Cold water storage tanks are an integral part of most cold-water distribution systems. They provide resilience when the supply from your water supplier or undertaker becomes interrupted, for whatever reason, so business as usual may be maintained until the supply is reinstated.
Legionella
Nearly all domestic, commercial, and healthcare water systems have cold water storage facilities. The water supplied by your undertaker will be of wholesome quality; that is defined by the DWI (Drinking Water Inspectorate) Regulation 4 as the drinking water must not contain any microorganism, parasite, or substance, whether alone or in conjunction with any other substance, at a concentration or value that would constitute a potential danger to human health.
It also lists a wide range of chemical, microbiological, and physical parameters that must not be exceeded for the water to be deemed wholesome.
That said, a wide range of organisms naturally occur within an aquatic environment, Legionella being one such organism. Your water supply is the means of transportation these organisms use to enter your water system.
Who is responsible for water?
Once water enters your building, (in the UK water safety then becomes the responsibility of the building landlord/owner) the environment in which it finds itself changes dramatically. The nature of large-diameter delivery pipework and associated velocity, which create a shearing action within, make it difficult for organisms to take hold and become established. The smaller diameter pipework within the built environment and cold-water storage facilities create a much more hospitable environment for any organisms to thrive. By this point, the presence of residual disinfectant used by the supplier will have also degraded significantly.
How can we prevent the proliferation of Legionella?
There are several guidance documents and publications available to us that give clear guidance and direction as to how we manage the potential risk of microbiological proliferation in our cold water systems;
ACOP L8 – The control of Legionella bacteria in water systems
HSG 274 Part 2 – The control of Legionella in hot and cold-water systems
HTM-04/01 Part A: Design, installation, and commissioning (Healthcare)
HTM 04-01 Part B: Operational management (Healthcare)
It is important that when carrying out a new build or major refurbishment program the predicted consumption/usage is calculated so as not to oversize the proposed storage capacity which will potentially create stagnation due to lack of turnover. A Project Design Review of proposed projects can greatly assist in getting it right, first time.
Cold water storage tanks must also comply with Schedule 2 of the current UK Water Regulations;
Every storage cistern shall be fitted with –
- An overflow pipe with a suitable means of warning of an impending overflow which excludes insects
- A cover positioned to exclude light and insects
- Thermal insulation to minimise freezing and undue warming
Every storage cistern shall be so installed as to minimise the risk of contamination of the stored water. The cistern shall be of appropriate size, and the pipe connections to the cistern shall be so positioned, as to allow free circulation to prevent areas of stagnation from developing.
*(HSG 274 2.36)
So now we have our tanks in place, what do we need to do to ensure that we control the risk of potential proliferation from Legionella and other water-borne pathogens?
HSG 274 Part 2 section states
2.57 An annual inspection of the cold-water storage tank should be done to check its condition inside and outside, and the water within it. Figure 2.12 demonstrates the condition of cold-water storage tanks and when action should be taken. The lid should be closely fitted and in good condition. The insect and vermin screen on the overflow and warning pipes and any vents should be intact and in good condition.
The thermal insulation should be in good condition so that it protects from extremes of temperature. The water surface should be clean and free from any visible, significant contamination. The cold-water storage tank should be cleaned, disinfected and any faults rectified. If debris or traces of vermin are found, the inspection should be carried out more frequently.
2.58 Whenever the building use pattern changes, a record of the total cold water consumption over a typical day should be established to confirm that there is reasonable flow through the tank and that water is not stagnating
2.59 Monitoring for temperature or any disinfectant concentration in cold water should be carried out at sentinel draw-off points, selected to represent the overall building water system. In a simple cold water system, the sentinel points are typically the furthest tap (far sentinel) and the nearest (near sentinel) to the supply or storage tank. In deciding which outlets to identify as sentinels, the layout of the distribution system should be considered rather than the location of the outlet. More complex systems are likely to have several far sentinels, such as the extremity of each of several risers or down services. Any parts of the system not represented by far and near sentinels should be identified and additional outlets selected for monitoring that represent the excluded section.
2.60 Maintaining regular movement of cold water in sections prone to stagnation and guarding against excessive heat gain by using insulation on the cold water tanks and pipework is the most effective legionella control measure in CWDS. For most buildings, carrying out these measures is all that is required.
2.126 The risk from exposure to legionella should be controlled by keeping the water system and water in it clean and free from nutrients, including those arising from contamination and corrosion; and maintaining its cleanliness. Hardness scale may also trap nutrients, encouraging biofilm formation and so form a barrier to disinfectants.
(*The above narrative is extracted from HSG274 Part 2)
Conclusion
The water supplied by our water undertaker is of wholesome quality. Once in our charge, after entering our premises, we have a duty of care to manage our stored water to prevent the proliferation of any organism that may be present to protect the end user, our staff, and the public.
By following and implementing a planned preventative maintenance regime from the guidance that is available to us, we can mitigate potential issues and have a positive impact on maintaining water quality, from the point of storage to the point of delivery, keeping everyone safe.
If you would like to discuss any of the issues raised above, please click here to get in touch.
Editors Note: The information provided in this blog is correct at the date of original publication – November 2024.
© Water Hygiene Centre 2024
