Waste Water Collection

7/30/2019 Waste Water Collection

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Waste water Collection


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Types of Systems

Sanitary sewer

Storm sewer

Combined sewer


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Sanitary sewers

Most sewers are designed to flow by gravity(water flows down-hill)

Includes sewer pipe from home to septic tank orto a municipal collector pipe

Gravity sewers must follow the topography of theland

Where gravity flow is not possible, pumps are

used Individual unit pump

Large municipal lift (pump) station


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Pump stations

Pump (lift) sewage from low to higherelevation, generally from end of onegravity sewer section to another,

higher section

Consist of a wet well and pumps

Wet well forms a place for wastewaterto collect and be pumped from


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Gravity

Suited to;

Gently sloping terrain towards one side

of the townshipAreas with good excavation conditions

Reasonably dense housing (i.e. not

sparsely spaced blocks)

Remote areas where system responsetimes are likely to be long

Areas with a hi h robabilit of


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Pressure

Suited to:

Areas where excavation conditions are

difficultAreas with high ground water

Townships that are elongated such as

those that follow coastlines or rivers

Areas with sparsely located houses

Areas with significantly undulatingterrain


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Vaccum

Suited to:

Areas where excavation is difficult

Areas of high ground waterTownships with over 100 connections

Gently undulating sites

Townships that are elongated such as thosethat follow coastlines or rivers


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Haga clic para modificar el estilo de texto del patrn

Segundo nivel Tercer nivel

Cuarto nivel Quinto nivel


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Combined Sewers

Sanitary sewer also collects storm water runoff

Quantity is highly variable and site specific

CSO: Combined Sewer Overflow

Wastewater flows greatly increase during a stormIf capacity of sewer or treatment systems areexceeded, some of the combined waste is dischargedwith minimal to no treatment


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Haga clic para modificar el estilo de texto del patrn

Segundo nivel Tercer nivel

Cuarto nivel Quinto nivel


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Types of sewers

Wastewater collection systems areresponsible for collection andtransmission of liquid wastes to a

central treatment facility.

Like a distribution system for watersupply, the collection system resembles

a tree that branches out from thetreatment plant to collect thewastewater from individuals.


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Wastewater from individual homesenters the collection system from a

service line.These services attach to the lateralwith a wye connection or a tap and

saddle connection.Branch lines or laterals usually rundown the street, collecting the flow

from individual services.They, like tributaries in a watershed,flow into larger lines called mains.

Mains carry the flow into the largest


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Collection system

appurtenances


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Manholes

Manholes are installed in sewer lineswhenever there is an intersection,change of direction, or change the in

elevation or slope of a line.

They are needed to provide access tothe system for cleaning, inspection,and clearing stoppages. Although theyshould be large enough for operatorsto enter and work in, they can contain

hazardous atmospheres that can


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Avoidance of Submerged Conditions:Manholes should not be located inareas subject to flooding, from

floodplains, surface runoff, or ponding


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Junction chambers: Man-holeconstructed at the intersection of twolarge sewers.

Drop man-hole: When the differencein elevation of the invert levels of theincoming and outgoing sewers of the

man-hole is more than 0.5m, theinterception is made by dropping theincoming sewer vertically outside and

then it is jointed to the man-hole


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Basic considerations in the

design of sewersDesigning a sanitary sewer involvesestimation of waste flow rates for thedesign data and evaluation of any

local conditions, which may affect thehydraulic operation of the system;

The selection of the hydraulic-design

equation, alternative sewer pipematerials and

Minimum and maximum sizes,minimum and maximum velocities and


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Design flow

Peak hourly flow and peak infiltrationallowances for the entire service areaare used for the design of new sanitary

sewers.

The size of the pipe, the type of pipe,and the downhill grade of the line

determine the amount of flow that aline can handle. The average percapita flow that is used to size the

system is usually about 100


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Infiltration and inflow are also concernswhen designing a collection system.

Infiltration occurs when groundwaterenters the system through broken pipeor leaking joints in wet weather.

Inflow enters the system directly andmay come from runoff that floodsstreets, entering through submergedmanhole covers, or illegal serviceconnections that direct storm flows


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Hydraulic design Equation

Manning Equation

V = velocity (ft/sec)n = coefficient of roughness(dependent upon pipe

material/condition)n- 0.013 for new and existing well-

constructed sewers.

n- 0.015 for older sewers.

