1. Secondary Clarifier 1

2. 2

3. Design Considerations
Overflow rate or surface-settling rate
Detention period
Weir loading rate
Tank shape and dimensions
Solid-loading rate
Influent structure
Effluent structure
Sludge collection and removal
Two Major Functions of Secondary Clarifiers
Provide clarification to produce high quality effluent
Provide thickening of settled solids in the underflow

4. Typical Design Values
Overflow rates at average and peak design flows: 15~32 and 40~48 m3/m2·day
Solids loadings at average and peak design flows: 49~144 and 100~220 kg/m2·day
Tank shape: circular, rectangular, or square Circular tanks: 10~60 m in diameter (< 5ⅹSWD) Depth: 4~6 m for circular and rectangular tanks
Influent and effluent structures and sludge collection equipment – Refer to ‘Primary Sedimentation Tank Lecture Note’

5. Hindered or Zone Settling Behavior5

6. Design Based on Single-Batch Test Data6

7. Design Based on Solids Flux
Gg = VgCi
Gu = VuCi
Gt = Gg + Gu
where:
Gg = solids flux caused
by gravity, kg/m2·h;
Gu = solids flux caused
by underflow, kg/m2·hr;
Gt = combined flux
caused by gravity and
underflow, kg/m2·hr;
Ci = concentration of
solids, mg/L;
Vg = hindered settling
velocity, m/hr; and
Vu = downward velocity
due to underflow, m/hr.
GL: Limiting solids concentration 7

8. Secondary Clarifier Design Criteria
1. Provide four circular clarifers, each clarifier shall have independent operation with respect to the aeration basin
2. Design the clarifiers for average design flow plus the recirculation
3. Design the influent and effluent structures, and check the hydraulics at peak design flow.
4. Return sludge from each
clarifier shall have an
independent sludge withdrawal
arrangement with flow
measurement and control
devices.
5. The design of the clarifer shall be
based on the solids-settling rate
obtained from laboratory results.

9. Secondary Clarifier Design Criteria - continued
6. The surface area of the clarifier shall be large enough to meet the clarification as well as the thickening requirements for the effluent and the underflow, respectively.
7. The water depth of the clarifier shall be sufficient to provide an adequate clearwater zone, thickening zone, and sludge storage zone.
8. The overflow rates at average and peak flow conditions shall not exceed 15 and 35 m3/m2·day, respectively.
9. The solids-loading rates at average and peak design flows shall not exceed 50 and 150 kg/m2·day, respectively.
10. Scum baffles and scum collection system shall be provided.
11. The effluent weir shall be designed to prevent turbulence.
The weir loading shall not exceed 124 m3/m·day (10,000 gpd/ft) and 372 m3/m·day (30,000 gpd/ft) at average and peak design flows, respectively.

10. Design Calculations Surface area of secondary clarifier
1. Establish design flow
Design flow to the secondary
clarifier = Average design flow
+ RAS - MLSS wasted
= m3/sec
m3/sec m3/day ×
day/86,400 sec = m3/sec
Design flow to each secondary
clarifier = 0.769/4
= m3/sec
2. Prepare flux curves
X, g/m
Vi, m/hr
X·Vi, kg/m2·hr

11. Design Calculations - continued
3. Determine limiting solids-loading rate
Sludge flux (SF) = 2 kg/m2·hr
= 48 kg/m2·day
= 9.81 lb/ft2·day
4. Calculate area and diameter
of the secondary clarifier
A = QX/SF
Q = m3/sec × 3600
sec/hr = 691 m3/hr
A = 691 m3/hr × 3.75 kg/m3
 2 kg/m2·day
= 1,296 m2
Actual area = /4 × = 1,301 m2

