13 Lecture 2

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CIVL 3700 Environmental

Engineering Design

Primary clarifer in Winnipeg’sNorth End Water Pollution Control Center

CIVIL3700 Oleszkiewicz

Jan A. OleszkiewiczPEng, CEng(UK); BCEE, FCAE, IWA Fellow

Lecture 2


Types of treatment levels

• Preliminary 1800’s• Primary 1900’s ‐Winnipeg 1935• Advanced (Enhanced) primary • Secondary 1970’s• Secondary with nutrient removal (BNR) 1980’s• Tertiary (residual solids removal) 1990’s• Advanced: 2000 –now

– Oxidation of organics– Adsorption of organics– Membrane filtration

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Traditional wastewater treatment plant removes cBOD. CEPA 1999: must disinfect (US always did). Since Cl2 banned only UV used in Canada

Bar Racks

UV Disinfection

Activated sludge

Secondary Clarifier

To lake Ontario

Influent

This plant ‐ Ashbridges ‐Toronto (860 ML/d) adds chemicals to activated sludge to precipitate phosphorus.Discharge to L Ontario

Fine screen

Grit tanks

Prim. clarifier

Effluent

Yr 2000 plants remove CBOD, nutrients and residual solids

UV Disinfectio

n

BNR

Activated sludge

Secondary Clarifier

Fine screen

Grit tanks

Prim. clarifie

r

Filtration

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Why preliminary/primary treatment?

• Poor screening:• Floatables on tank surfaces• Coarse solids may ruin sludge recycle pumps

• Poor grit removal:• Grit destroys all moving equipment – scrapers, pumps…• Grit accumulates in sludge digesters and plugs them with concrete‐like deposit

• Primary clarification keeps inert and slowly degradable solids from biological process (biomass can only ingest soluble food):

• Can run lower biomass concentration and biomass has higher active fraction


Preliminary and primarytreatment

Coarse screensBar racks 25+ mmRaw wastewater logs & dogs

O&G removalGreaseH2S

screenings

Grit removal

ParticlesSG>2

Primary settlingParticlesSG=1.01;P removal?

Coagulants?

PE Primaryeffluent

BOD: 30% removed

SS: 60+% removed (w/o chemicals)

In‐line EqualizationOPTIONAL

Equalization option

Fine screens <6mm

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Screening

Coarse 6‐150 mm Microscreens(drum) 10‐50 µm

Fine screens <6 mm

stationaryPerforated drums

Step (moving) screens

Design criteria: Qmax h and Qmin h

Bar racks

Perforatedplate


Coarse Bar Screens

• Removal of large objects (i.e. rags, paper, plastic, etc)• Vertical or inclined steel bars equally spaced across a channel, mechanically cleaned.

• Design criteria– Opening between bars = 10 ‐100 mm (25mm typical)– Dimensions based on velocity and peak design flow rate

• Typical velocity through openings > 0.75 m/s scour velocity– Quantity of screenings = 20 m3 /106 m3 of wastewater (average), & 36 m3 /106 m3 of wastewater (maximum)

• Head loss Example 5‐1 p. 321

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Coarse bar screens

Manual on by‐passMechanically cleaned

30 L of Screeningsper 1000 m3


Coarse screens 10+ mm• Velocity through 0.5 – 1.2 m/s

• Headloss activates removal

• Step‐screens, travelling scraper

• (Comminutors (grinders) not used anymore)

Brandon, old: manual

Winnipeg NorthNote the conveyor

Fordon PL, Note: lime silo for disinfection

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Fine screens 6 mm

• Screenings; 44‐100

L/1000 m3

Less if preceded by

a coarse screen

• Disinfection with lime

and land disposal

Step screenLethbridge AB

Dublin Bay Inlet Works 1000 MLd.

6 mm “bar” screen

At Ringsend, Huber was chosen to supply seven automatic raked‐bar screens and seven screenings handling units, again along with associated control panels for all the machines. The units supplied were the Huber RakeMax screen and WAP SL Washpress. Each RakeMax screen has its own dedicated WAP SL unit, with screenings being delivered to the WAP SL’s via launder chutes. The RakeMax and WAP SL units were installed in a rolling program; replacing existing 6mm perforated plate escalator screens.The RakeMax units have 6mm bar spacing and the bars do not have the traditional flat bar rakes but have a tear drop shaped bar, to allow increased throughput through the screen. Each screen has been designed to pass a flow of 4700 l/s. VSD’s fitted to the control panels allow for the rakes to clean at a higher speed in the event of a high screen differential being reached. A Pulsar Ultra 5 differential ultrasonic level controller is used to detect the level in each of the seven inlet channels.Washpress units, shown left, are designed to intensively wash up to 8m cubed per hour of wet screenings.

