CEE 371 Lecture #29 12/9/2009

Lecture #29 Dave Reckhow 1

CEE 371Water and Wastewater

Updated: 9 December 2009

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Water and Wastewater Systems

Lecture #29

David Reckhow CEE 371 L#29 1

Wastewater Treatment: Sewer DesignReading: Chapter 10, pp.329-359

PBS: Poisoned Waters

Sewers: General CategoriesSanitary Sewery

Carries domestic sewage, and frequently industrial, commercial sewage

Storm SewerCarries stormwater, replaces natural drainage

Combined SewerDoes bothNo longer designed & installed; still found in older cities, especially in the Eastern US

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Storm Sewers IInlets from street drainage to storm sewers

A – verticalB – horizontal

Storm Sewer DepthShallow to minimize excavation, but >2-4 ft below

d f t d h l

H&H, Fig 10-1, pp. 330

road surface to reduce wheel loadings

Storm Sewer Velocity3-10 ft/secSelf-cleaning, but not too erosive

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Storm Sewers IIStorm Sewers expected to surcharge; sanitary should p g ynotCircular concrete pipe often usedStorm-water retention ponds

Attenuate high storm flowsAllow for settling of sediment and therefore reduction of gpollutant loadAllow growth of macrophytesGroundwater infiltration

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Sanitary Sewer Principles IMost are “open channel” gravity flowp g y

Requires gentle slopeWhen topography doesn’t allow, use pumps

Force MainVacuum Sewer

Design Periods

Advantage: any slope can be accommodated, can use smaller pipesDisadvantage: must have power and maintain pumps

g

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Component Design Period (yrs)Wastewater Treatment Plant 20-25Pump Stations 25Sewers: Mains, Interceptors 40-50Sewers: Laterals etc <15” Full development

Sanitary Sewer Principles IITerminology

Building Sewer(service connection) ⇒ Lateral Sewer ⇒ Main Sewer⇒ Trunk Sewer ⇒ Intercepting Sewer ⇒WWTP

Velocity ≥ 2 ft/secFor self-cleansingSlightly lower velocities OK if high depth of flowSpecial provisions if >10ft/sec

DepthBelow frost lineLaterals in street are set ≥11 ft below top of house foundation

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Sanitary Sewer Principles IIICapacityp y

Based on normal I/I, flowing-full capacityH&H approach

Laterals & submains: 400 gpcdMain and trunk: 250 gpcdInterceptors: 350% of average dry-weather flow

Alternative approachAlternative approachUse 120 gpcd and population-based peaking factor

LayoutAll but large sewers (>24in) should be in straight sections between manholes

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Sanitary Sewer Principles IVRecall: mass balance at junctionsj

Inverted siphonsDrop to avoid obstructions

Q2,c2Q1,c1

Q3,c3

213 QQQ +=21

22113 QQ

cQcQc++

=

Must keep high velocities (>3 ft/s)2 or more pipes in parallelInlet box directs flow to smallest pipe first

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Sanitary Sewer Principles VInverted siphons p(cont.)

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From Viessman & Hammer, 6th ed., Figure 6.9, pp. 218

Flow in Sewers IManning Equation 5.067.049.1 SARQ = 5.067.049.1 SRV =org qExample

12” Vitrified Clay pipe, n=0.013; @0.405% gradeWhat is V and Q when pipe is full?

nQ

( ) ftDDwp

ARD

25.04

22 ====

ππ

nor

sec85.2)00405.0()25.0(013.049.1 5.067.0 ftV == ( ) sec24.285.2

322

1sec

ftVAQ ftft === π

What is V and Q when pipe is half full?V happens to be the same, whereas Q is half

What is depth of flow for V=2ft/sec?Use graph, with V/Vfull = 2/2.85 = 0.70

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Flow in Sewers IIGraphical psummary

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Sanitary Sewer Layouts

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H&H, Fig 10-2, pp.333

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Sanitary & Storm ComparisonWhat is the max 5.067.049.1 SARQ =

population that can be served by an 8in sanitary sewer assuming 400 gpcd?What would the

n

diameter of the storm drain be for this same population?

