Slow sand filter
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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).
Slow sand filters differ from all other filters used to treat drinking water in that they work by using a complex biological film that grows naturally on the surface of the sand. The sand itself does not perform any filtration function but simply acts as a substrate.
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. Slow sand filters now are also being tested for pathogen control of nutrient solutions in hydroponic systems.
Method of operation
Slow sand filters work through the formation of a gelatinous layer (or biofilm) called the hypogeal layer or Schmutzdecke in the top few millimetres 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 metabolised 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.
Slow sand filters slowly lose their performance as the Schmutzdecke grows and thereby reduces the rate of flow through the filter. Eventually it is necessary to refurbish the filter. Two methods are commonly used to do this. In the first, the top few millimetres of fine sand is scraped off to expose a new layer of clean sand. Water is then decanted back into the filter and re-circulated for a few hours to allow a new Schmutzdecke to develop. The filter is then filled to full depth and brought back into service. The second method, sometimes called wet harrowing, involves lowering the water level to just above the Schmutzdecke, stirring the sand and thereby suspending any solids held in that layer and then running the water to waste. The filter is then filled to full depth and brought back into service. Wet harrowing can allow the filter to be brought back into service more quickly.
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.
- Cleaning is traditionally done by use of a mechanical scraper, which is usually driven into the filter bed once the bed 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, since it is desirable for the maximum required throughput of water to be achievable with one or more beds out of service.
- Slow sand filters require relatively low turbidity levels to operate efficiently. In summer conditions and in conditions when the raw water is turbid, blinding of the filters occurs more quickly and pre-treatment is recommended.
- 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.:38–41 
While many municipal water treatment works will have 12 or more beds in use at any one time, smaller communities or households may only have one or two filter beds.
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.
- 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, may be created diy. DIY-slow sand filters have been used in Afghanistan and other countries to aid the poor.
- 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."
- Due to the low filtration rate, slow sand filters require extensive land area for a large municipal system.:38–39 Many municipal systems in the U.S. initially used slow sand filters, but as cities have grown they subsequently installed rapid sand filters, due to increased demand for drinking water.
- Centre for Affordable Water and Sanitation Technology, Biosand Filter Manual: Design, Construction, & Installation," July 2007.
- National Drinking Water Clearinghouse (U.S.), Morgantown, WV. "Slow Sand Filtration." Tech Brief Fourteen, June 2000.
- United States Environmental Protection Agency (EPA)(1990). Cincinnati, OH. "Technologies for Upgrading Existing or Designing New Drinking Water Treatment Facilities." Document no. EPA/625/4-89/023.
- HDR Engineering (2001). Handbook of Public Water Systems. New York: John Wiley and Sons. p. 353. ISBN 978-0-471-29211-1. Retrieved 2010-03-28.
- DIY slow sand filter
- "Learn More: Water (slow sand filter)". Refugee Camp Project -. Doctors Without Borders. Archived from the original on 2007-07-28. Retrieved 2007-03-27.
- "Slow Sand Filtration", World Health Organization, 1974 ISBN 92-4-154037-0
- "UN High Commissioner for Refugees (UNHCR) Water Manual for Refugee Situations", Geneva, November 1992. Slow sand filters recommendations listed on, p. 38.
- "Small System Compliance Technology List for The Surface Water Treatment Rule", United States Environmental Protection Agency, EPA 815-R-97-002 August 1997. Slow sand filtration is listed on, p. 24.
- " Filter bed ". Collier's New Encyclopedia. 1921.
- Modelling slow sand filters: PhD Thesis, Luiza Campos, Imperial College, 2002. PDF in 5 parts: Part 1, Part 2, Part 3, Part 4, Part 5