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.

The conditions that help it grow

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.

Where it tends to hide in a home

The tank is the most obvious spot, but not the only one. A house has other places where water can sit warm and still for weeks: a water heater running at a low temperature, showers and taps in rooms that are hardly ever used, dead-end pipe sections left behind after a renovation, rooftop tanks heating up in the sun, or garden hoses that spend the summer full of water in direct sunlight. All of them combine the two risk factors: little turnover and lukewarm temperatures.

Hard water adds a twist that's typical of Mallorca. The limescale that builds up on shower heads, tap aerators, water heater elements and tank walls creates rough surfaces where biofilm takes hold more easily and disinfectants struggle to reach. So descaling shower heads and aerators from time to time isn't just about water pressure or appearance: it also cuts down the nooks where bacteria can shelter.

How infection happens

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.

What the regulations say and who they apply to

In Spain, the prevention and control of legionnaires' disease is governed by Royal Decree 487/2022, which sets out the health requirements for higher-risk installations, such as those in hotels and other buildings in collective use, or cooling towers. These installations must have a maintenance, cleaning and disinfection programme, with records to prove it. The quality of drinking water, for its part, is governed by Royal Decree 3/2023.

Installations in a private home generally fall outside those formal obligations, but the principles are the same and well worth applying: clean tanks, water that keeps moving, and no warm, stagnant sections left for months. If you run tourist accommodation or a building with shared installations, the sensible step is to ask your adviser or the authorities which specific obligations apply to you.

Prevention: what actually works

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.

Reopening a house or holiday rental after months closed

For many homes on the island, the riskiest moment is reopening: the seasonal house opened up again in spring, or the holiday apartment welcoming its first guests after winter. Before that point, the sensible thing is to run the cold and hot water at every tap and shower for several minutes, opening them gradually to avoid splashing and with the bathroom ventilated, and to empty and clean the tank if it hasn't been refreshed for months. Our checklist for preparing a tank before cleaning explains how to organise it.

If the property is let, it's worth building these tasks into each season's set-up along with the rest of the plumbing, as we describe in our guide to preparing a holiday rental. And if you notice a smell, cloudiness or an odd colour when you turn on the tap, go through the signs of a contaminated cistern before assuming the water is fine.

Staying ahead always pays off

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, from flats in Palma to holiday apartments in Calvià, 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.