David Reckhow CEE 371 L#29 13H&H, Table 10-1, pp. 331

Comparison (cont.)Constraints

Density = 30/acreRunoff coeff = 0.4020 min duration

10 yr frequencyVelocity = 5 ft/s

Intensity = 4.2 in/hr

Velocity = 5 ft/s

David Reckhow CEE 371 L#29 14H&H, Fig 4-30, pp. 122

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Comparison (cont.)Calculations

Sanitary @ 8inStorm

AreaFlow

( )( ) peoplegpcd

gpmQ dhr

hr 1100400

310 24min60==

acresacrepeoplepeopleA 36/30

1100 ==

36)2.4(4.0 hrin acresCIAQ ==

1acre-ft=43,560ft3

Diameter

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sec33 61542,21904.548.60 ft

hrft

hrftacre

hrinacre ==== −−

inftVQD

ft

ft

479.34.3

45

614sec

sec3

====π

42 2DVrVVAQ ππ ===

Pick 48in

Sanitary Sewer Design IDesign Period: see tablegDesign Flows are sum of:

Industrial FlowDomestic Flow & I/I

Use 120 gpcd as average (includes normal I/I)Adjust to peak daily flow

Design & Construction of Sanitary and Storm Sewers, Water Pollution Control Federation Manual of Practice No.9

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Adjust to peak daily flow using MOP9 graph:

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Sanitary Sewer Design IISelect Pipe Materialsp

Type of Pipe Sizes (in)

Typical Manning “n”

Description

Asbestos Cement (AC) 4-36 0.011 Susceptible to acid corrosion, curing with steam at high pressure may improve this

Ductile Iron (DI) 4-54 0.011 Can support very high loads: good for river crossings, resistant to root intrusion, may be corroded

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Reinforced Concrete (RC) 12-144 0.012 Susceptible to corrosion like AC

Prestressed Concrete (PC) 16-144 0.012 Best for long transmission mains, otherwise like AC

Polyvinyl Chloride (PVC) 4-15 0.009 Resistant to corrosion, light weight

Vitrified clay (VC) 4-36 0.013 Most widely used pipe in small to medium sizes, corrosion resistant, brittle & subject to breakage

Sanitary Sewer Design IIIPipe Sizes H&H, Table 10-1, pp. 331p

4” for most service connections8” minimum for sewers

SlopesUse Manning’s equation

, , pp

for V≥2 ft/s:

Recall: Hydraulic Radius, R = Area /wetted-perimeterAvoid V>12 ft/s

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5.067.049.1 SARn

Q =

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Sanitary Sewer Design IVPrepare Layout Manholes

General plan: 1 in = 200 ftProfile and detailed plan

Horizontal: 1 in = 50 ftVertical: 1 in = 5 ft

DepthLow enough to receive sewage from basements

At end/beginning of each lineAt all changes in slope, size, or alignmentAt intersectionsOtherwise at regular intervals

≤400 ft for diameters of ≤15 insewage from basements

AlignmentStraight between manholes, unless >24 in diameter

≤500 ft for diameters of 18-30 in

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Summary of Design Steps1. Obtain Plan (streets, etc) and topographic data for the are2. Locate the area outlet3. Sketch a preliminary pipe system following the topography

and streets to determine manhole locations4. Establish preliminary sizes

Consider present and future developmentUse gravity, but avoid excessive excavation (not >25 ft)

5. Revise layoutoptimize flow carrying capacity at minimum cost

6. Final ProductProfiles & plan drawings: pipe sizes, materials, peak flows, slopes, Q and V at various depths of flow

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Role of TopographyPreliminary Design follows ground elevationsy g g

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Example system from V&HBasic info

40 persons/acreDomestic Q =100 gpcdInfiltration = 600g/acre//d

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600g/acre//d

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V&H example (cont.)s

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V&H example (cont.)s

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Construction methodsOpen-cut trenchpTrenchless - Tunneling

