Collection System Fundamentals & Components

Learn wastewater collection-system fundamentals, including gravity sewers, laterals, manholes, interceptors, pump stations, force mains, flow characteristics, access structures, maintenance, safety, and troubleshooting.

A wastewater collection system carries sewage from homes, businesses, industries, and other users to a treatment facility. Reliable collection depends on gravity sewers, service connections, manholes, pump stations, force mains, and other components working together while limiting blockages, overflows, infiltration, inflow, odors, and structural failures.

Collection-system operators need to understand how wastewater moves through the system, why pipe slope and capacity matter, how access structures support inspection and maintenance, how pump stations serve areas that cannot drain completely by gravity, and how failures in one part of the system can affect customers and treatment-plant operation.

Purpose of a Wastewater Collection System

The primary purpose of a collection system is to convey wastewater safely and reliably to treatment.

A well-operated system should:

provide adequate flow capacity;
minimize blockages and backups;
limit sanitary sewer overflows;
reduce excessive infiltration and inflow;
provide access for inspection and maintenance.

Major Collection-System Components

Common components include:

building sewers and service laterals;
gravity sewers;
manholes;
cleanouts;
interceptors and trunk sewers;
pump stations;
force mains.

Gravity Sewers

Gravity sewers carry wastewater downhill because of elevation difference and pipe slope.

Gravity systems depend on:

adequate slope;
proper pipe alignment;
sufficient capacity;
clear pipe interiors.

Pipe Slope

Slope provides the energy needed to move wastewater through a gravity sewer.

If slope is too low, flow velocity may decrease and solids can accumulate.

If hydraulic conditions become unfavorable, operators may see:

deposits;
blockages;
odor;
reduced capacity.

Flow Does Not Always Fill the Pipe

Gravity sewers usually operate partially full during normal conditions.

Air occupies the space above the wastewater.

During high-flow conditions, the depth can increase and the sewer may become surcharged.

Surcharge

Surcharge occurs when wastewater rises above the normal open-channel level and the sewer begins operating under pressure.

Surcharging can result from:

high wet-weather flow;
downstream restrictions;
blockages;
insufficient pipe capacity.

Building Sewer and Service Lateral

A service lateral carries wastewater from an individual property toward the public sewer.

Depending on local terminology and ownership boundaries, portions may also be called:

building sewer;
house lateral;
service connection.

Service Connections Can Affect the Main Sewer

Problems in laterals can contribute to:

root intrusion;
groundwater infiltration;
stormwater inflow;
debris entry.

Cleanouts

Cleanouts provide access to certain smaller sewer lines or laterals for:

inspection;
cleaning;
blockage removal.

Manholes

Manholes provide access to the sewer system for inspection, maintenance, cleaning, and flow observation.

They are commonly located at:

changes in direction;
changes in slope;
changes in pipe size;
junctions;
selected intervals along sewer lines.

Manhole Components

A manhole can include:

frame and cover;
chimney or upper structure;
barrel;
bench;
channel or invert.

Manhole Channel

The channel directs wastewater through the bottom of the manhole and should provide a smooth flow path between connecting sewers.

Manhole Bench

The bench is the sloped surface beside the channel.

Proper configuration helps keep wastewater in the flow channel rather than allowing solids to accumulate across the manhole floor.

Manhole Defects

Common problems include:

cracked structures;
leaking joints;
corrosion;
damaged covers;
infiltration;
root intrusion;
debris accumulation.

Manhole Covers

Manhole covers should remain:

secure;
accessible;
structurally sound.

Damaged or displaced covers can create public-safety and inflow problems.

Direct Inflow Through Manholes

Stormwater can enter through:

pick holes;
damaged covers;
poorly sealed frames;
low-lying structures.

Trunk Sewers

Trunk sewers collect wastewater from multiple smaller sewers and convey larger flows through the system.

Interceptors

Interceptors are large sewers that receive wastewater from major portions of the collection system and carry it toward treatment.

Increasing Flow Downstream

As tributary sewers combine, downstream flow generally increases.

