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Questions and Answers
What is the primary purpose of filtration in water treatment plants, as described in the text?
What is the primary purpose of filtration in water treatment plants, as described in the text?
In a typical filtration process, what is most commonly used as the primary filtering media?
In a typical filtration process, what is most commonly used as the primary filtering media?
What happens to the rate of filtration as solids accumulate and form a filter cake?
What happens to the rate of filtration as solids accumulate and form a filter cake?
What is the 'backwashing' process in the context of filtration?
What is the 'backwashing' process in the context of filtration?
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In filtration, what term describes the volume of filtrate collected over time ($dV/dt$)?
In filtration, what term describes the volume of filtrate collected over time ($dV/dt$)?
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Which of the following is NOT typically removed by filtration?
Which of the following is NOT typically removed by filtration?
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What is the primary function of mechanical straining in filtration?
What is the primary function of mechanical straining in filtration?
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If the rate of filtration is maintained as a constant, what will be the consequence according to the text?
If the rate of filtration is maintained as a constant, what will be the consequence according to the text?
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In the context of filtration, how do the interstices between sand grains contribute to the sedimentation process?
In the context of filtration, how do the interstices between sand grains contribute to the sedimentation process?
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What is the filter cake described in filtration?
What is the filter cake described in filtration?
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What role does the gelatinous coating formed on sand grains play in filtration?
What role does the gelatinous coating formed on sand grains play in filtration?
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What is the primary function of biological metabolism during water filtration?
What is the primary function of biological metabolism during water filtration?
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What is the 'Schmutzdecke'?
What is the 'Schmutzdecke'?
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Why is the ‘Schmutzdecke’ important in filtration?
Why is the ‘Schmutzdecke’ important in filtration?
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Besides breaking down organic impurities, what other crucial role do bacteria play in the filter?
Besides breaking down organic impurities, what other crucial role do bacteria play in the filter?
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What happens during the continuous voids of filter media operation?
What happens during the continuous voids of filter media operation?
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What is the primary mechanism by which filtration removes impurities from water based on the provided text?
What is the primary mechanism by which filtration removes impurities from water based on the provided text?
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What material is NOT explicitly mentioned as a commonly used filter material in the text provided?
What material is NOT explicitly mentioned as a commonly used filter material in the text provided?
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What action is required to restore the filtering capacity of sand filters after extended use?
What action is required to restore the filtering capacity of sand filters after extended use?
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What is a key characteristic that differentiates Slow Sand Filters from Rapid Sand Filters in terms of filtration rate?
What is a key characteristic that differentiates Slow Sand Filters from Rapid Sand Filters in terms of filtration rate?
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Which type of filter relies on surface scrapping for cleaning?
Which type of filter relies on surface scrapping for cleaning?
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Which of the following is a suitable situation for using a Slow Sand Filter?
Which of the following is a suitable situation for using a Slow Sand Filter?
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Based on the information provided, what contributes to classifying filters?
Based on the information provided, what contributes to classifying filters?
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Which of the following filter types can operate under pressure, as indicated in the text?
Which of the following filter types can operate under pressure, as indicated in the text?
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Study Notes
Water Quality Control - Filtration
- Filtration processes may not remove all particles in water, requiring prior treatment steps like sedimentation and coagulation.
- Fine flocs, suspended and colloidal particles, color, dissolved minerals, microorganisms, and bacteria require removal before public supply.
- Filtration passes water through layers of porous media, often sand, to remove remaining impurities.
Introduction to Filtration
- Filtration is a solid-liquid separation process.
- It involves liquid passing through porous media to remove fine suspended solids.
- Water treatment plants use filtration as a polishing step to remove small flocs and particles not removed during settling.
Actual Filtration and Backwashing
- Filtration applies water containing suspended matter to the top of a filter bed.
- As the water passes through porous medium, suspended matter is removed through various mechanisms.
- Backwashing is a cleaning process to clear the filter medium and prepare it for reuse.
