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ENVE 301
Environmental Engineering Unit Operations
Chapter 11
Secondary Clarifiers
Assist. Prof. Bilge Alpaslan Kocamemi
Marmara University
Department of Environmental Engineering
Istanbul, Turkey
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Hindered (Zone) settling refers to suspensions of intermediate
concentration
(in which interparticle forces are sufficient to hinder the settling of
neighbouring particles)
The particles tend to remain in fixed positions with respect to
each other and mass of particles settles as a unit.
.
SECONDARY CLARIFIERS
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Because of the high concentration of particles :
The liquid tends to move up through the interstices of contacting
particles
As a result,the contacting particles tend to settle as a zone or
“blanket” maintaining the same relative position with respect to
each other.
As the particles settle, a relatively clear layer of water is produced
above the particles in the settling region.
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DESIGN CRITERIA
Overflow rate(surface loading)
Solids loading (Where X= MLSS conc. mg/l)
Weir loading (less critical then overflowrate)
. 4
A
Q
XA
QQ R
L
Q
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Sedimentation tanks can be divided into 4 different functional zones;
1. Inlet zone
2. Settling zone
3. Sludge zone
4. Outlet zone
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are designed to uniformly distrubute the influent suspension over the cross section of the settling zone. For Rectangular Basins full width inlet channels effective spreading of flow introduce a vertical
velocity component into sludge hopper that may resuspend sludge.
•should dissimate influent energy
•distribute the flow
•mitigate density currents
•minimize sludge blanket disturbance
Inlet Structures
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For circular tanks circular tanks radial flow to achieve a radial flow patterninfluent is introduced in the center of the tank or around the periphery of the tank Central feed water enters a circular well designed to distribute the flow equally in all directions
D of feed well = 15-20 % of tank diameter Depth= 1- 2.5m Velocity through the orificies on feed well 0.075- 0.15 m/sec
Inlet Structures (continue)
entrance pipe suspended from bridge OR encased in concrete beneath the tank floor
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For peripheral feed (not as uniform as central feed)
orifice channel around periphery of the tank
from the channel the flow discharges through the orifices into sedimentation tank
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Settling Zone It depends on the following desing parameters:
Settling characteristics of the suspended matter
Surface loading (over flowrate)
Width / length ratio OR diameter
Detention time
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Sludge Zone Rectangular tanks the bottom is slightly sloped to facilitate sludge scraping a pair of endless conveyor chains bridge – type mechanism continously pulls the settled material into a sludge hopper where it is pumped out periodically.
Motion of scraper momentarily resuspend lighter particles a few cm above the scraper blades
Excessive horizontal velocity ( for the case of rectangular basins) move these materials towards outlet zone. To prevent this, horizontal velocity - < 9 m/hr for light flocculant suspensions ≈ 36 m/hr for heavier discrete suspensions
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Bridge type mechanism travels up and down the tank one or more scraper blades are suspended from the bridge
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Circular tanks
The bottom of the tank is sloped to form an inverted cone and the sludge is
scraped to a relatively small hopper located near the center of the tank
Velocity or scraper important
Very high velocty resuspension of settled particles (<5mm/sn)
Travelling bridge with sludge suction headers and pumps not very good
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Outlet Zone weir channels are used Checked by weir loading (m3/m.day)
L
Q
Large weir loading resuspension of particles settled near to effluent launders
Effluent weirs placed as far from the inlet as possible
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to increase weir length (i.e to decrease weir loading) double-sided weirs can be used
Typical weirs 90o V notch metal plates bolted onto the effluent collection through
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May be placed at the opposite and of the rectangular basins
around the perimeter of center – feed circular tanks
at the center of peripheral feed circular tanks
if the weir loading causes the required weir length to be greater than tank width
the channel may be extended to a length of 1/3 the basin length (Reynolds)
through the entire width of tank
through the length of the tank
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Effluent Influent
Orifices
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Example:
Design circular secondary clarifiers for an extended aeration activated sludge
system receiving a flow of 250 000 m3/d average, 375 000 m3/d peak flow.