2/13/2486.1SR

nV =


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R = hydraulic radius = area/wettedperimeter (ft)

S = hydraulic slope (assumed to beslope of pipe) (ft/ft)


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Sewer Pipe Materials

The most common type of wastewaterpiping is vitrified clay pipe or VCP. It ismade of fired clay and is constructed

with bell and spigot connections.


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VCP has sufficient strength to withstandmost trench loads and is almostimpervious to corrosion caused by acids

that form when sewer gases aregenerated.

VCP is available from 4" to 36" in

diameter. It is very heavy in sizes above18".


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Reinforced concrete pipe, or RCP, isused for larger lines from 18" to 60" indiameter. It is lighter than VCP at these

sizes.

The problem with concrete pipe iscorrosion of the pipe crown area (top

inside of the pipe) from sewer gasesand organic acids.


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Cast iron pipe (CIP) and ductile ironpipes (DIP) are not used often incollection systems, but have specific

applications.

They are used in areas where hightrench loading exists like crossing

under a railroad track or a dirt roadthat carries heavy equipment.

Iron pipe is also used for inverteds hons where collection lines run


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Acrylonitrile butadiene styrene (ABS)pipe is made from plastic. It islightweight and flexible.

It is impervious to corrosion fromsewer gases, acids and bases, andinorganic salts found in wastewater. It

is softened by petroleum products,which are not normally found indomestic wastewater.

It can't withstand heav trench loadin


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Polyvinyl chloride (PVC) piping is alsolightweight and easy to install.Petroleum products do not affect it, but

trench loading is still a problem.


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Minimum Velocity

The flow velocity in the sewers shouldbe such that the suspended materialsin sewage do not get silted up; i.e. the

velocity should be such as to causeautomatic self-cleansing effect.

The generation of such a minimum self

cleansing velocityin the sewer, atleastonce a day, is important, because ifcertain deposition takes place and is

not removed, it will obstruct free flow,


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Maximum Velocity

The smooth interior surface of a sewer pipegets scoured due to continuous abrasioncaused by the suspended solids present in

sewage.It is, therefore, necessary to limit themaximum velocity in the sewer pipe. Thislimiting or non-scouring velocity will mainly

depend upon the material of the sewer


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Effects of Flow Variationon Velocity in a Sewer

Due to variation in discharge, thedepth of flow varies, and hence thehydraulic mean depth (r) varies.

Due to the change in the hydraulicmean depth, the flow velocity (whichdepends directly on r2/3) gets affected

from time to time.

It is necessary to check the sewer formaintaining a minimum velocity ofabout 0.45 m/s at the time of


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The designer should also ensure that avelocity of 0.9 m/s is developed at leastat the time of maximum flow and

preferably during the average flowperiods also.

Moreover, care should be taken to see

that at the time of maximum flow, thevelocity generated does not exceed thescouring value


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Minimum slopes

Minimum slopes are often used toavoid extensive excavation where theslope of ground surface is flat.

As the pipe sizes increases beyond 600mm, the minimum practicable slopefor construction is about 0.0008 m/m.


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MAINTENANCE OFSEWERS


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Roots, Grease, AndCorrosion

Root intrusion is a major concern forcollection system operators.

Tree roots that find their way intosewer lines through leaking joints orcustomers' service lines can causechronic stoppage and backup

problems. Roots must be cut to clearthe line. The problem is that cuttingroots is similar to pruning shrubs

Use of little co er sulfate in sewer is


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Lines that carry large amounts ofgrease may experience clogging when

it accumulates on the crown of thepipe. The result is that the carryingcapacity of the pipe is greatly reduced.

This restriction will cause backupsduring peak flow conditions.

Flushing, Use of special tools attached

to cables or jointed rods and pushed orpulled through the sewer frommanhole or other point of entry to

remove grease from pipelines.


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Corrosion

Corrosion in sewer lines can be theresult of chemicals from industrialdischarges.

It can also be caused by septicconditions that release hydrogensulfide gas (H2S). Sulfuric acid is

created when the hydrogen sulfide gasreacts with the moisture on the insideof the pipe wall. This can cause serious

corrosion problems in large concrete


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It is less of an issue in vitrified claypipe. Acids do not affect PVC pipe.

Aerated lift stations and the additionof chlorine in the collection system aretwo ways of minimizing hydrogensulfide production.


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Waste Water Collection