag.org/downloads.php?technology=43 from the rotation action group and http://www.surferswithoutborders.org/Slow_Sand_Filter.html from surfers without borders. Surfers w/o borders and ESM takes no responsibility for illnesses resulting from the misuse of this design. Please educate yourself before you begin by going to the referred websites.