What are sewer vents for and how do they work?

What are sewer vents for and how do they work?

sewer vent

Inspecting sewer vent

Transporting more than wastewater

The process of transporting wastewater from houses to treatment facility is a journey through pipes and pumps of various type and size. When designing and optimising the transport of this fluid, there is another key factor that must be considered by engineers – ventilation of the sewer network. 

As wastewater is pumped up and flows down through the different pipes in the system, there is also a movement of air. Due to the atmospheric pressure differentials at pump stations and within the headspace in the top half of gravity pipes, the network itself needs to breathe. The system must move air, as well as fluid.  

Why do we need sewer vents?

Venting of the wastewater network is important for several reasons, this includes: 

  • Providing airflow and pressure equalisation between the home and the pipe network 
  • Reducing and controlling odour issues above ground.
  • Avoid creating corrosive environments below ground that reduce asset life 

The ins and outs of vents

House vent – Otherwise known as a drainage waste vent (DWV) consisting of a 50mm (2”) PVC for a single dwelling and up to 200mm (8”) PVC for multiple dwellings, which extends above a roof of a building. This is usually installed at the head (furthest point from the main drain) of your property’s drainage system to allow enough air movement for you to flush your toilets and use all fixtures within your house and still maintain the water seals within each fixture 

Pump Station Vents (induct & educt) and Discharge manhole ventAt the pump station, a vent stack will also be present. This usually extends above the tree line, or nearby property roof level.  Vents are often constructed at the discharge points of the rising/forced mains. With wastewater being forced into the gravity system between two points, both locations need to have an open flow of air as from one end it draws in and the other it exhausts as the wastewater is displaced from one to the other.  

sewer vent

Image: Sewer Ventilation Clearance Requirements 

Forced ventilation (e.g., tunnels)Mechanical ventilation using a powered fan generates airflow at a controlled rate to ensure a sufficient volume of fresh air is circulated to ensure critical structures do not experience accelerated material corrosion. . 

Odour control units

In instances within sewer networks where odour is problematic, activated carbon filters can be used as a replacement for vent stacks. Vent stacks can become an occupational health and safety risk due to their deteriorating condition over their lifetime. Activated carbon neutralises gasses before they exit the vent pipe. In some cases, they are reducing odours up to 99%.  

There is a move to replace vent stacks due to ageing infrastructure and the risk they can pose to the public if they fail/collapse. An example of this is here, where a vent stack located near Muswellbrook High School (Australia) was replaced following ongoing odour complaints and the risk that it posed to the surrounding neighbourhood. 

Installing vent stack

Vent stack replacement. Muswellbrook High School, Australia (before and after)

There are several different options available to suit the needs of the situation. However, the principle is the same. Air flows in or out of the sewer through the replaceable filter media removing unpleasant odours without restricting the air requirements for the sewer to operate as intended.

Examples of types of vent replacement options.

These units ensure that harmful gasses within our sewer networks are ventilated and treated, helping keep us safe and extend the life of the underground assets for the utilities of your local area. 

About the author Anthony Woodhouse

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