12. Design Calculations - continued
5. Check the overflow rate at average design flow
Overflow rate = Q/A = m3/sec × 86,400 sec/day1,301 m2
= 12.8 m3/m2·day (= 314 gal/ft2·day)
< 15 m3/m2·day
6. Check the clarifier area for clarification requirement
Calculated overflow rate = 12.8 m3/m2·day = m/hr
MLSS conc. at m/hr settling rate = 4,400 mg/L > 3,750 mg/L; thus, the area for clarification will be sufficient.
7. Check the overflow rate at peak design flow
At peak design flow plus recirculation, the flow to each clarifier
= ( ) m3/sec  4 = m3/sec
Overflow rate = m3/sec × 86,400 sec/day 1,301 m2
= 26.8 m3/m2·day (satisfactory) < 35 m3/m2·day

13. Design Calculations - continued

14. Design Calculations - continued
8. Calculate the solids loadings
The limiting solids loading at average design flow = m3/sec ×
3,750 g/m3 × kg/1,000 g × 86,400 sec/day  1,301 m2
= 47.8 kg/m2·day < 48 kg/m2·day (satisfactory)
Solids loading at peak design flow = m3/sec ×
= kg/ m2·day < 150 kg/m2·day (satisfactory)
Solids loading at peak design flow when three clarifiers are in operation = m3/sec × 3,750 g/m3 × kg/1,000 g × 86,400 sec/day 
1,301 m2 = 134 kg/m2·day < 150 kg/m2·day (satisfactory)
Depth of secondary clarifer
Liquid depth of the secondary clarifier = depth of clear water zone + depth of thickening zone + depth of sludge storage zone
1. Determine clearwater and settling zones
The clearwater and settling zones are generally 1~1.5 m and 1.5~2 m, respectively. Provide 3 m clearwater and settling zones.

15. Design Calculations - continued
2. Compute the depth of thickening zone
Assume that under normal conditions, the mass of sludge retained in the clarifier is 30% of the mass of solids in the aeration basin, and the average concentration of sludge in the clarifier is 7,000 mg/L.
Total mass of solids in BNR reactor = 3,750 g/m3 × kg/1,000 g ×
(2,631 m3 + 2,631 m3 + 11,560 m3) = 63,083 kg
Total mass of solids in each clarifier = 0.3 × 63,083 kg/4 = 4,731 kg
Depth of thickening zone = Total solids in the clarifer 
(Concentration × Area) = 4,731 kg × 1,000 g/kg 
(7,000 g/m3 × 1,301 m2) = 0.52 m  0.5 m
3. Compute the depth of sludge storage zone
The sludge storage zone is provided to store the sludge in the clarifier. Provide the sludge storage capacity for one day under sustained peak flow rate and BOD5 loadings. Assume that the sustained flow rate and sustained BOD5 factors are 2.5 and 1.5, respectively.

16. Design Calculations - continued
Total volatile solids produced under sustained loadings
= 1.5 × 2.5 × 2,820 kg/day (#35 in AS Design slides) = 10,575 kg/day
Provide one-day storage for solids
Total solids stored per clarifier = 20,575 kg/d/4 = 2,644 kg
Total solids stored in each clarifer = 2,644 kg (storage zone) + 4,731 kg
(thickening zone) = 7,375 kg
Clarifier depth for solids storage = 7,375 kg × 1,000 g/kg (7,000 g/m3 ×
1,301 m2) = 0.8 m
4. Compute total depth of clarifier
Total depth of clarifier = 3.0 m m m = 4.3 m
Provide average side water depth in the clarifier = 4.5 m (14.8 ft)
For additional safety provide a free board of 0.5 m
Total depth of clarifier = 5 m
Detention time
1. Calculate the volume of the clarifier
Average vol. of the clarifier = /4 × m2 × 4.5 m = 5,855 m3

