Sand Used for Filters | Filtration | Water Treatment | Water Engineering

In this article we will discuss about:- 1. Properties of Sand 2. Effective Size of Sand 3. Uniformity 4. Depth and Grading of Sand Bed 5. Gravel for Filter.

Properties of Sand:

The sand used for filters should have the following properties:

(i) It should be obtained from hard rocks such as basalt, trap and quartz.

(ii) It should be free from clay, loam, lime and organic matter etc.

(iii) It should be of uniform size and nature.

(iv) It should be resistant and hard.

(v) If placed in hydrochloric acid for 24 hours, it should not lose more than 5% of its weight.

Effective Size of Sand:

Very fine sand is clogged quickly and rate of filtration is reduced. On the other hand very coarse sand permits suspended particles and bacteria to pass through the sand bed. Therefore, the size of sand for filter bed should be properly selected.

The size of sand particles is specified by the effective size, which is the sieve size in millimetres that permits 10% of sand by weight to pass. The selection of the correct effective size is very important, because if very fine sand is used it will clog the filters frequently and reduce the rate of filtration.

On the other hand too large size sand will not filter the desired impurities and will allow fine suspended matter and bacteria to pass through the filter.

Table 13.1 below gives the percent size distribution of filter and grains:

[Note: A percent size of 60 means that 60% of the sand is smaller than the size given]

Uniformity of Sand:

It is specified by the uniformity coefficient. Which is the ratio between the sieve size that will pass 60% to the effective size. Now-a-days sand with an effective size laying between 0.30 and 0.55 and having uniformity coefficient between 1.30 and 1.75 are used.

Depth and Grading of Sand Bed:

The depth of sand bed should be between 60 cm to 90 cm.

Grading of Sand Bed:

The sand bed consists of graded sand in various layers. For slow sand filters, sand having effective size between 0.3 and 0.35 mm and uniformity coefficient of 1.75 is used in Indian practice. For rapid sand filters sand having effective size between 0.35 and 0.5 mm and uniformity coefficient of 1.6 is used.

Uniform grading or sand bed is very essential because it decreases the void spaces and makes bottom and top of filter bed equally effective, and increases the rate of filtration.

Gravel for Filter:

The sand bed is supported on the gravel bed. Gravel bed has several functions, due to which it is provided below the sand bed. It supports the sand and allows the filtered water to move freely towards the under-drains. It also allows the wash water to move upward uniformly on the sand. The gravel is placed in 5-6 layers having finest size on the top.

Common grading of gravel level is shown in Table 13.2.

The gravel used in filtration plant should be clean, hard, durable and rounded. It should be free from clay, loam, shells and other foreign matters. It should not contain flat, thin or long pieces. It should have a density of about 1600 kg/m³.