Which of the following is false about rapid gravity type filters used for water purification?

Skilled supervision is essential
Operational cost is high
Depreciation of plant is high
Coagulation is not essential

LohaHridoy

5 views

2025-01-04 04:42

1 Answers

AhmedNazir

Option 4 : Coagulation is not essential

Explanation:

Slow sand/gravity filter and rapid gravity/sand filter:

	Slow sand filter	Rapid sand filter
Pre-treatment	Effluents from plain sedimentation coagulation, not req.	Coagulation, flocculation, and sedimentation are a must, as the size of the particle is less and they don't get enough time to settle down on their own
Depreciation	Relatively low	Relatively high
Supervision	No skilled supervision	Skilled supervision required
Operational cost	Less	More
Filter Sand	D₁₀ b/w 0.2 and 0.4 mm C_u ⇒ 1.8 to 3. Gravel: 30-75 cm thick	D₁₀ = 0.35 mm - 0.55 mm C_u = 1.2 to 1.8 Gravel: 60-90 cm thick
Size of each unit	Large such as 30 m × 60 m. An area from 100 - 2000 m2. Less flexible on shock loading	Small 5 m × 8 m. Area < 100 m2 More flexible on shock loading
Rate of filtration	100 to 200 l/hr/m2	3000 to 6000 l/hr/m2
Efficiency	Very efficient in removing bacteria but less efficient in removing color.	Less efficient in bacteria (80 - 90 %), very efficient in color removal.
Post-treatment	Disinfection is not a must. Almost pure water was obtained. Turbidity removal up to 50 ppm	Disinfection is a must. Turbidity up to 35 ppm.
Method of cleaning	Scrapping and removing the top 1.5 to 3 cm thick layer and washing down by hoses.	Backwashing with or without compressed air.
Qty of wash water req.	0.2 to 0.6 % of total filtered water is req. in every 1 to 3 months	A large amount of total filtered water is req. in every 1 to 3 days (1 - 5 %)

Pressure filter

The difference between the rapid sand filters and pressure filters:

The flow of water is not under gravity. This implies that water inside the closed chamber is under pressure.
The diameter of the tank is 1.5 m - 3 m and the height or length is 3.5 to 8 m.
Operation is the same as rapid gravity filter.
Rate of filtration is 6000 - 15000 l/m2/hr.

5 views Answered 2022-09-04 04:29

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