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Pine Creek for Calgary (100 MLd) has perforated plate step 6 mm

stainless screens

A microscreen < 50 µm. Replaces primary clarifier

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Nordkanal D: 50 µm microscreensprotecting ultrafiltration membranes downstream

Sluicing channels rather than a conveyor take screenings from 4 screens to washer and

compactor for solids

Screen

Sluice channel

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Screens at L’Assomption QB 25 MLd (manufactured by Claro, Montreal)

screen

Shaftless screw conveyor

compactor

Compactedscreenings into bags

screen

In your design: all flows require screening

• Select a reliable manufacturer of screens and only go with fine screens 3 mm

• Calculate the head loss from the book

• Show room for all the screens on the plan

• Show screenings press and place for disposal bins

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Management of Wet Weather Flows WWF

Land drainage WWF in separate systems


Requirement: treat all flows. Options in practice

• Equalize all flow and then design plant for equalized flow (e.g. Chicago TARP) – the treatment process suffers during storm

• Split influent during storm‐flows and treat portion (e.g. 2*ADWF) in the main stream and overflow chemically in the side‐stream

ADWF = average dry weather flow

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In your design take care of WWF. Designer options

– microscreens for solids

– CEPT – chemically enhanced primary treatment during storms

– Acti‐Flow = ballasted floculation, DensaDeg and other proprietary equipment on a side stream

• Page 416, fig 5‐50

An example of plant flows (MLd) and Dry weather flow operation (1 MLd = 1000 m3/d)

Dry Weather – Wet weather to 2 Q

2008 CIVIL3700 Oleszkiewicz

Grit

Grit

Bio-Reactor

420MLd

120 Bypass screened wastewater to disinfection

PrimaryCEPT

Up to 150

DisinfUV

150

DisinfCl2

FinalClarifier

300

270

150

Bypass after enhanced primary

Winnipeg South: ADWF = 75 MLdBioreactor = 2ADWF=150 Red River

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Equalization‐ used to push all flow through bioreactors• Equalization useful for small and large

municipalities - San Diego North City 100 MLd• Question: is the flow more variable:

• for large or small plants?• For arid or wet climate plants?

• Equalization up to now rarely used in municipal WWTP. Recent requirements to treat all weather flows forces retention - W End

• Industrial WWTPs • Off-side versus in-line equalization

San Diego “North City”


Equalization tanks‐ above ground

• 11 000 m3 D 48 m; H 5.25 m;

• Designer Black & Veatch

• 16 000 m3; Diameter 68 m;

• Height 4 m;

• Designer: CDM

These tanks serve to alleviate CSO – before the plants had by‐passed tothe river 10+ times/year. Off‐line tanks remain empty until there is a large surge of wastewater. US EPA demands that CSO discharges are less than 4 times/year. Winnipeg currently exceeds 18/year

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Equalization of influent wastewater

• In‐line or off line? – fig 5‐10

• Volume? Based on load

• Deposition of solids?

• Odor control

• Where? Influent. Sludge liquor

• Example fig 5‐10 and 5‐11 p.334


Equalization is typically more expensive than phys‐chemical treatment of side‐stream

wastewater during storm.

Water reuse, equalization at the end: storage of treated wastewater for

groundwater recharge in St Petersburg FL South Cross Bayou plant

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West End Winnipeg: final effluent temperature

equalization

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 4 8 12 16 20 24

Hours

Da

ily v

ari

ati

on

co

mp

are

d w

ith

a

ve

rag

e lo

ad

flow

COD-Load

NH4-Load

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 4 8 12 16 20 24Hours

Da

ily v

ari

ati

on

co

mp

are

d w

ith

a

ve

rag

e lo

ad

flow

COD-Load

NH4-Load

Supernatant

Strong sludge liquor equalization – Zurich CH. Feeding in the valleys (liquor from processing sludge has 1000 mg/L

NH4‐N compared to 60 mg/L in sewage)

Compare Fig. on page 342

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Theory of settling p. 363

• Fg = Fd Gravity force = Frictional drag force Newton’s law for terminal velocity of a particle with SG and size in laminar or turbulent regime.

• in laminar region becomes Stokes law:

– vp ≈ g(sgp – 1)dp2/(18ѵ)

–sgp = sp. gravity

–ѵ = viscosity of liquid

–dp = diameter

Liquid T = 25oC vs 8oC?Where will solids settle better?

