Clarification and Sedimentation Flashcards
Define sedimentation.
A unit operation that separates unstable suspended solids and destabilized colloidal solids from the liquid phase using the force of gravity.
How does sedimentation remove solids from water?
It removes solids by allowing heavier-than-water suspended solids (SS) to settle under gravity.
When is sedimentation not a necessary step in clarification?
Low turbidity water of less than 10 NTU
Name the four types of settling phenomena.
- discrete particle (free settling)
- flocculant
- hindered or zone settling
- compression
Describe how a particle settles in terms of the forces on the particle.
Particle settles if
F g > F buoyancy + F drag
What is discrete particle settling?
- Occurs when particles settle INDIVIDUALLY without interacting with neighbouring particles.
- Typically in suspensions with low solid concentrations.
- Particles do not flocculate.
Give an example of discrete particle settling.
Removal of grit and sand during headworks or preliminary treatment in wastewater treatment.
What is flocculant settling?
Occurs in dilute suspensions where particles coalesce or flocculate.
Which settling is faster- flocculant or discrete?
Flocculation causes the particles to increase in mass and hence settle at a faster rate.
What are the three processes in flocculation?
Brownian diffusion
Stirring
Differential settling
Where is flocculant settling commonly used?
Removes SS in primary sedimentation
Upper portions of secondary settling facilities
Remove chemical floc in settling tanks
What is hindered or zone settling?
Occurs in suspensions with intermediate concentrations, where inter-particle forces hinder the settling of neighboring particles, causing them to settle as a single mass.
(Interparticle forces make particles settle together)
What forms at the top of the settling mass in hindered /zone settling?
A solids-liquid interface develops at the top of the settling mass.
Where is hindered / zone settling commonly used?
In secondary settling tanks
(used in conjunction with biological treatment facilities).
What is compression settling?
Occurs at high particle concentrations where particles form a structure compressed by their own weight. and further settling happens only by compression of the structure due to the weight of particles added by sedimentation.
What causes compression in compression settling?
Compression is caused by the weight of particles that continuously settle and add to the structure
Why is compression settling important in sludge management?
Reduces sludge volume and improve handling.
Where does compression settling occur?
Occurs in the lower layers of deep sludge masses, such as the bottom of deep secondary settling tanks and in sludge thickening processes.
When is sedimentation used in potable water treatment?
Before filtration of surface water
Before filtration of coagulant-flocculant water
After lime and soda ash is added in softening water (industrial)
What is the purpose of primary sedimentation in wastewater treatment?
Removes settleable solids from raw wastewater
Achieves 60% SS and 30% BOD removal.
It reduces oxygen demand and operational problems in secondary treatment.
What is the role of secondary sedimentation in wastewater treatment?
Removes MLSS in the activated sludge process and HUMUS after trickling filters.
How are settled solids removed in sedimentation tanks?
Settled solids are collected by mechanically driven scrapers
Pushes sludge towards a hopper at the base of the tank
Then, pumped to sludge processing.
How is oil and grease removed in sedimentation tanks?
Oil and grease form a scum layer on the surface
Removed by skimming from the surface of the primary treatment tank.
Name three factors that affect sedimentation.
Particle characteristics
Water temp
Current
How do particle parameters affect sedimentation?
- SIZE and TYPE of particles (denser particles settling faster than colloidal particles).
- The MASS and VOLUME of the particles (Dense particles settle faster)
- Particle SHAPE (Round particles settle more easily compared to irregularly shaped ones)
- Net CHARGE (Particles with the same charge repel each other, hindering sedimentation)
How does water temperature influence sedimentation?
As the temperature of water decreases, the rate of particle settling slows down.
To compensate, increase detention time and adjust coagulant dosages.
What role do currents play in sedimentation?
Eddy currents, created by water flowing in and out, promote flocculation.
However, may distribute flocs unevenly, leading to inconsistent settling.
What affects density of current?
- Weight of solids in tank.
- Concentration of solids
- Temp of water
How do water quality parameters and treatment requirements inform decision-making in sedimentation unit choices?
Evaluating factors (particle size, water temperature, and currents) helps in:
choosing the appropriate sedimentation unit and configuring the treatment process.
