wwt secondary clarifier.pdf

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Dr. Abdel Fattah Hasan

An-Najah National University

Masters Program of

Water and Environmental Engineering

461652, Spring 2011

WWT

3- Secondary Clarifier

Secondary Clarifier

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Secondary Clarifier

Objectives To separate biomass from liquid and then return

biomass to the activated sludge process so that SRT >> HRT

To meet total suspended solids discharge limit To thicken sludge in the underflow (less volume to

pump for removal) Tank shape

Circular: small to medium/large size plants (up to 200 MGD)

Rectangular: huge plants (> 200 MGD)3

Selection of Tank Shape

Circular clarifier

High TSS stream due to better sludge removal capability

Easy to collect sludge (less distance to carry)

More space required

More complex piping for inlet and outlet

Rectangular clarifier

Lower TSS stream since

sludge has to be carried for

a long distance for removal

Difficult to collect activated

sludge due to the density

close to water

Less space required (shared

common walls)

Straight-forward piping for

inlet and outlet

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Circular Secondary Clarifier (1)

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Circular Secondary Clarifier (2)

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Effluent Collector Bridge

Influent Sludge concentrator Collector

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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

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Typical Design

Values Overflow rates

Average flows: 500~1,100 gal/ft2day (15~32 m3/m2day) Peak design flows: 1,300~1,600 gal/ft2day (40~48

m3/m2day)

Solids loadings Average flows Peak design flows: 49~144 and 100~220 kg/m2day

Tank shape: circular, rectangular, or square Circular tank diameter: 30~200 ft (10~60 m) (< 5SWD) Depth: 13~20 ft (4~6 m) for circular and rectangular

tanks

Influent and effluent structures and sludge collection equipment Refer to Primary Sedimentation Tank Lecture Note

8SWD: Side water depth

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Hindered or Zone Settling

Behavior

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1234

Design Based on Single-Batch Test

Data

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Design Based

on Solids Flux

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Gg = VgCiGu = VuCi

Gt = Gg + Guwhere:Gg = solids flux caused

by gravity, kg/m2h;Gu = solids flux caused

by underflow, kg/m2hr; Gt = combined flux

caused by gravity and underflow, kg/m2hr;

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

TSS Better settling

Secondary Clarifier Design Criteria (1)1. Provide multiple (four in this example) 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.

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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/m2day, respectively.

9. The solids-loading rates at average and peak design flows shall not exceed 50 and 150 kg/m2day, 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/mday (10,000 gpd/ft) and 372 m3/mday (30,000 gpd/ft) at average and peak design flows, respectively.

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Secondary Clarifier Design Criteria (2)

Design Calculations (1)Surface area of secondary clarifier1. Establish design flow

Design flow to the secondary clarifier = Average design flow

+ RAS - MLSS wasted= 0.486 m3/sec + 0.292

m3/sec - 752 m3/day day/86,400 sec = 0.769 m3/sec

Design flow to each secondary clarifier = 0.769/4

= 0.192 m3/sec2. Prepare flux curves

X, g/m3 1000 1500 2000 3000 4000 5000 6000 7000 8000 9000 Vi, m/hr4.4 4.2 2.8 1.3 0.67 0.34 0.2 0.1 0.05 0.03XVi, kg/m2hr 4.4 6.3 5.6 3.9 2.7 1.7 1.2 0.7 0.4 0.3

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3. Determine limiting solids-loading rateSludge flux (SF) = 2 kg/m2hr = 48 kg/m2day= 9.81 lb/ft2day

4. Calculate area and diameterof the secondary clarifierA = QX/SFQ = 0.192 m3/sec 3600

sec/hr = 691 m3/hrA = 691 m3/hr 3.75 kg/m3

2 kg/m2day= 1,296 m2

Actual area = pi/4 40.72 = 1,301 m2ft) (134 m 40.7

4m 1,296Diameter2

=

=

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Design Calculations (2)

5. Check the overflow rate at average design flowOverflow rate = Q/A = 0.193 m3/sec 86,400 sec/day1,301 m2

= 12.8 m3/m2day (= 314 gal/ft2day) < 15 m3/m2day OK

6. Check the clarifier area for clarification requirement Calculated overflow rate = 12.8 m3/m2day = 0.533 m/hrMLSS conc. at 0.533 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 flowAt peak design flow plus recirculation, the flow to each clarifier= (1.321 + 0.292) m3/sec 4 = 0.403 m3/secOverflow rate = 0.403 m3/sec 86,400 sec/day 1,301 m2

= 26.8 m3/m2day < 35 m3/m2day OK

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8. Calculate the solids loadingsThe limiting solids loading at average design flow = 0.192 m3/sec

3,750 g/m3 kg/1,000 g 86,400 sec/day 1,301 m2= 47.8 kg/m2day < 48 kg/m2day OK

Solids loading at peak design flow = 0.403 m3/sec 3,750 g/m3 kg/1,000 g 86,400 sec/day 1,301 m2

= 100.4 kg/ m2day < 150 kg/m2day OKSolids loading at peak design flow when three clarifiers are in operation = 0.538 m3/sec 3,750 g/m3 kg/1,000 g 86,400 sec/day

1,301 m2 = 134 kg/m2day < 150 kg/m2day OKDepth of secondary clarifier

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 zonesThe clearwater and settling zones are generally 1~1.5 m and 1.5~2 m, respectively. Provide 3 m clearwater and settling zones.

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2. Compute the depth of thickening zoneAssume 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 kgTotal mass of solids in each clarifier = 0.3 63,083 kg/4 = 4,731 kgDepth 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 zoneThe 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.

