Deals with zone settling and hindered settling, secondary clarfiiers and design of secondary sedimentation tanks by limiting solids flux method and by Thalmadge and Fitch method.

1. Secodary Settling/Clarification

2. Hindered or Zone Settling
In systems with high SS, in addition to discrete and flocculent
settling, both hindered/zone settling and compression
settling also occur
– Liquid tends to move up through the interstices of the
contacting particles
– Contacting particles tend to settle as a zone or blanket
– Clear layer of water is produced above the blanket
– Rate of settling in the hindered settling zone is a function of
concentration of solids and their characteristics
– As settling continues, compressed layer of particles begin to
form on the bottom
– Above the compressed layer concentration of solids is
successively lower upto the interface

3. Approaches for design, and settling tests
Two design approaches to find out area required for the
settling and thickening facilities
• Data needed is obtained from the batch settling tests
• First approach uses data derived from one or more batch
settling tests
• Second approach (solids flux method) uses data obtained from
a series settling tests conducted at different solids concentration
• The methods are rarely used in the design of small treatment
plants

4. • In addition to discrete and flocculant settling, hindered/zone and
compression settling also occur in secondary clarifier
• Final overflow rate for a secondary clarifier is selected based on
the consideration of
– Area needed for clarification
– Area needed for thickening
– Rate of sludge withdrawal
• In Talmadge and Fitch method, Data from a single settling test
is used for the design of a secondary clarifier
• Both area required for thickening and area required for
clarification are found out and whichever is greater is
considered in the design of the secondary clarifier
• Area required for thickening is usually greater than the area
required for clarification
First Approach
(Talmadge and Fitch method)

5. • Area required for thickening
• Tu corresponds to Hu and obtained through
• Find critical concentration controlling the sludge handling
capability
– Draw tangents to the initial and final legs of the settling curve
– Bisect the angle of intersection of the tangents and extend the
bisector to settling curve to get Cc
• Find tu (time at which sludge concentration is Cu)
• Draw tangent through Cc
• Locate Hu on y-axis, extend horizontal line to the tangent through Cc
and draw vertical from the intersection to obtain Tu
o
u
t
H
Qt
A =
First Approach
(Talmadge and Fitch method)
u
oo
u
C
CH
H =
Co is initial concentration of TSS and Ho column height
Hu is sludge height for Cu TSS concentration

7. Area for clarification
Interface subsidence velocity
• Slope of the tangent on the initial leg of the settling curve
is taken as subsidence velocity
Clarification rate
• Taken as proportional to the liquid volume above the
critical sludge zone and computed as
First Approach
(Talmadge and Fitch method)
v
Q
A c
c =
Here Qc is clarification rate
V is interface subsidence velocity
o
c
c
H
H
QQ =
Here Hc is depth of critical sludge zone
Q is flow rate of mixed liquor into the clarifier

8. Solids flux method
• Area required for thickening the mixed liquor solids
applied depends on the limiting solids flux that can be
transported to the bottom of the settling tank
• Solids flux depends on the characteristics of the sludge
(column settling tests can be used to determine
relationship between sludge concentration and settling
rate and solids flux)
• Within the settling tank downward flux of solids is brought
about by gravity settling and by bulk transport occurring
due to sludge withdrawal from bottom
• Solids flux due to gravity
ugt SFSFSF +=
SFg is solids flux due to gravity
SFu is solids flux by bulk transport
iig VCSF = Ci is concentration of solids at the point in question
Vi is settling velocity of the solids at Ci concentration

9. • Solids flux by bulk transport
• Vi of sludge at different concentrations is obtained through
multiple settling tests at different concentrations
– Slope of the initial portion of the curve is taken as Vi
• Solids flux due to gravity (SFg)
– At low concentration (<1000 mg/l) settling velocity is high and
decreases with increasing concentration due to hindered settling
– Initially SFg is very small and first increases with increasing
concentration and then decreases
• Flux by bulk transport linearly increases with increasing
concentration (Ci)
• Total flux increases initially, then drops to limiting solids
flux and then increases with increasing withdrawal rate
Solids flux method
A
QC
UCSF ui
biu ==
Ub is bulk underflow velocity
Qu is underflow rate of sludge
A is cross sectional area of the sludge

10. Area for thickening
• Draw horizontal line tangent to the low point on the total
flux curve – point of intersection with the vertical axis
represents limiting solids flux (SFl, the flux that can be
processed in the settling basin)
– If solids loading is greater than limiting solids flux then solids
will build up in the settling basin and ultimately overflow
• Underflow sludge concentration (Cu) corresponding to
limiting solids flux is obtained by dropping vertical to x-axis
from the intersection of the horizontal line with the
underflow flux line
• Area required for thickening
• For a desired underflow concentration one can increase or
decrease the slope of the underflow flux line
Solids flux method
LSF
CQ
A 0
=
Q is mixed liquor flow rate
C0 is MLSS concentration
SFL is limiting solids flux

