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How ‘ice pigs’ keep our sewers clean and unblocked

Pumped sewers, or rising mains, make up around 5% of our sewer network. This may not sound a great deal but in the Severn Trent Water Region this equates to 2500km of pipe. Over time these rising mains can develop operational issues such as blockages and maintenance may be required. This is difficult due to the nature of the pipe, when compared with a non-pressurised gravity sewer, as rising mains are usually hundreds if not thousands of meters long, can change direction suddenly with tight bends and have very few access points along their length. If maintenance is not carried out, the potential impacts on customers and the environment can be significant.

 

A new technique, Ice Pigging, uses ice slurries as semi-solid ‘Pigs’ as the cleaning force. It is similar to conventional pigging using foam swabs, however it is much easier in that enabling works, such as the construction of launching and collection stations are not needed. Instead of having several sizes of swab for a pipe, the ice slurry mixture inserted will form to the pipes topography, navigating bends with ease.

The method requires the isolation of the rising main. Ice is fed into the pipe at the pumping station and either the pumps are switched back on or a separate delivery tanker is used depending on site conditions, which then drives forward the pig using the pressure of water behind it. The pig travels through the length of pipe collecting debris and suspending it within the ice, scouring and cleaning the pipe walls it goes through. The pig and associated waste is then removed from the downstream end of the pipe which is then returned to service.

A major advantage in using this technology is that in regular pigging, a swab has the potential to get stuck, whereas using the ice slurry mixture, if the ice were to get stuck it would simply melt. The slurry is pumped like a liquid, yet behaves as a solid in the pipe, which in turn provides an enhanced mass ensuring the pipe walls are cleaned simultaneously. Ice pigging has the ability to navigate tight bends, which other industry techniques, such as jetting, are not able to do without adding extra entry points for the equipment.  The technology is also less aggressive than some other current methods used such as air scouring.

For example, at a particular pump station in Shropshire, the site was struggling to pass forward flow and an expensive capital scheme was being considered to relocate it. Using ice pigging to clean the rising main increased capacity in the pipe and improved the pump rate from 1l/s to 3.2l/s, solving the issue and ensuring the capital scheme was no longer needed, making considerable cost savings.

There are now a number of more extensive trials over a wider range of sites with varying site conditions to prove the technology further and assess which sites are suitable. In particular, Severn Trent Water is currently carrying out more extensive trials as part of their AMP6 programme and we are keenly looking forward to seeing the results.

James Trafford is project manager for asset planning at Mouchel 

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