||MonitoringThe ACOP says that the risk from exposure to legionella should be prevented or controlled and that the precautions taken should be monitored to ensure that they remain effective. The following section on monitoring offers guidance on how to do this in cooling systems.
The composition of the make-up and cooling water should be routinely monitored to ensure the continued effectiveness of the treatment programme. The frequency and extent will depend on the operating characteristics of the system, the minimum recommended frequency being once a week to ensure that dosage and bleed rates are correct (see Table 1).
Table 1: Typical on-site monitoring checks recommended for good operating practice
The identification of changes in the water chemistry such as pH, dissolved and suspended solids, hardness, chloride and alkalinity allows any necessary corrective actions to be taken to the treatment programme or system operating conditions. In addition, chemical treatment reserves such as scale and corrosion inhibitors and oxidising biocides should be measured. Routine on-site determination of the concentration of non-oxidising biocides is not practical. The amount of non-oxidising biocide required is therefore calculated from the volume and half-life of the system. Other aspects of the treatment programme such as corrosion rates and microbiological activity will also need to be monitored.
The monitoring programme should also include the routine sampling and testing for the presence of bacteria, both general (aerobic) bacterial species and legionella bacteria. Since the detection of legionella bacteria requires specialist laboratory techniques, routine monitoring for aerobic bacteria is used as an indication of whether microbiological control is being achieved.
The most common method to measure microbiological activity within a cooling system is to use a dip slide. These are commercially available plastic slides which are coated with a sterile nutrient agar, a medium on which many micro-organisms will grow but not legionella. They are dipped into the water and incubated for 48 hours. Any bacteria in the cooling water will grow and form colonies. Comparision with a chart will indicate the number of bacteria in the water. Dip-slides should be dipped in the system water as near to the heat source as possible. If a drain cock is used, it is important that any residual water is run off before the slide is dipped. The dip-slide should then be replaced into its container and incubated for a minimum of 48 hours in an incubator, usually at 30ºC. The incubation period and the temperature should be the same each time the test is performed.
Cooling tower water should be tested, using dip-slides (or similar) on a weekly basis. The timing of dip-slides and other microbiological sampling is important. Sampling should not be carried out if biocide has been recently added. Neither should the visible condition of the water be taken as a good indicator of the need for sampling; there are a number of chemical additions which render the water opaque. Conversely, relatively clear water may be heavily contaminated with bacteria.
Table 2 lists microbiological counts and the appropriate action that should be taken in response to them. While the number of micro-organisms is itself important, it is also necessary to monitor any changes from week-to-week, particularly if there are any increases in the numbers of micro-organisms detected. This should always result in a review of the system and the control strategies. A graphical representation of these date will often help to monitor any trends.
If the control strategy is effective, the dip-slide counts should be consistently low. If an unusually high result is obtained, the test should be repeated immediately and, if confirmed, appropriate action taken (see Table 2). Consistently high microbiological counts using dip-slides should be checked by laboratory-based total viable (TVC). The laboratory should be accredited by the United Kingdom Accreditation Service (UKAS).
Table 2: Action levels following microbial monitoring for cooling towers
Alternative techniques for determining microbiological activity have been developed for on-site use. It is important that such methods can be clearly related to the results achieved by traditional counting methods and that appropriate action levels can be set to inform decisions on the necessary control measures.
Monitoring for legionella
In addition to the routine sampling for aerobic bacteria, the routine monitoring scheme should also include periodic sampling for the presence of legionella bacteria. This should be undertaken at least quarterly unless sampling is necessary for other reasons, such as to identify possible sources of the bacteria during outbreaks of Legionnaires’ Disease. More frequent sampling should be carried out when commissioning a system and establishing a treatment programme. Sampling should be carried out on a monthly basis until it can be shown that the system is under control. If a legionella-positive sample is found as a result of routine sampling, more frequent samples may be required as part of the review of the system/risk assessment to help establish when the system is back under control. The sampling method should be in accordance with ISO 11731:19988 and the biocide neutralised where possible. Samples should be taken as near to the heat source as possible. They should be tested by a UKAS accredited laboratory that takes part in the Public Health Laboratory Scheme for the isolation of legionella from water. The laboratory should also supply a minimum theoretical mathematical detection limit of less than, or equal to, 100 legionella bacteria per litre of sample.
Legionella bacteria are commonly found in almost all natural water sources, so sampling of water systems and services may often yield positive results and the inter-operation of any results of sampling should be carried out by experienced microbiological experts. Failure to detect legionella bacteria should not lead to the relaxation of control measures and monitoring. Neither should monitoring for the presence of legionella bacteria in a cooling system be used as a substitute in any way for vigilance with control strategies and those measures identified in the risk assessment.