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Collection System Design and Layout

© American Water College 1

© American Water College

Waste Collection Systems

System Design and Layout


Design and Layout Training Objectives

To gain an understanding of:

Collection system vocabulary

Collection system design criteria

The function of collection system components

How to estimate flow velocity within a sewer


Vocabulary

Appurtenance

Machinery, appliances, structure and other parts of the

main structure necessary to allow it to operate as

intended, but not part of the main structure




Vocabulary

Backwater Gate

A gate installed at the end of a drain or outlet pipe to

prevent the backward flow of water or wastewater.

Generally used on storm sewer outlets into streams to

prevent backward flow during times of flood or high tide.


Vocabulary

Barrel

(1) The cylindrical part of a pipe that may have a bell

on one end.

(2) The cylindrical part of a manhole between the cone

at the top and the shelf at the bottom.


Vocabulary

Bedding

The prepared base or bottom of a trench or excavation

on which a pipe or other underground structure is

supported.




Vocabulary

Blockage

(1) Partial or complete interruption of flow as a result of

some obstruction in a sewer.

(2) Commonly called a Stoppage.


Vocabulary

Branch Sewer

A sewer that receives flow from a relatively small area

and discharges into a main sewer serving more than

one branch sewer area.


Vocabulary

Building Sewer A gravity-flow pipeline connecting a building wastewater collection system to a lateral or branch sewer. The building sewer may begin at the outside of the building's foundation wall or some distance (such as 2 to 10 feet) from the wall, depending on local sewer

ordinances.




Vocabulary

Catch Basin

A chamber or well used with storm or combined sewers

as a means of removing grit which might otherwise

enter and be deposited in sewers.


Vocabulary

Cleanout

An opening in a wastewater collection system used for

inserting tools, rods or snakes while cleaning a pipeline

or clearing a stoppage.


Vocabulary

Combined Sewer

A sewer designed to carry both sanitary wastewater and

storm or surface water runoff.




Vocabulary

Concentric Manhole Cone

Cone that tapers uniformly from the barrel to the

manhole cover.


Vocabulary

Eccentric Manhole Cone

Cone that tapers from the barrel to the manhole cover

with one side usually vertical.


Vocabulary

Curb Inlet

A chamber or well built at the curb-line of a street to

allow gutter flow into the storm water drainage system.




Vocabulary

Flap Gate

A hinged gate that is mounted at the top of a pipe or

channel to allow flow in only one direction. Flow in the

wrong direction closes the gate.


Vocabulary

Flow Line

The top of the wetted line, the water surface or the

hydraulic grade line of water flowing in an open channel

or partially full conduit.


Vocabulary

Flusher Branch

A line built specifically to allow the introduction of large

quantities of water to the collection system so the lines

can be "flushed out“ with water. Also installed to provide

access for equipment to clear stoppages in a sewer.




Vocabulary

Force Main

A pipe that carries wastewater under pressure from the

discharge side of a pump to a point of gravity flow

downstream.


Vocabulary

Grade Ring

A precast concrete ring 4 to 12 inches high which is

placed on top of a manhole cone to raise the manhole

cover frame flush with the surface grade. Sometimes

called a "spacer."


Vocabulary

Grit

The heavy mineral material present in wastewater such

as sand, coffee grounds, eggshells, gravel and cinders.

Grit tends to settle out at flow velocities below 2 ft/sec

and accumulate in the invert or bottoms of the pipelines.




Vocabulary

Hydraulic Grade Line (HGL)

The surface or profile of water flowing in an open

channel or a pipe flowing partially full. If a pipe is under

pressure, the hydraulic grade line is at the level water

would rise to in a small tube connected to the pipe.


Vocabulary

Hydrogen Sulfide Gas (H2S)

Hydrogen sulfide is a gas with a rotten egg odor. This gas is produced under anaerobic conditions. Hydrogen sulfide gas dulls the sense of smell so that you don't notice it after you have been around it for a while. The gas is very poisonous to the respiratory system, explosive, flammable, colorless and heavier than air.


Vocabulary

Infiltration

The seepage of groundwater into a sewer system,

including service connections. Seepage frequently

occurs through defective or cracked pipes, pipe joints,

connections or manhole walls.




Vocabulary

Inflow

Water discharged into a sewer system and service

connections from such sources as: rain gutters, cellar

sump drains, yard and area drains, foundation drains,

cooling water discharges, cross connections from

storm and combined sewer systems.


Vocabulary

Infiltration / Inflow

The total quantity of water from both infiltration and

inflow without distinguishing the source.

Abbreviated I & I or I/I.


Vocabulary

Intercepting Sewer

A sewer that receives flow from a number of other large

sewers or outlets and conducts the waters to a point for

treatment or disposal. Often called an "interceptor."




