Filtration | Types | Construction of filter | Cleaning | Slow - Rapid - Pressure Filter

Filtration | Type of filtration in water treatment plant | Construction and working of filtration | Cleaning of filter box

The process involves water treatment plant is screening, sedimentation, coagulation, filtration, disinfection and distribution. First process of the water treatment plant is screening; Screening is the process  of the water treatment plant removing the floating particles such as glasses, wood, dead animals, bottles etc are removed by the screening process. After the screening process the raw water flows from the sedimentation tank. The working principle of a sedimentation tank is to remove the suspended particles such as sand, silt, clay etc. Sedimentation is a natural process (Gravity). Again sedimentation processes are done then raw water flows from the coagulation tank; The working principle of the coagulation tank is to remove the colloidal particles. Sometime sedimentation and coagulation are done simultaneously is called plain sedimentation, after the coagulation process raw water flow from filtration tank;

Filtration in water treatment plant

Filtration is the process of removing the dissolved particles from the raw water with the help of a filter.

Filter consists of filter box, Granular medium / filter medium and gravel. Generally granular mediums are granular sand, graphite sand, anthracite coal.

Filtration | Types | Construction  of filter | Cleaning | Slow - Rapid - Pressure Filter
Filtration Box

Rate of filtration

Rate of filtration is defined as the amount of water filtered per unit time for a given surface area. It is denoted by ROF

Unit of rate of filtration is Lit / hr / m2.


Types of filter

Based on the rate of filtration, Three types of filter are available.

      1. Slow sand filter 
      2. Rapid sand filter 
      3. Pressure filter


Slow sand filter 

    • Slow sand filter is a single medium gravity filter.
    • Rate of filtration of slow sand filter is lies between 100 to 200 Lit / hr / m2.
    • Efficiency of a slow sand filter is 98% to 99%.
    • If we use a slow sand filter, No need of prior and post treatment.


Construction and working of slow sand filter in water treatment plant

Filters consist of filter box, filter medium and gravel particles.


Filter box

    • Filter boxes are open watertight rectangular tanks.
    • The depth of the filter box lies between 2.5 m to 3.5 m.
    • The surface area of a slow sand filter box is 100 to 200 m2.


Filter medium

    • Filter mediums are used in slow sand filter is river fine sand.
    • The effective size of sand particles are 0.2 to 0.4 mm.
    • The Uniform coefficient of sand particles are 1.8 to 3.
    • Thickness of the filter medium is 90 to 110 cm.


Gravel bed

    • It is used to support the filter medium
    • Thickness of the gravel bed is 30 cm to 35 cm.
    • Particle size of gravel used in slow sand filters is 4.75 mm to 60 mm.


Cleaning of the slow sand filter.

Cleaning of the slow sand filter is done every 1 to 3 months. The interval between two successive cleaning depends on impurities present in raw water and size of filter medium.

Cleaning or a slow sand filter is done by scrapping and washing.


Design of a slow sand filter

 Total surface area of filter = Discharge / R.O.F (Lit/hr/m2)

 Number of slow sand filter used = Total surface area of slow sand filter / Surface area of one slow sand filter.

Note - Generally 0.2% to 0.6% of filter water is used for washing of slow sand filters.


Rapid sand filter

    • Rapid sand filter is also a single medium gravity filter.
    • Rate of filtration of rapid sand filter lies between 3000 to 4000 Lit /  hr /  m2.
    • Efficiency of rapid sand filters is 80% to 90%.
    • If we used a rapid sand filter, there would be a need for prior and post treatment.


Construction and working of rapid sand filter in water treatment plant

Filters consist of filter box, filter medium and gravel particles.


Filter box

    • Filter box is an open Watertight rectangular tank which is made of masonry.
    • Surface area of a rapid sand filter is 10 to 80 m.
    • Depth of the filter box is 2.5 to 3.5 m.


Filter medium

    • Generally river coarse sand is used in rapid sand filters.
    • The Uniform coefficient of sand lies between 1.2 to 1.8.
    • Size of the filter medium is 0.4 to 0.6 mm.
    • Thickness of the filter medium is 60 to 90 cm.


Gravel bed

    • The thickness of the gravel bed in the rapid sand filter is 30 to 60 cm.
    • Size of the gravel is 4.75 mm to 40 mm.


Cleaning of rapid sand filter in filtration

    • Filter should be clean every 24 to 40 hr’s.
    • Percentage filter water used for back washing is 2% to 6 %.
    • Cleaning of the rapid sand filter is done by back washing only.


Design of rapid sand filter

Total surface area of filter = Discharge / R.O.F.

No of filter = Total surface area of filter / S.A. of one rapid sand filter

% filtered watered used for back washing = R.O.B. X tb / R.O.F. X tf

Where, ROB - Rate of back washing 

  tb - Time back

Pressure filter

    • It is also a single medium filter.
    • Rate of filtration is pressure filter lies between 6000 to 15000 Lit / hr / m2.
    • Efficiency of the pressure filter is 60% to 70%.
    • Pressure filter is generally used in emergency work.
    • Pressure filters are the filters that treat the water by applying the pressure with the help of a pump.

FAQ's

Question 01. What is filtration process?

Answer - Filtration is the process of removing the dissolved particles from the raw water with the help of a filter. Filter consists of filter box, Granular medium / filter medium and gravel. Generally granular mediums are granular sand, graphite sand, anthracite coal.

Question 02. What are the 3 types of filtration?

Answer - Types of filtration - Based on the rate of filtration, Three types of filter are available. 01. Slow sand filter 02. Rapid sand filter 03. Pressure filter

Question 03. Cleaning of the slow sand filter

Answer - Cleaning of the slow sand filter is done every 1 to 3 months. The interval between two successive cleaning depends on impurities present in raw water and size of filter medium. Cleaning or a slow sand filter is done by scrapping and washing.
Note - Generally 0.2% to 0.6% of filter water is used for washing of slow sand filters.

Question 04. Working of filtration

Answer - Filters consist of filter box, filter medium and gravel particles.
Filter box
Filter boxes are open watertight rectangular tanks.
The depth of the filter box lies between 2.5 m to 3.5 m.
The surface area of a slow sand filter box is 100 to 200 m2

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