Treatment and Control Programmes

Guidance

The ACOP says that the risk from exposure to legionella should be prevented or controlled; precautions should include the use of water treatment techniques. The following section on treatment programmes offers advice on how to treat water in hot and cold water systems.
It is essential that the system is kept clean (see section on cleaning and disinfection) because the efficacy of the control method (both temperature and biocide activity) may be reduced substantially in systems that are fouled with organic matter such as slimes or inorganic matter such as scale.

Temperature regime

This is the traditional approach to legionella control. It is recommended that hot water should be stored at 60C and distributed so that it reaches a temperature of 50C within one minute at outlets. Care is needed to avoid much higher temperatures because of the risk of scalding. At 50C the risk of scalding is small for most people but the risk increases rapidly with higher temperatures and for longer exposure times. However the risk, particularly to young children or the handicapped or elderly and to those with sensory loss, will be greater. Where a significant scalding risk has been identified, the use of TMVs in baths and showers should be considered to reduce temperature. These need to be placed as close to the point of use as possible.
To ensure the correct function of fail-safe TMVs, there needs to be a minimum temperature differential between the hot and cold water supplies and the mixed water temperature. Users should refer to the manufacturers' operating instructions to ensure these devices are working safely and correctly.

Monitoring the temperature regime

As well as the routine monitoring and inspection outlined in paragraphs

Biocide treatments

Where biocides are used to treat water systems, they, like the temperature regime, will require meticulous control if they are to be equally effective. In such situations, if hot water is not needed for other reasons (e.g. for kitchens or laundries), there is no requirement to store hot water at 60C (or distribute at 50C), although this is not currently permitted in NHS premises. However, if water temperatures are reduced, any lapses in the biocide control regime would leave the system vulnerable. It is, therefore, recommended that the control system is checked at least weekly to ensure that it is operating correctly and so continuing to control legionella.

Chlorine dioxide

Chlorine dioxide is an oxidising biocide capable of reacting with a wide range of organic substances. Levels of 0.5 mg/l can, if properly managed be achieved.

Frequency Check

Standard to meet Notes Cold Water Hot Water Monthly sentinel taps
(see glossary)The water temperature should be below 20C after running the water for up to two minutes. The water temperature should be at least 50C within a minute of running the water. This check makes sure that the supply and return temperatures on each loop are unchanged, i.e. the loop is functioning as required. If fitted, input to TMVs on a sentinel basis: The water supply to the TMV temperature should be at least 50C within a minute of running the water. One way of measuring this is to use a surface temperature probe. Water leaving and returning to the calorifier: Outgoing water should be at least 60C, return at least 50C. If fitted, the thermometer pocket at the top of the calorifier and on the return leg are useful points for accurate temperature measurement. If installed, these measurements could be carried out and logged by a building management system six-monthly. Incoming cold water inlet (at least once in the winter and once in summer). The water should preferably be below 20C at all times (but see paragraph 156). The most convenient place to measure is usually at the ball valve outlet to the cold water storage tank annually. Representative number of taps on a rotational basis: The water temperature should be below 20C after running the water for two minutes. The water temperature should be at least 50C within a minute of running the water. This check makes sure that the whole system is reaching satisfactory temperatures for legionella control, effective against planktonic and sessile legionella in hot water systems. The Drinking Water Inspectorate prescribes a maximum value for total oxidants in drinking water supplies which is the combined chlorine dioxide, chlorite and chlorate concentration. This should not exceed 0.5mg/l as chlorine dioxide. There are a number of commercial systems available that release chlorine dixoide into water systems and it may be necessary to contact the local company in order to check that the installations complies with the requirements of the water regulations and, for Scotland, the Water Supply (Water Quality) (Scotland) Regulations and the Private Water Supplies (Scotland) Regulations 1992, as amended. It should be noted that maintaining total oxidant levels below 0.5mg/1 at outlets may be difficult in systems with a low turnover of water. Suppliers of commercial chlorine dioxide systems will need to consider these problems and, when choosing a system, these points should be checked to ensure that they have been addressed satisfactorily by the supplier.

Monitoring the chlorine dioxide regime

For most systems, routine inspection and maintenance will usually be sufficient to ensure control if the following areas are checked at regular intervals and remedial action taken when necessary, with details of all actions being recorded:
* the quantity of chemicals in the reservior;
* the rate of addition of chlorine dioxide to the water supply;
* on a monthly basis, the concentration of chlorine dioxide should be measured at the sentinel taps; The concentration should be at least 0.1mg/1. On an annual basis, the chlorine dioxide concentration at a representative number of outlets; the concentration should be at least 0.1mg/1.

Ionisation

Ionisation is the term given to the electrolytic generation of copper and silver ions for use as a water treatment. Copper and silver ions concentrations maintained at 400mg/1 and 40mg/1 respectively can, if properly managed, be effective against planktonic legionella in hot water systems. If, however, the water is softened, silver ion concentrations between 20-30mg/1 can also be effective, provided a minimum concentration of 20mg/1 is maintained. This level of silver still requires copper ions to complete the synergy.
The application of ionisation will need to be properly assessed, designed and maintained as part of an overall water treatment programme. The water supply (water quality) regulations and private supply regulations prescribe a maximum value for the level of copper and silver ions in drinking water supplies. It is important that installers of ionisation systems are aware of the need to avoid any breach of these regulations and maintain copper and silver levels below the maximum allowable concentration. The local water company may need to be consulted to check that the installation complies with the requirements of the water regulations.
It should be noted that, in hard water systems, silver ion concentrations can be difficult to maintain due to build-up of scale on the electrodes and the high concentration of dissolved solids precipitating the silver ions out of solution. For both hard and soft water, the ionisation process is pH 7,6. The build-up of scale and concentration of dissolved solids therefore needs to be carefully controlled so that suitable ion levels are consistently maintained throughout the system. This may need extra treatments.

Monitoring the ionisation regime

For most systems, routine inspection and maintenance will usually be sufficient to ensure control if the following parameters are also monitored at regular intervals and remedial action taken when necessary, with details of all actions being recorded:
* The rate of release of copper and silver unions into the water supply;
* The silver ion concentrations at sentinel outlets should be checked monthly - this should be at least 20ug/l at outlets;
* The measurement of silver ion concentrations at representative taps selected on a rotational basis once each year - this should be at least 20ug/l at outlets;
* The condition and the cleanliness of the electrodes; and the pH of the water supply.

Ozone and UV treatment

The strategies previously described are dispersive, i.e. they are directly effective throughout the water system downstream from the point of application. A number of other strategies are available (for example, UV irradiation or ozone). These systems are not intended to be dispersive and are usually designed to have their effect at, or very close to, the point of application. This usually results in the active ingredient not being directly measurable in the circulating system. In large systems, it may be necessary to use a number of point applications of these treatments and the system suppliers will be able to advise appropriately.

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