17. Design Calculations - continued
2. Calculate detention time under different flow conditions
Detention time under average design flow plus recirculation
= 5,855 m3  (0.192 m3/sec × 3,600 sec/hr) = 8.5 hrs
Detention time under peak design flow plus recirculation
= 5,855 m3  (0.403 m3/sec × 3,600 sec/hr) = 4.0 hrs
Influent structure
Consists of a central feed well. An influent pipe is installed across the clarifier that will discharge into the central feed well. The influent will pass under the baffle and then distribute uniformly throughout the tank.
Effluent structure
Consists of effluent baffle, V-notches, effluent launder, effluent box, and a pressure outlet pipe.
1. Select weir arrangement, and dimensions of effluent launder, effluent box, and outlet sewer
Provide 90° standard V-notches on the weir plate that shall be installed on one side of the effluent launder

18. 18 316 SS 90° V-notchs around the weir plate @ 39.5 cm c/c
40.7 m diam.
Scum baffle
Walkway.
Outer sewer
Scum trough
Effluent box
2 m × 2 m
Scum line.
Rake arm
Effluent launder
Influent pipe
Influent pipe
Sludge pipe
Scum trough
Scum line
Drive
Skimmer assembly
Water level
Center
shaft
Scraper arm
Influent baffle
Effluent
pipe
Sludge
line
Concrete tank
Center scraper 18

19. Design Calculations - continued
Provide width of launder = 0.5 m
Length of effluent weir plate =  ( ) m = m
Provide 8 cm deep 90 V-notches at 39.5 cm center-to-center
Total # of notches = m  (39.5 cm/notch × m/100 cm) = 316
2. Compute head over V-notch at average design flow
Average design flow from the clarifier = Average design flow to
aeration basin - MLSS wasted
= m3/sec - (75.2 m3/day × day/86,400 sec) = m3/sec
Flow per clarifier = m3/sec  4 = 0.12 m3/sec per basin
Flow per notch at average design flow = 0.12 m3/sec  316 notches
= m3/sec/notch
Head over V-notch

20. 20 Weir head at peak design flow when one clarifier is out of service
39.5 cm
Weir head at average
design flow
23.5 cm
Weir notches and
effluent launder
6 cm
4 cm
88 cm
Effluent box
84 cm
0.88 m
80 cm
0.84 m
0.31 m
0.61 m
Invert of the launder
0.3 m
2 m
Detail of the 90° V-notchs and effluent launder
Section AA showing V-notches, effluent launder
on both sides, and effluent box
Average side
water depth 4.5 m
Water surface at
average design flow
Invert of effluent box 3.36 m
4.52 m
Invert of effluent launder 3.66 m
Effluent box
Influent well
Top of weir 4.54 m
0.84 m
3.97 m
0.61 m
0.3 m
Outlet
pressure
pipe
0.5 m
0.00
Section BB showing outlet pipe, effluent box,
effluent launder and water depth at average design flow
Hydraulic profile secondary clarifier at peak
design flow when one clarifier is out of service 20

21. Design Calculations - continued
3. Compute head over V-notch at peak design flow when one unit is out of service
Flow per notch at peak design flow when one unit is out of service
= m3/sec  (3 clarifiers × 316 notch) = m3/sec
4. Compute actual weir loading
Weir loading at average design flow = 0.12 m3/sec × 86,400 sec/day
 m = 83.1 m3/m·day < design loading of 124 m3/m·day
Weir loading at peak design flow = m3/sec × 86,400 sec/day
 (4 × m) = 229 m3/m·day < design loading of 372 m3/m·day
5. Compute the depth of the effluent launder
Width of effluent launder = 0.5 m
Provide effluent box 2 m × 2 m

22. Design Calculations - continued
Provide 0.8-m-diameter outlet pressure pipe. The pipe is an inverted siphon connected to a common junction box. The water surface elevation in the junction box is kept such that the depth of flow in the effluent box at peak design flow is maintained at 0.61 m. Provide invert of the effluent launder 0.3 m above the invert of the effluent box.
y2 = Depth of water in the effluent box - invert height of effluent
launder above the invert = 0.61 m m = 0.31 m
b = 0.5 m; N = 1
Half of the flow divides on each side
of the launder; therefore, flow on each
side of the launder = m3/sec 
(2 × 4 clarifiers in operation)
= 0.17 m3/sec

