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 and are rectangular in cross section. The length and breadth of the tanks are determined by a number of factors:-

1. Cleaning is generally by mechanical scraper which is usually driven into the filter bed once it has been dried out.
2. The maximum required throughput of water must be achievable with one or more beds out of service.
3. In summer conditions and in conditions when the raw water is turbid, blinding of the filters occurs more quickly

In general most water treatment works will have 12 or more beds in use at any one time.

In the base of each bed are a series of herring-bone drains which 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.

Slow sand filters work through the formation of a gelatinous layer called the hypogeal layer or Schmutzedecke 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 Schmutzedecke ages more algae tend to develop and larger aquatic organisms may be present including some Endoprocta , Snails and Annelid worms. The Schmutzedecke 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 Schmutzedecke, 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

Slow sand filters slowly lose their performance as the Schmutzedecke grows and this reduces the rate of flow through the filter. Eventually it is necessary to refurbish the filter. To do this, 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.