Slow sand filters are used in water purification for treating raw water to produce a potable product. They are typically 1 to 2 metres deep, can be rectangular or cylindrical in cross section and are used primarily to treat surface water. The length and breadth of the tanks are determined by the flow rate desired by the filters, which typically have a loading rate of 0.1 to 0.2 metres per hour (or cubic metres per square metre per hour). Although they are often the preferred technology in many developing countries, 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. Slow sand filters now are also being tested for pathogen control of nutrient solutions in hydroponic systems.
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.
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;
For municipal systems there usually is a certain degree of redundancy, it is desirable for the maximum required throughput of water to......