Types of Sand Filters

Sand filters are composed of layers of different grades of sand.

Rapid Sand Filters
The most common physical treatment of water involves passing flocculated water through a sand filter to strain out the floc and the particles trapped in it. When unpleasant tastes and odors are present, the sand filter may include a layer of activated carbon to remove them.

In time, sand filters become clogged with floc and must be backwashed or pressure-washed to clean them. Backwashed water is run into special tanks for settling to allow the floc to precipitate out of the water. The water is then disposed of as wastewater, which may be used as a soil conditioner in some countries.

Slow Sand Filters
Slow sand filters, which require much land and space, use biological treatment processes rather than physical filtration. They are nonpressurized systems carefully constructed of graded layers of sand, with the finest layer at the top and the coarsest layer at the bottom. They are rectangular or cylindrical in shape. The length and width are determined by the loading rate, which is generally 0.1 to 0.2 meters per hour, or one cubic meter per square meter per hour.

At the bottom of each sand bed are drains in a herring-bone design that carry the water away for disinfection. These drains are covered by a layer of pebbles which, in turn, is covered with a layer of coarse gravel. Further layers of sand are placed on top of these layers. Finally, all of these layers are topped by a thick layer of fine sand. The filter material may come to more than 1 meter in depth. Most of this will be fine sand.

Small communities may need only one or two beds. Municipal systems often have 12 or more beds in service at one time, with one or more beds out of service for cleaning. In summer conditions and when the raw water is turbid, pretreatment is important to prevent rapid binding of the filters.

When a slow sand filter bed is new, raw water is slowly poured onto the sand until a depth of 1 to 3 meters, depending on the size of the bed, is reached. The water is recirculated for the first few hours and not put into supply. As it completes recirculation, a film composed of bacteria, aquatic insect larvae, protozoa, fungi, rotifera, and algae can be seen on the surface of the sand. This is called the Schmutzdecke layer, where all impurities are removed. As the Schmutzdecke ages, algae and larger aquatic organisms including ectoprocta, snails, and annelid worms may be present. A carefully constructed slow sand filter may remain effectively in service for many weeks or even months, especially if pretreatment has been thorough. An excellent quality of water is produced that is rarely achieved by physical methods.

As the Schmutzdecke biological film grows and reduces the rate of flow, the filter has to be refurbished in one of two methods. In the first, a few millimeters of fine sand are carefully scraped off the top layer by a mechanical scraper, exposing a new layer of clean sand. Water is then slowly poured back into the filter and recirculated for a few hours to allow a new Schmutzedecke layer to develop. The sand filter is then refilled to full depth. In the second method, called wet harrowing, the water level is lowered to just above the Schmutzdecke layer, and the sand is stirred to suspend any solids. The water is then run to wastewater and the sand filter is filled to full depth.

The Schmutzdecke layer is vital in providing effective water purification. As water passes through the Schmutzdecke layer, particles become trapped, and dissolved organic materials are absorbed and metabolised by the bacteria, fungi, and protozoa.

The water produced from a well-managed slow sand filter bed is of exceptional quality, with no bacterial content. The World Health Organization, Oxfam, and the United States Environmental Protection Agency recognize slow sand filters as a superior technology for the treatment of surface water sources.

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