Rapid Sand Filtration

Rapid sand filters evolved at the end of the 19th century in the United States and quickly gained popularity. By the 1920s, they were widely used as a major water purification method, since necessary facilities required less land area compared to slow sand filters. Today, a combination of flocculation and coagulation, sedimentation, filtration and disinfection (e.g. chlorination, ozonation) is the most widely applied water treatment technology for treating large quantities of drinking water in industrialised countries (SDWF n.y.) (see also treatment plant set-ups). 

Rapid sand filtration, in contrast to slow sand filtration, is a purely physical treatment process. As the water flows through several layers of coarse-grained sand and gravel, relatively large particles are held back safely (DIJK & OOMEN 1978). However, RSFs never provide safe drinking water without adequate pre-treatment and final disinfection. Usually, coagulation and flocculation and chlorination are applied for that purpose.

The major parts of a gravity rapid sand filter are:

• Chamber: filter tank or filter box
• Filter media (sand)
• Gravel support
• Under drain system
• Wash water troughs

The filter chamber is usually made out of reinforced concrete, filled with sand and gravel to the height of 1.5-2 metres. The water is supplied to the top of the sand-bed and filtered as it flows through the layers of graded sand and gravel. A system of perforated pipes on the bottom drains the chamber (WHO 1996). The filter chamber can be constructed as open tanks (rapid gravity filters) or closed tanks (pressure filters).

This filtering process is determined by two basic physical principles. First, relatively large suspended particles get stuck between the sand grains as they pass the filter medium (mechanical straining). Second, smaller particles adhere to the surface of the sand grains caused by the effect of the van der Waals forces (physical adsorption). A chemical filter-aid (i.e. coagulant or flocculant) might be added to promote additional adhesion (SCHMITT & SHINAULT 1996).

In the course of these processes, more and more particles accumulate in the filter medium, increasingly causing clogged filters and decreased performance. Initial filtering performance can be re-achieved through a cleaning of the filter bed. This is usually conducted through backwashing: the flow of water is reversed, so that treated water flows backwards through the filter. The sand is re-suspended and the solid matter is separated in the surface water. Often, air is injected additionally to support the cleaning process (WHO 1996). As soon as most particles are washed out and the backward flowing water is clear, the filter is put back to operation. Clearly, relatively large quantities of sludge are generated through backwashing and require some form of treatment before discharge into the environment (UNEP 1998).

Rapid sand filtration is a highly effective method to remove turbidity if it is correctly applied (BRIKKE & BREDERO 2003). Equally, solids formed during pre-treatment, i.e. coagulation-flocculation, are filtered. A well-operated RSF reduces turbidity to less than 1 NTN and often less than 0.1 NTU (WHO 1996). Regarding the removal of most other contaminants, the RSFs are ineffective. If combined with adequate pre-treatment measures and final disinfection, rapid sand filtration usually produces safe drinking water.

Typical treatment performance of rapid sand filters if freshwater has been pre-treated with coagulation-flocculation. Adapted from: BRIKKE & BREDERO (2003), DEBOCH & FARIS (1999), SDWF (n.y.) and WHO (n.y.)

Construction

The construction of a RSF requires the supervision of a competent engineer and highly skilled workers. It involves the construction of reinforced concrete fundaments and walls and many technical installations. Only a very precise realisation guarantees a functioning water treatment.

Operation & maintenance

Operation of a rapid sand filter consists of flow control, regular backwashing and cleaning. The period between backwashes depends on the quality of the influent water and normally lies between 24 – 72 hours (UNEP 1998).The cleaning process requires an interruption of the purification process of 5 - 10 minutes per filter bed. Several parallel filter units are required to guarantee constant water supply. The backwash process must be observed carefully; in particular the rate of flow must be controlled to avoid erosion of the filter medium. Periodic repacking of the filter bed may be required at infrequent intervals to ensure efficient operation (UNEP 1998). Operation and maintenance thus requires skilled and highly reliable workers.

Construction costs

The construction cost of rapid sand filters is determined primarily by the cost of materials such as cement, building sand, gravel, reinforcing steel, filter media, pipes, and valves. However, the cost of land and transport of materials could add substantially to the total cost (UNEP 1998).

Operation & maintenance costs

The cost of energy required to operate a rapid sand filter and the costs for treatment of generated sludge during backwashing may add significant costs (UNEP 1998). Although operation is usually conducted automatically, frequent inspection by a well-educated worker is necessary to ensure proper treatment. Maintenance costs will include repairs of the filters and replacement of equipment. In general, construction, operation and maintenance costs for rapid sand filters are significantly higher than costs for slow sand filters (UNEP 1998).