Have you ever wondered how clean your drinking water supply is? In this guide the water quality specialists at Water Treatment Services explain the processes around drinking water sampling and microbiological testing. The guide looks at the common types of testing, how to take a sample correctly, what happens in the laboratory, and concludes with a summary outlining how to interpret the test results once you receive them from the lab.
Drinking water quality
Would it surprise you to find out that the drinking water coming out of your kitchen tap isn’t sterile?
In the UK any people assume that because we accept that the mains water supply is safe to drink, that it’s also bacteria-free … but that’s not the case.
Water companies in the UK have a statutory duty to make sure that the water they provide to us is of a high quality (often referred to as wholesome) and that means removing bacteria which could be harmful.
However, some other bacteria which have no impact on health may remain in the water.
Although they won’t cause illness, high levels of non-harmful bacteria can become an issue should they start to breed.
The only way of assessing the level of bacteria in drinking water is to implement a process of microbiological testing carried out in a specialist laboratory.
Types of microbiological testing
There are many different reasons why businesses decide to start sampling their drinking water and testing it for the presence of bacteria.
How to collect water samples
The main consideration when collecting drinking water samples is to ensure that there’s no way of contamination getting into the sample bottle, and people in charge of collecting the water will require clear guidance or even proper training where circumstances dictate.
Firstly, carefully take the lid off the special sample bottle as late as possible, take the water sample, and replace the lid straight away.
Lids should not be put down on any surfaces as the sample is taken as this could lead to cross-contamination.
Water sample bottles are sterile, and contain a special chemical to neutralise any chlorine which has been put into the mains water by your supply company.
Once the samples have been collected you should label each bottle clearly with when and where the sample was taken before placing them into a cool box to take back to the water testing laboratory.
It’s important to remember that samples from hot water outlets should be stored separately from cold water samples.
What happens at the water testing laboratory?
As soon as the drinking water samples arrive in the laboratory, they will be logged into the Laboratory Information Management System, or LIMS, and coded with a unique sample number.
This LIMS code tracks the water sample as it moves through the testing system … most laboratories will tend to group the samples into batches for testing.
Testing for microbial contamination in drinking water is not a complex procedure.
Some of the water is mixed with agar jelly in a special container called a petri dish, which is then put into an incubator set at the ideal growth temperature, and left to see what happens.
After a pre-determined time period, the petri dishes are removed from the incubator and examined and the bacteria (if any) is counted and recorded.
That’s the simplest way of microbiological testing, but there are often additional steps needed too.
Testing for Legionella bacteria in water can be more complicated still.
A litre of water is typically required for testing and if the water is contaminated with silt, it may have to be filtered several times before it can be used.
Filters are put into a plastic container filled with sterile liquid, agitated and then mixed up to make sure any bacteria caught on the filters are forced into the liquid.
This process condenses the usual number of bacteria in one litre of water, into a much smaller sample volume.
Tests for Legionella bacteria can then be carried out on the smaller size water sample.
- The first test takes 5ml from the concentrated sample and adds it to 5ml of sterile solution. 0.25ml of this solution is then put onto a special Legionella growth agar, which is designed to let Legionella bacteria develop and hinder the growth of other bacteria. This process is carried out with two separate samples.
- Then the tester will add 5ml of acid into 5ml of the concentrate, leave it for five minutes, and take 0.25ml out and placed on two separate agar plates.
- Finally, testers take 5ml of the concentrate and mix it with 5ml of sterile solution before warming it up to 50°C for 30 minutes. Two samples are again prepared using 0.25ml of heated liquid and the legionella agar.
At the end of this process, there are six separate samples, all of which are incubated.
As it can take up to 10 days for Legionella bacteria to develop in the lab, the plates are reviewed after 4 days, 7 days and then 10 days for the final reading.
The presence of Legionella in drinking water can cause serious risk, so it is standard practice to let clients know as soon as Legionella is detected rather than waiting until the end of the 10 day testing period – this is called a presumptive positive and can act as an early warning that something is not right.
Testing for E.coli and coliforms
This test is different in that a larger volume of sample water is needed, and it’s not practical to mix 100ml of water with agar.
Testers therefore take the sample and pass it through ultra-fine filters which capture any bacteria on the surface.
The filter is then put onto the agar jelly plate, allowing the E.coli and coliform bacteria to grow but not other types of bacteria.
The sample is put into an incubator to grow for a set period pf time after which a count takes place.
Total Viable Count (TVC)
One millilitre of water is taken from the water sample and put in a petri dish labelled with the unique number.
The water sample is then carefully mixed with melted agar and then left to solidify.
The sample is then left to incubate in an attempt to grow any bacteria present in the drinking water, before the numbers of bacteria are counted.
What do the microbiological test results mean?
Laboratories produce reports identifying both the total number of Legionella bacteria in a litre of water, and the exact species which has been found.
If there’s nothing present, then the result will be reported as “not detected”.
Any result which indicates Legionella at levels of over 100 colony forming units per litre (cfu/litre) means the client should take action right away – in healthcare environments any presence of Legionella should be actioned (refer to HTM 04-01)
UK law requires that all drinking water is free from E.coli and other forms of coliform bacteria.
Any levels of these bacteria is classed as a fail on the water test.
If there’s a fail, the first step is to thoroughly clean and disinfect all local outlets, followed by re-testing of the water to assess the effectiveness of the cleaning.
Because of the higher risk from cooling towers the limits that apply are different.
A TVC under 1000 colony forming units per millilitre (cfu/ml) is satisfactory, between 1000 and 10,000 requires further checks and investigation, and over 100,000 requires the client to take immediate action.
Microbiological testing & drinking water analysis solutions
Water Treatment Services offer a comprehensive range of microbiological testing and drinking water analysis solutions.
Our experts can provide advice and support to help you identify the most appropriate strategies for the safe management of your drinking water supplies.
With offices in London serving the South and South East England, Manchester (North West), Birmingham (Midlands), Bristol (South East England and Wales), Leeds (North and North East) and Glasgow (Scotland), supported by regional teams of specially trained technicians, in-field specialists and consultants we can offer professional, cost effective water testing and laboratory analysis solutions across the whole of the UK and internationally.
Contact us today to learn how our water quality analysis solutions can help keep your water supplies safe.
Learn more about our drinking water testing.