How to Minimize Legionella Risks in Hot & Cold Water Systems

Many of us are unaware that even clean-looking water can present hidden dangers. Legionella, the cause of Legionnaires’ Disease, is a waterborne bacterium commonly found in natural water sources, usually in small volumes.

However, it can pose a health risk when it enters man-made water systems. Where conditions are right legionella bacteria can quickly proliferate in hot and cold water systems and could lead to an outbreak.

Those in control of premises have a legal obligation to control the risk and ensure the safety of people who may use such water systems or come in contact with them.

Favourable Conditions for Legionella Growth

With the right environmental conditions, any water system could be a source for Legionella growth.

Legionella can survive in a wide range of water temperatures, with an optimal growth range between 20°C- 45°C. These optimal temperatures aren’t often found in drinking water distribution systems but can be common in building water systems, specifically hot water and non-potable water, where Legionella can grow and become a persistent problem.

In addition to appropriate temperatures, Legionella bacteria require nutrients to enable them to multiply. These nutrients are found in stagnant water systems where biofilms are present along with other common microorganisms, sludge, scale and sediment.

Low disinfectant residuals, typically caused by high water age can further contribute to favourable growth conditions.

Typically, Legionella wouldn’t be detected in the cold water of a municipal distribution system. However, growth conditions such as sediments in water storage tanks, sections of distribution systems with persistently low chlorine residuals, and warm water zones due to tank stratification or low-use mains can contribute to Legionella growth in a water system.

What do You Need to Do?

Those with the responsibility for managing buildings are required by law to control this risk, through proper assessment, planning and prevention measures.

Employers or people in control of the premises are responsible to reduce the risks of exposure to legionella and must understand how to:

  • Identify and assess sources of risk
  • Manage the risk
  • Prevent/ control the risk
  • Keep records of the steps taken to prevent/control the risk

The first step to achieving this and staying compliant is a legionella risk assessment.

The purpose of a risk assessment is to:

  • Identify and assess the risk of exposure to Legionella bacteria from work activities and water systems on the premises
  • Establish any necessary preventive and control measures
  • Provide direction on prioritising the risks

A risk assessment should include:

  • Management responsibilities, named competent person/s and duty holders amongst others responsible for the health of your system
  • Evaluation of the competence and training of key personnel
  • A description of your system
  • Any identified potential risk sources
  • Recommendation for means of preventing the risk or controls in place to control risks
  • Monitoring, inspection and maintenance procedures
  • Reference to records of the monitoring results and inspection and checks carried out
  • Arrangements to review the risk assessment.

Here are some of the practical measures that should be taken when it comes to controlling legionella in hot and cold water systems:

Control the Temperature

One of the main control measures for legionella bacteria in the water is temperature or thermal disinfection.

As a general rule of thumb you should remember the following:

  • Keep hot water hot
  • Keep cold water cold

Legionella bacteria thrive at temperatures between 20°C and 45°C. Below 20°C, the bacteria won’t die but will remain dormant. Temperatures over 50°C will kill the bacteria.

Water services should be operated at temperatures that prevent Legionella growth:

  • Hot water storage cylinders (calorifiers) should store water at 60°C or higher
  • Hot water should be distributed at 50°C or higher (thermostatic mixer valves need to be fitted as close as possible to outlets, where a scald risk is identified).
  • Cold water should be stored and distributed below 20°C.

Read More: How Temperature Affects Growth of Legionella

A competent person should routinely check, inspect and clean the system, in accordance with the risk assessment.

Sentinel outlets (furthest and closest to each tank or cylinder) should be checked monthly for their distribution temperatures. Competent persons should also check the hot water storage cylinder temperatures every month and cold water tank temperatures at least every six months.

NOTE: Water heated and stored at sufficiently high temperatures to control and kill legionella bacteria can also cause severe scalding injuries in a matter of seconds. This is a real risk to children, the elderly, and other susceptible individuals in healthcare settings. Therefore, to restrict the temperature of hot water at an outlet, a Thermostatic Mixing Valve (TMV) should be fitted locally to the outlet (wash hand basin, bath, or shower) to prevent scalding.

Don’t Allow Water to Stagnate

Water with little or no movement can be referred to as stagnant.

Most buildings will have a constant, ready stream of water that may be used for drinking, washing, bathing and/or air conditioning. If these supplies aren’t properly monitored or aren’t used for a period of time, the water can become stagnant, causing harmful bacteria to multiply.

When water does not flow well, the resulting areas of stagnation encourage biofilm growth, reduce water temperatures to levels that allow Legionella to grow, and reduce levels of disinfectant.

Stagnation is more likely in dead legs and dead ends of pipes where outlets have been removed but the pipework left in place.

Other dead-legs may be caused by infrequent or intermittent use of outlets such as unoccupied rooms or rooms and other services that are otherwise not in use e.g. services subject to seasonal use.

To prevent water stagnation, it is recommended that the following are done:

  • Removal of dead legs and dead ends in-pipe work
  • Flushing of outlets weekly
  • Cleaning and descaling of showerheads and hoses every three months
  • Coldwater storage tanks should be cleaned periodically and water should be drained from hot water cylinders to remove debris

Get the Design Right

It is much harder (and costlier) to rectify problems once the water systems are installed than it is to make any required adjustments beforehand – whether these are at the design stage or during the commissioning stage.

So, to avoid potentially costly remedial works, the design of new buildings and their water systems is controlled in order to ‘get it right first time’ essentially to ‘design out’ risk, leading to the safe operation of a building’s water system

Water tanks that are too large in comparison to the demand for water downstream will not experience the required throughput of freshwater to maintain hygienic conditions. Such tanks will suffer from stagnation and this may lead to the conditions in which the number of waterborne bacteria can quickly grow out of control.

Outbreaks of Legionnaires Disease have been associated with the faulty installation of equipment used in buildings. New water systems may contain high nutrients derived from the surfaces of new materials and dirt & debris entering the system while under construction.

Good engineering design is key to successfully mitigating risks from legionella bacteria.

Those designing and maintaining systems should seek to minimise Legionella growth, by:

  • Keeping pipework as short and direct as possible;
  • Adequately insulating pipes and tanks
  • Using materials that do not promote the growth of Legionella (avoid fibre washers, hemp, natural rubber, linseed oil-based compound jointing).
  • Preventing contamination by fitting tanks with lids and insect screens
  • Hot water vessels should have a valve to remove accumulated sludge at the base of the tank, for annual removal of build up.
  • Avoid multiple linked storage tanks because of potential operational difficulties, possible unequal flow rates and resulting stagnation.

Expert Legionella Risk Management

Successful control of legionella bacteria is a combination of good technical engineering design and client-side control management.

For companies of all shapes and sizes, legionella compliance must always start with a legionella risk assessment and by the appointment of a competent individual who is knowledgeable in this critical element of health and safety.

At Water Treatment Ireland Ltd, we offer a comprehensive range of environmental solutions for the control of legionella bacteria in building water systems. Our team of experts support duty holders to keep their facilities safe and comply with relevant health and safety legislation.

If you would like to speak with one of our specialists about managing legionella risks in your facility, feel free to reach out to us.