CE 428 Water and Wastewater Treatment Design
Secondary Clarifiers
Dr. S.K. Ong
Design of secondary clarifier for biological systems is slightly different from primary clarifiers because of
the high solids contents entering the clarifier from the biological aeration tank.
Incoming solids concentrations are in the range of 2,500 to 5,000 mg/L. Under this situation, interactions
between particles become important and particle settling is hindered. Settling is classified as Type III
settling as the solid suspension tends to settle en masse. There is usually a distinct clarified zone showing a
liquid-solid interface.
H
t1
t2
t3
Type IV
- compression
There are several approaches used in the design of secondary clarifiers.
In the absence of solids settling data, literature data is generally used (see attached Table 10-12 and Ten
States Standards). The three important design criteria are:
Surface overflow rate
Solids loading rate
Weir overflow rate.
Treatment Process
Trickling filter
Conventional
Completely Mixed
Extended aeration
Surface Overflow
Rate at Design Peak
Hourly Flow* (gal/d/ft2)
1,200
1,200**
1,000
Peak Solids
Loading Rate***
(lb/d/ft2)
50
35
Weir Overflow Rate Design Peak Hourly Flow
(gal/d/ft)
20,000
(< 1mgd)
30,000
(> 1 mgd)
* based on influent flow
** to meet 20 mg/L, reduce surface overflow rate to 1,000 mg/L
*** based on the design maximum day flow rate plus the design maximum return sludge rate and the
design MLSS under aeration
For Ten States Standard - the peak hourly flows are used for both surface overflow rate and solids loading
rate. The larger of the surface area for either surface overflow or for solids loading is selected.
If solids data are available, the following experiment is conducted to size the secondary clarifier. The test
use is the solids flux method.
In a typical secondary clarifier, there are essentially two zones that control the design of the clarifier.
Zone 1 - sludge blanket, settling velocity of the particles should control the design for this zone. The
mass of solids per unit area per unit time (g/m2/s) is defined as the solids flux. In this zone the
solids flux is called the gravity flux.
Zone 2 - concentrated sludge zone, thickening of the sludge occurs, the rate of drawoff of the sludge
controls, if not there will be a build up of sludge
In this zone the solids flux is defined as underflow flux.
Inflow
1
2
Sludge drawoff
Solids Flux Method
As in the earlier above (page 1), a jar of the wastewater is mixed evenly to uniformly distribute the solids
throughout the length of jar. As the solids settle, the interface between the liquid and the solids are
measured with respect to time. Various tests for different initial solids concentrations are conducted. A
typical curve (height of interface vs time) is plotted:
Zone Settling
Height of
Interface
Transition
Compression settling
Time
The slope of the hindered settling region is the settling velocity, V i which is equivalent to the surface
loading rate. For each initial solids content, we have different settling velocities, V 1, V2, V3, ….
In Zone 1, the gravity flux, Ngi = Ci VI
Where Ngi = gravity flux
Ci = initial solids concentration
Vi = hindered settling velocity
Computing the gravity flux for different initial solids concentration, and plotting the gravity flux against the
initial concentration, we have the following curve:
gravity flux, Ng
(g/m2/s)
Underflow
Flux, Nu
Concentration, C
Concentration, C
Drawoff from the bottom or undeflow flux is given by N u = CuUb
Where Cu = concentration at the bottom of the clarifier
Ub = underflow velocity = Qu/As = volumetric underflow rate/surface area of clarifier
A plot of Nu for various Ub's is shown above.
The total flux through the clarifier is given by N = N g + Nu. Combining the two curves, we have the
following diagram.
Total Flux
Concentration, C
The minimum on the curve indicates the minimum or limiting solids handling capacity of the clarifier for
the given suspension. The limiting flux is given by N L.
Therefore, the clarifier for thickening must be able to handle:
As NL > Q Co
Where Q = influent flow rate
C0 = influent solids concentration
Design area for thickening As > QCo/NL
Since mass in must equal to mass out, then Q Co = Qu Cu = As NL
Cu is found by extending the limiting flux line until it intersects the underflow flux line for Co. Based on
this Qu can be found.
Design area for hindered settling or clarification is given by As = Q/V i where Vi is the settling velocity for
th given inlet solids concentration.
Select the larger of the two areas.
Simplified Approach
 Plot the solid flux curve for gravity flux only
 Decide on a concentration, Cu, of thickened sludge for the underflow
 Locate Cu on the concentration lines, draw a line such that it is a tangent to the solids flux curve
 Intersect of this line with the Y-axis is the limiting flux
 Use the limiting flux in your design.
Example
Design a secondary clarifier for a flow rate of 4200 m3/d, solids content of 2,000 mg/L, and a thickened
sludge of 6,000 mg/L. Given the following data.
Concentration (mg/L)
1000
2000
3000
4000
5000
6000
Settling velocity (m/h)
2.8
1.4
0.4
0.2
0.1
0.06
Solids flux (kg/m2.h)
2.8
2.8
1.2
0.8
0.5
0.36
Draw tangent to locate limiting flux
NL = 2.3 kg/m3day
Total solids loading to clarifier
4200 (m3/d)  (d/24h)  (2000 mg/L)  (kg/106 mg)  (1000 L/m3) = 350 kg/h
Surface area of clarifier for thickening
= 350/2.3
= 152.2 m2
For clarification with a solid concentration of 2,000 mg/L, the settling velocity is 1.4 m/h.
Surface area needed for clarification = 4200 (m3/d)  (d/24 h) /1.4
= 125 m2
Therefore clarifier is limited by thickening.
Diameter of the clarifier is given by [(152.2)  4/3.142]1/2
= 13.9 m, use 14 m
Single Batch Settling Curve Approach
1.
2.
3.
4.
5.
6.
7.
8.
Determine the slope of the hindered settling region, Vo. This is the settling velocity for
clarification (surface loading rate).
Extend tangents from the hindered settling region and the compression region. Bisect the angles
between the intersect of the two tangents, to locate point 1.
Draw a tangent to the curve at point 1.
The initial sludge concentration is Co and the initial height of the solids is Ho. The total mass of
sludge per unit area is equal to Ho Co.
If the undeflow concentration is Cu and the height of this sludge at the bottom of the clarifier is
Hu, then by mass balance understeady state conditions, we have
HoCo = Hu Cu
If a Cu value is selected for the underflow concentration, then Hu can be computed.
Using the computed Hu value, a horizontal line is draw. The intersection of this line and the
tangent drawn from point 1 will give the time, tu, required to reach the desired underflow
concentration Cu.
Area required for thickening At = (Q= Qr) tu/Ho
Use a safety factor of between 1.5 for the thickening surface area needed.
Use a safety factor for the hindered settling area needed, i.e.,
Ac = 2 (Q/Vo)