34 Questions
What is the primary purpose of filtration in water treatment?
To remove suspended particles and bacteria
Which mechanism of filtration involves particles moving randomly due to thermal gradients?
Brownian diffusion
What is the term used to measure and express the size of sand filter media?
Effective size
Which type of particles experience interception when passing through granular filter media?
Large particles
In filtration, what is the force responsible for causing sedimentation of particles?
Gravity
For which type of particles does Brownian diffusion play an important role in the filtration process?
< 1 micron particles
What is the method used to clean SSF?
Scraping and removing the top layer
What is the initial loss of head for RSF cleaning?
0.3m
Which equation is used to calculate head loss through a porous medium?
Carman-Kozeny equation
What is the porosity factor 'n' in the Carman-Kozeny equation?
0.73
What can happen if filter runs are too long?
Media packs down making it difficult to expand during backwash
What kind of water is generally used for the backwash cycle?
Treated water from storage
What is the condition that indicates a filter needs to be backwashed?
'Floc' starts breaking through into the effluent
'Air Binding' in filters occurs due to:
Frictional resistance exceeding static head of water
'Mud Balls' formation in filters is primarily due to:
'Mud' sinking down into the sand bed
'Cracking of Filters' occurs due to:
'Shrinkage Cracks'
What is the standard design practice for the spacing of holes in a Rapid Sand Filter?
6 mm holes at 7.5 cm c/c
What is the maximum loss of head in a Rapid Sand Filter according to the text?
2 to 5 m
In the design problem mentioned, what percentage of filtered water is allowed for backwashing?
0.5%
What is the velocity of flow in the manifold for washwater in a Rapid Sand Filter?
1.8 m/s
How many troughs are assumed to run lengthwise in the washwater gutter per bed?
3 troughs
What is the formula provided for calculating the discharge of each trough in the washwater gutter?
$(Q/3)$
What is the clear water reservoir's capacity required for a 4-hour filter capacity based on the text?
1725 m3
What is the slope mentioned for the bottom of a Rapid Sand Filter?
$1:40$
What is the formula provided in the text for calculating the total area of holes in an underdrainage system?
$(0.2 \times \text{bed area})$
What is the purpose of the uniformity coefficient in sand filters?
To express the size variations in sand particles
What is the main difference between slow sand filters and rapid sand filters regarding base material?
The size of the base material
How are slow sand filters cleaned compared to rapid sand filters?
By scraping away the top layer of sand
What is Anthrafilt and how does it differ from sand as a filter media?
Made from anthracite coal-stone, cheaper and smokeless when burnt
Which process contributes to water purification mainly during the first few days in a slow sand filter?
Mechanical and physical-chemical processes
What is the function of 'Schmutzdecke' in a slow sand filter?
To act as a living quarters for micro-organisms that break down organic material
In which part of the slow sand filter are most impurities, including bacteria and viruses, removed from raw water?
Filter skin and organic layer
Which statement accurately describes the rate of filtration between slow sand filters and rapid sand filters?
Slow sand filters have a smaller rate of filtration than rapid sand filters
What is required to obtain almost pure water from slow sand filters?
Slight disinfection of water to make it safe.
Study Notes
Sand Filters
- D10 is defined as the size of the sieve in mm through which 10% of the sample of sand by weight will pass.
- Uniformity coefficient (D60/D10) measures the uniformity in size or degree of variations in sizes of particles.
Types of Filters
- Slow Sand Filter: consists of fine sand, supported by gravel, captures particles near the surface of the bed, and is usually cleaned by scraping away the top layer of sand.
- Rapid-Sand Filter: consists of larger sand grains supported by gravel, captures particles throughout the bed, and is cleaned by backwashing water through the bed.
- Multimedia Filter: consists of two or more layers of different granular materials, with different densities, providing a more versatile collection of particles.
Principles of Slow Sand Filtration
- Impurities in water are removed by a combination of processes: sedimentation, straining, adsorption, and chemical and bacteriological action.
- Water is purified mainly by mechanical and physical-chemical processes during the first few days.
- A thin layer of sediment and organic matter forms on the sand surface, which remains permeable and retains particles even smaller than the spaces between the sand grains.
- Most impurities, including bacteria and viruses, are removed from the raw water as it passes through the filter skin and the layer of filter bed sand just below.
Sand Filters vs. Rapid Sand Filters
- Base material: SSF (3-65 mm, 30-75 cm depth), RSF (3-40 mm, 60-90 cm depth)
- Filter sand: SSF (0.2-0.4 mm, 1.8-2.5 uniformity coefficient), RSF (0.35-0.55 mm, 1.2-1.8 uniformity coefficient)
- Rate of filtration: SSF (100-200 L/h/sq.m), RSF (3000-6000 L/h/sq.m)
- Flexibility: SSF (not flexible), RSF (flexible)
- Post-treatment required: SSF (almost pure water, occasional disinfection), RSF (disinfection required)
Filtration Mechanisms
- Sedimentation: due to force of gravity and the associated settling velocity of the particle
- Interception: for large particles, particles hit the media grain and are captured
- Brownian Diffusion: for very small particles, particles move randomly about within the fluid and are captured
- Inertia: attachment by inertia occurs when larger particles move fast enough to travel off their streamlines and bump into media grains
Filter Materials
- Sand: measured and expressed by the term called effective size
Backwashing of Rapid Sand Filter
- For a filter to operate efficiently, it must be cleaned before the next filter run
- Treated water from storage is used for the backwash cycle
- The filter backwash rate has to be great enough to expand and agitate the filter media and suspend the floc in the water for removal
When is Backwashing Needed
- When the head loss is so high that the filter no longer produces water at the desired rate
- When floc starts to break through the filter and the turbidity in the filter effluent increases
- When a filter run reaches a given hour of operation
Operational Troubles in Rapid Gravity Filters
- Air Binding: when the frictional resistance offered by the filter media exceeds the static head of water above the and bed, causing the formation of bubbles that stick to the sand grains
- Formation of Mud Balls: when the mud from the atmosphere accumulates on the sand surface and sinks down into the sand bed, forming mud balls
- Cracking of Filters: when the fine sand contained in the top layers of the filter bed shrinks, causing the development of shrinkage cracks in the sand bed
Test your knowledge on the process of water filtration and the mechanisms involved in removing impurities through sedimentation and granular material beds. Learn about how filters work to make water cleaner and safer to drink.
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