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Slow sand filter
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).
Additional recommended knowledge
Slow sand filters have a number of unique qualities:
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.
How it works
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. 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 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, absorbed 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 no detectable bacterial content.
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. Wet harrowing can allow the filter to be brought back into service more quickly.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Slow_sand_filter". A list of authors is available in Wikipedia.|