The Approved Code of Practice and Guidance (L8) describes the risk from exposure to legionella should be prevented or controlled; precautions include maintaining the cleanliness of the system and the water in it.

Domestic Water Systems

There are an increased number of applications where annual inspections and disinfection proves to be ineffective over the year; with high levels of bacterial counts and legionella positives. In these instances it is prudent to consider an additional secondary disinfection mechanism as a primary control strategy to minimise the risk of legionellosis. Potable water biocides are often added to the water, usually this biocide is locally generated chlorine dioxide maintaining a level less than 0.5ppm in the domestic hot or cold water system.

Some form of scale control is desirable in hard water areas as there is a risk of calcium being deposited at the base of the calorifier at temperatures greater than 60ºC. The presence of scale makes it more difficult to generate hot water efficiently and reduces the effectiveness of any treatment or disinfection measures. Corrosion control may be required if low-corrosion materials (copper, plastic, stainless steel etc) have not been used in the system.

Conventional showers have been implicated as the source of a number of legionella infections and special attention should be made to the risks from these common devices; L8 describes how shower heads and hoses should be dismantled, cleaned and descaled, quarterly or as necessary (normally more frequently). However, the new self-purging showers significantly reduce this requirement.
Cold water storage tanks should be checked annually. The water should be clean and not contain any debris or contamination. Cold water storage tanks should be cleaned, disinfected and faults rectified, if considered necessary.

Cooling Systems

Many cooling systems operate on a continuous basis where process conditions preclude total system shutdown except infrequently. Other measures, such as side-stream filtration, more frequent microbiological monitoring, continuous biocide addition, etc. which are reasonably practicable should be applied and monitored carefully.
Disinfection and cleaning of cooling towers should be undertaken at least twice a year, but more frequent cleaning may be necessary depending on local environmental conditions such as dirty atmospheres and the conclusions reached in the risk assessment. Cooling systems that have a short operating period may only need to be cleaned at the beginning and end of that period. If, on inspection, the system shows signs of a significant build up of deposits or slime, then disinfection and cleaning should be carried out. The use of chlorine or other oxidising biocides to disinfect the tower is an effective approach, provided they are used correctly.

Cleaning staff who carry out water-jetting should wear suitable respiratory protective equipment. Staff who use this equipment should be adequately trained and the equipment properly maintained.

Accessible areas of the tower and its packs should be adequately washed but cleaning methods that create excessive spray, e.g. high-pressure water jetting, should be avoided. If this is not possible, the operation should be carried out when the building is unoccupied or, in the case of permanently occupied buildings, windows in the vicinity should be closed, air inlet blanked off and the area that is being water-jetted should be tented. The area should be isolated and consideration should also be given to other occupied premises in the immediate areas as well as members of the public who may be in the vicinity during cleaning.

Adherent scale or other deposits on the tower and distribution system that have not been removed by the above method can be dissolved using chemical descalents carefully chosen to avoid damage to the fabric of the system. If this is not possible, routine inspection and testing of water quality should be particularly thorough.
The importance of manual and chemical cleaning and disinfection of water systems should never be underestimated.

Article by:
Mike Hunter
Director
Maplin Environmental Ltd
(A division of Waterchem Ltd)
T: +44 (0)1905 23669
E: mike.hunter@waterchem.co.uk