Slow sand filter

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 nonpressurized 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 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.
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.