Theory of Filtration
- The filter medium retains solid particles and permits fluid to pass through (the filtrate).
- The rate of filtration (dV/dt) is the volume of filtrate collected per unit time.
- During filtration, solid particles accumulate on the filter medium forming a filter cake.
- As the filter cake thickens, resistance to filtrate flow increases, and the filtration rate decreases.
- Filtration can be operated at a constant rate or a constant pressure.
- Various mechanisms remove particles during filtration.
Mechanisms of Filtration
- Mechanical straining: particles larger than filter void size are removed.
- Sedimentation: particles settle in the interstices of the filter media, acting as a miniature sedimentation tank.
- Adsorption: particles adhere to filter media.
- Impaction: particles collide with the filter media and become trapped.
- Interception: particles follow a path through the pores, and get trapped as the fluid flow encounters the filter media.
- Adhesion: particles attach to each other and stick to the media.
- Biological metabolism: organic matter is metabolized into harmless substances by microorganisms that colonize in the filter.
- Electrolytic changes: charges of suspended matter neutralize and change the chemical properties of water.
Efficiency of Slow Sand Filters (SSF)
- High efficiency in removing bacterial load (98-99.9%).
- Removes turbidity up to 50 NTUs.
- Less efficient in color removal (around 20-25%).
- Not effective against colloidal matter, but can remove associated odors and tastes.
Efficiency of Rapid Sand Filters (RSF)
- High bacterial removal efficiency (80–90%).
- Effective in removing turbidity (up to 35–40 NTU).
- Effective in color removal (can remove up to 3 on a cobalt scale).
- Effective in removing colloidal matter.
Types of Filters
- Slow Sand Filters (SSF): use gravity and are suitable when land and labor are readily available and cost-effective.
- Rapid Sand Filters (RSF): use pressure and are suitable for areas with limited land.
- Pressure Filters (PF): use a pressure vessel to increase filtration rate, but not as easy to maintain.
- Various filter materials like sand, anthracite, garnet, crushed glass, rice husk, and gravel are used.
Components of SSF
- Enclosure tank: a watertight basin with a sloped floor.
- Filter media: layers of graded sand (different grain sizes).
- Base material: gravel layer supporting the filter media.
- Under drainage system: collects filtered water and distributes wash water.
- Appurtenances: supporting structures like wash water troughs, air compressors, and rate control devices.
Components of RSF
- Enclosure tank: a watertight basin with a sloped floor.
- Filter media: layers of graded sand (different grain sizes).
- Base material: gravel layer supporting the filter media.
- Under drainage system: collects filtered water and distributes wash water.
- Appurtenances: supporting structures like wash water troughs, air compressors, and rate control devices.
Working of SSF
- A constant water depth is maintained in the enclosure tank.
- Water is allowed to flow through the filter media from sedimentation tank.
- Treated water is collected through the under drainage system.
Working of RSF
- Coagulated raw water is fed directly to RSF without mixing, flocculation, or sedimentation.
- Alum or chemicals are kept in pressure containers to be added to water.
- Similar to SSF, water moves through filters and collected in under drainage system.
Cleaning of SSF
- Water in the tank is drained off.
- Scrabbed, washed, and dried top layer.
- Refill with filtered water from below.
- Wash water is used to prevent air entrapment.
Cleaning of RSF
- Cleaning is based on head loss across the filter.
- Washing water is used to remove accumulated particles at the surface and inside the filter.
- Air is used when necessary to remove accumulated particles.
Filter Problems
- Air binding: water is unable to pass through filter because of gas bubbles.
- Mud ball formation: clogging and formation of large clumps.
- Sand Incrustation: accumulation of sticky material in the filter.
- Jetting and sand boils: water flows through points of least resistance.
- Sand leakage: smallest gravels get shifted during backwashing.
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Description
Test your knowledge on the principles of filtration in water treatment plants. This quiz covers key concepts such as filtering media, the backwashing process, and the role of biological metabolism in filtration. Dive into the mechanisms behind effective water purification.