The activated sludge system has MLSS concentration of 4160 mg/l. Return
activated sludge recycle ratio is 1
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Operational problems in secondary clarifiers
1)Bulking sludge
2)Rising sludge
3)Nocardia foam
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Bulking sludge
MLSS with poor settling characteristics.
In a bulking sludge condition ,the MLSS floc does not compact or settle well,
and floc particles one discharged in the clarifier effluent.
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Filamenteous bulking
caused by the growth of filamenteous organisms
predominant form of bulking
low DO, low F/M, complete mixing conditions favor the growth of
filamentous organisms
Viscous bulking
caused by an excessive amount of extracellular biopolymer, which
produces a sludge with a slimy,jellylike consistency)
Usually found with nutrient limited systems or in a very high loading
condition with wastewater having a high amount of rbCOD
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Checklist in the control of bulking
1)Wastewater characteristics
The nature of components found in wastewateror the absence of
certain components, such as trace elements.
2) DO concentration
Limited DO oxygen
3)Process loading / Reactor configuration
Low F /M, complete mix systems with long SRT
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4) Internal plant overloading
Recycle loads
(centrate or filtrate from sludge dewatering, supernatant from
digesters)
5) Clarifier operation
Poor settling is a problem in center-feed circular tanks
Where sludge is removed from tank directly under the point where
the mixed liquor enters
Sludge may actually be retained in the tank many hours rather than
desired retention time cause localized septic condition
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Temporary control measures
Chlorination of return sludge
for low θ (5 to 10 hr) systems 0.002 -0.008
Good solution:
Use of selectors specific bioreactor
favors the growth of floc-forming bacteria
instead of filamenteous bacteria.
dMLSSkg
Clkg 2
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Selectors high F/M ratio
small tank (20-60 min contact time)
a series of tanks in which the incoming
wastewater is mixed with return sludge.
Selectors kinetics based (high F/M selector)
metabolic based selectors(anoxic anaerobic)
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Kinetics based selectors:
Filamenteous bacteria more efficient for substrate utilizaiton
at low substrate conc.
Floc- forming bacteria have higher growth rates at high
soluble substrate concentration
A series of reactors at relatively low θ values is used to provide
high soluble substrate conc.
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Metabolic based selectors:
Anoxic or anaerobic metabolic conditions
favor growth of floc-forming bacteria.
Filamenteous bacteria can not use NO2 , NO3 for an electron
acceptor
yielding a significant advantage to denitrifiying floc-
forming acteria
Filamenteous bacteria do not store polyphosphate
can not consume acetate in the anaerobic contact zone
giving an advantage for substrate uptake and growth to PAOs 65
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Rising Sludge
Sludge that has good settling characteristics will be observed to
rise or float to the surface after a relatively short settling period
The most common cause Denitrification
As N2 gas is formed in the sludge layer sludge mass becomes
buoyant and rises or floats to the surface.
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Rising sludge can be differentiated from bulking sludge by noting
the presence of small gas bubbles attached to the floating solids.
Common in short SRT systems where the temperature
encourages the inititation of nitrification and the mixed liquor is
very active due to low θc .
Rising Sludge (continue)
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Rising sludge problem may be overcome by :
Increasing the RAS withdrawal rate from the clarifier to reduce the
detention time of sludge in clarifier.
Increasing speed of the sludge collecting mechanism in settling
tank.
Decreasing the SRT to bring the active sludge out of nitrification
Pre-anoxic (Anoxic Aerobic) process is preferred for
denitrification
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Nocardia Foam
Two bacteria genera Nocardia
Microthix parcivella
are associated with extensive foaming in activated sludge process
These organisms have hydrophobic cell surfaces and attach to air
bubbles, where they stabilize the bubbles to cause foam
More pronounced with diffused air systems.
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Methods that can be used to control Nocardia foam :
avoiding trapping foam in the secondary treatment process
avoiding the recycle of skimmings into the secondary treatment
process.
using chlorine spray on the surface of the nocardia foam.
addition of small conc of cationic polymer.
reducing the oil and grease content from discharges to collection
systems from restaurant etc.
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