The Three Types of Municipal Sewer Systems

In this article, you will find the definition, characteristics and illustrations of the three types of municipal sewer systems commonly used in Quebec:

Combined sewer systems
Pseudo-separate sewer systems
Separate sewer systems

schéma de tous les types de réseaux d'égouts — *A single community may have several types of sewer systems.*

What is a sanitary sewer system?

Sanitary sewer systems collect domestic wastewater from household plumbing fixtures (sinks, toilets, showers, bathtubs, dishwashers, washing machines, etc.) as well as industrial effluents, and carry them to the wastewater treatment plant.

The wastewater collection system is made up of:

Manholes
Pipes
Pumping stations
Overflow structures
Flow regulators

What is a municipal wastewater treatment works?

Municipal wastewater treatment works (OMAEU) are used to collect, store, convey and treat wastewater before returning it to the environment. The wastewater handled by these works can be of domestic, industrial or mixed origin.

The collective wastewater treatment system is made up of:

Wastewater treatment plants
Diversion structures

What is a storm sewer system?

Storm sewer systems collect runoff generated by rain and snowmelt and carry it back to the environment.

In separate sewer systems, stormwater and runoff are not treated at the wastewater treatment plant, as domestic and industrial wastewater are. Instead, they are collected and carried by the storm sewer system to a receiving water body.

The storm sewer system is made up of:

Storm inlets
Catch basins
Pipes
Gutters
Ditches
Stormwater management facilities

Many of these stormwater management facilities can be used to improve the quality of stormwater and runoff before it returns to the natural environment.

What is a stormwater management facility?

Stormwater management facilities (OGEP) are used to collect, store, convey or treat stormwater and runoff. They are generally built to offset the increase in impervious surfaces caused by urban development.

Among the stormwater management facilities most commonly used in Quebec are retention ponds, vegetated swales, bioretention systems and infiltration chambers.

What is a combined sewer system?

Combined sewer systems collect domestic wastewater, industrial wastewater and runoff, particularly from roads and parking lots, and carry it all to the wastewater treatment plant.

Characteristics of combined sewer systems

More compact
Less expensive
Self-cleaning effect of stormwater flowing through the system
Lower risk of hydrogen sulfide emissions in the system
Oversized pipes
Oversized treatment plant
Diversion structure often required at the treatment plant
Overflow structures required on the pipes
High risk of wastewater overflows into the natural environment

This type of system can therefore carry very high flows and volumes of wastewater, especially during intense rainfall events such as summer thunderstorms.

What is a pseudo-separate sewer system?

In pseudo-separate sewer systems, the sanitary sewers collect and carry domestic and industrial wastewater, water from foundation drains and, in some cases, water from roof drains.

Runoff generated by rain and snowmelt is directed to a separate storm sewer system.

This type of system therefore carries much less water than combined sewer systems, but it can still carry significant flows and volumes of wastewater, especially in spring when the groundwater table is high.

What is a separate sewer system?

Separate sewer systems collect and carry domestic and industrial wastewater through a sanitary sewer system that is distinct and separate from the storm sewer system, which receives only runoff from rain and snowmelt and water from foundation drains.

Characteristics of separate sewer systems

Stormwater does not pass through the wastewater treatment plant
Low load variation at the treatment plant
Treatment costs are generally lower
Additional cost for stormwater collection infrastructure
The absence of stormwater in the pipes can eliminate the self-cleansing effect and lead to greater fouling, or even blockage, of the pipes
The reduced load in wastewater pipes can also cause unpleasant odours

This type of system normally carries a much lower flow than combined or pseudo-separate sewer systems, unless there are illicit connections capturing rainwater and snowmelt (e.g., catch basins, flat-roof drains, gutters or other interconnections with the storm system) or other structural defects in the system (cracks, offset joints, etc.). These improper connections collect extraneous water that can overload the system and cause sewer backups and wastewater overflows into the environment.

Sewer system diagnostics

Many problems can result from malfunctioning municipal sewer systems. Identifying their cause requires specific diagnostic tools:

1. Continuous sewer flow and precipitation monitoring

Used to determine the flows carried by the system at strategic points and to assess the flow response to rainfall events (assessment of extraneous flows).

2. Smoke or dye testing

Used to detect sources of surface extraneous water entering the system.

3. Night-time instantaneous sewer flow measurements

Used to identify pipe sections affected by groundwater infiltration through structural defects in the system.

4. Sump pump and foundation drain inspections

Used to identify illicit connections of foundation drains to a separate sanitary sewer system and to plan disconnections.

Beyond these diagnostic tools, it is above all the expertise of sewer system diagnostic specialists that will help you resolve problems with infiltration, extraneous water, cross-connections or sulphur odours.

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The Three Types of Municipal Sewer Systems