Vocabulary

Internal Inflow

Non-sanitary or industrial wastewaters generated inside of a domestic, commercial or industrial facility and being discharged into the sewer system. Examples are cooling tower waters, basement sump pump discharge waters, continuous-flow drinking fountains, and defective or leaking plumbing fixtures.


Vocabulary

Invert

The lowest point of the channel inside a pipe or

manhole.


Vocabulary

Inverted Siphon

A pressure pipeline used to carry wastewater flowing in

a gravity collection system under a depression such as

a valley or roadway or under a structure such as a

building.




Vocabulary

Lateral Sewer

A sewer that discharges into a branch or other sewer

and has no other common sewer tributary to it.

Sometimes called a "street sewer'' because it collects

wastewater from individual homes.


Vocabulary

Lift Station

A wastewater pumping station that lifts the wastewater to a higher elevation when continuing the sewer at reasonable slopes would involve excessive depths of trench. Also, an installation of pumps that raise wastewater from areas too low to drain into available sewers.


Vocabulary

Outfall

(1) The point, location or structure where wastewater or drainage discharges from a sewer, drain, or other conduit.

(2) The conduit leading to the final disposal point or area.




Vocabulary

Peaking Factor

Ratio of a maximum flow to the average flow. For

example: maximum hourly flow or maximum daily flow

to the average daily flow.


Vocabulary

Pig

Pig Refers to a poly pig which is a bullet-shaped device

made of hard rubber or similar material. This device is

used to clean pipes.


Vocabulary

Sanitary Sewer

A pipe or conduit intended to carry wastewater or waterborne wastes from homes, businesses, and industries to the wastewater treatment plant. Storm water runoff or unpolluted water should be collected and transported in a separate system of pipes or conduits and discharged to natural waterways.




Vocabulary

Sedimentation

The process of settling and depositing suspended matter carried by wastewater. Sedimentation usually occurs by gravity when the velocity of the wastewater is reduced below the point at which it can transport the suspended material. (2 fps)


Vocabulary

Septic

A condition produced by anaerobic bacteria. If severe,

the wastewater produces hydrogen sulfide, turns black

and gives off foul odors. Septic wastewater has a high

oxygen demand.


Vocabulary

Sewer Gas

(1) Gas in collection lines that results from the decomposition of organic matter in the wastewater. (2) Any gas present in the wastewater collection system, even though it is from such sources as gas mains, gasoline, and cleaning fluid.




Vocabulary

Sewerage

System of piping with appurtenances for collecting,

moving and treating wastewater from source to

discharge.


Vocabulary

Siphon

A pipe or conduit through which water will flow above the hydraulic grade line (HGL) under certain conditions. Water (or other liquid) is first forced to flow or is sucked or drawn through the pipe by creation of a vacuum.


Vocabulary

Slope

The slope or incline of a sewer trench excavation is the

ratio of the vertical distance to the horizontal distance,

also called the "rise over run."




Vocabulary

Storm Sewer

A separate pipe, conduit or open channel (sewer) that

carries runoff from storms, surface drainage, and street

wash, but does not include domestic and industrial

wastes.


Vocabulary

Tap

A small hole in a sewer where a wastewater service line

from a building is connected into a lateral or branch

sewer.


Vocabulary

Thrust Block

A mass of concrete or similar material placed around a

pipe to prevent movement when the pipe is carrying

water. Usually placed at bends and valve structures.




Vocabulary

Tide Gate

A gate with a flap suspended from a free-swinging

horizontal hinge, usually placed at the end of a conduit

discharging into a body of water having a fluctuating

surface elevation.


Vocabulary

Trunk Sewer

A sewer that receives wastewater from many tributary

branches or sewers and serves a large area and

population.


Vocabulary

Weir

A wall or plate placed in an open channel and used to measure the flow of water. The depth of the flow over the weir can be used to calculate the flow rate, or a chart or conversion table may be used to convert depth to flow.




Vocabulary

Wetted Perimeter

The length of the wetted portion of a pipe covered by

flowing wastewater.


Purpose

Purpose of Collection System

To collect and convey the wastewater from

homes and industry at a flow velocity greater

than 2 feet per second to a wastewater

treatment plant.




Method of Conveyance



Force used to move wastewater

Gravity

Low Pressure

Vacuum



Gravity





Low Pressure



Vacuum

Bilfinger Airvac Water Technologies Inc.