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H&H, Fig 10-3, pp. 334

Trenchless MethodsBoring MachinegMicrotunnel

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H&H, Fig 10-4, pp. 335

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ManholesPurposep

Access point for maintenance of sewerConnection point between sewer lines of different sizes, elevations and directions

Dimensions4 ft inside diameter is typical21 inch cover

Special considerations for junctionsSimilar elevation: Merge at crown or 80% depth2 ft or greater: Drop hole structures

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ManholesWith and without dropp

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H&H, Fig 10-5, pp. 336

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Service ConnectionsStraight linegMin 2% slope4 inch or 6 inchBedding

Crushed stone or coarse sand

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H&H, Fig 10-6, pp. 337

sa d

Connected to sanitary sewer by T-branch @ 45o

Flow Measurement IImportant for I/IImportant for I/I assessment

Use Manning formula

Current meter

5.067.049.1 SARn

Q =

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H&H, Fig 10-7, pp. 338

FlumeCompare “H” to rating curve

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Flow Measurement II

P t bl d i

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H&H, Fig 10-9, pp. 340

Portable device

SamplersISCO type composite sampleryp p p

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H&H, Fig 10-10, pp. 341

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Sampling stationCombines flow and Quality measurementsQ y

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H&H, Fig 10-11, pp. 342

Sewer pipeStandard sizes Materials

4-12 inches, increments of 2 inches12-36 inches, increments of 3 inches36-144 inches, increments of 6 inches

Joints

Vitrified Clay pipe (VCP) –most common

Max size is 42 inchesConcrete (precast)

Non-reinforced up to 2 ftGood for storm sewers

PlasticJointsBell & spigot (VCP)Tongue & groove (concrete)Solvent weld (Plastic)

Polyvinyl chloride (PVC)Max is 30 in

Polyethylene (PE)Underwater crossings

Ductile IronForce mains and other difficult situationsDavid Reckhow CEE 371 L#29 34

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Corrosion IssuesElectrochemical corrosionChemical reaction

Crown corrosion by hydrogen sulfide minimized by steep gradients of venting

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H&H, Fig 10-12, pp. 343•Vitrified clay or plastic are most resistant•Protective layer of coal tar, vinyl or epoxy with concrete

Joints

& sp

igot

Up

to 4

% d

efle

ctio

nB

ell U

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H&H, Fig 10-13, pp. 344

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Bedding and BackfillMust support o erb rdenoverburdenNative materials are usually sandy loam; readily compactable

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H&H, Fig 10-14, pp. 345

compactable

Installation IOpen trench excavationp

Soil types and allowable vertical slopes

H&H, Table 10-2, pp. 346

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Installation IISloped sidewalls por Trench box

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H&H, Fig 10-15, pp. 348

Use of laser

Installation III

levels for achieving the desired slope

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H&H, Fig 10-16, pp. 349

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Soil CompactionModified Proctor Test

Adjusting moisture content to get to 90%+ of the maximum density

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H&H, Fig 10-17, pp. 350

y

Testing New Sewer lines for leaks I

Exfiltration with waterExfiltration with waterUsed where water table is below sewerMethod

10 ft max pressureLet stand for 24 hr

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H&H, Table 10-3, pp. 352

Max allowable ranges from 100-500 gal/in-diameter /mile/day

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Testing New Sewer Lines IILow pressure air testingp g

Time for pressure to drop from 3.5 to 2.5 psiAcceptable values range from 0.3 min (4”) to 7.3 min (42”) - see Table 10-4

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H&H, Fig 10-20, pp. 353

Lift stationsNeeded where gravity flow is no longer feasible g y g

A. submersible pumpB. self-priming pump

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H&H, Fig 10-21, pp. 355

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EndTo next lecture

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Hw problemGraphical psummary

d/D =55%

David Reckhow CEE 371 L#29 46V/Vfull =35%

d/D =12%

Q/Qfull =57%

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d/D =55%

V/Vfull =35%

d/D =12%

Q/Qfull =57%