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Intergroup is cleaning the underground pipes and using special cameras to look for tree roots and fatbergs, which are gooey masses of solidified fat and wet wipes.
Ashburton District Council service delivery manager Neil McCann said letters were being delivered to residents in the Belt Road, Racecourse Road and West Street areas advising them about the work, which included cleaning with a high-pressure water jet.
People needed to keep toilet lids down when not in use because on rare occasions the air pressure used in the cleaning process generated a blow back of water.
“There is always a slight risk it could blow out of the toilet.”
A heavy object, like a brick or book, should be placed on the toilet lid to keep it down.
The cleaning and inspection programme is part of council’s ongoing work to survey the condition of the wastewater network, which includes some concrete pipes 80 years old.
Mr McCann said cameras inserted into the system allowed staff to see the condition of the pipes, which had a lifespan of 100 years or more. Pipe renewal was scheduled based on the results.
He said tree roots sometimes grew into cracked pipes and snagged objects like rags or wet wipes, which people should not be flushing. The cameras also looked for fatbergs, which grew in the colder weather as fat solidified.
The wastewater pipes carry water from showers and dishwashers as well as sewerage from toilets.
Mr McCann said fat or grease should not be disposed of down household drains. Businesses and other commercial operators have special grease traps to make sure it does not get into the wastewater system.
He said there was an ongoing and nationwide issue with people flushing wet wipes, and these could cause big, expensive problems.
Contractors are only inspecting council sewer mains, not individual connections.