Calgary’s 100 MLd Pine Creek WWTP clarifiers

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Discrete settling: sand and PS

• Rectangular, radial and vertical – Fig. 5‐22 p 368

• v = Q/A = (m3/d)/(m2) = depth/time = OR

• Fraction of particles removed = Xr = vp/vc

vh

vsc

vs

vh

Sludge zone

Flocculent settling: biological solids

• Agglomeration makes flocs settle fast

Sludge zone

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Flocculent particles settling, Fig 5‐24 p.369 and e.g. 5‐7 p.373

V

250

1000

60%44%

55%

47%

41%

40%

35%

30%

time

heig

ht

h1

h2

h3

h4

h5 t1 t2 t3 t4

Types of settling tanks

• Horizontal (rectangular)

• Radial

• Vertical

• Lamella

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Lamella and tube settlers: 45o‐60o angle

• See graphs on page 375 –

• Why does it work?

Activated sludge settling in the final clarifier• Flocculent and then hindered

or zone settling

• Simplest method based on 1 batch test:

• Athick = Q*tu/Ho note that Q/ Athick = Ho/tu

• Determine tu, Hu from e.g. 5‐8; p. 381

• A for clarification is from the settling portion of the subsidence curve

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Grit removal objective: separate SG ±2 from SG<1.1

• Removal of grit = sand, gravel, cinder or other heavy solid material

• Design criteria– HRT=2‐5min (3min is typical)1) Establish the peak hourly flow rate for design

2) Peak flowrate = Qavg,d* 2.75 p. 3913) Grit chamber volume = peak flowrate * HRT*1/24) Dimensions: Depth = 2‐5m; length‐to‐width ratio (2.5:1) ‐ (5:1) 5) Air supply 0.3 m3/mLmin6) Grit quantity (m3/1000m3 of influent):

‐Separate sewer system 0.004‐0.037‐Combined sewer system 0.004‐0.18

NOTE: (you need at least 2 chambers)

0.5 m/s>Vscour>0.2 m/s


Aerated grit chamber. Not used in new WWTP in America. Still used

in EU1. air is pumped in to maintain velocity

allowing settling of grit but keeping the VSS in

2. Retention time 2‐3 minutes3. Slowly phased out as the new vortex

ones are smaller and more effective

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Nordkanal D: aerated grit tank and grease trap. Microscreens (sieves) in

the building protected by the grit tank!

Vortex

www.huber.de/products/grit-separation-and-treatment/circular-grit-traps/huber-vortex-grit-chamber-vormax.html

Grit/sand

Influent

Effluent

Hydraulic Residence time HRT – 30‐50 seconds

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Tea‐cup grit

separator

HRT – 30‐50 secondsGrit/sand

Effluent

Effluent

New grit tank superior as it has the smallest settling depth. HeadcellTM

A modular, multiple‐tray settleable solids concentrator that removes grit as small as 50 microns with minimal headloss. The high efficiency flow distribution header evenly distributes influent over multiple conical trays. Tangential feed establishes a vortex flow pattern where solids settle into a boundary layer on each tray, and are swept down to the center underflow collection chamber. These settled solids are continuously pumped to grit separation, classification, and dewatering system.

The HEADCELL™ captures very fine particles due to the large surface area and short settling distances. Evenly split flow eliminates thermal short circuiting which reduces the performance of conventional grit basins. The HEADCELL™ provides high performance fine grit removal in a small footprint. This allows future capacity increases using existing space. For new headworks grit control systems, the HEADCELL™ can be installed in a poured‐in‐place concrete basin at grade, with a footprint much smaller

than conventional grit removal systems. Does not use air!

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1 MGD = 3.78 MLd

S. Cross Bayou, St Pete FL

200 ML/d

Why does grit stay settled in a grit tank if the wastewater is still swirling around?

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Removed grit is washed, dewatered and sent to landfill

Grit washing: provide a pipe with final effluent

Grit washing

Fig 5-38 p. 396

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Primary settlingGrit: d=0.2 mm; vs = 2 cm/s; vh= 25 cm/s

Organic: d=0.2 mm; vs = 0.5 cm/s; vh= 5 cm/s

• Removes settleable organics with SG ≥ 1.1

• Will remove sand and grit if not removed earlier

• Floatable material generated

• Older plants (in Winnipeg) send biological sludge (Waste activated sludge) here in a now phased out process called co‐thickening

Fig. 5‐46 – page 405

• 60‐65% TSS removal

• 25‐30% BOD5 removal

Pine Creek AB. Primary Clarifiers are covered for odor control

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Primary clarifier design

• Surface overflow rate

– At average flow SORavg = 30‐50m3/m2d

– SOR = Q/A [m/d]

• Determine area of clarifiers

– Determine the number of clarifiers

diameter 25 m ‐ 50 m Can be rectangular as well!

