An inquiry into an outbreak of Cryptosporidium in water supplies in Northamptonshire is continuing.
Anglian Water isolated the source of the bug on Friday and has been treating it with ultra-violet light and flushing out the water system.
The firm found the location of the outbreak at Pitsford Treatment works, but was unable to confirm the cause.
Customers were told to boil water, but most of the 100,000 homes hit should be able to use tap water by Friday.
More than 250,000 customers in the Daventry and Northampton area were told to boil their water for drinking and food use during the outbreak which began on 25 June.
Praise for firm
Anglian Water said successful flushing of water mains meant about 75% of homes would be able to use tap water by 4 July, and water supplied to homes in the Daventry area should be back to normal on 11 July.
The firm was praised in a statement on the Defra website for working well following the outbreak.
In the statement, Hilary Benn, Secretary of State for Environment, Food and Rural Affairs, said: "I am pleased to report that all relevant agencies appear to have worked extremely well together to respond to this notification."
Jeni Colbourne, Chief Inspector of the Drinking Water Inspectorate, which is carrying out the inquiry, told the BBC the investigation would not be particularly long.
Mike Templeton, lecturer in environmental engineering at Imperial College London, and member of the Chartered Institute of Water and Environment Management, said: "UV treatment kills Cryptosporidium fairly easily.
"Most outbreaks are in unfiltered water. Warm blooded animals are commonly a problem for unfiltered supplies."
He said within water treatment works, filtration removes any Cryptosporidium.
The parasite causes diarrhoea in animals and humans.
An Anglian Water spokesman said he was unable to confirm the exact source of the outbreak at Pitsford.
"We think we know what happened but don't know how," he said.
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