Sand Filter Working: Components, Operation, Benefits

For purifying huge amounts of drinking water, developed nations frequently use the rapid sand filter method. It’s a somewhat complex procedure that often calls for power-operated pumps to clean the filter bed and backwash it, as well as flow control for the filter output.

When using raw water with low turbidity, a constantly working filter will often need to be backwashed every one to three days. Where turbidity is high, pretreatment of the raw water with a chemical flocculation agent in conjunction with setting tanks is usual. In general, water is permitted to pass from coagulation and sedimentation to the quicksand gravity filter.

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Components of a rapid sand filter

Enclosed tank

rectangular tank made of concrete, stone, or brick that is open and waterproof. The enclosing tank is between 2.5 and 3.5 inches deep. The tank’s surface size or plan area can range from 10 to 50 metres, and its length-to-width ratio is typically maintained between 1.25 and 1.35.

The range of the filtration rate is between 3000 and 6000 lit/hr/m2.

Filtering media

It is made up of a layer of sand that is 60 to 75 cm thick and has an effective thickness (Dio) of 0.45 to 0.7 mm. Filter media’s uniformity coefficient (Cu), which ranges from 1.3 to 1.7 but is typically 1.5, is variable.

Base element

The foundation material (gravel) of a 45 to 60-cm thick bed supports the filter medium. Graded gravel is spread out in layers that are each 15 cm thick.

Drainage system underneath

There are two functions of the quicksand filter beneath the drainage system;

1. I) To gather purified water; and
2. ii) To distribute the backwash water evenly.

There are several different under-drainage methods for these filters, but the two that are most frequently used are explained here.

System of perforated pipes

This system is made up of a central drain or manifold that is joined on either side by a number of lateral drains. The lateral drains have holes and are spaced 15 to 30 cm apart. This method is cost-effective and easy to use. However, additional water must be pumped at a high rate of speed in order to backwash the filter.

System of pipes and strainers

A central drain or manifold, to which a number of lateral drains are linked on either side, is also a component of this system. At the top of the laterals, holes are bored, and each hole has a strainer in it. On top of the lateral drains, the strainers are either screwed or permanently fastened. Strainers are typically spaced 15 to 30 cm apart.

Appurtenances

Certain equipment, such as appurtenances such as wash water troughs, air compressors, valves, head loss monitoring devices, flow regulators, etc., are known during construction for the effective and proper operation of the filter.

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Rapid sand filter operation and cleaning

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Only valves 1 and 2 are opened during the quicksand filter’s regular operating stage; the remaining valves are kept closed. In order to transfer filtered water to the storage tank, valve 2 (exit) is opened and valve 1 (inlet) is opened to allow water from sedimentation with coagulation.

Between one and two metres of water are above the sand bed. Backwashing (a process used in the maintenance of various filtration systems, including water filtration systems, air filters, and some pool filtration systems. The purpose of backwashing is to remove accumulated debris, dirt, and other contaminants from the filter media, which helps to maintain the efficiency and effectiveness of the filtration system) is often performed when the head loss reaches 2.5 to 3 m or filter cleaning is performed every one to three days.

Typically, backwashing takes 10 to 15 minutes. Nonetheless, the restart procedure might take a few minutes in total. The first step in backwashing a filter is to shut off valve 1 and allow the water level to drop to the edge of the wash water troughs.

The next step is to close valve 2 and then open valve 6 so that compressed air may flow upward for 2 to 3 minutes, breaking up surface scum and loosening the dirt. To let water through, close valve 6 and open valve 4.

To remove filthy wash water through the wash water drain, open valve 3. Now shut off valves 4 and 3, and let the items on the sand’s surface some time to settle. Now let out filtered water through valves 1 and 5 for a few minutes to clean the water drain.

Last but not least, close valve 5 and open valve 2. Filter may now be used at its typical filtering rate.

Rapid Sand Filter efficiency:

1. The efficacy of removing microorganisms is expected to be between 80 and 90 percent.
2. Water turbidity can reduce an amount by 35 to 40 ppm.
3. These filters are very effective at removing colour, with a cobalt scale value below 3.

Rapid sand filters are commonly used in water treatment operations because they are effective at removing a wide range of impurities from water, including suspended solids, bacteria, and algae. There are several reasons why rapid sand filters are well-suited for these types of applications:

• High filtration rate: Rapid sand filters are designed to have a high filtration rate, meaning they can filter large volumes of water in a relatively short amount of time.
• Effective at removing impurities: The filter bed in a rapid sand filter is made up of a fine sand that effectively removes impurities from water.
• Easy maintenance: Rapid sand filters are relatively simple to maintain and clean, making them a popular choice for water treatment operations.
• Durability: Rapid sand filters are designed to be durable and long-lasting, which is important in water treatment operations where the filters are in constant use.
• Cost-effective: Compared to other types of filtration systems, rapid sand filters are relatively inexpensive and cost-effective to operate, making them an attractive option for water treatment facilities.

Benefits of rapid sand filters

The fact that this method is tried-and-true, efficient in removing suspended materials, and only needs a small amount of land for setup and operation are its benefits. as opposed to laborious sand filters.

Rapid-sand filters’ drawbacks

Rapid sand filters have significant startup and running expenses, which might go up even more if raw water needs to be pretreated.

The method consumes energy for pumping and necessitates a considerable amount of plant operator training.

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