manhole cover in slovokia

Basics of Sewer Manholes

Basics of Sewer Manholes

Picture of sewer manhole

What are manholes?

Sewer manholes or also called maintenance holes are formal access points within the sewer pipe network that provide maintenance teams a chance to get access to maintain the sewer pipe network. They can come in many different shapes and sizes depending on how deep they go into the ground and what the surrounding ground conditions are like.

Why do you need manholes?

Once a blockage or a break in the sewer pipe is confirmed through a CCTV inspection, maintenance teams need to get access to remedy the issue. Without the presence of manholes, any remediation would be complicated and expensive.

Where can you find manholes?

The spacing of the manholes depends on a couple of factors. If a sewer pipe is running in a straight line in an area where access is not an issue, then they are usually placed every 80-100 metres (260-330 feet) along a sewer pipe. This spacing is determined by the practical length of water jetting equipment to reach the full length of pipe, regardless of whether the water jetting was done from the upstream manhole, or the downstream manhole.

That being said, manholes can also be built at shorter or longer lengths. For example, if the pipe needs to have bends in it, the design engineer might want to install extra manholes to account for the risk of blockage at the change point in the flow of sewage.

Manholes can also be placed within the network at irregular locations when the pipe network runs under a highly urbanised area. Placing manholes in the middle of roads, or in the middle of someone’s property is not advisable.

There are serious safety issues with placing manholes too close to live roads and having a manhole under a concrete floor slab doesn’t really serve anyone either. For this reason, the configuration of the network and the spacing of manholes might vary to account for the above ground infrastructure.

Drain spotting

You can identify the presence of manholes by the manhole covers on roads, footpaths and even in parks. The manhole covers themselves can come in many different shapes and sizes also, although most are round. They are usually made of metal to withstand the weight of heavy vehicles.

There is a great #drainspotting hashtag that you can browse to see what others have contributed from all over the world. Perhaps on your travels, you might feel compelled to contribute some interesting manhole designs and locations and help educate others on the weird and wonderful world of our underground sewer networks.

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sewer overflow

What causes sewer blockage and how to get it cleared?

What causes sewer blockage and how to get it cleared?

Image of sewer overflow due to blockage

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.

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