Vacuum Station

Vacuum Interface Unit


Type of Waste Conveyed




Type of Waste

Sanitary Sewer System

Storm Water system

Combined System


Type of Waste

Sanitary Sewer System


Type of Waste

Storm Water system




Type of Waste

Combined System


Sewer System Capacity


System Capacity

Considerations:

Current Flows

Residential, Commercial, Industrial

Future Flows

Residential, Commercial, Industrial

Infiltration and Inflow

Groundwater and Storm Water

Flow Velocity

Sufficient to carry solids and organic matter




System Capacity

Flow Variations:

Minimum flow = early morning

Maximum flow = middle of day


System Capacity

Flow Velocity:

Measured with instrumentation

Estimated by dye test

Estimated by float test


System Capacity

Full Pipe Instrumentation:

Magnetic

Ultra Sonic

Pressure Differential




System Capacity

Open Channel:

Flume

Weir

Surface Velocity and Depth


System Capacity

Dye test

1. Insert dye through manhole and record time

2. Record time of first sighting of dye at

downstream manhole

3. Record time dye is no longer visible at

downstream manhole

4. Determine average time

5. Divide length between manholes by average

time (velocity = feet per second)


System Capacity

Dye Test

Flow Velocity = Distance Avg. Time

Distance in feet




System Capacity

Float Test

1. Insert float at manhole and record time

2. Record time float arrives at downstream

manhole

3. Divide distance between manholes by time in

seconds (velocity = feet per second)

4. Subtract 10%-15% due to pipe wall friction


System Capacity

Float Test

Flow Velocity = Distance Avg. Time

Distance in feet


System Capacity

Plotting Flow

Good tool to identify Inflow or infiltration

problems when evaluated regularly.




Gravity Sewer System


System Components

Interceptor Sewer

WWTP

High Point

Low Point

A sewer that receives

flow from a number of

other large sewers or

outlets and conveys the

water to a point for

treatment or disposal.


System Components

Trunk Sewers

WWTP

High Point

Low Point


wastewater from many

tributary branches or

sewers and serves a

large territory and

contributing population.




System Components

Main Sewers

WWTP

High Point

Low Point

A sewer line that

receives wastewater

from many tributary

branches and sewer

lines and serves as an

outlet for a large

territory or is used to

feed an intercepting

sewer.


System Components

Branch Sewers

WWTP

High Point

Low Point


wastewater from a

relatively small area and

discharges into a main

sewer serving more

than one branch

sewer area.


System Components

Lateral Sewers

WWTP

High Point

Low Point

A sewer that discharges

into a branch or other

sewer and has no other

common sewer tributary

to it. Sometimes called a

"street sewer'' because it

collects wastewater from

individual homes.




Gravity System Components

Building Sewer Systems


System Components

Lift Stations

A wastewater pumping

station that lifts the

wastewater to a higher

elevation when continuing

the sewer at reasonable

slopes would involve

excessive depths of trench.

These stations may be

equipped with air-operated

ejectors or centrifugal

pumps.


Low Pressure Sewer System




Low Pressure System Components

Low Pressure systems are

installed in areas where

the topography of the land

and ground conditions are

not suitable for a gravity

system.

These may include:

Flat terrain

Rocky ground

High water table



Advantages:

Fewer stoppages

No root intrusion

No infiltration

No deep trenches

No inverted siphons



Disadvantages:

Higher energy cost

Greater cost for pump

stations





Gravity Sewer

The line from the house

or building to the Pump

Chamber, also called a

Pressurization Unit, is a

gravity line that is

vented to atmospheric

pressure.



Holding Tank

Serves as reservoir and

holds about 50 gallons.

Contains Grinder Pump.



Grinder Pump

Grinds solid waste to

prevent downstream

plugging. Also raises

water pressure to move

wastewater.





Pressure Main

Convey wastewater to

the treatment plant. They

follow the contour of the

land since they do not

rely on gravity to move

the water.

Pressure mains have

access points for “PIG”

insertion for line cleaning


Vacuum Sewer System


Vacuum System Components

Vacuum systems are

being installed for the

same reasons as Low

Pressure Systems

Operation/Maintenance

is more difficult than for

Gravity and Pressure

systems.





Gravity sewers

The line from the house

or building to the

Vacuum Interface Unit.



Vacuum Interface Unit

Connects the gravity

line to the vacuum line

and seals the system to

maintain vacuum in the

system. Accumulates

about 3 gallons of water

before actuating a valve

to allow flow into the

system.



Vacuum Mains

Convey wastewater to

the treatment plant.

They follow the contour

of the land since they

do not rely on gravity to

move the water.





Vacuum Pumps

Installed at a central

station adjacent to the

treatment plant. They

maintain the proper

vacuum in the system.


Sewer System Appurtenances



Backflow Preventers

Type “A”

Used when elevation at top of

device is lower than floor

elevation of building.

Allows wastewater to dump

to ground before entering

building.

May not dump fast enough to

prevent backflow into

building.





Backflow Preventers

Type “B”

Used when ground elevation

along building sewer is above

floor elevation.

Uses a swing check or a ball

check valve to prevent

backflow.

Requires maintenance to

ensure seating surface is not

fouled with debris.



Backflow Preventers

Automated Type

Uses a flexible diaphragm

and level sensor in building

sewer.

