Innovative approach to sanitary sewer cleaning
Manhole system designed to access concrete pipe is a Canadian first
Civil engineers could be looking at London as a template for hard-to-access sewer cleaning jobs. The Ontario municipality recently installed three specialized manholes with sealed access flanges into one of its longest and oldest sewers in order to clean the pipe, after a camera probe two years ago identified it as being partially filled with solids and grit that would eventually restrict flows.
The 1.2 m-diameter concrete pipe used in the Horton sanitary sewer is over 70 years old and 1.2 kilometres-long. Normally sanitary sewers have manholes that serve as access points for sewer cleaning, but for reasons that are unclear, the Horton sewer was built without any manholes. The likely explanation is the fact that the sewer cuts through Greenway Park, which was originally swampy land near the Thames River that was initially used as a landfill site. Soils there are very poor and the original designers probably feared that manholes could let groundwater into the pipe because of its proximity to the river.
Many years later, that oversight created a big dilemma for city staff as they tried to figure out a way to clean the sewer. They turned to R.V. Anderson Associates Limited, a consulting engineering firm, for help.
R.V. Anderson determined that a traditional flushing wasn’t feasible because the sewer had been converted from a gravity to a siphon system 40 years ago. Siphon systems use hydrostatic pressure to move the flow without needing a pump. The advantage of siphons is they save municipalities on pump electricity, but the downside is they can get build-up of solids throughout the pipe.
According to David Evans, associate director, regional manager with the London branch of R.V. Anderson, adding traditional manholes to the sewer wasn’t an option for cleaning, so the company looked at unconventional “tall” manholes that would extend two to three metres above grade. But there were a number of issues associated with that option, including the fact that the liquid would rise up in the manholes and create an odour problem. Not an attractive scenario considering the sewer is located in a park.
Another option was to twin the pipe, but that was expensive, costing at least $1.5 million. Plus the sewer didn’t really need the capacity for a twin.
At that point R.V. Anderson started thinking outside the box, and came up with the idea of building specialized manholes that could “go with the flow,” as it were.
Normally sanitary sewers have manholes that serve as access points for sewer cleaning, but for reasons that are unclear, the Horton sewer was built without any manholes. Many years later, that oversight created a big dilemma for city staff as they tried to figure out a way to clean the sewer.
“We thought ‘what if we attach a saddle with a cleanout hatch on it, onto the pipe which we would use when we needed to?’” said Evans. “That morphed into ‘we’re building these specialized manholes that have got these hatches attached onto the pipe, with blind flanges on them to keep them closed during normal operation and then at periods of low flow we can actually divert the flow down into the low-level wet well at the outlet, and turn it back into a gravity sewer.’”
Opening up the flanges during low flow facilitates access for sewer cleaning.
“We can go down into our special manhole, take the access cover off and flush it out just like a normal sewer,” Evans said.
The operation to install the new manholes took about a month, and much of the work was conducted at night during low flow.
Evans said one of the benefits of the project was being able to check the condition of the decades-old concrete sewer pipe. When engineers cored a three-inch-diameter test hole into the pipe they found the concrete to be of sufficient condition and thickness, plus evidence of steel mesh – showing the pipe was strong enough to keep using without relining.
“With that test hole, we basically were reassured about the strength and condition of the pipe and that we would not have a lot of risk moving forward with the project.” CUI