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Filtration is a process in which water is passed through the beds of fine granular materials like sand. The bed of fine granular materials is known as a filter and it helps in removing impurities to a great extent. The purpose of filtration after screening and sedimentation is to attain palatable and potable water in the end. It helps to remove bacteria, odor, color, and turbidity from water.

Rapid sand gravity filters and slow sand filters are two types of filters that are commonly used for treating water. The slow sand gravity filter (or slow sand filters) removes a larger percentage of bacteria and impurities but has a very slow rate of filtration and is expensive. On the other hand, rapid sand gravity filters (or rapid gravity filters) are universally adopted as they help to disinfect and remove the color from the water that is fed directly from the sedimentation plant.

Differences between Rapid Gravity Filters and Slow Sand Filters

Sl. No.ParticularRapid Gravity FiltersSlow Sand Filters
1Pre-treatment requirementThe water must undergo coagulation, flocculation, and sedimentation.Raw water either without any treatment or having undergone sedimentation can be fed.
2Base materialThe sand is supported by the gravel base. The base also distributes the water uniformly on the sandy surface. It ranges from 0.6 to 0.9 m in depth and 3 to 40 mm in size.The sand is supported by the gravel base. It ranges from 0.3 to 0.75 m in depth and 3 to 65 mm in size.
3Filter sandThe effective size varies from 0.35 mm to 0.55 mm and the uniformity coefficient is between 1.2 to 1.8.The effective size varies from 0.2 mm to 0.4 mm and the uniformity coefficient is between 1.8 to 3.
4Size of each unitThe area occupied is small and is about 10 to 80 sq.m (5 m x 8 m).The area occupied is huge and is about 100 to 2000 sq.m (30 m x 60 m).
5Ease in constructionComplicated to construct.Simple to construct.
6Rate of filtrationLarge capacity i.e. 3000 to 6000 lit/hr per sq.m of filter area.Small capacity i.e. 100 to 200 lit/hr per sq.m of filter area.
7Loss of headInitial loss is 0.3 m, and approximately 2.5 to 3.5 m when cleaning is required.Initial loss is 0.1 m, and approximately 0.8 to 1.2 m when cleaning is required.
8FlexibilityThe filter can sustain reasonable variations and thus is very flexible.Cannot meet demand variations and thus not flexible.
9EfficiencyThis filter is well known for handling turbid water. Although it removes only 80% to 90% of bacteria, it efficiently removes color and turbidity.This filter removes bacteria effectively up to 98% to 99% but not very effective when it comes to eliminating color and turbidity.
10Under-drainage systemThe under-drainage system is laid not only to receive filtered water but also to pass water for carrying out backwashing.The under-drainage system is laid to receive filtered water.
11Post-treatment requiredIt is a must to carry out disinfection. In addition to this, other miscellaneous treatments can be carried out if needed.The water obtained is very pure but disinfection can still be carried out. Any other treatment is not required.
12EconomyIt is economical and cheap as the initial cost is low. Operation and maintenance (O&M) costs are high.  The initial cost of the land and material is high but O&M costs are low
13Depreciation costHigh depreciation cost.Low depreciation cost.
14Requirement of skilled supervisionSkilled supervision is essential.No need for skilled supervision.
15CleaningFrequent cleaning has to be done within a span of 1 to 3 days.
1% to 5% of water filtered is required for cleaning. Cleaning is comparatively easier as sand grains have to be agitated and backwashing can be done with/without compressed air.
Cleaning has to be done within a span of 1 to 3 months.
0.2% to 0.6% of water filtered is required for cleaning. Cleaning is comparatively laborious as 1.5 to 3 cm thick layer has to be scraped and removed and washed down by hoses.
Table-1: Comparison between rapid gravity filters and slow sand filters
Rapid sand filtration (and backwashing process)
Figure-1: Rapid sand filtration (and its backwashing process)
Slow sand filtration
Figure-2: Slow sand filtration

FAQs

What is filtration?

Filtration is a process in which water is passed through the beds of fine granular materials like sand it helps in removing impurities to a greater extent.

What are the different types of filters?

Rapid gravity filters, slow sand filters, and pressure filters are the types of filters.

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