When backflow is detected,

air valve is actuated to close

the diaphragm which closes

the building.

Air is supplied by a small

compressor and storage tank.

Alarm is activated when

system is actuated.



Building Sewer Cleanouts Allow access to remove solid debris causing blockage.

Location established by local building codes.

At a minimum, cleanout should be located 3 feet from building foundation and another at the property line.

In cold climates, the building sewer cleanout may be installed in the inside wall of the basement, below the frost line.





Building Sewer

Connections

Referred to as “Taps” to

street sewers.

Installed during initial

construction or as new

service is needed.

Can be source of

groundwater infiltration

or system blockage if

not installed properly.



Lateral and Branch Sewer Cleanouts

Provide a location to insert cleaning equipment or water for flushing the line other than at a manhole location.

Slant riser provides easy access but does not hold up well to street traffic.



Manholes

Installed in lateral, main,

trunk and interceptor

sewers.

Provides access point

for People or equipment

to perform maintenance,

repairs or inspections.

Usually spaced 300-500

feet apart.





Drop Manhole

Used when elevation

difference between

inflow and outflow is

greater than the slope of

the manhole channel.

To avoid using Drop

Manholes, Steeper

slopes entering the

manhole are preferred.



Junction Structures

Used to join large

diameter trunk sewers.

Usually has a way to

regulate or divert flow

within the structure



Interconnector Sewers

Short lines that connect

adjacent sewers and allows

them to run in parallel.

Allows for automatic diversion

of flow from one to the other if

blockage occurs.

Periodic inspections are

required to detect and correct

blockage.





Inverted Siphons

Lines installed below the

normal sewer elevation to

allow crossing under a

depressed roadway or

watercourse.

Upstream water pushes

downstream water through a

completely filled pipe.

Low flow velocity may cause

accumulation of solids a low

point of siphon.



Air Jumpers

Installed with inverted

siphons to allow air flow

between the manholes on

either end of the siphon.

Prevents the accumulation of

hydrogen sulfide in the

upstream manhole.

Hydrogen sulfide is very toxic

and corrosive.



Flow Regulators

Used to divert flow from

one line to another to

optimize operation of the

system.

Weirs are used in

manholes and junction

structures to divert flow.

Hydro-Brakes are also

used to control flow.




Sewer System Design


Sewer System Design

Size and Slope

Considerations:

Follow slope of land if practical

Adequate to provide gravity flow

Adequate to achieve scour velocity (2 fps) 8 inch pipe slope = 0.40 foot per 100 feet of pipe

16 inch pipe slope = 0.14 foot per 100 feet of pipe

36 inch pipe slope = 0.046 foot per 100 feet of pipe

Half full at peak dry weather flow

Barely full at peak wet weather flow


Sewer System Design

House sewers minimum 4 inches

Commercial and industrial minimum 6 inches

Lateral and Branch sewers 6 to 8 inches




Sewer System Design

Location and Alignment

Lateral, Main and Trunk lines

centered in road

Separated from water lines as

required by Health Department

Curved sewers are not

desirable

Minimum radius of curvature is

200 feet to facilitate use of

modern cleaning equipment


Sewer System Design

Depth

Lateral and Main sewers are

usually buried to a depth of 6

feet.

Allows for sewer lines to

pass below most

underground utilities.

Trunk and Interceptor sewer

depth will depend on depth

of connecting main sewers.


Sewer System Design

Loading

The depth and with of the

trench, the type of

backfill material, and the

compaction method will

determine the surface

load placed on the sewer

pipe.

Surface Load

Resistance to

surface load




Sewer System Design

Piping Material

Materials selected based on application

Piping can be rigid or flexible

Considerations: Gravity or force main?

Wastewater abrasive?

Potential for internal or external corrosion?

Loading requirements?

Infiltration or exfiltration a potential problem?

Connection requirements?

Cost and life expectancy?

Handling requirements and ease of installation?


Sewer System Design

Piping Material

Rigid:

Asbestos Cement Pipe (ACP)

Cast Iron Pipe (CIP)

Reinforced Concrete Pipe (RCP)

Pre-stressed Concrete Pressure Pipe (PCPP)

Vitrified Clay Pipe (VCP)


Sewer System Design

Piping Material

Flexible:

Ductile Iron Pipe (DIP)

Steel Pipe

Acrylonitrile Butadiene Styrene (ABS)

ABS Composite

Polyethylene (PE)

Polyvinyl Chloride (PVC)

Reinforced Plastic Mortar (RPM)

Reinforced Thermosetting Resin (RTR)




Sewer System Design

Piping Joints

Gasket

Flanged

Cement Mortar

Solvent Cement

Rubber Coupling with Compression Bands

Welded

Heat Fusion

Bituminous

Elastomeric

Mastic

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