23. Design Calculations - continued
Provide 16% losses for friction, turbulence, and bends, and provide 31 cm additional depth to ensure free fall.
Total depth of the effluent launder = (0.42 m × 1.16) m = 0.8 m
The water surface elevation in the clarifier at average design flow is kept ( ) = 0.84 m above the invert of the effluent launder.
Sludge collection system and skimmer
Consists of a rotating rake structure with scraper blades that will scrape the settled sludge from the tank bottom to a sludge pocket located near the center of the basin. The fixed access bridge shall house the drive machinery and shall be supported by a column at the center of the tank. The skimmer shall remove the scum and deposit it into the scum trough.
Return sludge pumps
Four return sludge variable speed pumps each having a rated pumping capacity of m3/d 4×1.5 = m3/sec (150% of design average flow per basin); independent operation of one clarifier; an identical pump (fifth pump) as a standby unit and cross-connected to serve all four clarifiers; a magnetic flow meter; and a sonic sludge blanket meter

24. Common Operating Problems
1. A slightly pungent odor in the anaerobic reactor is the characteristic smell. A strong putrid, or hydrogen sulfide odor is an indication of an excessive anaerobic zone, settling of solids, or trapped scum. Increase the return sludge and the mixer speed.
2. The ORP (oxidation-reduction potential) in the anaerobic zone should be well below -200 mV. Failure to reach low ORP is an indication of insufficient anaerobic conditions. Reduce return sludge flow and adjust mixer speed to reduce surface turbulence.
3. Poor P release in the anaerobic tank (< 20 mg P/L) may be caused by (a) insufficient SCFAs (short chain fatty acids) in influent, (b) the presence of an aerobic or anoxic condition, (c) insufficient HRT, and (d) low BOD5) in the mixture of influent and return flow. Provide an anaerobic fermenter, reduce return flow, and reduce mixer turbulence.
4. An NO3--N conc. > 0.1 mg N/L is an indication of insufficient denitrification. Possible causes are high recycle ratio, insufficient HRT, excessive turbulence, low pH, and insufficient biolodegradable organic carbon. Reduce the recycle ratio and adjust mixer speed to minimize surface turbulence.

25. Common Operating Problems - continued
5. Sludge floating to the surface of the clarifiers (bulking of sludge) may be due to growth of filamentous organisms. Often denitrification occurring in the secondary clarifier may be the cause of nitrogen bubbles attaching to sludge particles and sludge rising in clumps. The problem may be overcome by increasing the sludge return rate and DO in the aeration basin, and reducing the sludge age.
6. Turbid effluent (pin point floc in effluent) but good SVI may be due to excessive turbulence in the aeration basin or overoxidized sludge. Reduce aeration or agitation, increase sludge wasting, or decrease sludge age.
7. Sludge blanket uniformly overflowing the weir may be due to excessively high solids loading, peak flows overloading the clarifiers, unequal flow distribution on clarifiers, excessively high MLSS, and inadequate return sludge.
4. Sludge blanket discharging over the weir in one portion of the clarifier may be the result of unequal flow distribution. Level the effluent weirs.

26. Operation and Maintenance
1. Remove accumulations from the influent baffles, effluent weirs, scum baffles, and scum box each day.
2. Observe sludge return from individual clarifier, and adjust the flow rate as required from laboratory tests.
3. Determine sludge level and adjust waste sludge pump as necessary.
4. Observe operation of scum pump and provide hosing as necessary.
5. Clean daily all inside exposed vertical walls and channels by a squeegee.
6. Inspect distribution box and clean weirs, gates, and walls as necessary and remove all settled solids. Also check flow to all clarifiers.
7. Inspect effluent box, and clean weir and walls as necessary. Measure the head over the weir daily.
8. Hose down and remove wastewater sludge and spills without delay.
9. Check electrical motors for overall operation, bearing temperature, and overload detector twice each day.
10.Check oil level, grease reducer and rollers on skimmer each week.