Sampling 1: Considerations for your water sampling strategy

by Water Hygiene Centre, on 01-02-2018
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Sampling 1: Considerations for your water sampling strategy imageSampling of water systems for Legionella, Pseudomonas aeruginosa or bacteriological can be somewhat daunting, thoughts of ‘do I need to sample’, ‘how to take a sample’ and ‘where do I need to send the samples’.

This blog is aimed to provide some background on the sampling process…

Introduction to Water Sampling

The accuracy of the results obtained from water sampling relies principally on ‘doing the basics well’. Whilst it’s imperative to understand the; how, what, why, where and when we take water samples, these considerations are underpinned by two key requirements:

  • The sampler must follow the approved technique when sampling from an outlet;

  • As far as reasonable and practicable, think about all aspects of the sampling environment that may pose a cross-contamination risk.

By following the approved methodology/sampling technique coupled with the adoption of good sampling practice – to mitigate factors that may otherwise cross-contaminate the sample taken, it’s possible to consistently obtain results that accurately represent the water quality within the outlet sampled. 


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Sampling Processes

Guidance is available from British Standards and the Environment Agency, which offers both practical advice on how to sample and monitor from hot and cold water services (BS 8554:2015) and how to sample specifically for Legionella bacteria in water systems (BS 7592:2008) as well as laboratory advice surrounding the determination/detection and enumeration of aquatic bacteria (Microbiology of Drinking Water – ‘Blue Book’).

Therefore ‘sampling processes’ may be defined by two parts:

  • Operational considerations when sampling outlets;

  • Laboratory methodologies and associated diagnostics when determining the presence of the ‘target organism’.

This blog will focus on part 1 and now outline the operational considerations of taking a water sample in accordance with the aforementioned British Standards.

Sampling Rationale

For legionella sampling, it is advised that water samples are undertaken to demonstrate the effectiveness of the established control scheme and is not considered a control measure itself. Therefore, superfluous sampling (‘fishing for results’) is not advised. HSG274 and HTM04-01 offer similar guidance on when to sample for Legionella, for example:

  • In systems where the exposed population is particularly at risk;

  • In systems that are “out of control”, e.g. when control parameters defined in the scheme cannot be consistently achieved;

  • In systems that operate under an alternative control strategy, other than temperature;

  • In systems that have been implicated in a confirmed or suspected case of Legionnaires’ disease. 

For Pseudomonas aeruginosa, it is advised in the HTM04-01 that samples are taken every 6 months from outlets in augmented care units. The water outlets to be sampled should be those that supply water which:

  • Has direct contact with patients;

  • Is used to wash staff hands; or

  • Is used to fill or clean equipment that will have contact with patients as determined by risk assessment.

Although, guidance for Scotland ‘Guidance for neonatal units (NNUs) (levels 1, 2 & 3), adult and
 paediatric intensive care units (ICUs) in Scotland to minimise the risk of Pseudomonas aeruginosa infection from water’ does not advocate routine sampling for Pseudomonas aeruginosa.

For Bacteriological monitoring, it is advised in the HTM04 that routine samples are not necessary as no direct association with the presence of waterborne pathogens. Although testing may be considered necessary where there are taste or odour problems.


Sampling Plan

Before water sampling begins there should first of all be a strategy/rationale for the works and HTM 04-01 Part B Paragraph 10.2 provides guidance in this regard. Moreover, it’s advised that water samples should be taken from nearest and farthest points on a water distribution system, known as the sentinel outlets.


Sampling from outlets at the extremities of a distribution system offers insight in respect to the microbiological quality of water between these points, thus removing the need to sample every outlet in between. The sampling plan should also consider the need to sample any additional sentinel outlets, such as those that represent remote parts of the system e.g. an outlet positioned at the end of an unusually long pipe run. 


Additional sampling may also be required if systems are considered to be susceptible to colonisation or if there has been a loss of control (in accordance with BS 7592 guidance). 


Collecting Samples

When collecting a water sample, the aim is that these waters should be representative of the sample location at the time of collection; hence the importance of using the approved sampling technique and mitigating any risk of cross-contamination.


Equally important is the handling of the sample once the sample has been taken in order to minimise change before the laboratory analysis begins. Therefore careful consideration should be given to the following:


  • Choice of sampling point;

  • The possible presence of biocides, such as chlorine;

  • Whether or not there is a need to disinfect the sampling point;

  • Location & timing of the sample – in relation to normal operating conditions & control measures of the system. Especially if secondary disinfection is commonplace ;

  • The type & quantity of sample to be taken – considering the requirement for different volume samples dependent upon the target organism. For example; Legionella samples are usually collected in either 500ml or 1-litre containers whereas Pseudomonas analysis only requires a 100ml sample and may be collected in a 100ml to 500ml container depending on the laboratory;

  • Storage of the samples [this is detailed in Part 2 of the blog - Water sampling technique];

  • Transportation of the water samples [this will be detailed in Part 2 of the blog - Water sampling technique].


Sampling Tips to Remember

Sampling can provide valuable information on the efficacy of the control scheme, of which pre-planned maintenance (PPM’s) such as temperature monitoring may form a part of. However, this will only be true if the sampling programme is properly planned and executed in accordance with the available guidance.


Random ‘fishing for results’ is not recommended. Sampling should be carried out in response to a specific need with clear underlying rationale.


When sampling has been carried out as a result of a previous loss of control, once it’s been accepted that control has been regained, then sampling in accordance with the original regime should resume – supported by the site-specific risk assessment as part of the organisational Water Safety Plan (WSP).


Water sampling technique and the chain of custody associated with microbiological testing will be discussed in Sampling Processes - Click here to read part 2.


Editors Note: The information provided in this blog is correct at date of original publication - February 2018. 

© Water Hygiene Centre 2019

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About the author

Water Hygiene Centre

The Water Hygiene Centre was established in 2009 to address the lack of independent water hygiene consultancy within the industry. From our humble beginnings, we have established ourselves as a market leader, helping clients identify and minimise the risk of waterborne contamination and disease, whilst improving compliance performance.

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