Water undertakers have a responsibility to ensure that ‘wholesome’ water is delivered to the point of entry to buildings, this does not necessarily mean that such waters are free from microorganisms.
What's in our drinking water?
Moreover, whilst drinking water should be free from parasites (cryptosporidium) and faecal contaminants (coliforms/E. coli), it’s common for wholesome water to contain aquatic bacteria, as such bacteria make up the ‘normal’ microbiology of drinking water.
Therefore, bacteria such as Pseudomonas aeruginosa and Legionella live preferentially in the environment and these soil-derived bacteria frequently reside within water supplied to buildings.
Planktonic bacteria are bacteria that are mobile and float freely in water, exploring their environment for nutrients. The term comes from the Ancient Greek word planktos, which means "wanderer" or "drifter".
Drinking water distribution systems (DWDS) are known to harbour biofilms, even in the continuous presence of a disinfectant. These biofilms are a source of planktonic bacteria, which will remain present when the water is delivered through a consumer's tap.
Moreover, usually hot water temperatures of 60˚C and greater will cause cell lysis (breaking the bacterial cell wall) typically after 2 minutes of exposure time to this temperature hot water temperatures over 70˚C will kill ‘planktonic’ bacterial cells upon contact.
Conversely, cold-water temperatures of less than 20˚C will typically encourage waterborne bacteria to stay dormant (not multiplying/metabolically active), which is why temperature is the recognised ‘traditional’ method of control.
Why do we have waterborne bacteria in our drinking water?
Inadequate water usage within the distribution system may encourage the settling of nutrients within water systems which in-turn will provide a ‘food source’ for waterborne bacteria.
As nutrients accumulate within a water system - waterborne bacteria will adhere to the inner surface of pipework to utilise these nutrients for growth and eventual proliferation. This is essentially the early stages (conditioning phase) of biofilm formation and therefore underpins the importance of maintaining good system pressure and adequate water usage – to ensure that nutrients don’t settle, and all freely-suspended particles and microorganisms are flushed out of the system, thus mitigating the risk of biofilm formation.
From a ‘practical perspective,’ this supports the importance of identifying and establishing a Legionella flushing regime to flush through infrequently used outlets as well as identifying and remediating dead-legs and blind ends. As well as ensuring adequate circulation of water through the distribution system in order to mitigate the risk of stagnation within a water system.
Biofilm bacteria are different from free-floating (planktonic) bacterial cells in that they may gain protection from being within the biofilm.
As such, it’s accepted that the use of temperature (heat) is not the most robust treatment strategy as biofilms offer protection against temperature as the biofilm doesn’t increase bacterial tolerance, rather it acts as a shield.
In addition, biofilm bacteria may demonstrate phenotypic differences to their previous (planktonic) form and develop antimicrobial resistance characteristics by virtue of propagating (growing) within a biofilm containing bacteria of varying antimicrobial resistance.
The ‘pili’ structures on the surface of the (Gram negative) bacterial cell wall have been associated with binding to the inner surface of pipework (mentioned above) but have also been associated with the exchange of resistance characteristics (genes) between bacteria, therefore increasing the ‘virulence’ of waterborne bacteria and making them more difficult to treat.
How can you control bacterial contamination in your water system?
For these reasons, careful consideration should be given to the choice of remediation strategy when controlling bacterial contamination within water systems as some will be more effective than others – depending on the condition of the water system being treated and the strategy of use employed.
Key considerations may also include; the age of the water system and associated dilapidation – including the presence of corrosion scale, materials of construction of water systems, level of organic material and particulate that may support the growth of waterborne bacteria, as well as the nature of bacterial contamination when detected (biofilm or planktonic).
The Responsible Person (Water) / Authorised Person (Water) with responsibility for buildings, of which the majority will have complex distribution systems (i.e. Hospitals, Universities, Hotels, Care Homes , etc.) will need to ensure such conditions are avoided to reduce the risk of waterborne bacteria establishing.
How does Legionella grow?
A broad summary of the ‘principal conditions’ that support the growth of waterborne bacteria, including Legionella within water distribution systems, are;
- Water temperature between 20-45˚C;
- Poor circulation and water flow;
- Available nutrients.
Whilst understanding the ecology of waterborne bacteria and the limitations of a given water system will undoubtedly help with choosing an effective treatment regimen, choosing an appropriate water treatment regimen requires careful consideration that should not be neglected as this may result in an unsatisfactory outcome – either by impacting upon the integrity of the water system being treated and/or by demonstrating poor efficacy in terms of biocidal effect (bacterial cell death and or removal from the water system not achieved).
Click here to read Part 2 - How to control Legionella: the risks and options…
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Editors Note: The information provided in this blog is correct at date of original publication - July 2018. (Revised July 2024)
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