But by using a remotely operated robot -- known as a VR600 and which bears a striking resemblance to the Disney-Pixar character Wall-E - Panton McLeod were able to carry out the cleaning work without any need to take the reservoir out of service or with any risk of interrupting supplies to consumers.
The firm cleaned a total of six tanks in the Dunoon area, including service reservoirs at Rashfield and Bishop's Glen, in order to reduce manganese levels in the structures and ensure that the purity of the water continued to be exceptionally high.
Graeme Connor, Scottish Water's operations team leader in the Dunoon area, said: "We were delighted to work with Panton McLeod as part of our efforts to tackle the problem of manganese in the network.
"We chose to work with them because of their expertise in tank cleaning and because this novel way of carrying out the work helped minimise disruption to our customers in the area.
"The tank cleaning has now been completed and we are very pleased with Panton McLeod's work. We have worked with them before and look forward to doing so again."
Paul Henderson, Operations Director with Panton McLeod, added: "We regularly carry out work for some of the biggest water firms across the UK, and we often use our underwater robots to ensure these big storage tanks are clean.
"Inspections and cleaning projects like these are standard practice and necessary to ensure that water supplies remain of the highest possible quality.
"In this instance, the tanks in Dunoon could not be taken off-line easily, which meant there was no chance of sending in a team to drain and manually clean the tank with chemicals.
"However, our technology means we can carry out a thorough clean while ensuring the supply through the reservoir is totally unaffected. We remotely control the VR600 and it cleans the tank while the water is still in use.
"We've used this technology for Scottish Water before and we're delighted they brought us in to help with their latest project. We look forward to working closely with them on similar jobs in the New Year."
Panton McLeod, which has offices in Nottingham and the Scottish Borders, is one of the best-known names in the industry, working with the biggest water companies on the inspection, cleaning and repair of drinking water structures.
The firm's VR600 robot is a large-tracked machine that is manoeuvred along the floor of a service reservoir and cleans away any sediment or impurities in the water. It can also be used to inspect the condition of the water tanks, including checking the walls and pipework for corrosion or damage.
The unit is remotely operated and fitted with camera and lighting equipment, which allows staff controlling the sub to assess the interior of the tanks. It is also meticulously treated to ensure it can be safely used in the public water supply, and Panton McLeod conducts rigorous tests before and after each inspection.
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