Deals with primary sedimentation tanks for the primary treatment of sewage. settling column test, settling profile graph construction and use of the settling profile graph for the design of primary sedimentation tank. both circular and rectangular settling tanks are described here.

1. Primary Sedimentation
Dr. Akepati S. Reddy
School of Energy and Environment
Thapar University, Patiala
PUNJAB, INDIA

2. Flocculent Particle Settling
• Particles do not settle as discrete particles – instead during
sedimentation they coalesce and flocculate
• Extent of coalescence or flocculation depend on
– Overflow rate
– Depth of settling basin
– Velocity gradients in the system
– Concentration of particles
– Range of particle sizes
• Settling characteristics of a suspension of flocculent particles
can be obtained by a settling column test
• Settling column
– Height: equal to the depth of the proposed settling tank
– Diameter can be anything – influences of the wall should be
taken care off – sample required should be small enough
– Provided with sampling ports

3. • Settling column test
– To begin with ensure uniform distribution of particle size from
top to bottom of the column
– Duration of test should be equal to the settling time in the
proposed tank
– Draw samples from the ports at various time intervals, analyze
for TSS and compute % removal
– Plot % removal against time and depth and construct curves of
equal % removal
– Overflow rates for various settling are determined by noting
where the curve intersect the x-axis
Flocculent Particle Settling





 





 
 


2
(%) 1
1
nn
n
h
n RR
H
h
R

4. Settling Column Settling Profile Graph

6. Primary Sedimentation Tank
• Purpose is removal of readily settleable solids and floating
material from wastewater
• Usually give 50-70% suspended solids removal efficiency and
25-40% BOD removal (for municipal sewage)
• Rectangular or circular sedimentation tanks are used
– Two or more cylindrical or rectangular sedimentation tanks may
be used for allowing maintenance and repair
– Multiple rectangular tanks require less land area than multiple
circular tanks and common walls reduce construction cost
• Sedimentation tanks should include
– Inlet section and Outlet section
– Facilities and provisions for the settled sludge scrapping,
collection and removal
– Facilities and provisions for the floating material entrapment,
skimming, collection and removal
• Sedimentation tank can be considered to include Settling
zone and Sludge zone

7. Flow pattern is radial with central feed arrangement
• Has a center well of 15-20% of the tank diameter and 1.0 to 2.5 m
depth is used
• Center well has a tangential energy dissipating inlet
• wastewater is discharged into the well within the upper 0.5-0.7 m
at 0.3 to 0.45 m/sec flow velocity
Solids removal mechanisms
• Smaller tanks have hopper bottom with >45 inclination with the
horizontal
• 3.6-9.0 m diameter tanks have solids removal equipment supported
on beams spanning the tank
• Larger diameter tanks have a central pier supporting the
mechanism and is reached by a walkway or bridge
• Bottom of the tank is sloped at 1 in 12 to 64 or lesser
• Solids are scrapped to the center and removed
Clarified liquid is skimmed off over weirs into a collection trough
Circular Primary Sedimentation Tank

9. Clarified sewage
Collection trough
Scum retaining baffle
Distribution baffle
of central well
Scrapper arm
Skimmer bladeCentral well
Central pier
Influent pipe Sludge draw-off pipe
Central rotating
equipment
Sludge trough
Tangential openings in the pier
for the distribution of the influent
Slope 1 in 12
Side wall
of clarifier
0.5m
0.3m
0.3m
0.3m
0.6m Central pier Sludge trough
0.5 m
1.5 m
0.5 m
Sludge draw-off pipe
Influent pipe
Primary clarifier
Section to show side wall with
clarified sewage collection trough Bottom sludge trough and outlet
3.0 m

10. Inlet
• Full width inlet channels with inlet weirs – can introduce vertical
velocity component spreading into the sludge hoppers (provided at
the inlet end)
• Inlets with submerged ports/orifices (velocities in the ports/orifices
are in the range of 3-9 m/min.)
• Inlet channels with wide gates and slotted baffles
• Inlet baffles can be used to reduce high initial velocities and
distribute flow width wise – usually extend from 150 mm below and
300 mm above the water surface
Outlet
• Clarified effluent is collected into a collection trough through an
overflow weir
• Floating material or settled suspended solids should not be washed
out with the clarified effluent
• Floating material retaining or entrapping baffle is provided
• Weir length is designed to avoid washout out of sludge
Rectangular
Primary Sedimentation Tanks

11. Facilities and provisions for the settled sludge scrapping,
collection and removal
• Settled sludge is collected at the inlet end
• Chain and flight solids collectors
– Have a pair of endless conveyor chains
– Scraper flights of wood or fiberglass, running full width of the
tank and attached to the chains at 3 m interval
– The settled solids are scrapped
• to solids hoppers In case of small tanks – multiple hoppers are
used in larger tanks
• to a transverse trough in case of large tanks - troughs in turn have
cross collectors
Rectangular
Primary Sedimentation Tank

