Drinking Water Sampling & Microbiological Testing

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.

Legionella testing

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.