Give the overflow rate equation for sedimentation design.
vO = Q / Ap
overflow rate = flow rate / SA
= to settling velocity of smallest particle removed in tank
~ 20 m3/m2 d
Give detention time equation.
tR = V / Q
What is the % removal of particles?
(X0 - Xt) * 100
In reality, why do particles not settle uniformly?
Particles smaller than assumed size:
- will not settle out in time
- may settle if enter tank at low height
What are settling column tests used for in the context of industrial wastewater?
Estimate design parameters for industrial wastewater treatment and evaluate the settling characteristics of solids in the water.
What scale-up factors are used when converting lab results to prototype design for settling column tests?
For velocity of operation (VO): Scale-up factor of 1.25 to 1.75.
For detention time: Scale-up factor of 1.5 to 2.0.
How are settling characteristics evaluated in settling column tests?
Evaluated by using a sampling port to study discrete settling.
SS are well-mixed, then allowed to settle, with samples taken at timed intervals.
How are removals determined from settling column test data?
Plot depth vs. time graph
Removal isolines plotted
Read the change in depth (∆ depth) between removal isolines to find fraction of solid removed at given detention time.
What are the advantages of using a rectangular settling basin?
- Hydraulically stable and easy flow control for large volumes.
- Simple design that allows horizontal water flow from one end to the other end, ensuring uniform flow.
- Better hydraulic characteristics in long, narrow tanks due to reduced short-circuiting.
What is the typical length-to-width ratio of a rectangular settling basin?
Length is 3 times the width
(Between 2-4)
How is sludge managed in a rectangular settling basin?
Bottom of the tank is slightly sloped
slow-moving mechanical chain-drive sludge scraper continuously pulls settled material into a sludge hopper
Hopper pumped out periodically
How is grease and oil removed in a rectangular settling basin?
A skimmer is used to remove grease and oil,
Grease and oil float to the surface, by raising and collecting these substances.
What is the purpose of baffles in a settling tank?
Reduce vortices and rotational flow
Prevents short-circuiting of the tank and avoid the creation of dead zones.
Flow control devices to ensure the water follows the intended path through the basin.
Define short-circuiting.
When water in a settling tank takes a faster route to the outlet, bypassing the main flow path.
Reduces detention time for the particles to settle properly, reducing the efficiency of the settling process.
It can happen when water flows across the top or along the bottom of the tank, instead of moving slowly through the entire tank.
How do inlet and outlet baffles affect flow in a settling tank?
Minimising turbulence
Ensuring more uniform flow.
How can baffles increase hydraulic retention time?
Promotes more uniform and controlled water flow, allowing particles more time to settle.
How does the flow regime in a circular settling basin differ from that in a rectangular basin?
Circular: flow enters at centre and is baffled to flow radially towards the perimeter. (Centre to outer)
Rectangular: water flows horizontally from one end to the other.
What is the effect of the radial flow in a circular settling basin?
As flow moves from centre to perimeter, horizontal velocity of water decreases.
Particle path is parabolic
More detention time and enhanced settling
What are the advantages of circular settling basins over rectangular ones in terms of maintenance and cost?
Circular
- simpler sludge removal mechanisms
- less maintenance
- lower capital costs compared to rectangular tanks.
What is the importance of overflow rate in circular settling basins?
- critical for tank size selection
- loading rate depends on the effluent characteristics and the solids content
(determined from the settling column tests)
How does flow work in a solids contact unit?
Flow in upward direction
through a sludge blanket or slurry of flocculated suspended solids
allowing particles to settle as the water moves upward.
What are two typical uses of solids contact units?
- Lime-soda ash process for settling out floc formed during water softening.
- Cold climates where sedimentation must occur indoors to avoid freezing.
Are settling column tests necessary for designing primary (1º) settling tanks for municipal or domestic sewage?
No
How is the design of a settling tank determined?
Selecting appropriate overflow rate (surface loading)
Primary or secondary settling tank
Sludge return included?, and
Determine required tank area.
Determine detention time using the average depth.
Is there a limit to detention time in settling tanks?
Avoid detention time over 2.5 hours, especially in warm climates,
because induce anaerobic conditions undesirable for the treatment process.
