How to Clean Stagnant Water: Guidance on Managing Legionella Bacteria

June 19, 2020
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If you’re an employer, someone in control of premises (such as a landlord), or self-employed, you have a responsibility to ensure that workplace risks are controlled so that they don’t compromise anyone’s health and safety. Stagnant water is one of these risks that needs managing as it may cause people to contract Legionnaires’ disease. As workplaces and buildings start to reopen after a sustained period of closure due to COVID-19, it is important to consider and manage this danger to prevent anyone from getting ill.


What is Stagnant Water & What Causes it?

Stagnant water is water that sits in place for hours or longer with little or no movement. The phrase can be used to describe bodies of water that have stopped flowing, such as rivers, as well as water that is contained in systems within buildings. For example, 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. 

Hand under water tap

Sometimes, you can identify stagnant water by the smell it gives off. However, just because there is no smell does not mean that the water isn’t stagnant and that harmful bacteria such as Legionella is not present. If you are reopening premises that have been closed due to the COVID-19 pandemic, you are responsible for ensuring water supplies haven’t become stagnant and don’t pose a risk to human health.

This article will focus on water supplies that are stored and used in buildings for various purposes and how they should be treated to be made safe.


What are the Risks of Stagnant Water?

For the last few months, water supplies have potentially been sitting in pipes and water systems in unoccupied buildings without any use. Due to the water being stagnant, serious risks to health and safety can arise. The main risk is to human health, which you have a legal responsibility to manage and reduce as a duty holder.You must consider whether there’s an increase in the bacterium Legionella pneumophila and other related bacteria that can thrive in purpose-built water systems, particularly when water is stagnant. 

People contract Legionnaires’ disease through the inhalation of small water droplets that contain the bacteria. For example, this could be done by someone washing their hands in a restaurant bathroom sink who inhales the small droplets unknowingly, or someone who rinses vegetables under a tap before eating them. Once this bacteria has multiplied, it can make those who then come into contact with the water supplies seriously ill. A form of pneumonia, Legionnaires’ disease can be potentially fatal. You can find out more about the disease in our article Symptoms and Treatment of Legionella and Legionnaires’ Disease.

Unwell woman

In addition, there is a risk of water supplies being contaminated with heavy metals from the piping, which can also cause people to fall ill. If your premises has lead piping, then there’s a risk of lead poisoning with symptoms including headaches, abdominal pain and nausea. However, this type of piping is only likely to be used in buildings with systems pre-dating the 1970s and many lead water pipes have since been replaced.


What are My Responsibilities to Control Legionella Bacteria in Water Systems?

If you are an employer, self-employed or someone in control of premises, such as landlords, you have a duty to identify and control risks associated with Legionella. This is a legal requirement under general health and safety law including the Health and Safety at Work, etc. Act 1974 and the Management of Health and Safety at Work Regulations 1999 (MHSWR). In addition, the Control of Substances Hazardous to Health Regulations 2002 (COSHH) provides a framework of actions explaining how to assess, prevent or control the risk of bacteria such as Legionella. You need to make sure that you take suitable precautions to prevent or control the risk of people being exposed to Legionella and to protect their health and safety. This will depend on the building and its use, but may include people who are employees, tenants, customers, patients, and/or other members of the public.

Legionella Risks During COVID-19

As mentioned, it is going to be particularly crucial that you take steps to control Legionella bacteria in your water systems after a period of disuse due to COVID-19. The HSE has issued guidance regarding this to remind businesses, which are now starting to reopen, what their duties are. To ensure you comply with this, you must review your risk assessment when restarting some types of air conditioning and/or any water system at the premises.

You are responsible for carrying out this risk assessment, or for nominating a competent person to do so on your behalf. You must take the necessary steps to ensure that the risk of Legionella exposure is minimised by:

  • Identifying and assessing sources of risk.
  • Managing any risks.
  • Preventing or controlling any risks.
  • Keeping and maintaining the correct records.
  • Carrying out any other duties you may have.

For more information on how to carry out a Legionella risk assessment, our article on Legionella Control may be of use.


How to Clean Stagnant Water

While you may be able to safely maintain some water and air conditioning systems yourself, it is likely that you will have to employ a qualified specialist to come to your premises and routinely check, inspect and clean most systems. You may also rely on a competent person or specialist to assist you with identifying and implementing controls. If you are unable to clean stagnant water yourself, you have a responsibility to ensure that someone who is competent does it on your behalf. As getting specialist assistance during the COVID-19 pandemic may be challenging, you may have to consider the possibility of stopping the operation of certain systems until their safe use can be guaranteed.

Man fixing air conditioning unit

There are, however, some steps that you can take yourself to help reduce the negative impact of stagnant water, such as an increase in Legionella bacteria. Although you may not be carrying out deep cleans yourself, you still need to know what treatment and maintenance your water systems require. You should also be aware that the person performing these cleaning duties may require personal protective equipment (PPE), particularly respiratory protective equipment (RPE). It may be your responsibility to provide this protective equipment to ensure their health and safety isn’t compromised.

We have listed the steps that must be taken to clean, as well as prevent, stagnant water, for the most common types of water supplies below.


Hot and Cold Outlets

Hot and cold water outlets, such as taps and shower heads, must be flushed weekly if they are used infrequently. If doing this hasn’t been possible, you must arrange for a qualified specialist to come in and perform a deep clean. This will likely include the water system undergoing a more thorough flushing and disinfection process before the building can be reopened and facilities used.


Air Conditioning

Some types of air conditioning units have a source of water, which can result in the Legionella risks that we have outlined here. If so, you need to assess the extent of this risk before the air conditioning can be used again (if not used for a sustained period or used infrequently). It is most likely that larger units will pose this risk if they contain areas where bacteria can thrive, such as in humidifier or evaporative sections, or condensate trays where water can stagnate. Smaller units with a closed cooling system shouldn’t present this risk, but you should still carry out an assessment to ensure this is the case.

During your review of your risk assessment, you may determine that your air conditioning units need to be thoroughly cleaned before use.


Commercial Spa Pools and Hot Tubs

If commercial spa pools and hot tubs are used regularly, you should maintain the existing control measures and procedures that you have put in place. If they have been out of use then you must ensure that they are drained, cleaned and disinfected correctly. The process of cleaning and disinfecting the spa pool or hot tub and surrounding area should be repeated before they are used.

Spa pool

Cooling Towers and Evaporative Condensers

If cooling towers or evaporative condensers are installed and have been out of use for up to one month, you should isolate the fans and circulate biocidally-treated water around the system for a minimum of an hour every week. If they have been out of use for longer than this, you must ensure that the systems are drained, cleaned and disinfected before use. The process of cleaning and disinfection should be repeated before they are then refilled and used.


As a duty holder, you have legal responsibilities under several health and safety laws to manage the risks caused by stagnant water, namely Legionnaires’ disease. As many buildings have been vacant and largely unused during the COVID-19 pandemic, so have their water supplies. You must revisit your risk assessment for your water supplies and ensure that they are correctly treated and cleaned before your building reopens.


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