Why does water stagnation cause bacterial proliferation?
You won’t be surprised to hear that microorganisms (bacteria, fungi etc.) multiply exceptionally quickly. If we use Escherichia coli (E. coli) as an example, under the right conditions it will double in number every 20 minutes. This means that every 2 hours x1 bacteria transforms into 512 bacteria. Bacteria multiply quicker when they are not stressed. What are stress conditions for bacteria? – temperature, chemicals, toxic metals, lack of nutrients and a high flow all cause stress conditions. As far as flow is concerned, the lower the flow is (stagnation) the less stressed the microorganisms are, and the better they multiply. On the other hand, when water is flowing quickly, it is more difficult for bacteria to attach to surfaces, release nutrients and start building biofilms. It can be concluded then that avoiding stagnation in all areas (eg by engineering out) is an essential factor in successful water installation management to minimize microbial contamination risk.
Do materials exist, that cannot be colonized by biofilms?
It’s a very clear ‘no‘ here. With water interfaces (pipes, tanks, seals etc), independent of material, biofilms will grow sooner or later. How quickly and how thick & stable they become, depends on different factors; temperature, nutrient availability, water flow rates and materials will all impact here. Biofilms have been shown to grow on copper and stainless steel pipes. Even glass, which is almost completely smooth will develop a biofilm over time.
What can I do to eliminate biofilms within a water installation?
You will never manage to fully eliminate biofilm, because 1. it is too difficult and 2. it will start to grow back again immediately. In reality, when a water installation is colonized with biofilm, the only way to completely remove it is to mechanically scrub it from the whole inner surface. Even if this was managed, as soon as the pipes were filled again with water and the installation turned on, the first layers of new biofilms would colonize the inner pipe surface and within a short period of time (several weeks) a stable biofilm will be reestablished again. Biofilm scientists suggest the best way to treat biofilm is to simply ‘manage‘ it so that it doesn’t cause problems within a building installation.
Does Legionella die at 60 °C?
It depends on all the conditions involved. Different strains of Legionella pneumophila exposed to 30 minutes at 70 °C have been shown not to die, but rather enter a dormant state (VBNC-viable but non culturable). When these were co-incubated with amoeba (their natural host), they were resuscitated or awakened and then were able to infect human lung cells.
When temperature is applied as a control measure for the growth of Legionella, different facts have to be considered:
> Does the set temperature (eg 60°C at the boiler) reach every part of the water installation including all outlets? (taps and showers)
> Do I have stagnating areas?
> How long can a continuous 60 °C treatment be ensured, without temperature loss?
> Is my water installation colonized with stable biofilms or is it a very new, untreated installation?
> Are all my water installation materials compatible with continuous treatment at 60 °C and for how long?
> Do I have the respective documentation / validation from the manufacturers?
> What is the nutrient concentration within my water installation?
> Are amoeba present?
All these factors (and likely more) play a role on how successful a continuous treatment at 60 °C will be in killing and/or controlling Legionella (planktonic Legionella). This is because Legionella living within biofilms is much more resistent than planktonic (Legionella in the water phase) as the biofilm offers protection against mechanical, chemical and thermal stress. In conclusion, even if you manage to eliminate planktonic Legionella at a continuous temperature of 60 °C, Legionella within biofilm will be released at some point and recolonize the water phase.
Several papers have shown that a continuous PWH (potable water hot) temperature at 60 °C is an important factor to control Legionella growth within water installations.
Please contact us here for respective literature and further comments.