Best residence time for rectangular tank basin?
2 hours
Name the four basin zones.
Inlet
Settling
Outlet
Sludge
What are the primary purposes of the inlet zone in a sedimentation basin?
Distribute water evenly.
To control the incoming water flow’s velocity.
To prevent turbulence in the water.
Why must the incoming flow be evenly distributed in a sedimentation basin?
Prevent short-circuiting, ensuring the water moves through the tank properly and allows for effective settling.
What happens if the water velocity exceeds 0.5 ft/sec (0.15 m/s) in the inlet zone?
Floc in the water will break up due to agitation.
This reduces the efficiency of settling.
What is the function of a stilling wall in the inlet zone?
Baffle wall with evenly spaced holes that spans the entire basin from top to bottom and side to side.
Promoting uniform flow and minimises turbulence of water entering settling zone
What happens to the water when it enters the settling zone of a sedimentation basin?
Water velocity is greatly reduced, allowing for settling to occur.
What occurs in the settling zone of a sedimentation basin?
Bulk of floc settling
Makes up the largest volume of the sedimentation basin.
What is required for optimal performance in the settling zone?
Slow, even flow of water to allow sufficient time for particles to settle.
What is a simple design suggestion for ensuring optimal performance in the settling zone?
A large expanse of open water to allow for slow, even flow and efficient settling
What are lamella plates and how are they used in the settling zone?
Slanted plates in the settling zone where water flows along them.
The floc settles on the plates and drifts down to the bottom of the tank, while clarified water passes between the plates and exits the basin.
What are the benefits of using lamella plates in a settling basin?
- Increased settling efficiency and speed.
- Increases surface area where settling can occur.
- Useful when site area is limited or when increasing the capacity of shallow basins.
What is the purpose of the outlet zone in a sedimentation basin?
Controls the flow of water leaving the basin
Ensures the water has minimal floc suspended in it.
How is the best quality water obtained in a sedimentation basin?
Best quality water found at top becuse most particles settle to the bottom.
Water is skimmed from top of basin.
What is the function of a baffle in the outlet zone?
A baffle is placed in front of the outlet to stop floating material from escaping the basin and clogging filters.
What are the main components of the outlet zone?
Launder – A TROUGH that collects and directs water to the next stage.
Weirs – WALL with holes or slits that CONTROL WATER flow into the launder.
Effluent piping – Pipes that carry water to the next treatment process.
How do weirs improve water flow in the outlet zone?
Weirs skim water evenly off the surface and prevent uncontrolled flow into the launder.
Where is the effluent structure located in different sedimentation basin designs?
Rectangular tanks – At the end of the basin.
Circular tanks – Around the edges of the clarifier.
What is the sludge zone in a sedimentation basin?
Bottom of the basin, where sludge collects temporarily before being removed.
Why should the velocity in the sludge zone be very slow?
To prevent re-suspension of sludge
How is sludge removed from the sedimentation basin?
Through a drain at the bottom of the tank, which may be automated or manually cleaned.
How does the tank design help with sludge removal?
Tank bottom slopes toward the drains, making it easier for sludge to flow to the removal points.
When is the best time to clean the sedimentation basin?
When water demand is low, typically in April and October.
Why do many plants have at least two sedimentation basins?
One basin can remain in operation while the other is being cleaned, maintained, or inspected.
To facilitate for high treatment demands during heavy rainfall (wastewater treatment)
What problems can occur if sludge is not properly removed?
Reduced sedimentation efficiency due to a smaller usable volume.
Sludge may decay anaerobically, causing septic conditions.
Septic sludge can cause taste and odor problems or float to the top as scum.
Re-suspension of sludge may carry over into the next treatment stage.
What is flotation?
Another method of clarification
Uses gas bubbles to attach to SS, increasing buoyancy so they float instead of settling.
When is flotation used?
Flotation is better for particles that do not easily settle.
E.g. low density particles (oil, algae), hydrophobic, colloidal
Disadvantages of flotation?
High CAPEX and OPEX.
Mechanically complex, requiring more maintenance and expertise.
How does flotation work?
Floc particles are combined with air bubbles to increase buoyancy and make them float to the surface for removal.
