Legionella is an opportunistic waterborne pathogen associated with Legionnaires’ disease and Pontiac fever.
Legionella bacteria are found naturally in freshwater environments, like lakes and streams. The bacteria can become a health concern when they grow and spread in man-made building water systems like showers, cooling towers, decorative fountains, hot tubs, hot water tanks, large & complex plumbing systems etc.
Three factors determine the likelihood of legionella bacteria colonizing a water system which are:
- Availability of Nutrients
- Flow of water
We have already discussed how to manage legionella risk by temperature control.
In this blog, we discuss how stagnant water leads to legionella growth in hot and cold water systems and what you can do to minimize this risk.
What is Stagnant Water?
Stagnant water is water that sits in place for hours or longer with little or no movement.
Most buildings will have a constant 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.
This is exactly what happened when buildings were closed due to the coronavirus pandemic. The unused water systems became a large dead leg with no water flow, thereby increasing the risk of legionella bacteria.
Stagnant water conditions may also be formed due to the presence of dead legs in water systems. Dead legs may be intentionally installed to facilitate future expansion of a building’s potable water plumbing system to avoid a complete building outage.
How does Stagnant Water Influences Legionella Growth?
During stagnation hot water temperatures cool to ambient temperatures at the point of use outlets. The biggest problem with stagnant water is the formation of biofilms.
Biofilms are communities of surface-attached bacteria surrounded by a sticky, gel-like secretion often called slime. Although biofilms start out microscopic in size, they can grow into visible biofouling deposits in just a matter of days.
Biofilms only grow in wet areas. The faster the moving water is, the thinner the biofilm. Conversely, the slower the moving water, the thicker the biofilm.
Without flow, the water in a dead leg does not receive disinfectant or biocide treatment. This allows bacteria to attach to system surfaces to start the biofilm formation process.
Water use after periods of stagnation can release opportunistic pathogens that have grown or become detached from the biofilm.
Stagnation also reduces the effectiveness of the residual disinfectant often added to inhibit the growth of waterborne pathogens. Once the disinfectant is consumed, and no other water and disinfectant is introduced into the piping due to lack of flow, Legionella and other waterborne pathogens can quickly colonize the water system.
Biofilms Provide Safe Haven For E.Coli & Legionella
For microorganisms such as amoebas, protozoans, algae and bacteria, living as part of a biofilm has its advantages.
The biofilm facilitates nutrient and gaseous exchange and protects microorganisms not only from biocides but also from periodic increases in temperature and attempts at physical removal, especially in areas where surfaces are scaled or corroded.
Bacteria living as a group are more resilient to stressors, such as dehydration, ultraviolet light, high or low pH or to the presence of bacterial toxic substances, such as antibiotics, antimicrobials, disinfectants or heavy metals.
This is why once established, biofilms and associated microorganisms provide a protective environment for any Legionella entering a building with the source water to multiply to high levels.
Uncontrolled biofilms can occlude pipework, resulting in areas of poor flow and stagnation which further contributes to Legionella growth.
How to Prevent Water Stagnation
Preventing low flow rates and stagnation of water is an essential and important control measure, and the system should be designed to minimize areas of stagnation and low flow.
Care should be taken to ensure that any modifications to the system do not introduce areas of stagnation and low flow. Where such areas are unavoidable, design and operation should aim to at least reduce stagnation and low flow.
1. Remove Dead Legs
If dead legs are present in the system, the first choice should be to remove them. When removing dead legs, it is critical to consider the benefits versus the cost.
Some dead legs present a greater risk than do others. Some are expensive to correct; others are not.
The following rules are good practice:
In areas where dead legs are accessible, the cost of removal will typically be minimal, so remove them. For example, if water heaters are abandoned, remove all the piping associated with the installation to the point of flow, rather than simply capping the lines.
If a dead leg cannot be removed without tearing out a wall, then leave it in the wall but cut and cap it where it tees into the main. For example, if a sink is removed, cut and cap the water lines serving the sink where they tee into the main, rather than at the wall.
If a dead leg is not accessible, and it cannot be cut at the main, then try other methods of controlling Legionella bacteria before going to the expense of tearing out walls to remove dead legs.
2. Establish a Flushing Regime
Flushing is the process of replacing stagnant water with the introduction of fresh cold water containing residual disinfectant. Flushed water carries away waterborne Legionella and dislodges biofilm lining the inside of the pipes and fittings, which harbours Legionella.
The flushing should be done in such a way as to control the production of aerosols. Outlets should be fitted with a length of hose into the sink drain and be opened slowly to run without excessive splashing.
Normally a period of five minutes flushing per outlet will be sufficient.
The frequency of flushing depends on the susceptibility of the occupants of the building.
In healthcare premises such as hospitals, nursing homes, and care homes where people with higher susceptibility would be present, the water outlets should be flushed at least twice a week.
The risk assessment may indicate a need for more frequent flushing.
3. Use backup water supplies regularly
Large buildings usually have a backup water supply that may go several days or months without use.
These lines can build up foul water that will be distributed throughout the facility when the line is used. If backup supply lines are not kept open, they should be flushed before each use, which may require adding a valve and drain at the downstream end, just before the building.
Reopen Safely After COVID-19 Lockdown
With buildings shut down or infrequently used during the COVID-19 pandemic, water is not circulating through plumbing systems and has become stagnant. As a result, disinfectants in the water dissipate, potentially allowing microorganisms, bacteria and pathogens to grow within plumbing systems.
Such systems may have been out of use for a significant time and in most cases cannot simply be used straight away. The system may require recommissioning as if new (that is thorough flushing, cleaning and disinfection and/or controlled flushing of outlets such as taps, showers and toilets) prior to returning to use and reopening of the building.
Risk assessment review and legionella testing should also be considered as part of the recommissioning plan.
Addressing stagnant water concerns may seem daunting to building owners who have never had to consider these impacts before.
Celtic Water Solutions provides water hygiene solutions to a range of businesses in the Republic of Ireland helping them stay compliant with health and safety laws.
If you need more information on our legionella risk management, feel free to get in touch with our team of experts.
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