12. Alternative graphical method for SFL
• Uses only the gravity flux curve
• One should decide the underflow sludge concentration
• draw tangent to gravity flux curve through underflow sludge
concentration on X-axis and extend to the Y-axis
• The point of intersection on Y-axis gives SFL
Solids flux method

13. Settling and thickening characteristics of the mixed liquor
measured by either SVI or ZSV can be used as basis
SVI below 100 is desired and above 150 typically indicates
filamentous growth
Surface over flow rate for a secondary clarifier is related to
zone settling velocity as shown below
Here Vi is zone settling velocity (ZSV)
SF is safety factor and taken as 1.75 to 2.5
Design of Secondary clarifier on the basis
of SVI and ZSV
SF
V
rateoverflowSurface i
=

14. MLSS, ZSV and SVI/DSVI are related
Here x is MLSS concentration in g/l
DSVI and SVI in ml/g (dilution of sludge till settled sludge
volume is reduced to 250 ml after settling for 30 min.)
Fluctuations in wastewater and return sludge flow rates and MLSS
concentration should be considered in the design
– Safety factor used is meant for this purpose
Solids loading rate is a limiting parameter and affects effluent
concentration of TSS
– Effluent quality remains unaffected over a wide range of surface
overflow rates (upto 3-4 m/h)
xSVIVi )001586.01646.0(871.1)(ln +−=
xDSVIVi )002555.0103.0(028.2)(ln +−=
Design of Secondary clarifier on the basis
of SVI and ZSV

15. Secondary Sedimentation Tank
Much of the information presented for the design of primary
clarifier is applicable
Inadequate size of the settling tank and/or sludge pumping
capacity can increase the thickness of the sludge blanket
and sludge overflow can occur during peak flows
Mixed liquor has the tendency to show density current and
interfere with separation of solids and thickening of sludge
Tank types: Either circular or rectangular tanks (Oxidation
ditches use intra-channel clarifiers)
Circular tanks
• Diameter is 3 to 60 m (typical is 10 to 40 m) - radius
should preferably be < 5 times of the side wall water depth
• Use revolving mechanism to transport and remove sludge
from the bottom
– Sludge may either be plowed to a central hopper for removal
or it is removed directly from the tank bottom by suction
orifices
– Removal is either hydrostatically or by pumping

17. Rectangular tanks
Maximum length < 10 times the liquid depth (up to 90 meters
were also used)
Width should be <6m (if multiple sludge collection
mechanisms are used then up to 24 m is used)
Travel flights or traveling bridges are used as sludge
collectors
Sludge can be collected at the influent or effluent end of the
tank – very long tanks can have two sets of flights and
have a central hopper to minimize sludge transport
distance
A traveling bridge may have sludge removal system (a
scraper or a suction manifold) – the sludge is discharged
to a collection trough running the length of the tank
Secondary Sedimentation Tank

19. Clarifier performance is affected by
• Liquid depth
• Inlet design
• Type of sludge removal equipment
• Sludge blanket depth
Liquid depth
• Side wall liquid depth (in case of rectangular tanks depth
at the effluent end)
• Minimum 3.5 m for large clarifiers and max. upto 6 m
• Deeper tanks provide greater flexibility of operation and a
larger margin of safety
Secondary Sedimentation Tank

20. Tank inlet design
• Jetting of influent can increase formation of density
currents and scouring of settled sludge
• Tank inlet should
– dissipate influent energy
– distribute the flow evenly in horizontal and vertical directions
– mitigate density currents
– minimize sludge blanket disturbance
– promote flocculation
• Circular center feed tanks use cylindrical baffles
– Minimum diameter is 25% of tank diameter (typical 30-35%)
– Bottom of the feed well should end well above the sludge
blanket interface
Secondary Sedimentation Tank

21. Weir placement and loading
• In larger tanks circular weir trough placed at 2/3rd
to 3/4th
radial distance from the center is considered
• In smaller tanks circular weir at the perimeter is used
• A baffle may be provided to deflect density currents away
from the effluent overflow weir
• Up-flow velocity in the vicinity of weir should be 3.5-7 m/hr
• Weir loading rates should be
– < 375 m3
/m.day
– <250 m3
/m.day if located in the density current upturn zone
– <125 m3
/m.day for average flow and <250 m3
/m.day for
maximum flow in smaller tanks
• Usually designed for overflow rates during peak flow
– Alternatively can be designed for average flow and loading
conditions, and checked for peak flow and loading
conditions
Secondary Sedimentation Tank

22. Flow distribution
• Imbalanced distribution can cause under-loading or over-
loading of individual units and affect overall performance
• Weirs, flow distribution boxes, flow control valves,
hydraulic distribution using hydraulic symmetry, and feed
gate or inlet port controls are used
Scum removal
• Very little scum is usually formed in secondary clarifiers
• Scum removal may be needed when primary clarifiers are
not used
• Scum removal equipment include beach and scrapper
type, rotating pipe-through skimmer, and slotted pipes
• Scum should not be taken for treatment – microorganisms
responsible for foaming will be recycled
Secondary Sedimentation Tank