Staff at Scottish Water have accused the government-owned company of withholding pay rises as strikes began last week.
Over the next three months, hundreds of employees will participate in the 48-day walkouts.
The row revolves around a new grading structure unions claim could cost the lowest-paid up to £5,000.
Workers began three days of walkouts last week.
Scottish Water’s chief operating officer has expressed their disappointment at the strike, highlighting a recent eight percent pay rise offer.
However, talks but talks have failed, and new negotiations are scheduled.
Unison’s Patricia McArthur has criticised the company for attempting to impose an unilaterally-devised pay and grading system.
She said: “We’re on strike because Scottish Water are basically holding our cost-of-living rise to ransom.
“They’re trying to force the staff to accept a new pay and grading system that’s not been collectively bargained by the unions and we have some serious concerns about the content of it.”
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She added: “We are just looking to get a decent cost-of-living rise before Christmas, we’re looking to work with Scottish Water to try and come up with a pay and grading system that is going to be good for everyone.”
Actions are “template to demoralise staff”
GMB Scotland organiser Claire Greer said: “It has managed to infuriate a committed workforce and drive them to strike action, when the dispute could have easily been resolved months ago.
“Scottish Water has repeatedly been given a clear choice between doing the right thing and making the situation worse, and has taken the wrong decision every single time.”
She added: “Its actions over the last six months are a template for how to demoralise staff and demolish industrial relations.
Kevin Roy, Scottish Water’s customer service general manager, added: “It is essential we can continue to provide customers with the high standard of service they have come to expect from Scottish Water.
“We have looked in detail at how our vast range of activities might be impacted by this action and have put measures in place to minimise any disruption.
“Among the work we are doing ahead of the strikes is the development of the Scottish Water website to provide customers with information on how we will be continuing to support their essential services.”