Main Sewer

Published: | Updated: May 14, 2021

Definition - What does Main Sewer mean?

The main sewer can be defined as the primary pipeline in a sewerage system running under the public street collecting wastewater from lateral connections. These lateral pipelines are smaller pipelines connecting homes and businesses to the main sewer line. The main sewer carries the sewage from these branches to the sewage treatment plant where the wastewater is treated before being safely discharged.

Main drains are usually of sufficient capacity to cater to the load from the neighborhood it serves. However; with increasing population, and the need for newer buildings, more connections are joined to the main sewer, increasing the load without an increase in its capacity. Problems like backflow can occur when main drains are overworked or clogged.

Other reasons for failure of main sewers are the collapse of pipes, tree root intrusion, flushing of solid objects down toilet drains and other sorts of clogs. Trenchless rehabilitation methods like cured-in-place pipe (CIPP), mechanical spot repair, pipe bursting, sliplining, shotcrete, etc. can be used to rectify problems in sewer lines.

The main sewer line is the responsibility of the municipality but the lateral lines are to be maintained and repaired by individual homes and businesses.

Clean out of branch sewers from the main sewer can be done by using drain snakes. However; when main sewers get clogged it requires professional clean up. Regular inspection can help prevent clogs and blocks and keep the main drain in working condition.

Sewer mains should not be confused with force mains. Force mains are pipelines that work under pressure to convey wastewater to a higher elevation or where gravity flow is not enough to push sewage forward. The energy required is provided by pumps or compressors located in a lift station.

Trenchlesspedia explains Main Sewer

Main sewers from different locations interconnect and flow towards the wastewater treatment plant. The main sewers work under gravity unless the terrain does not permit gravity to work effectively. In such cases, a grinder pump or lift station is utlilized which are usually built away from residences as odor and fumes from the sewage are released.

Types of Sewer Systems

There are three types of sewer systems:

1. Sanitary sewers – They carry wastewater from homes and businesses to wastewater treatment plants. Water from toilets and sinks is carried through lateral pipes into the main sewer and onward to the treatment plant for treating the waste and removing pollutants before releasing it to the environment.

2. Storm sewers – These sewers carry rainwater and melting snow runoff from roofs and roads through manholes, storm drain inlets, and ditches. Wastewater is not allowed to enter this system as the water from storm drains is directly directed into streams, rivers and waterways.

3. Combined sewers – These sewers carry wastewater and stormwater runoff to the wastewater treatment plant. This system is becoming obsolete because during monsoon, the stormwater runoff can overwhelm the system leading to sewers to overflow or back up.

Treatment of Wastewater

Wastewater delivered to the treatment plant from the network of main sewers undergoes primary, secondary and tertiary treatment before it is released back into the environment or used for landscaping or non-edible crop irrigation. Primary treatment removes large solids and the water is settled to allow sediments to sink or float. Secondary treatment is done using aeration tanks and bacteria to clean the wastewater.


Once the bacteria have done their job, the water flows into settling tanks to allow the bacteria to settle. In the tertiary treatment, disinfecting chemicals such as chlorine are used to remove nitrogen and phosphorous from the water. The water is then filtered through activated carbon that removes a major percentage of chemicals and toxins.

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