Slow sand filter beds 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 one cubic metre per square metre per hour). Water purification, or drinking water treatment, is the process of removing contaminants from surface water or groundwater to make it safe and palatable for human consumption. ... Raw water is water taken from the environment, and is subsequently treated or purified to produce potable water in a water purification works. ...

Slow sand filters have a number of unique qualities:

1. Unlike other filtration methods, they use biological processes to clean the water, and are nonpressurized systems.
2. 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;
3. For municipal systems, it is desirable for the maximum required throughput of water to be achievable with one or more beds out of service;
4. In summer conditions and in conditions when the raw water is turbid, binding of the filters occurs more quickly, and pre-treatment is recommended.

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. Raw water is water taken from the environment, and is subsequently treated or purified to produce potable water in a water purification works. ... Turbidity standards of 10, 100, and 1000 NTU Turbidity is a cloudiness or haziness of water (or other liquid) caused by individual particles that are too small to be seen without magnification, thus being much like smoke in air. ... Water treatment may mean: sewage treatment for the treatment of sewage or other wastewater water purification for the treatment of drinking water to make it potable or suitable for human consumption industrial water treatment for the treatment of water used in industry, often in closed systems This is a disambiguation...

In the base of each bed is a series of herring-bone 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. The liquid, containing soluble compounds, that is left behind after a mixture is centrifuged or precipitated. ...

Slow sand filters work through the formation of a gelatinous layer called the hypogeal layer or Schmutzdecke in the top few millimetres of the fine sand layer. This layer 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 ectoprocta, 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 absorbed and metabolised by the bacteria, fungi and protozoa. The water draining from a well-managed slow sand filter can be of exceptionally good quality with no detectable bacterial content. In botany, a seed is described as hypogeal when the cotyledons of the germinating seed remain non-photosynthetic, inside the seed shell, and below ground. ... Schmutzdecke (German, grime or filth cover, sometimes incorrectly spelt Schmutzedecke) is a complex biological layer formed on the surface of slow sand filter beds. ... Phyla/Divisions Actinobacteria Aquificae Bacteroidetes/Chlorobi Chlamydiae/Verrucomicrobia Chloroflexi Chrysiogenetes Cyanobacteria Deferribacteres Deinococcus-Thermus Dictyoglomi Fibrobacteres/Acidobacteria Firmicutes Fusobacteria Gemmatimonadetes Nitrospirae Omnibacteria Planctomycetes Proteobacteria Spirochaetes Thermodesulfobacteria Thermomicrobia Thermotogae Bacteria (singular, bacterium) are a major group of living organisms. ... Divisions Chytridiomycota Zygomycota Ascomycota Basidiomycota The Fungi (singular: fungus) are a large group of organisms ranked as a kingdom within the Domain Eukaryota. ... Protozoa (in Greek protos = first and zoon = animal) are single-celled eukaryotes (organisms with nuclei) that show some characteristics usually associated with animals, most notably mobility and heterotrophy. ... Classes Seisonoidea Bdelloidea Monogononta The rotifers make up a phylum of microscopic, pseudocoelomate animals. ... Classes Phylactolaemata Stenolaemata Gymnolaemata The Ectoprocta are a phylum of lophophorate animals. ... Classes Caudofoveata Aplacophora Polyplacophora - Chitons Monoplacophora Bivalvia - Bivalves Scaphopoda - Tusk shells Gastropoda - Snails and Slugs Cephalopoda - Squids, Octopuses, etc. ... Classes and subclasses Class Polychaeta (paraphyletic?) Class Clitellata    Oligochaeta - Earthworms and others    Acanthobdellida    Branchiobdellida    Hirudinea - Leeches Class Myzostomida Class Archiannelida (polyphyletic) Class Echiura *Some authors consider the subclasses under Clitellata to be classes The annelids, collectively called Annelida (from Latin annellus little ring), are a large phylum of animals, comprising...

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 very carefully scraped off using mechanical plant and this exposes 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 Schmutzedecke 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. A physical plant or mechanical plant refers to the necessary infrastructure used in support of a given facility. ...

Slow sand filters are recognized by the World Health Organization, Oxfam, and the United States Environmental Protection Agency as being superior technology for the treatment of surface water sources.