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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 solidsTotal solids stored per clarifier = 20,575 kg/d/4 = 2,644 kgTotal solids stored in each clarifer = 2,644 kg (storage zone) + 4,731 kg

(thickening zone) = 7,375 kgClarifier depth for solids storage = 7,375 kg 1,000 g/kg (7,000 g/m3

1,301 m2) = 0.8 m4. Compute total depth of clarifier

Total depth of clarifier = 3.0 m + 0.5 m + 0.8 m = 4.3 mProvide average side water depth in the clarifier = 4.5 m (14.8 ft)For additional safety provide a free board of 0.5 mTotal depth of clarifier = 5 m

Detention time1. Calculate the volume of the clarifier

Average vol. of the clarifier = pi/4 40.72 m2 4.5 m = 5,855 m3

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2. Calculate detention time under different flow conditionsDetention time under average design flow plus recirculation

= 5,855 m3 (0.192 m3/sec 3,600 sec/hr) = 8.5 hrsDetention time under peak design flow plus recirculation

= 5,855 m3 (0.403 m3/sec 3,600 sec/hr) = 4.0 hrsInfluent 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 structureConsists 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 sewerProvide 90 standard V-notches on the weir plate that shall be installed on one side of the effluent launder

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40.7 m diam.

Scum baffleWalkway.

Scum line.

Influent pipeSludge pipe

Scum trough

Rake arm

Influent pipe

Outer sewerEffluent box

2 m 2 mEffluent launder

316 SS 90 V-notchsaround the weir

plate @ 39.5 cm c/c

DriveScum troughScum line

Skimmer assembly

Scraper armCentershaft Influent baffle

Water level

EffluentpipeSludge

line Center scraperConcrete tank 22

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Provide width of launder = 0.5 m Length of effluent weir plate = pi (40.7 - 1) m = 124.7 mProvide 8 cm deep 90 V-notches at 39.5 cm center-to-centerTotal # of notches = 124.7 m (39.5 cm/notch m/100 cm) = 316

2. Compute head over V-notch at average design flowAverage design flow from the clarifier = Average design flow to

aeration basin - MLSS wasted= 0.486 m3/sec - (75.2 m3/day day/86,400 sec) = 0.477 m3/sec

Flow per clarifier = 0.477 m3/sec 4 = 0.12 m3/sec per basinFlow per notch at average design flow = 0.12 m3/sec 316 notches

= 0.00038 m3/sec/notchHead over V-notch

( ) ( )cm 4 cm 3.7 m 0.037

290tanm/sec 9.82584.08/15

/secm 0.00038

2tan2gC8/15

QH2/5

2

22/5

d

==

=

=

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Weir head at peakdesign flow when one

clarifier is out of service

Weir head at averagedesign flow

Weir notches andeffluent launder

39.5 cm23.5 cm

6 cm4 cm

84 cm88 cm 80 cmEffluent box

0.61 m

2 m0.3 m

0.31 m 0.84 m0.88 m

Water surface ataverage design flow

0.84 m

0.5 m0.3 m0.61 m

Effluent box

Average sidewater depth 4.5 m

Influent well4.52 m

Invert of effluent box 3.36 mInvert of effluent launder 3.66 m

Top of weir 4.54 m3.97 m

Outletpressure

pipe0.00

Detail of the 90 V-notchs and effluent launderSection AA showing V-notches, effluent launder

on both sides, and effluent box

Section BB showing outlet pipe, effluent box,effluent launder and water depth at average design flow Hydraulic profile secondary clarifier at peakdesign flow when one clarifier is out of service

Invert of the launder

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3. Compute head over V-notch at peak design flow when one unit is out of serviceFlow per notch at peak design flow when one unit is out of service

= 1.321 m3/sec (3 clarifiers 316 notch) = 0.00139 m3/sec

4. Compute actual weir loadingWeir loading at average design flow = 0.12 m3/sec 86,400 sec/day

124.7 m = 83.1 m3/mday < design loading of 124 m3/mdayWeir loading at peak design flow = 1.321 m3/sec 86,400 sec/day

(4 124.7 m) = 229 m3/mday < design loading of 372 m3/mday5. Compute the depth of the effluent launder

Width of effluent launder = 0.5 mProvide effluent box 2 m 2 m

( ) ( ) m 0.063290tanm/sec 9.82584.08/15/secm 0.00139

2tan2gC8/15

QH2/5

2

22/5

d

=

=

=

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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 - 0.3 m = 0.31 mb = 0.5 m; N = 1Half of the flow divides on each side of the launder; therefore, flow on each side of the launder = 1.321 m3/sec

(2 4 clarifiers in operation)= 0.17 m3/sec

m 0.42 m 0.31m) (0.5m/sec 9.81

.17)0(2m) (0.31y 22

22

1

=

+=

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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) + 0.31 m = 0.8 mThe water surface elevation in the clarifier at average design flow is kept (0.80 + 0.04) = 0.84 m above the invert of the effluent launder.

Sludge collection system and skimmerConsists 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 pumpsFour return sludge variable speed pumps each having a rated pumping capacity of 0.486 m3/d 41.5 = 0.182 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

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Common Operating Problems (1)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 fermentor, 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 biodegradable organic carbon. Reduce the recycle ratio and adjust mixer speed to minimize surface turbulence.

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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 over-oxidized 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.

8. Sludge blanket discharging over the weir in one portion of the clarifier may be the result of unequal flow distribution. Level the effluent weirs.

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Common Operating Problems (2)

Operation and Maintenance1. 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.

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wwt secondary clarifier.pdf

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