Water stagnation – a hygiene problem

Water is a vital part of our lives: whether we’re cooking, cleaning, washing or drinking, we come into contact with water in a variety of ways, every day. We assume it’s clean and safe, but even drinking-water can be contaminated by harmful bacteria, such as legionella. These harmful bacteria can breed, and multiply, in our water systems. One of the main causes for this is stagnation - water remaining stationary in the water-system for an extended period of time.

What is water stagnation?

The Health & Safety Executive guidance document1, HSG274, defines water stagnation as “the condition where water ceases to flow and is therefore liable to microbiological growth”.The document is useful for gaining an understanding of water stagnation, and the risks it poses.

HSG274 explains that, “Duty-holders are required to prevent or control the risk from exposure to legionella. Precautions include physical methods such as regular movement of hot and cold water in distribution pipework, regular flushing of outlets to ensure water cannot stagnate in the hot and cold-water systems…”.

Regarding shared premises and residential accommodation, it gives this advice for Landlords2: “The risk [of legionella proliferation] may increase where the property is unoccupied for a short period. It is important that water is not allowed to stagnate […] hot- and cold-water systems should be used at least once a week, to maintain a degree of water flow and minimise the chances of stagnation”.

If we were to summarise the recommendations into a single statement, it would read:


“Drinking water must flow”.


Where does water tend to stagnate?

Water stagnates wherever taps or showers are left unused, preventing the water from flowing. This happens when buildings are left empty, because of renovation work, holidays, under-utilisation or seasonal factors. Therefore, stagnation is particularly likely in facilities such as schools, hotels, residential buildings, day-nurseries, hospitals, medical practices, care homes and leisure centres.

But under-utilisation of taps and showers is not the only problem. Drinking water can also stagnate if the water pipes are too large and water consumption is reduced, if sections of pipe are not used, or if long periods of time elapse between the installation of pipes and the activation of the pipe network.

What problems does water stagnation cause?

When our water-systems are under-utilised, microorganisms can colonise the surfaces of pipework and fittings causing biofilms to develop. Water stagnation also allows the cold water temperature to increase which further encourages the growth of these biofilms.

Biofilm consists of a layer of slime-like matter known as EPS (extracellular polymeric substances) in which different micro-organisms settle and form a community. These micro-organisms can include pathogens, such as Pseudomonas aeruginosa and legionella bacteria.


These bacteria will thrive and reproduce when the perfect conditions are created for them: increased water temperatures – particularly in the range from 20 to 45°C – provide the optimal conditions for microbial growth.

Such conditions can be caused by badly-insulated pipes, the hot water temperature dropping, or inadequate water flow. If a tap is turned on after a long period of stagnation, these pathogens can be released, creating a health risk for anyone using the water.

There is also a risk that contamination may not be contained in a small area. If stagnation occurs, the pipe leading to an unused outlet is the first part of the water system to be colonised by microorganisms. These microorganisms can then spread throughout the whole water system, thus contaminating other outlets that are not primarily affected by stagnation.

How can water stagnation be avoided?

As we summarised, earlier, “drinking water must flow”.

Water stagnation can be avoided by a combination of good design and careful system management. The initial system design and any subsequent alterations to the pipework system should avoid any potential for dead-legs, where water is inclined to stagnate. In addition to this, a proactive programme of flushing unused or under-used outlets will prevent the risks caused by periods of vacancy.

One commonly-used method is to flush the water pipes by turning on the taps, manually, at regular intervals. However, this is far from an ideal solution, since the personnel costs are high, the time taken is considerable and there is the added risk that the flushing regime may not be conducted exactly to the prescribed duration and frequency.

A more successful flushing approach is to use available technology, such as automatic washbasin taps. These can automatically flush the system, based on either a timed cycle or water temperature. Whilst being relatively expensive to buy and install, such solutions will ensure the regular movement of water within the system, thus avoiding contamination risks.


1Health & Safety Executive guidance document, HSG274: ‘Legionnaires’ disease Part 2: The control of legionella bacteria in hot- and cold-water systems’.
2Shared premises and residential accommodation: Landlords - paragraph 2.144.