This means larger downstream pipes often carry:

more wastewater;
greater peak flow;
greater wet-weather flow.

Separate Sanitary Sewers

A separate sanitary sewer is intended primarily to carry wastewater rather than stormwater.

Stormwater entering a sanitary sewer can consume capacity and increase treatment-plant flow.

Infiltration

Infiltration is groundwater that enters the sewer system through defects such as:

cracked pipes;
defective joints;
leaking manholes;
damaged service connections.

Inflow

Inflow is water entering the collection system through relatively direct connections or openings.

Possible sources include:

storm drains;
roof drains;
sump pumps;
foundation drains;
open or defective manhole covers.

Why Infiltration and Inflow Matter

Excessive infiltration and inflow can:

increase sewer flow;
reduce available capacity;
increase pump-station operation;
increase treatment-plant hydraulic loading;
contribute to overflows and backups.

Dry-Weather Flow

Dry-weather flow generally consists mainly of normal wastewater contributions plus relatively steady infiltration.

Wet-Weather Flow

Wet-weather flow may increase because of:

inflow;
increased groundwater infiltration;
illegal or unintended stormwater connections.

Flow Patterns

Wastewater flow commonly varies during the day.

Daily patterns may show:

lower flow overnight;
morning peaks;
evening peaks.

Peak Flow

Collection systems must be able to handle short periods of flow significantly higher than average flow.

Velocity

Wastewater velocity affects:

solids transport;
deposit formation;
hydraulic capacity.

Low Velocity

Low velocity can allow:

grit accumulation;
grease deposition;
settleable solids buildup.

High Velocity

High velocity can improve solids transport but may also increase:

abrasion;
hydraulic turbulence;
downstream energy effects.

Continuity Concept

A basic hydraulic relationship is:

Flow = Area × Velocity

For the same flow, a smaller flowing area requires a higher average velocity.

Flow Example

If wastewater flows through an effective cross-sectional area of 2 ft² at 3 ft/s:

Flow = 2 × 3

Flow = 6 ft³/s

This relationship helps connect sewer size, flow depth, and velocity.

Blockages

Common causes of sewer blockages include:

grease;
roots;
rags and wipes;
debris;
collapsed pipe;
settled solids.

Grease

Fats, oils, and grease can cool and accumulate on sewer walls.

Over time they can:

reduce pipe area;
trap debris;
create blockages.

Roots

Roots can enter through cracks or defective joints.

Root intrusion can:

restrict flow;
capture debris;
damage pipe further.

Rags and Wipes

Fibrous materials can accumulate around:

roots;
rough pipe surfaces;
pump components;
other obstructions.

Sanitary Sewer Overflow

A sanitary sewer overflow occurs when wastewater escapes from the sanitary collection system before reaching the treatment plant.

Possible causes include:

blockages;
excessive wet-weather flow;
pump-station failure;
pipe collapse;
downstream hydraulic restriction.

Backup Versus Overflow

A sewer backup may send wastewater toward:

building plumbing;
basements;
private property.

An overflow may discharge wastewater from a manhole or another system opening.

Early Warning Signs

Possible warning signs include:

rising upstream manhole levels;
reduced downstream flow;
repeated customer backups;
pump-station high-level alarms;
unusual SCADA trends.

Pump Stations

Pump stations are used where wastewater cannot continue to flow by gravity to the next part of the system.

They lift wastewater to a higher elevation or pressure so flow can continue.

Common Pump-Station Components

A pump station can include:

wet well;
pumps;
motors;
level controls;
check valves;
isolation valves;
electrical controls;
alarms;
backup power.

Wet Well

The wet well receives wastewater before it is pumped.

Wet-well level controls can start and stop pumps according to wastewater level.

Wet-Well Problems

Problems can include:

grease accumulation;
ragging;
odor;
corrosion;
failed level sensors.

Lead and Lag Pumps

Many stations use multiple pumps.

A common arrangement includes:

lead pump;
lag pump;
alternation between pumps.

Lag Pump

The lag pump can start when:

flow exceeds lead-pump capacity;
wet-well level continues rising;
additional pumping is required.

