Software and Systems Engineering Standards: Verification and Validation of Sy...
sedimentation
1.
2. Sedimentation
• Water contains variety of particles- mineral as well as
organic.
• Some particles may be colloidal, very fine or coarse.
• Also, the sp. gravity of the particles may range from
1.01 to 2.65.
• This indicates that different particles requires different
time for their settlement.
• Sedimentation is the process of removal of suspended
particles of impurities by gravitational settling, it occurs
when particles are heavier than water.
3. Principle of Sedimentation
• The very fundamental principle under lying the process of
sedimentation is that, the suspended matter present in water, is
having specific gravity greater than that of water.
• In still water these particles will therefore tend to settle down by
gravity, where as in a flowing water, they kept in suspension
because of turbulence.
• Hence as soon as turbulence is retarded by offering storage to the
water or reducing the flow velocity, the impurities tend to settle at
the bottom of the tank. This the principle of sedimentation.
4. • The basin in which the flow of water is retarded is called as
settling tank or sedimentation tank.
• Detention Time: The theoretical average time for which the
water is detained in the tank is called the detention period.
OR detention time of sedimentation tank may be defined as
the average theoretical time required for the water to flow
through the tank.
• Purpose of Settling:
• To remove coarse dispersed phase.
• To remove coagulated and flocculated impurities.
• To remove precipitated impurities after chemical treatment.
• To settle the sludge (biomass) after activated sludge process
/ tricking filters
5. Types of sedimentation
•Plain Sedimentation: When impurities are separated from
water by action of natural forces alone i.e. gravitational
and natural aggregation of settling particles, the process is
called plain sedimentation. Suspended particles of size
0.01mm and above and having sp. gr. greater than 1 are
removed by plain sedimentation.
•Sedimentation with coagulation: When chemicals or
other substances are added to induce or hasten aggregation
and settling of finely divided suspended matter, colloidal
substances and large molecules, the process is called
sedimentation with coagulation or simply clarification.
6. Factors Affecting Sedimentation
• Shape, Size, Density and Nature of particles
• Viscosity, density and temperature of
water/wastewater
• Surface overflow rate
• Velocity of flow
• Inlet and outlet arrangements
• Detention period
• Effective depth of settling zone
7. TYPES OF SEDIMENTATION
Type -I: Discrete particle settling
• No interaction between particles
• Settling velocity is constant for individual particles
• Dilute solid’s concentration
• Examples: pre-sedimentation in water treatment, grit
removal in wastewater.
Type –II: Flocculent settling
• Particles collide and adhere to each other resulting in
particle growth
• Dilute solid’s concentration
• Examples: coagulation/flocculation settling in water
treatment and primary sedimentation in wastewater
treatment
8. Type III: Hindered or Zone Settling
• Particles are so close together movement is restricted
• Intermediate solids concentration
• Solids move as a block rather than individual particles
• Fluidic interference causes a reduction in settling
velocity
• Distinguishable solids liquid interface
• Example: settling of secondary effluents
Type IV: Compression Settling
• Particles physically in contact
• Water is squeezed out of interstitial spaces
• Volume of solids may decrease
• High concentration of solids (sludges)
9.
10. • Types of Sedimentation Tanks
Depending upon various factors sedimentation tanks
are classified as follows.
1. Based on methods of operation
a. Fill and draw type tank
b. Continuous flow type tank
2. Based on shape
a. Circular tank
b. Rectangular tank
c. Hopper bottom tank
3. Based on location
a. Primary tank
b. Secondary tank
11. • Fill and Draw Type of
Sedimentation Tank:
• In case of fill and draw type
sedimentation tank, water is stored
for some time in the tank after it is
filled completely. The time may be
24 hours. In that time, the
suspended particles are settled at
the bottom of the tank. After 24
hours, the water is discharged
through outlet.
• Then settled particle are removed.
This removal action requires 6-12
hours. So, one complete action of
sedimentation requires 30-40
hours in case of fill and draw type
sedimentation tank
12. • Continuous Flow Type
Sedimentation Tank
• In this type of thank, water is
not allowed to rest.
• The flow always takes place
but with a very small velocity.
• During this flow, suspended
particles are settle at the
bottom of the tank.
