Sewer Blowbacks – Releasing the pressure!
What is a Sewer Blowback?
A sewer blowback is when a combination of air/water/wastewater is ejected from a household wastewater fixture, such as a basin, floor drain, gully trap, and, most commonly a toilet.
This can occur when routine or reactive maintenance is being carried out on the utility’s sewer network with high-pressure water jetting equipment. Water jetters are used to clear roots, debris and blockages in the sewer system and ensure flow occurs uninhibited.
Blowbacks occur due to difference in pressure in both the main sewer line and the attached house plumbing. When jetting equipment is in use, water is forced into the sewer main under high pressure, this requires the displacement of a similar volume of air. The space behind the jetter is an area of high pressure, while there can be a negative pressure zone in front of the jetter nozzle. In most cases this air is forced along the sewer main or out private property vents. However, if there is insufficient ventilation, or a blockage in a house vent, this pressure will move to take the path of least resistance. This can result in pipe odors rising through floor drains, and if there is enough positive pressure the water seal in the toilet bed can be blown with force out of the fixture.
The inverse of a ‘blowback’ can occur where negative pressure occurs in property plumbing, and the water seal in fitting bends can be sucked out, resulting in an atmospheric opening to the sewer causing the odour from the sewer to flow through the fixture into the room that it is located. In these cases, it can be rectified by adding additional water to the water seal
Why do sewer blowbacks occur?
There are two factors that can be attributed to a sewer blowback within the property drain itself.
A drainage vent can become blocked or restricted over time which can increase the likelihood of a sewer blowback.
Blockages within the sewer main can also cause water to build up on the upstream side of them; when the jetter nozzle goes past the property connection the water can be forced up the connection pressurizing the property drain unless it has somewhere to go.
How can we prevent or minimise sewer blowbacks happening?
There are several factors that contribute to sewer blowbacks that happen during the cleaning of the sewer main, some can be minimised/mitigated, and others cannot.
- Communicating to property owners that sewer main cleaning is scheduled, and it is recommended that they keep toilet lids closed.
- Opening the upstream manhole to allow additional flow into the main being cleaned.
- Using lower pressure and/or flow from the jetting truck.
Most blowbacks consist of only the contents of the water volume within the trap/water seal and is usually clean water that is replaced on a regular basis by flushing the toilet and running a tap. Some utilities will keep a blowback register of historical occurrences. It is recommended to contact them if this happens to help inform future maintenance programs and minimise the potential of it happening again.
I think everyone can agree there is a vacuum then pressure while jetting smaller sewers. Has anyone done any study to determine what the vacuum and pressure actually is in an 8″ sewer?