Until last Thursday, every one of them at the Warrington lab has been negative – for at least the last 15 years.
So when the moment happened mid-morning, and traces of the nasty little bug cryptosporidium were discovered in one filter from Garstang, you might have expected pandemonium to break out.
“Not at all,” insisted analytical technical manager Jeanette Williams, one of the team who carried out the tests which, for the past five days, have disrupted life in large parts of Lancashire and sparked panic-buying in the shops.
“These scientists are very professional and they took it all in their stride. Everyone was calm, no running around waving arms in the air. We just got on with confirming what the tests were telling us.
“It’s a rare happening - the first time in the North West since at least 2000 - but we had a process to go through to establish what the risk was to the public before any warning could be issued.”
Once that alert was flashed out across the region later that day the world came crashing down for the 2,000 or so people who work for United Utilities.
“We are all devastated this has happened,” said company media spokesman Helen Apps. “That’s because we pride ourselves on delivering great water and services. We just want to get that back to normal as quickly as we can.”
Estimates of how long more than 300,000 households in Preston, South Ribble, Chorley, the Fylde Coast and parts of Blackburn will be under threat from the parasite vary, from as early as tomorrow to the “worst case scenario” of the end of the week.
Staff have been working around the clock, some through the night doing double shifts, since the scare was first revealed. The company has received two-and-a-half times more calls from customers during the past five days than it would normally handle.
“That peaked last Friday to 4,500 on that day alone,” said Helen. “They were mainly people wanting to know more about the problem and what the advice was. We even had to open an overflow contact centre in Liverpool to deal with the volume of enquiries.”
Dr Kate Brierley, lead for the health protection team for Public Health England for Lancashire and Cumbria, insists the warning to boil water - even for cleaning your teeth and washing salads - is not an over-reaction.
“We have been trying to get this message out to prevent people becoming ill,” said the former Preston Girls’ Grammar School student who was bombarded with requests for TV, radio and newspaper interviews in the wake of the scare.
“Cryptosporidium can cause an unpleasant illness. It can cause watery diarrhoea that can last for a few days - even longer for some people. Those at opposite ends of the age spectrum - the very young and the old - are the most vulnerable. So too are people whose immune system isn’t working quite as well as it should, like patients having cancer treatment.
“We were called in as soon as United Utilities confirmed there was a problem. They took a filter off at the Franklaw Treatment Plant near Garstang - as they do three times a week - and found traces. They put another on to reassure themselves that it was a correct reading and then contacted us straight away.
“The cause of it is still under investigation. It is obviously connected with the Franklaw Plant. There were some huge reductions in the bugs in the first instance and the water that is coming through is now clear.
“I would anticipate it is going to be another few days before this is flushed through the system. The worst case scenario would be the end of the week.”
But while the water is beginning to run clear, the threat from cryptosporidium may not be fully over just yet.
Dr Brierley said: “There have been no clinical cases to date. But obviously we are in a timeline where there may be infections that occur in the next few days. That is obviously something we will be looking out for.”