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CEE 371 Lecture #29 12/9/2009
Lecture #29 Dave Reckhow 1
CEE 371Water and Wastewater
Updated: 9 December 2009
Print version
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
David Reckhow CEE 371 L#29 2
CEE 371 Lecture #29 12/9/2009
Lecture #29 Dave Reckhow 2
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
David Reckhow CEE 371 L#29 3
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
David Reckhow CEE 371 L#29 4
CEE 371 Lecture #29 12/9/2009
Lecture #29 Dave Reckhow 3
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
David Reckhow CEE 371 L#29 5
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
David Reckhow CEE 371 L#29 6
CEE 371 Lecture #29 12/9/2009
Lecture #29 Dave Reckhow 4
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
David Reckhow CEE 371 L#29 7
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
David Reckhow CEE 371 L#29 8
CEE 371 Lecture #29 12/9/2009
Lecture #29 Dave Reckhow 5
Sanitary Sewer Principles VInverted siphons p(cont.)
David Reckhow CEE 371 L#29 9
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
David Reckhow CEE 371 L#29 10
CEE 371 Lecture #29 12/9/2009
Lecture #29 Dave Reckhow 6
Flow in Sewers IIGraphical psummary
David Reckhow CEE 371 L#29 11
Sanitary Sewer Layouts
David Reckhow CEE 371 L#29 12
H&H, Fig 10-2, pp.333
CEE 371 Lecture #29 12/9/2009
Lecture #29 Dave Reckhow 7
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
CEE 371 Lecture #29 12/9/2009
Lecture #29 Dave Reckhow 8
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
David Reckhow CEE 371 L#29 15
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
David Reckhow CEE 371 L#29 16
Adjust to peak daily flow using MOP9 graph:
CEE 371 Lecture #29 12/9/2009
Lecture #29 Dave Reckhow 9
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
David Reckhow CEE 371 L#29 17
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
David Reckhow CEE 371 L#29 18
5.067.049.1 SARn
Q =
CEE 371 Lecture #29 12/9/2009
Lecture #29 Dave Reckhow 10
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
David Reckhow CEE 371 L#29 19
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
David Reckhow CEE 371 L#29 20
CEE 371 Lecture #29 12/9/2009
Lecture #29 Dave Reckhow 11
Role of TopographyPreliminary Design follows ground elevationsy g g
David Reckhow CEE 371 L#29 21
Example system from V&HBasic info
40 persons/acreDomestic Q =100 gpcdInfiltration = 600g/acre//d
David Reckhow CEE 371 L#29 22
600g/acre//d
CEE 371 Lecture #29 12/9/2009
Lecture #29 Dave Reckhow 12
V&H example (cont.)s
David Reckhow CEE 371 L#29 23
V&H example (cont.)s
David Reckhow CEE 371 L#29 24
CEE 371 Lecture #29 12/9/2009
Lecture #29 Dave Reckhow 13
Construction methodsOpen-cut trenchpTrenchless - Tunneling
David Reckhow CEE 371 L#29 25
H&H, Fig 10-3, pp. 334
Trenchless MethodsBoring MachinegMicrotunnel
David Reckhow CEE 371 L#29 26
H&H, Fig 10-4, pp. 335
CEE 371 Lecture #29 12/9/2009
Lecture #29 Dave Reckhow 14
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
David Reckhow CEE 371 L#29 27
ManholesWith and without dropp
David Reckhow CEE 371 L#29 28
H&H, Fig 10-5, pp. 336
CEE 371 Lecture #29 12/9/2009
Lecture #29 Dave Reckhow 15
Service ConnectionsStraight linegMin 2% slope4 inch or 6 inchBedding
Crushed stone or coarse sand
David Reckhow CEE 371 L#29 29
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 =
David Reckhow CEE 371 L#29 30
H&H, Fig 10-7, pp. 338
FlumeCompare “H” to rating curve
CEE 371 Lecture #29 12/9/2009
Lecture #29 Dave Reckhow 16
Flow Measurement II
P t bl d i
David Reckhow CEE 371 L#29 31
H&H, Fig 10-9, pp. 340
Portable device
SamplersISCO type composite sampleryp p p
David Reckhow CEE 371 L#29 32
H&H, Fig 10-10, pp. 341
CEE 371 Lecture #29 12/9/2009
Lecture #29 Dave Reckhow 17
Sampling stationCombines flow and Quality measurementsQ y
David Reckhow CEE 371 L#29 33
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
CEE 371 Lecture #29 12/9/2009
Lecture #29 Dave Reckhow 18
Corrosion IssuesElectrochemical corrosionChemical reaction
Crown corrosion by hydrogen sulfide minimized by steep gradients of venting
David Reckhow CEE 371 L#29 35
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
David Reckhow CEE 371 L#29 36
H&H, Fig 10-13, pp. 344
CEE 371 Lecture #29 12/9/2009
Lecture #29 Dave Reckhow 19
Bedding and BackfillMust support o erb rdenoverburdenNative materials are usually sandy loam; readily compactable
David Reckhow CEE 371 L#29 37
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
David Reckhow CEE 371 L#29 38
CEE 371 Lecture #29 12/9/2009
Lecture #29 Dave Reckhow 20
Installation IISloped sidewalls por Trench box
David Reckhow CEE 371 L#29 39
H&H, Fig 10-15, pp. 348
Use of laser
Installation III
levels for achieving the desired slope
David Reckhow CEE 371 L#29 40
H&H, Fig 10-16, pp. 349
CEE 371 Lecture #29 12/9/2009
Lecture #29 Dave Reckhow 21
Soil CompactionModified Proctor Test
Adjusting moisture content to get to 90%+ of the maximum density
David Reckhow CEE 371 L#29 41
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
David Reckhow CEE 371 L#29 42
H&H, Table 10-3, pp. 352
Max allowable ranges from 100-500 gal/in-diameter /mile/day
CEE 371 Lecture #29 12/9/2009
Lecture #29 Dave Reckhow 22
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
David Reckhow CEE 371 L#29 43
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
David Reckhow CEE 371 L#29 44
H&H, Fig 10-21, pp. 355
CEE 371 Lecture #29 12/9/2009
Lecture #29 Dave Reckhow 23
EndTo next lecture
David Reckhow CEE 371 L#29 45
Hw problemGraphical psummary
d/D =55%
David Reckhow CEE 371 L#29 46V/Vfull =35%
d/D =12%
Q/Qfull =57%