• The flow may be either in
horizontal direction or vertical
direction.
13. • Circular Tank
• Circular sedimentation tanks are
mostly preferred for continuous
flow type sedimentation.
• In this case influent is sent
through central pipe of the tank
and radial flow takes place.
• Mechanical sludge scrappers are
provided to collect the sludge
and collected sludge is carried
through sludge pipe provided at
the bottom. But circular tanks
are uneconomical as compared
to rectangular tanks but they
have high clarification
efficiency.
14. • Hopper Bottom Tank
• In case of hopper bottom
tank, a deflector box is
located at the top which
deflects the influent coming
from central pipe to
downwards. Sludge is
collected at the bottom and it
is disposed through sludge
pump.
20. Vertical cross-section through a rectangular sedimentation
tank
H = depth of settling zone L = length of settling zone
B = width of settling zone V = volume of settling zone
Q = volumetric flow rate v = flow speed/velocity
T = detention time = hydraulic retention time
21. • Inlet Zone: The inlet distributes flow uniformly across the inlet to
the tank. The normal design includes baffles that gently spread the
flow across the total inlet of the tank and prevent short circuiting in
the tank.
• Settling Zone: The settling zone is the largest portion of the
sedimentation basin. This zone provides the calm area necessary for
the suspended particles to settle.
• Sludge Zone: The sludge zone, located at the bottom of the tank,
provides a storage area for the sludge before it is removed for
additional treatment or disposal. Sludge is removed for further
treatment from the sludge zone by scraper or vacuum devices which
move along the bottom.
• Outlet Zone: Outlet zone (or launder) should provide a smooth
transition from the sedimentation zone to the outlet from the tank. This
area of the tank also controls the depth of water in the basin. Weirs set
at the end of the tank control the overflow rate and prevent the solids
from rising to the weirs and leaving the tank before they settle out.
22. • Design Details
• Detention period: for plain sedimentation: 3 to 4 h, and for
coagulated sedimentation: 2 to 2.5 h.
• Velocity of flow: Not greater than 30 cm/min (horizontal flow).
• Tank dimensions: L:B = 3 to 5:1. Generally L= 30 m (common)
maximum 100 m. Breadth= 6 m to 10 m. Circular: Diameter
not greater than 60 m. generally 20 to 40 m.
• Depth 2.5 to 5.0 m (3 m).
• Surface Overflow Rate: For plain sedimentation 12000 to
18000 L/d/m2 tank area; for thoroughly flocculated water
24000 to 30000 L/d/m2 tank area.
• Slopes: Rectangular 1% towards inlet and circular 8%
23. Tube Settler
• Tube settlers are commonly used in rectangular
clarifiers to increase the settling capacity by reducing
the vertical distance a suspended particle must travel.
• High efficiency tube settlers use a stack of parallel
tubes, rectangles or flat pieces separated by a few
inches (several centimeters) and sloping upwards in the
direction of flow.
• This structure creates a large number of narrow parallel
flow pathways encouraging uniform laminar flow as
modeled by Stokes' law.
24. • Tube settler systems are an inexpensive solution for
drinking water and wastewater plants to increase treatment
capacity, reduce new installation footprints, improve
effluent water quality, and decrease operating costs.
• Constructed of lightweight PVC.
• tube settler modules can be easily supported with minimal
structures that often incorporate effluent troughs and
baffles. Modules are available in a variety of sizes to fit any
tank geometry and tube lengths to accommodate a wide
range of flows.
• Tube settler modules consist of a series of sloped, tubular
channels and structural wave sheets that reduce particle
settling distance and increase the effective settling area in a
clarifier or sedimentation basin.
25.
26.
27. • Very cost economical since it uses 1/10th of space required by conventional
clarifiers.
• No moving parts, operates totally on Gravity.
• No maintenance list.
• Laminar flow ensures excellent treated water quality.
• Wide choice of materials of construction such as steel, concrete/FRP.
• Non-corrosive tubes can be lifted easily for cleaning without shutdown.
• Flexible configurations
• Required Very Less Space
• Easy installation
• No Power Consumption
• Simple in Operation
• Easily Mobility
Advantages of Tube Settlers
