A notice telling to residents to boil water after the discovery of a parasite at a treatment works in North Somerset may not be lifted until Monday.
Bristol Water said its Clevedon works was immediately taken out of operation on Wednesday following the discovery of the cryptosporidium bug.
A "boil notice" was issued to 7,000 homes in BS21 and BS49 on Thursday.
The company said it waited 24 hours for test results to be confirmed before informing residents.
Clevedon School, St John the Evangelist School and Mary Elton Primary School, all in Clevedon, were closed on Friday.
Bristol Water said 50 of its staff delivered notices by hand about the precautionary measures to the affected residential properties.
The company said it was "aiming to be in a position to confirm the boil notice removal on Monday morning".
It said it was supplying bottled water to vulnerable customers in the meantime.
Compensation of £10 will also be paid automatically to all affected customers.
David Heath of the Consumer Council for Water said: "We're monitoring the situation closely to ensure Bristol Water continues to prioritise the needs of vulnerable customers.
"We expect the company to honour its pledge to compensate affected customers as soon as the boil water notice is lifted."
Public Health England said the level of cryptosporidium detected in the water supply was low and the advice to boil the water was a precaution.
Anyone with symptoms of diarrhoea and vomiting should contact NHS 111 or, if the symptoms become severe, contact a GP.
What is cryptosporidium?
- The bug, known as crypto, is a parasite that can cause extreme diarrhoea called cryptosporidiosis, also shortened to crypto
- It can affect all people but, for those with weakened immune levels, crypto can be severe and even life-threatening
- Millions of parasites can be released in a single bowel movement which are then passed on to others through accidental ingestion
- The most common symptom is diarrhoea but other warning signs include dehydration, nausea, vomiting, fever and weight loss
- Most people with healthy immune systems can recover without treatment by making sure they drink plenty of fluids