• Calculate the volume of each clarifier using HRT = 1‐2.5 h at Qavg,d, Depth must be between 3‐6 m

• Check at peak flows – SORmax must be between 70 ‐130 m3/m2d

San Juan PR


EXAMPLE:Typical SORavg = 35 m

3/m2d• ADWF Qavg d= 10,000 m

3/d with; Qmaxh = 30,000 m3/d (peaking factor PF = 3)

• Surface required = Q/SOR = 10,000/35 = 286 m2

• Circular clarifier Diam. = 19 m

• At Qmaxh 30,000/286 = 105 m3/m2d this value is within the peak value that will be tolerated by the clarifier

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Primary Clarifers

Lynetten‐Copenhagen500 000 PE(population equivalents)(built 1980’s)

Travelling bridge with vacuum pumpBurlington ON Skyway plant


Primary: rectangular or circular, but avoid squircles

San Diego, North CityJones Is, Milwaukee

Square circular clarifiers were dubbed “squircles” in Winnipeg and Calgary and work lousy. Structural engineers, who slept through hydraulicsClasses, convinced city officials to “save’ on walls. Errors were perpetuated!!!

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Rectangular counter‐current primary clarifiers:

1.5 h HRT at Average flow ADWF

Radial final clarifiers: 4 h HRT at ADWF

San Jose CA

San Jose CA


Primary sludge without chemicals

• Removals– 50‐65% removal of influent TSS (60% typical)

– BOD (35%); N = 10%; P = 5 ‐10%

• Sludge production– 60% influent TSS= [influent, Q (m3/d) * TSS (mg/L) * 0.6]

– 4‐12% total solids (TS) concentration (typical 4.5% TS)

• Simplified calculations assume SGH2O= 1 = SGsolids• Vo*TSo = V1*TS1 example 100m3*1% = 25m3*4%

• 100 m3/d at 1% TS 1000 kg TSdry in 100,000 kg slurry

• At 4% TS 1000 kgTS in 25,000 kg sludge or 25 m3

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Chemically Enhanced Primary Treatment CEPT

• Add coagulants and precipitate/settle 75-80% solids (TSS) and additionally remove 70-90% phosphorus

• Particulate BOD5 (and bpCOD) removal increased to 40-70%

• In Montreal that is all they

do because the raw

wastewater is very diluted

BOD =50 -70 mg/L

Montreal 8 000 000 m3/d

Primary clarification efficiency depends

on SOR.

SOR m3/m2d

Removal(%)

20

40

60

80

100

20 40 60 80 100

BOD5

Hydraulics is key to good performance at higher flows

This is without chemicals. Yousplit WWFlow to 2*Q through plantThen add chemcialsto the 2Q flow and treat the rest in dedicatedballasted flocculation Acti-Flo – Fig. 5-50

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Actiflo® (Veolia Water): Ballasted flocculation

. Fig. 5‐50 p. 516

P

SludgeSludge HydrocycloneHydrocyclone

PolymerPolymer

Micro-sandMicro-sand

MaturationMaturation

SettledwaterSettledwater

Lamella settlerLamella settlerRaw waterRaw water

InjectionInjection

Coagulation

Static mixing

coagulant


Clarifier hydraulics depends on baffles. The art: maintain laminar flow when flow input varies!

• No baffle

• Quick flow/dye

Breakthrough

See Fig. 5-47, p. 407

HRTtheor = 30’

Should be plug flow

In real plug flow dye in effluent after 30’

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With a simple baffle re‐directing the density current the flow/water stays much longer (on the average) in the clarifier. EDI Energy dissipating inlet design

Use flow metering to control your flow splits, bypasses, chemical dose etc: Parshall flume;

Venturi; magnetic meters

Reno-Sparks NV - Parshall flume

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Flow splitting is an art

Prague Cz Central WWTP: split into four “even” streams


Conclusion on preliminary/primary treatment

• Defines plant esthetics and makes sure downstream processes work:

• Poor screening:• Floatables on tank surfaces• Coarse solids may ruin sludge recycle pumps• Rags kill mixers

• Poor grit removal:• Grit destroys all moving equipment – scrapers, pumps…• Grit accumulates in sludge digesters and plugs them with cement‐like hard deposit

• Primary clarification keeps inert and slowly degradable solids from biological process (biomass can only ingest soluble food):

• Can run lower biomass concentration and biomass has higher active fraction

Design Settling velocity H/t or Q/A m3/m2·hGrit 2 cm/sPrimary sludge 0.5 cm/sSecondary sludge 0.05 cm/s

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Final or secondary clarifiers settle flocculent solids (zone settling). Usually radial. Deeper; 4 h HRT and designed

for solids loading; e.g. 4 kg SS/m2h and low SOR = 1 m/h (24 m3/m2d)

Innsbruck, Austria. Strass WWTP


Final ClarifierInnsbruck, Austria. Strass WWTP

Design Settling velocity H/t or Q/A m3/m2·hGrit 2 cm/sPrimary sludge 0.5 cm/sSecondary sludge 0.05 cm/s

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• Bonnybrook Calgary 600 MLd

13 Lecture 2

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Transcript of 13 Lecture 2