High-Level Alarm

A high-level alarm warns operators that wastewater level is approaching an abnormal condition.

Possible causes include:

pump failure;
power failure;
blocked force main;
excessive inflow;
failed controls.

Backup Power

Some pump stations use generators or other backup power to reduce the risk of overflow during utility outages.

Force Main

A force main is a pressurized pipeline carrying wastewater discharged by a pump station.

Gravity Sewer Versus Force Main

A gravity sewer normally operates because of elevation and slope.

A force main operates under pressure supplied by pumps.

Force-Main Components

Depending on the system, components can include:

isolation valves;
check valves;
air-release equipment;
cleanout or access features.

Force-Main Problems

Possible problems include:

air accumulation;
leaks;
corrosion;
blockage;
pressure transients.

Force-Main Failure

A force-main break can release wastewater under pressure and rapidly create:

service disruption;
environmental impact;
pump-station problems.

Air in Force Mains

Air can collect at high points and interfere with hydraulic performance.

Air-release equipment may be used where appropriate.

Odor and Corrosion

Wastewater that remains in a force main or wet well for long periods can become septic.

Septic wastewater can contribute to:

odor;
hydrogen sulfide generation;
corrosion.

Hydrogen Sulfide

Hydrogen sulfide can be produced under anaerobic wastewater conditions.

It is important because of:

worker toxicity;
odor;
corrosion potential.

Corrosion in Collection Systems

Corrosion can affect:

concrete;
metal pipe;
manholes;
pump-station equipment.

Collection-System Maps

Accurate maps help operators identify:

pipe locations;
pipe sizes;
flow direction;
manholes;
pump stations;
force mains.

Asset Records

Useful records can include:

pipe material;
diameter;
installation date;
inspection history;
cleaning history;
repair history.

Preventive Maintenance

Collection-system maintenance can include:

sewer cleaning;
root control;
manhole inspection;
pump maintenance;
force-main inspection;
condition assessment.

High-Frequency Maintenance Areas

Some sewer sections require more frequent attention because of:

grease;
roots;
low slope;
repeated blockages;
structural defects.

Use Maintenance History

Repeated cleaning of the same location can indicate an underlying problem rather than a random blockage.

Inspection

Collection-system inspection can use:

visual inspection;
closed-circuit television;
manhole inspection;
flow monitoring;
other condition-assessment methods.

Collection-System Safety

Collection work can involve:

confined spaces;
traffic;
excavation;
biological exposure;
hazardous gases;
moving equipment.

Manholes Can Be Hazardous

Manholes and related structures can contain:

low oxygen;
hydrogen sulfide;
methane;
other hazardous atmospheres.

Do Not Enter Without Appropriate Procedures

Opening a manhole for observation is not the same as entering it.

Entry should follow applicable confined-space procedures.

Traffic Hazards

Collection-system structures are frequently located in:

streets;
intersections;
parking areas.

Operators should use appropriate work-zone and traffic-control procedures.

Biological Exposure

Wastewater can contain disease-causing organisms.

Good practices include:

appropriate PPE;
handwashing;
avoiding hand-to-mouth contact;
cleaning contaminated equipment.

Example: Upstream Manhole High, Downstream Manhole Low

This pattern can indicate a restriction between the two locations.

Possible causes include:

blockage;
collapsed pipe;
heavy deposits.

Example: Several Manholes High During Rain

This pattern can indicate:

wet-weather infiltration and inflow;
downstream capacity limitation;
pump-station limitation.

Example: Repeated Blockage at the Same Location

Review:

roots;
grease;
pipe slope;
structural defects;
service connections.

Example: Pump Station Runs Much Longer Than Normal

Possible causes include:

higher incoming flow;
pump wear;
force-main restriction;
check-valve problem;
level-control problem.

Example: Wet-Well Level Rises While Pump Runs

Incoming flow may exceed pump output, or pump performance may be reduced.

Review:

pump flow;
force-main pressure;
valve position;
pump condition;
incoming flow.

Example: High-Level Alarm During a Power Outage

Review:

backup generator operation;
available wet-well storage;
portable pumping options;
overflow risk.

