Legionnaires Disease & WFP

What are the risks of Legionnaires Disease when storing pure water?


Legionnaires Disease
Legionella Bacteria can be found in low levels in most standard water sources, low-level presence of bacteria is in itself very unlikely to cause a health problem, it is when they begin to multiply that the risk increases. Legionella bacteria requires suitable nutrients to multiply -  these can be provided by sediment, scale, sludge and biofilms.  These nutrients can build up in the water filters used to purify water and if not replaced at specified intervals filters may become a suitable breeding ground for legionella bacteria.  The temperature of the water is a crucially important factor in the survival and multiplication of legionella. When the temperature of water rises above 20°C the bacteria begin to multiply with the optimum temperature of 37 degrees providing the most fertile conditions.

This means that, if the water filters are changed regulalry, for most cold water stored for use in WFP there is very little risk of the water growing dangerous levels of legionella bacteria. This is due to the cleanliness of the storage tank, the frequency with which the tank is emptied and renewed and the sub 20°C temperatures of stored water.

Spreading the Bacteria or Contracting the Disease
The disease is most usually contracted after the inhalation of the bacterium in small droplets (called aerosols).  Water-fed poles can produce aerosols not only at the point of water purification but also at the point of end use (spraying water in the air on windows). Under suitable wind conditions, viable bacteria can travel up to 500 metres. Legionella will not normally multiply in cold water stored WFP systems or even hot water WFP systems when the water is heated at point of use, or when the system is in regular use with the tank of heated water being emptied and replenished on a daily or weekly basis.

Important measures that should be taken to control the risk of legionella bacteria are:
•Replacement of water purficiation filters at recommended intervals
•Following RO manufacturers servicing recommendations
•Keeping the purficiation system and water storage tank in a cool place when it is not in regular use
•If the systemtank cannot be stored in a cool place then drain the tank and water filters whenever the system is to be left idle for more than one week during warmer months (usually inside a van the water in a tank will not reach as high as 20°C even in the summer)

This means that if the water filters are changed regularly and the stored water is kept below 20°C then the chance of the contracting the disease or spreading the bacteria is very limited. If however you heat the water to above 20°C in the storage tank (and especially over 37 degrees) - such as with immersion heater systems it would be important to use and replenish the water within three to four days rather than allowing the water to be stored once heated.


To sum up:

In order for legionnella bacterium to grow you need all of the following conditions:

1. Stagnant Water - This is a tank or body of water that does not move or have a through flow through it for long periods of time, such as 7-10 days or more.
2. Water that is heated to above 20°C or ideally above 37°C and then remains at this temperature for an extended period of time, such as 7-10 days or more.
3. Water that has a sludge, rust or bio-film available in it as a source of growth

Removing any one of these conditions will prevent legionella bacterium from multiplying.

So looking at a typical cold water WFP supply.

1. Stagnant Water - This is classed as water that does not move or have through flow in over a  week. Water in tank van of course does not stay stagnant as it moves around a lot.
2. Heated Water - even in a shed or a van a tank of water will virtually never be heated to 20°C and this temperature sustained for longer than a week. Even when an RO is left in the sun treating water, the flow of water through the unit will prevent the water temperature rising anywhere near the 20°C point and its very movement through the unit would prevent it being classed as stagnant.
3. Water that has sludge or rust etc. As long as the pre-filters in the RO are changed when advised the water supply reaching the RO membranes and the pure water tank will be free of 'sludge' and rust etc. becuase of this there is nothing for the bacterium to grow on.

Please note the above material is meant for information purposes and does not constiutue an absolute guide for the prevention or control of legionella bacteria. If in doubt please consult one of the HSE guides linked to below.

Important Information

The release of legionella is also under the Control Of Substances Hazardous to Health (COSHH) Regulations 2002
Used water filters should be disposed of in accordance with local authority guidelines - many supermarkets now have used water filter recycling points

HSE Guideline Documents

Approved Code of Practice and guidance on regulations 

Control of Legionella bacteria in water systems - Audit Checklists