A large part of Glasgow remains at risk of a repeat of the water contamination problem which affected thousands of homes last year, according to a report.
Residents were forced to boil their water
After a review of the cryptosporidium alert in August, NHS Greater Glasgow said the city would remain vulnerable until a new water filtration plant has been built.
However, Scottish Water said it has made "substantial progress" in cutting the risk of parasite contamination through a series of improvements.
Greater Glasgow's water supply remains at threat from cryptosporidium contamination and will remain at risk until a new water filtration plant is built
Cryptosporidiosis causes diarrhoea and sickness, which can have a severe affect on vulnerable groups including the very young and elderly.
The cryptosporidium incident was the largest to affect a UK water supply in 10 years, with tens of thousands of people in north Glasgow and outlying areas advised to boil water.
The parasite had been found in water from the Mugdock Reservoir in Milngavie, East Dunbartonshire, which receives water from Loch Katrine.
Tests on one of the aqueducts at Loch Katrine detected a high level of cryptosporidium but the source of the contamination has never been established.
Reviewing the handling of the problem, NHS Greater Glasgow's Incident Management Team said some lessons had been learned but key improvements must be made.
Dr Jim McMenamin, Consultant in Public Health, said: "The main conclusions are that Greater Glasgow's water supply remains at threat from cryptosporidium contamination and will remain at risk until a new water filtration plant is built.
"It is also essential that concerns about the 26-mile Victorian aqueduct linking Loch Katrine and Mugdock reservoir are addressed."
The problem affected Mugdock Reservoir
"Our main recommendations are that every effort should be made to achieve these two actions as quickly as possible."
The incident team said their review showed there were gaps in scientific knowledge and national guidance about the levels of parasite that will produce illness.
He said lessons had been learned about the need to improve communication when contamination occurs.
However, there was also a need for Scottish Water to manage a "more accurate database" of vulnerable groups such as hospitals, nursing homes and hotels.
Scottish Water said it has made improvements but that the report reinforced the case for a new water treatment works.
It also stated that is has moved to ensure better communication after it was accused of delays in warning a sizeable number of those affected.
Plans for a works at Mugdock were rejected last year but new proposals for a site at Milngavie have been put before East Dunbartonshire Council.
Scottish Water said its improvements include:
- cutting the risk of contamination at the aqueducts
- additional testing to provide more effective early warning
- ending sheep grazing at Loch Katrine - this was suspected as a possible source but never proved
- a revised communication plan for the public.
Chief Executive Dr Jon Hargreaves said: "These reports relate to a set of circumstances last August and do not reflect where we are today.
"We accept there were areas in the incident where we should have performed better.
"We have learned considerably from what happened and since then we have put into place a series of actions and improvements which mean that the bulk of the recommendations in the two reports have been met or are well advanced."