Thousands of homes in Ceredigion remain without water after dramatic changes in temperature caused a "mass outbreak of burst pipes".
Welsh water says it has 500 people working on the issue and hoped that most customers would have water again by Wednesday, although he could not promise that would be the case.
Read more: No end in sight for communities left without tap water
On Monday night Welsh Water said around 4000 homes were experiencing intermittent, or loss of water supply.
Nine schools are closed in the county on Tuesday due to the issue.
Speaking on BBC Radio Wales, Chief Executive of Welsh Water, Peter Perry said: "A huge apology to start with. We faced a massive temperature swing over the weekend from almost two weeks of sub-zero temperatures to temperatures up to eight, nine, ten degrees. We've been preparing for this right across the whole of last week- this didn't start for us over the weekend.
"We had our full incident team stood up from a week last Monday and we knew this was coming. We've managed to keep over 99% of our customers in supply across Wales because this thaw impacted the whole of the country.
"Unfortunately, where we have struggled is down in Cardigan and I apologise to customers there. Overnight we are making good progress - what we've seen with the amount of ground movement caused by the thaw is a mass outbreak of burst pipes, not only on our network but also things like outbuildings, outside taps to cattle troughs.
"We've been taking water with us and fixing leaks as we go to repressurise which has been successful overnight. I know Aberaeron should be back in water and we've got other areas which will be coming on through the rest of the morning.
"We've got a problem down in Llansteffan. We had a number of bursts down there which we've fixed but we've got some air locking in the mains down there and we've got a team of people already starting to release that air from the system to get people back in supply but it's that combination of the ground movement linked to the thaw, a huge amount of burst activity on our network plus the losses from outside pipes."
When asked if things will be back to normal by the end of Wednesday, Peter said: "That's what we're aiming for. I can't say categorically everybody because at the end of long rural water mains systems, once the system's empty you get this problem of air locking so isolated properties struggle but our intention is to do all we can to bring people back in supply today.
"Can't guarantee it, but every effort's been expended. We've got over 500 people now working down in Cardigan."
One Cardigan resident told the BBC: "It's the third day and it's been absolutely terrible. Can't even flush the toilets but the council have been very good with bringing us some water to flush the toilets but there's no water working in town at all."
Another added: "I really would like to flush the toilet. We've been putting buckets outside collecting water and thankfully it's been pouring down for a couple of days but every time I've come down to collect the bucket water from Dŵr Cymru there's been none available."
Rob Morgan from Bethania told WalesOnline: "We’ve got a small farm with water collection tanks so we’re using that to fill the toilets. We’re collecting drinking water from my daughter’s school in Llangeitho, that’s about the only place that has got it."
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