Rapid Sand Filter & Pressure Filter: Design It !

This article gives you an online Excel in which you can design the rapid sand and pressure filter. You don’t need to be an engineer to get a general design. Just input the value and get the design

First, let’s understand the rapid sand filter and its workings:

To overcome the defect of the slow sand filter, the rapid sand filter and pressure filter have been designed for faster filtration rates.

The main differences between a slow sand filter and a rapid sand filter are:

• The effective size of sand in a rapid sand filter is a little bigger
• The rate of filtration in rapid sand filter is higher
• Rapid sand filter needs frequent backwashing
• Bacteria removal is less in rapid sand filter

Also, the pressure filter is a type of rapid sand filter in which the pressure is applied to filter the water. Whereas, in a rapid sand filter the gravity flow occurs.

The processes of treatment happening in the rapid sand filter are: Straining, sedimentation, and adsorption.

1. Sizing of Filter Tank

The size of the tank is determined just by using the rate of filtration desired (m3/m2/hour). Selecting the shape of the tank, and the sizing of the tank can be easily done.

2. Sand depth determination

The minimum depth of sand required for treatment is estimated using Hudson’s formula for peak flow of water. Increasing the depth to sufficient factor of safety, we get the depth of sand to be used.

It should be noted that the filter sand needs to be prepared and not any sand can be used. Sand should be prepared for getting effective size (0.35 to 0.6 mm) and coefficient of uniformity (Cu: 1.2 to 1.7)

3. Gravel depth and grading determination

4. Underdrainage design

The water filter and the underdrains collect and accumulate the water from output. The underdrain consists of a main central pipe called as central manifold. The perforated pipe laid in branches is called as lateral pipe. The design of the underdrains is done empirically to sufficiently fulfill the flow capacity.

5. Initial system head loss in clean condition

The head loss that occurs due to the filter needs to be calculated. This is calculated with any of the two equations which are the Carman-Kozeny equation and Rose equation. The usual value of the clean bed head loss is 15-30 cm of water head.

The head loss increases with the operation of the filter due to dirt accumulation. After reaching a terminal head loss (Generally, 2.5-3.5 m for rapid sand filter and 1.5 meters for pressure filter), the filter needs to be back washed to clean bed head loss conditions.

There has been further development in the rapid sand filters as dual media filters and mixed media filters that use Anthracite coals and garnet sands. The advantages are its compact size and higher efficiency. However, the cost is also higher and suits for small-scale households and industrial treatments of water.

Performance of the rapid sand filter

The turbidity, color, and dissolved iron are removed efficiently in the rapid sand and pressure filter. However, the bacteria if present remains. Hence, chlorination of filtered water is a must to remove all the pathogens from the filtered water.

Have you noticed something unusual in the design of the rapid sand filter?

Well, the size of the tank is determined from the rate of filtration required only. Typically, the calculated sand depth and gravel depth have not much difference with the different parameters, which are not so different.

This implies that the filtration rate is the key factor. The media depths have near identical values with the parameters varied also. However, the operation and maintenance of these filters are essential to give a uniform efficiency in treatment.