What are sewer vents for and how do they work?
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 vent – At 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.
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.
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.
Hi Anthony,
Thanks for an excellent introduction to a critical and complex area.
As someone who has looked at this for some time, including involvement in the SCORe Project and its outcomes, I think a better understanding of this topic would help reduce Utilities long-term capital and maintenance costs as well as improve community and frontline worker safety. I especially appreciate your discussion of DWV’s and their role in the retic (“municipal”) wastewater network. As in many areas a whole of system approach is required between Utilities and customers / builders, which can be especially challenging.
In regards to Induct and Educt vent placement throughout the wastewater transport system, given the differences between locations I think it is very hard to generalise across locations within any given network. I would agree that passive OCU’s can work well to remove harmful gases from low pressure sewer networks and can be an excellent procurement option, as you mentioned. While I am not familiar with the particular product you referenced in your article, I would like to make a general point regarding Induct vents and the need to provide more replacement air into networks (which is critical but often overlooked). That is I do think the pressure drops across the OCU filter bed are sufficient in many instances to prevent air from entering the network as needed, especially during dry weather flows when we know that pressure drops are often extremely small. Of course this is also dependent on particular site variables such as liquid flow rates (i.e. negative pressure within the network), age of filter media and others.
I am interested in your view on this topic. Do you have experience using OCUs as Induct vents and under what conditions are they effective?
Thanks again for your interest in this area,
Duncan Reynolds
Research and Development Manager
SVSR Pty Ltd
Hi Duncan,
Thanks for the comments. I have had a range of experiences within sewer networks but not specifically relating to OCUs. As an asset manager in the area for around 10yrs within a water utility, I had input into how the sewer network was maintained. I completely agree with your sentiment around the importance of induct vents being an extremely vital part of an efficiently operating system and as a plumber prior to the asset management role, the principals of air flow into a network is critical for its function.