12. Traveling bridge type collectors
– Travel up and down the tank on rubber wheels or on rails
supported on the sidewalls
– Scrapper blades are suspended from the bridge (returning
scraper blades lifted free of the solids blanket)
Facilities and provisions for floating material entrapment,
skimming, collection and removal
• Floating scum is collected at the effluent end
• Floating scum is skimmed and carried to the effluent end and
entrapped for avoiding overflow into the clarified effluent trough
– Flights returning at the liquid surface can be used
– Water showers can be used to push forward the floating scum
– Baffle is provided in front of the overflow weir for the scum
entrapment until removed
Rectangular
Primary Sedimentation Tank

13. • In small tanks scum draw-down facility consisting of a
horizontal, slotted pipe that can be rotated by a lever or screw
can be used
– Limitation - creates large volume of scum liquor
• Transverse rotating helical wiper attached to a shaft can also
be used for scum removal
– Scum is removed from the water surface and moved over an
inclined apron for discharge to a cross-collecting scum trough
Other provisions and facilities
• Tanks may sloping bottom towards the influent end
• Influent end has sludge hoppers or sludge trench
• Scum pit is provided for storing the scum liquid removed
• Sludge pump, piping and tank may be there to remove the collected
sludge from the sludge hopper or trench
Rectangular
Primary Sedimentation Tank

15. Design of Primary Clarifier
• Overflow rate and hydraulic retention time obtained from the
settling test and settling profile diagram are used to find
surface area and depth of the clarifier
• Safety factors of 1.25 is usually used to take into account
– Inlet and outlet disturbances
– Disturbances created by the rotating mechanism to scrap the
settled sludge and to collect the floating scum
– Wind blow effects
– Temperature variations
– Flow short-circuiting
• Design also takes into account the following:
– Horizontal flow velocity (below the scour velocity)
– Weir overflow rate (below a limit to avoid density currents)
– Weir overflow balancing to avoid short-circuiting problems
– Flooding of collection trough

16. Detention time 1.5 to 2.5 hours (2.0 hr.)
Overflow rate
Average flow 30-50 m3/m2.day (40) – 24-32 (26)
Peak flow 80-120 m3/m2.day (100) – 48-70 (60)
Weir loading 125-500 m3/m.day (250)
Rectangular tanks
Depth 3-4.9 (4.3)
Length 15-90 (24-40)
Width 3-24 (4.9-9.8)
Flight speed 0.6-1.2 m/min (0.9)
Circular tank
Diameter 3-60 (12-45)
Bottom slope 1 in 16 to 1 in 6 (1 in 12)
Flight speed 0.02-0.05 rpm (0.03)
Primary Sedimentation Tank

17. Short circuiting and hydraulic stability
• Tracer studies can be used for determining short-circuiting
problems
• Method of influent flow distribution can also affect short circuiting
Temperature difference of 1C between incoming wastewater
and wastewater of the tank can cause density current
Wind blowing across the top of an open sedimentation tank can
cause circulation cell to form – this reduces effective
volumetric capacity of the tank
Surface loading rates (overflow rates)
• There can be regulations prescribing limits to surface loading rates
• Overflow rates must be set low enough to ensure satisfactory
performance at peak rates of flow
Primary Sedimentation Tank

18. Detention time
• Solids reaching the settling tank are susceptible to flocculation
• Flocculation is aided by eddying motion of fluid within the tank
• Level of flocculation depends on the time elapsed (detention time)
• In cold climates because of increase in water viscosity detention
time required also increases (1.38 time more HRT for 10C water
that when temperature is 20C)
Weir loading rates
• Have little effect on efficiency of primary settlign tanks
Primary sludge scum characteristics and quantities
• Factors influencing are
– Characteristics of untreated water (strength and freshness)
– Period of sedimentation
– Conditions of the deposited solids
– Period between solids removal operations
Primary Sedimentation Tank

19. Primary Sludge
• Primary clarifier is designed for a specified % removal of TSS
• From knowing the TSS and flow rate of the influent sewage
primary sludge generated can be quantified
• TSS of the sewage is comprised of inorganic TSS, VSS and
Biodegradable VSS
– Biodegradable VSS contributes BOD and this is 50-60% of the
total BOD for municipal sewage
• Primary clarifier through removing the Biodegradable VSS
contributes to BOD removal
– BOD removal in the primary clarifier is typically 35-45%
• If BOD removal in the primary clarifier is known, one can find
out the biodegradable fraction of the primary sludge
– BOD removal can be expressed as BODu removal and BODu
removal as TSS equivalent removal
– The fraction of biodegradable VSS in the TSS is assumed to
remain the same even in the clarified effluent
• If consistency of the primary sludge is known even volume of
the primary sludge can be calculated