Water Sampling for Legionella: Why, What and When.
Why
The overwhelming attitude to managing Legionella risks in Australia and globally is to take samples. Sounds good? Well it could be if you knew why you took the sample....?
Or if you knew what exactly you sampled....?
Even better if you knew what you'd do with the result once you got it....?
Water sampling has a place in a risk management based water safety plan. But where it belongs in the plan seems to be more of a problem.
Many jurisdictions recommend or require routine sampling for Legionella. Some advise a frequency for this. Others suggest you work out a frequency for yourself based on your risks. Both of these approaches don't really help you work out the answers to the questions above
What do I need to know before I take samples?
A golden rule about taking a sample is knowing what you will do with the result. You should clearly understand how to interpret the result when it lands on your desk. You should also have a clear idea of what actions will need to be taken depending on the result you get. If you take a sample without knowing these quite simple things it is likely to lead you to panic. Usually panic means expense, something most of us would like to avoid.
Understanding why you are taking a sample and what the results are likely to mean is an important first step. It is also likely to save time, money and stress!
What do samples mean?
Well, this is a tricky question. Samples are 'snapshots' of the condition of your system. They give you a taste of what is going on but not the full picture. It is important to understand that Legionella test results are notoriously inaccurate. This is one of the reasons they can't be part of the control measures in your plan. Understanding a positive result means understanding where it came from and what the result is. It is important to know exactly where the sample was taken from in the first place. So that you can go back to that location and check out what is happening. Different positive Legionella test results can mean different things in different locations. This is the subject of another blog on Legionella species.
Sample results are usually at least a week 'after the fact'. What they mean is that at the time you took them they support the notion that the sample controls you have in place are either working or not working. Or it means that you are not achieving the goals of the controls you have put in place. They do not give you any certainty just support for the effective operation of your plan. In legislative jargon this is called 'verification' or 'validation'.
How do samples help?
Sampling has a role to play in a Water Safety and Risk Management Plan. In potable water systems taking samples is part of the verification process in a multi-barrier system.
Samples do not give you any control - they tell you what happened last week! But they can reassure you that you most likely have control. So a sample result is useful, even though for Legionella they are very insensitive. A positive test result means a review of your control measures. It also usually means disinfection of the water system.
A negative result does not mean you are out of the woods. Remember, the sample result is only as good as your control measures. Legionella and other 'nasties' are most probably alive and well in your system. The best way to understand your results is to monitor them over time and look for a trend.
When do I take samples?
Another tricky question! In the first place taking a sample before you have control measures in place is putting you on a collision course with panic. This goes
back to the question - "what will I do with the result?"
Working out and applying your control measures and making sure they are in place is the starting point. Once you are confident that the control measures are happening then take samples. Of course the results only verify that what you have put in place is working. But sample results are not proof. The only certainty you get from a sample result is that it is an underestimate of what is actually there.
How often to take samples will depend on two things. Firstly the level of risk to the people exposed. Secondly, the previous test results. In some cases the risk level is very high. So frequent verification of your control measures is a good idea. For instance transplant recipients, people having dialysis or cancer patients are very high risk. In these cases you may want to take monthly samples.
If previous test results come back positive then review your plan and make adjustments. Take samples more frequently until the test history reassures you that the control measures are working. Once the results suggest things are back on track it is OK to take less frequent samples.
Finally, different jurisdictions may require a minimum frequency of sampling. In South Australia 6-monthly sampling from warm water systems is necessary. In Queensland quarterly reports of your sampling results must be sent to the Health Department. It follows that the minimum sampling frequency in Queensland would be at 3 monthly intervals. No-one wants to send a report that states 'we didn't take any'!
In cooling water systems (cooling towers) the regulations are different. In most States annual audits and sampling are required. Most jurisdictions require compliance with AS/NZS 3666. This standard has three parts. The first part is about design and operational requirements. The next two are options. Compliance with part two does not require a sampling protocol if maintenance requirements are met. Part 3 specifically requires monthly sampling for Legionella and heterotrophic plate counts or HCC. This part of the Standard is captured by the Building Code of Australia and is a legal obligation. However, it only applies if you choose to adopt part 3 instead of part 2.
Where would I take samples from?
An obvious starting point is knowing your system. If you don't know what you've got then it's very hard to pick a point to sample it. An up-to-date schematic diagram or better still hydraulic drawings of the system design is pretty much essential. Knowing what you've got and where is an important first step.
Samples need to be representative. They should include all parts of the system. To that end, samples from hot, cold and warm water is a good idea. It is a fallacy that Legionella are only in warm water. This is supported by enHealth guidelines for Legionella control, Queensland and South Australian Government legislation. So samples need to represent all of the potable water system regardless of temperature.
To be representative of the system does not mean sampling every outlet. This is not a requirement in any jurisdiction globally! Samples at the beginning and end of each water distribution system is sufficient. Another sample at the mid-point is also a good idea. For instance, on a hot water system take a sample close to the exit from hot water service and close to the end of the return to the hot water service. On a cold water system samples near the entry and exit points of the water is a good idea. A sample at the mid-point will provide some extra insight.
In cooling water systems (cooling towers) the sample point is much easier. Since the water is circulating constantly it is well mixed. A single sample from the basin of the cooling tower when it has been operating is considered representative. Taking a sample whilst the cooling tower is operating is not a good idea, unless you have appropriate PPE. And you can get very wet!!
All of the mentioned areas above are what we specialize in, our business delivers industry leading processes, systems, training and support as well as market leading disinfection products that will enable you to manage and reduce risk of waterborne infection. We are an ISO accredited business and hold Systems, Safety and Environmental certification, please feel free to contact us if you would like to talk more.
Further Reading
EnHealth (2015) Guidelines Legionella Control.
Queensland Health Department Web Site.
SA Health Public Health Fact Sheet #3-13
SA Government (2013) Guidelines for Legionella Control
UK HSE (2014) Legionella Guidance 274 part 2
