Extra equipment has been brought in to kill the cryptosporidium parasite which has disrupted water supplies to parts of Northamptonshire.
Meanwhile, experts have confirmed that tap water should not be used for washing up, although dishwashers are safe to use.
Some 37,000 two-litre bottles of water were delivered by Anglian Water engineers to schools, nursing care homes, hospitals, nurseries, those 321 people on the Water Care register and those not on the register but who fulfilled its criteria.
An Anglian Water spokesman said: "We are working round the clock to clear the water and extra resources have been called in.
"We are stripping our waterworks bit by bit to identify where the cryptosporidium could have come from and where in the system it is.
"It is killed by ultraviolet light and we have kits being shipped in from other parts of the region to use alongside the kits we already have.
"Naturally there will be an investigation."
The spokesman said there was currently no estimation of when the water system would be back to normal and tap water would be safe to drink.
He said: "We have no idea how long it will go on. We are asking people to boil their water and take no chances."
Cryptosporidium was detected at Anglian Water's Pitsford Water Treatment Works, near Northampton, on Tuesday night, the spokesman said. Its detection was part of the company's regular monitoring for the parasite.
Samples of water will be sent to Anglian Water's central laboratory in Huntingdon, Cambridgeshire, for analysis. The company said it monitored water for cryptosporidium "all the time".
The spokesman said cryptosporidium was a naturally occurring organism, which was often in rivers. It can get into ground water and can be in animal faeces.
The spokesman confirmed water deliveries were completed by 11am yesterday and said nurseries which did not receive water should contact the company because it may not have their details.
The water company said it had taken 9,700 calls on Wednesday, compared to its usual daily call rate of 1,200. Yesterday t had taken 3,250 calls by 5pm.
Most of the calls were basic inquiries about how customers were affected, a company spokesman said.
Anglian Water has launched a special website www.boilcheck.co.uk