Understanding Inverted Siphons and Sewer Air Jumpers in Sewage Systems
Most sewer pipe segments aim to maintain a relatively steady grade to keep sewage flowing downhill smoothly. However, there are occasions when existing infrastructure, commonly drainage culverts, or natural obstacles such as waterways can block what would be the usual design path of a sewer line. Sewer air jumpers are sometimes constructed when a gravity sewer main needs to cross beneath other infrastructure using an inverted siphon.
What are Inverted Siphons and Sewer Air Jumpers?
A sewer line can be constructed beneath existing obstructions using an inverted siphon. Here is where sewer air jumpers and inverted siphons come into play, aiding in crossing beneath other infrastructures. This is where the pipe drops to a lower relative level below the obstacle that’s in the way, before rising back to its previous level and grade. As the top of the siphon pipe drops below the hydraulic grade line, full flow will occur creating a short section where the air component in the headspace of the pipe can’t continue.
The Role of Inverted Siphons
An inverted siphon allows the sewer line to be constructed beneath existing obstructions. It involves dropping the pipe to a lower level below the obstacle before rising back to its original level and grade. This process creates a section where the air component in the pipe’s headspace cannot continue, necessitating the use of sewer air jumpers.
The Importance of Sewer Air Jumpers
Preventing Odour Complaints
Without an sewer air jumper, the air pressure will increase on the upstream side of the siphon and look to escape from manholes or other non-sealed parts of the nearby system. Often resulting in odour complaints from the public. The sewer air jumper design offers an alternative path for the sewer atmosphere to bypass and re-join the sewer flow once it returns to its normal level.
Above or Below Ground Installation
Depending on the infrastructure’s size and depth in the way, sewer air jumpers can be installed above or below ground, providing flexibility in their setup.
Guidelines for Sizing Your Sewer Air Jumper
There is only sparse literature and design guidance on sizing sewer air jumper pipework. Steve Deering and Steve Jepsen et al. presented a paper that discuses air jumper sizing in a paper that was presented at the 2006 WEFTEC Conference: Sewer Siphon Assessment and Air Jumper Design.
Addressing Deposit Build-up in Long Inverted Siphons
Due to the creation of a low point at the dip of the inverted syphon, there is a propensity for deposits to settle in the lower pipe component if self-cleansing flow conditions are not met. Designing an inverted syphon in such a way that achieves self-cleansing flow is the preferred approach; HR Wallingford put together a comprehensive report that goes into significant detail on this subject: Self-cleansing flow conditions for inverted siphons.
Other options used for removing deposit build-up include periodic cleaning, flushing, or forced pumping. There are also mechanical flush valves that are designed to temporarily block the siphon at the downstream end, allowing pressure to build-up with the resulting increase of flow velocity shifting settled deposits through the inverted siphon.
The Misnomer of the “Siphon”
Despite being termed a “siphon,” the fluid mechanics of an inverted siphon do not rely on suction or pressure differential to draw fluid uphill against gravity. The term is derived from its resemblance to a regular siphon that has been flipped or inverted.
About the Author
Mark Lee is the Business Development Manager (AUS/NZ) at VAPAR and a former Senior Asset Engineer who has spent more than a decade managing the asset lifecycle of infrastructure. He has extensive experience managing pipeline networks, including design, construction, condition assessment and decommissioning.