How Bacteria Like Legionella Form in Water Tanks

Legionella is a bacterium that exists naturally in water and soil, and under normal conditions it poses no problem at all. The risk appears when it finds an environment that allows it to multiply to high concentrations, and that environment has very specific conditions which, unfortunately, occur easily in poorly maintained water tanks: stagnant water, mild-to-warm temperatures, and the presence of organic matter or sediment that acts as a food source.

Temperature range is one of the most decisive factors. Legionella tends to thrive in warm water, neither too cold nor too hot, a range that a cistern or tank exposed to the sun for much of the year in Mallorca reaches easily, especially during the hotter months. Add water that is rarely refreshed, whether because the tank is used infrequently or because a property's water consumption has dropped, and you create exactly the conditions this bacterium needs to establish itself and grow.

Sediment that builds up at the bottom of the tank also plays a significant role. Over time, any cistern or tank accumulates a layer of organic matter, dust, mineral residue and biofilm — a kind of biological film that clings to the walls and is very difficult to remove through normal water use alone. That biofilm acts as a refuge for legionella and other microorganisms, protecting them even against disinfectants if the cleaning isn't thorough enough.

As for health risks, exposure to legionella happens mainly by inhaling small droplets of contaminated water in aerosol form — for example in showers, high-pressure taps or sprinkler irrigation systems — rather than by drinking it directly. Health organisations such as the World Health Organization have for years pointed to the importance of properly maintaining water systems as a general preventive measure against this type of bacteria, particularly in tanks, cooling towers and networks with sections of stagnant water.

The good news is that prevention is reasonably straightforward when done regularly. Periodic cleaning and disinfection of the tank, removing sediment from the bottom and biofilm from the walls, is the most effective measure. It also helps to avoid letting water sit stagnant for long periods by refreshing the tank fairly regularly even when a property's water use is low, and to make sure the system doesn't hold sections of water at warm temperatures for extended periods — for instance by checking the insulation on tanks that get a lot of direct sun.

You don't need to be an expert to suspect something is off: a cistern that hasn't been cleaned in a long time, water with an odour or cloudiness, or a property that's been closed up for several months are all enough reason to request a professional inspection before using the system normally again. At Tuberías y Aljibes we've been cleaning and disinfecting water tanks across Mallorca for more than fourteen years, and we always recommend treating this task as preventive maintenance rather than a reaction to a problem that's already visible. When it comes to bacteria like legionella, staying ahead of it is far simpler, and far cheaper, than dealing with it afterwards.