Key Points
- Microbiological sampling is a monitoring and verification tool within the ACoP L8 control regime — it confirms whether control measures are working.
- Legionella sampling results are expressed in colony forming units per litre (cfu/L) and assessed against the three action levels published in HSG274.
- Full potable microbiological sampling tests for TVC, E. coli, coliforms, and Pseudomonas aeruginosa.
- All samples are tested by UKAS accredited laboratories to ISO 17025 — providing legally defensible, traceable results.
- A quarterly sampling frequency provides trend data across the year and supports early detection of deterioration.
- Sampling alone does not manage Legionella risk. It must be part of a wider programme including cleaning, inspection, and temperature management.
Why microbiological sampling is required
ACoP L8 requires duty holders to monitor the effectiveness of the control measures defined in their written scheme of control. For cold water storage systems, microbiological sampling is the primary method of verifying that the physical control measures in place — cleaning, inspection, and temperature management — are preventing Legionella and other waterborne pathogens from reaching harmful concentrations.
Physical control measures such as annual cleaning and 6-monthly inspection can identify deterioration in tank condition and remove accumulated contamination. They do not, however, provide real-time confirmation of water quality at a point in time. A tank that passes a visual inspection may still carry a bacterial load that poses a risk. Sampling closes this gap by providing direct microbiological evidence of the water’s condition at the time the sample was taken.
Regular sampling also builds a trend record. A single elevated Legionella result may reflect a transient condition; a series of increasing results across quarters indicates a systematic failure in the control regime that requires investigation and correction.
Regulatory basis
Microbiological monitoring of cold water systems is required under ACoP L8 as part of the written scheme of control. HSG274 Part 2 provides guidance on Legionella sampling methodology and action levels. Healthcare premises must additionally comply with HTM 04-01, which sets specific monitoring requirements. All sampling by Hydraclean uses UKAS accredited laboratories.
Legionella culture sampling
What is tested
Legionella sampling from a cold water storage tank tests for the presence of Legionella bacteria — principally Legionella pneumophila, the species responsible for the majority of Legionnaires’ disease cases, but also other Legionella species depending on the laboratory method used.
The standard laboratory method is culture analysis, developed under ISO 11731. A water sample is collected by Hydraclean’s trained hygienists from an appropriate sample point (typically from within or immediately downstream of the cold water storage tank) using sterile sampling equipment and following the collection protocol required to produce a valid result. The sample is transported to a UKAS accredited laboratory where it is cultured at the appropriate temperature and incubation period before colonies are counted and identified.
How results are expressed
Legionella results are expressed in colony forming units per litre (cfu/L). A single colony forming unit represents a viable bacterium or cluster of bacteria capable of forming a colony under laboratory conditions. A result of, for example, 50 cfu/L means that approximately 50 viable Legionella organisms per litre were detected in the sample.
Culture analysis typically takes between 10 and 14 days to produce a result, due to the slow growth rate of Legionella bacteria in laboratory conditions. Results are reported to the client through Watercompliance.online when available from the laboratory.
HSG274 action levels
HSG274 Part 2 sets out three action levels for Legionella results from cold water systems. These levels guide the response required by the duty holder and the appointed responsible person.
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Result (cfu/L)
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Classification
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Required action
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|---|---|---|
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<100 cfu/L
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Satisfactory
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The system is considered under adequate control. Continue the routine monitoring programme. No additional action is required in response to the result alone, though any trends should be reviewed at the next risk assessment review.
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100–1,000 cfu/L
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Investigate
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The result indicates a potential failure in the control regime. Investigate the cause — review recent inspection findings, cleaning records, and temperature data. Take corrective action where deficiencies are found. Resample to confirm the effect of any corrective action. Inform the responsible person.
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>1,000 cfu/L
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Immediate action
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The result is unsatisfactory. Notify the responsible person and, where appropriate, the organisation's safety advisor. Review the risk assessment. Implement corrective measures without delay — which may include shock chlorination, flushing, or restriction of access to affected outlets. Obtain a confirmatory sample following corrective action. Do not return the system to unrestricted use until results are satisfactory.
