Trunk sewers – changes in inspection methods
What is a trunk sewer?
A trunk sewer is usually a large diameter pipe that wastewater from smaller diameter sewer pipes connect to downstream. These carry the wastewater from upstream catchments and carry the flow downstream to a discharge point. Depending on the size of the catchment and population upstream, the trunk sewers can be quite large in diameter, most of them are large enough for people to walk through.
Why are sewer trunk mains needed?
A trunk sewer is the most cost-effective way to transport large amounts of wastewater from upstream populations to a downstream discharge point. If we didn’t have trunk sewers we would need to take wastewater from every property all the way from the most upstream point in the network through to the discharge point, which is cost prohibitive. Due to the importance of these large pipes to urban centres, it is imperative that they are in good condition and maintained and inspected regularly.
Due to their sizer and flow capacity, trunk sewers often have a higher consequence of failure compared to the smaller upstream collection network. If a trunk sewer was to fail due to a blockage or a collapse the results can be extremely expensive to repair, damaging to the local environment, and disrupt the use and access to surrounding public and private infrastructure.
How are trunk sewers inspected?
Traditionally, because of large diameter nature of trunk sewers, trained and certified professionals should be used to carry out inspections, and they do these inspections by walking through the trunk sewer. These highly skilled and trained professionals not only carry out the condition inspection of the pipe, but also complete this in a safe manner. It is normal for these inspections to be carried out while the sewage is still flowing in the pipe which means the inspection should only be carried out in low flow times of the day (i.e., not during peak usage or after recent rainfall) and when the section of pipe to be inspected is well ventilated and monitored.
The airspace in the truck mains is considered to be a hazardous environment due to the H2S gas generated from raw sewage. Additionally, the pipes are ‘confined’ in nature (i.e., There is no free flow movement of air). For that reason, forced ventilation equipment is sometimes used and people inspecting the pipes wear gas masks and gas monitors for the duration of the inspection.
What are the changes in the trunk sewer inspection technology?
There is new technology available on the market that means that people don’t need to enter these hazardous and confined environments. This new technology improves the safety outcomes for the inspection process dramatically as they can reduce or eliminate the physical entry of personnel. The devices used can be tethered or untethered, meaning that the inspection device may or may not be connected to a particular location. An example of this technology is floatable cameras that undertake 3D imaging of the pipe as they flow through. There are also vendors that provide wheeled robotic inspections, whilst others provide drone hardware that fly through the pipe.