Why do sewers get blocked?

Why do sewers get blocked?


Dry weather sewer overflows caused by blockages can create significant issues for utility providers, the community, and the environment. What are some of the reasons sewer/wastewater pipes become blocked?

Tree roots

The number one cause of sewer blockages in most networks is tree roots.1 In addition to sunlight and carbon dioxide, trees require water and nutrients to grow and survive. The consistent flow through sewer pipes provides a rich and attractive source for trees. While the gravity pipe network would ideally be a closed system, there are numerous pipe joints and cracks where tree roots are able to squeeze through as they seek out nutrients. If enough of a gap exists, the root mass can become so large within the pipe that it can eventually block, causing a back-up of sewage flow and discharge at a point upstream. The point of discharge is commonly a sewer maintenance hole or house overflow relief gully. Dry weather overflows may persist for some time before being observed and reported for fixing.

FOG: fats, oils and grease

Another contributor to sewer blockages is FOG: fats, oils and grease. If you’ve ever wondered why you are asked not to pour these cooking by-products down the sink, this is the reason. Incorrectly managed trade waste from food establishments is also a significant contributor to FOG issues in a sewer network. These products increase the likelihood of a blocked sewer and overflow. Fats, oils, and grease can solidify after they cool and build-up on the inside of pipe walls, they have a tendency to coagulate with similar particles, or exacerbate already existing root intrusion issues, and can result in partial or complete blockage of a pipe. An internet search of ‘sewer fatbergs’ will provide graphical insight into the scale of the problem.

Foreign objects

A third cause of sewer overflows is the flushing of objects down the toilet that belong in the rubbish bin. The most recent offender on this list is wet wipes. While toilet paper is biodegradable and designed to break apart after flushing, wet wipes are not the same. This material interacts with both tree roots, fat deposits and other solid materials dramatically increasing the likelihood of blockages. Other common items that don’t belong in the sewer system and contribute to blockages include paper towels, sanitary items, condoms, hair, kitty litter, and cotton balls.

Other potential triggers for blockages in pipes are the build-up of sediment or broken pieces of pipe which reduce the cross-sectional area of a pipe and can lead to partial chokes and eventual blockage.

How are these defects identified during a CCTV inspection?

VAPAR uses artificial intelligence to automatically identify and categorise pipe defects including root intrusion, debris/deposits (incl. FOG build-up), and other obstructions within a pipe that can identify the potential risk of blockage. The VAPAR.Solutions platform provides defect level reporting that can be matched with historical work orders and blockage events to assist in pipe maintenance programs to reduce the risk of future blockages.

  1. Sewer performance reporting: Factors that influence blockages (Marlow et al., 2011)

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