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Important note on action levels
The HSG274 action levels are guidance values, not absolute safe thresholds. Even a result below 100 cfu/L should be considered in the context of the trend — a rising series of results within the “satisfactory” range warrants investigation of the control regime before the next result. Hydraclean reviews all results against the client’s trend record when uploading them to Watercompliance.online.
Full potable microbiological sampling
The full potable microbiological sample is a broader test of water quality that assesses the general hygienic condition of the water in the cold water storage tank. Unlike Legionella sampling, which targets a specific organism, potable microbiological testing uses several indicator organisms to assess whether the water is safe for human consumption.
TVC
Total Viable Count
A measure of the total number of viable bacteria in the water, incubated at 22 °C and 37 °C. TVC is a general indicator of microbial activity — a significantly elevated TVC suggests bacterial growth is occurring, which may indicate a cleaning deficit, biofilm, or a temperature management failure. TVC results alone do not identify specific pathogens.
E. coli / Total coliforms
Escherichia coli & Coliforms
E. coli is the definitive indicator of faecal contamination in water. Its detection in a potable water sample represents a serious failure of hygienic conditions, indicating that waste material has entered the water supply. Coliform bacteria more broadly are indicators of inadequate treatment or protection of the distribution system. Neither should be present in water from a properly maintained CWST.
Pseudomonas
Pseudomonas aeruginosa
Pseudomonas aeruginosa is an opportunistic pathogen associated with biofilm formation and a particular concern in healthcare settings and for immunocompromised individuals. Its presence in a CWST sample is an indicator of biofilm development and inadequate system hygiene. It is particularly significant in hospital and care home environments where patients may be especially vulnerable.
Why all four matter
Significance of the full test
Used together, these four parameters provide a picture of the water’s hygienic condition that cannot be obtained from any single test. A tank showing an elevated TVC, the absence of E. coli, and the presence of Pseudomonas points to a different problem from a tank with E. coli present. The combined profile supports more targeted investigation and corrective action.
UKAS accreditation: what it means and why it matters
UKAS — the United Kingdom Accreditation Service — is the single national accreditation body for the UK, designated by the government under European Regulation (EC) No 765/2008. UKAS assesses and accredits organisations that provide calibration, testing, inspection, and certification services against internationally agreed standards.
Laboratory accreditation to ISO 17025 (General requirements for the competence of testing and calibration laboratories) means that the laboratory has been independently assessed and found to have the technical competence, quality management systems, and measurement traceability required to produce accurate and reliable results for the specific tests covered by the accreditation scope.
For water testing, UKAS accreditation matters for three reasons:
- Reliability. Results from an accredited laboratory have been produced using validated methods under quality-managed conditions. The result is less likely to be an artefact of laboratory error.
- Legal defensibility. Where a result is used as the basis for enforcement action, insurance claims, or litigation, results from an accredited laboratory carry significantly more weight than results from an unaccredited source.
- Regulatory expectation. HSG274 and associated guidance expect that where sampling is carried out as part of the Legionella control programme, it should be done competently. UKAS accreditation is the objective evidence that the laboratory meets this standard.
All samples collected by Hydraclean Ltd under the Tricel aftercare packages are sent to independent UKAS accredited laboratories. Clients can confirm the UKAS schedule number of the laboratory from the sampling certificate uploaded to Watercompliance.online.
Why quarterly sampling is recommended
The Tricel–Hydraclean sampling packages operate on a quarterly (3-monthly) basis. This frequency is appropriate for cold water storage tanks in commercial and multi-residential buildings for several reasons:
- Quarterly sampling captures seasonal variation. Cold water temperatures in some plant rooms can approach 20 °C during summer months — a condition that increases Legionella risk — and sampling at this time of year is important for detecting any associated increase in bacterial growth.
- Four results per year allow trend analysis. A single annual result provides a snapshot; quarterly results provide a trend that can distinguish between a transient elevated result and a systematic deterioration in control.