Example: Strong Odor Near Force-Main Discharge

Possible causes include:

long wastewater detention;
septic conditions;
hydrogen sulfide generation.

Example: Treatment-Plant Flow Rises Rapidly During Rain

Review collection-system wet-weather response and possible infiltration and inflow.

Example: One Neighborhood Reports Multiple Backups

Investigate:

downstream main blockage;
capacity problem;
pump-station problem where applicable.

Collection-System Troubleshooting Strategy

Start by determining:

where the abnormal condition begins;
whether the problem is local or systemwide;
whether flow conditions changed;
whether equipment is operating normally.

Trace the Hydraulic Path

Maps and field observations can help determine:

upstream sources;
downstream restrictions;
alternate flow paths;
affected pump stations.

Common Collection-System Fundamentals Mistakes

Assuming all sewer flow is sanitary wastewater.
Confusing infiltration with inflow.
Ignoring service laterals as sources of system problems.
Ignoring repeated blockage history.
Assuming high treatment-plant flow must originate at the plant.
Ignoring pump-station alarms until overflow occurs.
Confusing gravity sewers with force mains.
Ignoring manhole condition and direct stormwater entry.
Entering manholes without appropriate confined-space procedures.
Failing to keep maps and maintenance records current.

A Practical Collection-System Review

Review current system flow.
Review weather conditions.
Review pump-station status and alarms.
Review known blockage locations.
Review manhole observations.
Review recent customer complaints.
Compare current conditions with normal trends.
Identify locations requiring inspection or maintenance.

A Practical Blockage Review

Identify the affected customers or manholes.
Determine upstream and downstream sewer conditions.
Locate the likely restricted segment.
Protect the work area.
Clean or remove the blockage according to facility procedures.
Verify restored flow.
Inspect for underlying structural or root problems.
Document the event and corrective action.

A Practical Pump-Station Review

Review wet-well level.
Review pump status.
Review pump run time.
Review discharge pressure or flow where available.
Review check and isolation valves.
Review electrical power and alarms.
Review force-main condition.
Compare performance with historical trends.

A Practical Wet-Weather Review

Review rainfall timing.
Review collection-system flow response.
Review pump-station run times.
Review high manhole or overflow locations.
Compare dry-weather and wet-weather flow.
Identify areas with excessive infiltration or inflow.
Document recurring problem areas for further investigation.

What to Remember for the Exam

Wastewater collection systems convey sewage from users to treatment facilities.
Major components include service laterals, gravity sewers, manholes, trunk sewers, interceptors, pump stations, and force mains.
Gravity sewers depend on slope and normally operate partially full.
Surcharge occurs when sewer depth rises and the pipe begins operating under pressure.
Low sewer velocity can contribute to solids, grit, and grease accumulation.
Manholes provide access for inspection, cleaning, maintenance, and flow observation.
Infiltration is groundwater entering through defects, while inflow enters through relatively direct stormwater connections or openings.
Excessive infiltration and inflow consume collection-system and treatment capacity.
Common blockage causes include grease, roots, wipes, debris, structural failure, and settled solids.
Sanitary sewer overflows can result from blockages, excessive wet-weather flow, pump failures, pipe failures, or hydraulic restrictions.
Pump stations lift wastewater where gravity flow is insufficient.
Wet wells, pumps, controls, alarms, valves, and backup power are common pump-station components.
A force main carries pumped wastewater under pressure.
Long detention in wet wells and force mains can contribute to septicity, odor, hydrogen sulfide, and corrosion.
Accurate system maps are essential for understanding flow paths and responding to failures.
Repeated maintenance problems should be investigated for underlying structural or hydraulic causes.
Collection-system work can involve confined-space, traffic, excavation, gas, and biological hazards.
High upstream levels with lower downstream levels can indicate a restriction between those locations.
Wet-weather flow increases can provide evidence of infiltration and inflow.
Good collection-system operation combines hydraulic understanding, inspection, preventive maintenance, pump-station reliability, safety, accurate records, and early response to developing problems.