- The quarterly interval falls between the annual cleaning and the 6-monthly inspection, meaning sampling results are available to inform the interpretation of inspection findings and vice versa.
- For healthcare premises, HTM 04-01 specifies higher monitoring frequencies. Quarterly sampling satisfies this requirement and Hydraclean can advise on sector-specific requirements.
Responding to unsatisfactory results
Where a Legionella or potable microbiological result is unsatisfactory, the following principles apply:
Act promptly.
The responsible person must be notified without delay. Where the result exceeds the HSG274 action level for immediate action (above 1,000 cfu/L for Legionella), do not wait for further samples or investigation before initiating corrective action.
Investigate before assuming.
An unsatisfactory result has a cause. Before implementing corrective measures, review the control record: when was the tank last cleaned? What did the last inspection report? Have temperature readings been within range? Has there been any system work, period of low demand, or occupancy gap since the last sample? The answer will guide the appropriate response.
Take appropriate corrective action.
Depending on the investigation, corrective action may include shock chlorination, increased flushing, remedial works to address physical deficiencies, or review of the temperature management regime. Corrective action should be proportionate to the result and the investigation findings.
Resample to confirm.
A confirmatory sample should be taken after corrective action to verify that the action was effective. The system should not be returned to unrestricted normal use on the basis of the corrective action alone — the confirmatory result is required.
Documentation.
A written report of all work carried out, including findings, chlorination details, and verification results, is issued to the client and uploaded to Watercompliance.online.
Frequently asked questions
Why is Legionella sampling required for cold water storage tanks?
Sampling provides direct microbiological evidence that the cold water system is under adequate biological control. It verifies that the physical control measures — cleaning, inspection, and temperature management — are preventing Legionella from reaching concentrations that present a health risk. ACoP L8 requires control measures to be monitored and their effectiveness verified; sampling is the primary verification method for cold water storage tanks.
What are the HSG274 action levels for Legionella in cold water systems?
HSG274 publishes three action levels: fewer than 100 cfu/L is satisfactory (continue routine monitoring); 100 to 1,000 cfu/L requires investigation and resampling; greater than 1,000 cfu/L is unsatisfactory and requires immediate action, including notification of the responsible person, investigation, corrective measures, and confirmatory resampling.
What does the full potable microbiological sample test for?
The full potable test covers: total viable count (TVC) at 22 °C and 37 °C; Escherichia coli (indicator of faecal contamination); total coliforms (indicator of system hygiene); and Pseudomonas aeruginosa (indicator of biofilm, particularly relevant in healthcare settings). Used together, these give a broad picture of the water’s microbiological condition.
What is UKAS accreditation and why does it matter?
UKAS is the United Kingdom Accreditation Service — the national body that assesses and accredits testing laboratories. Laboratory accreditation to ISO 17025 means the laboratory has been independently assessed and found competent. UKAS accredited results are reliable, traceable, and legally defensible. All samples collected by Hydraclean are sent to UKAS accredited laboratories.
Is sampling alone sufficient to manage Legionella risk?
No. Sampling is a monitoring and verification tool, not a control measure in itself. ACoP L8 requires a risk assessment-based approach that combines written schemes of control, physical maintenance (cleaning and inspection), temperature management, and monitoring (including sampling). Sampling confirms whether the control regime is working; it does not substitute for implementing that regime.
What happens after an unsatisfactory sample result?
Hydraclean notifies the responsible person immediately. Investigation of the control record is carried out to identify the likely cause. Appropriate corrective action is implemented — which may include shock chlorination, flushing, or remedial works — and a confirmatory sample is taken to verify the effectiveness of the action. All steps are documented on Watercompliance.online.
Why is quarterly sampling recommended rather than annual?
Quarterly sampling captures seasonal variation (including summer conditions when water temperatures may be higher), enables trend analysis across four data points per year, and aligns with the overall maintenance schedule to support informed interpretation of results alongside inspection and cleaning records. Annual sampling provides only a single data point and cannot reveal trends or seasonal patterns.
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