Cleaning and Disinfection
||The ACOP says the risk from exposure to legionella should be prevented or controlled; precautions include maintaining the cleanliness of the system and the water in it. The following section on cleaning and disinfection offers guidance on how to do this in cooling systems.The maintenance of an effective biocide regime will provide a hostile environment for microbial life (including legionella) and minimise biofouling. However, the use of biocides should not be considered in isolation but as part of the overall water treatment programme including the manual and chemical cleaning and disinfection of open cooling system and, in particular, the cooling tower.
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, cleaning and manual de-sludging 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.
In addition to this regular disinfection, cooling towers should always be cleaned and disinfected before being put back into service:
Routine cleaning and disinfection
Pre-cleaning disinfection The system should be disinfected using an oxidising biocide such as chlorine, bromine or chlorine dioxide to minimise health risks to the cleaning staff. This is done by adding either sodium hypochlorite solution or chloroisocyanurate compounds available as rapid-release tablets to achieve a measured residual of 5 mg/l free chlorine. Sodium hypochlorite solutions typically contain 10-12% available chlorine and rapid-release tablets contain 50-55% available chlorine. Such products should be handled with care and according to instructions given by the supplier. A bio-dispersant should also be used to enhance the effectiveness of the chlorination.
The chlorinated water containing 5mg/I free chlorine should be circulated through the system for a period of 5 hours with the fan off, maintaining a minimum of 5mg/I free chlorine at all times. However, if the system pH value is greater than 8.0, the measured residual will need to be in the range 15-20mg/I free chlorine in order to achieve the required disinfection level. An alternative procedure to provide more effective use of chlorine is to introduce a heavy bleed-off for several hours to both reduce the pH of the system water and its chlorine demand, before carrying out disinfection. The system should then be de-chlorinated (see paragraph 144) and drained.
Cleaning Manual cleaning operations can then be carried out, with all accessible areas of the tower, etc., being adequately cleaned. Where practicable, the packs should be removed at least once a year and preferably every six months. If this is not practicable, it may be necessary to apply supplementary strategies such as side-stream filtration, increased monitoring, etc. 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.
Cleaning staff who carry out water-jetting should wear suitable respiratory protective equipment such as a positive-pressure respirator with full face-piece or a hood and blouse. Staff who use this equipment should be adequately trained and the equipment properly maintained (see section on protection of personnel)
Adherent scale or other deposits on the tower and distribution system that
Finally, the system should be sluiced out until the water going to drain is clear.
Post-cleaning disinfection On completion of the cleaning operation, the system should be refilled and chlorinated to maintain a minimum level of 5 mg/1 of free chlorine for a period of 5 hours with than fan off. This needs to be checked hourly to ensure that a concentration of 5mg/1 is present for the total period. Again, the use of a bio-dispersant will enhance the effectiveness of this chlorination. If the system volume is greater than 5m3, the water should be de-chlorinated, drained, flushed and refilled with fresh water and dosed with the appropriate start-up level of treatment chemicals, including the biocides.
While the maintenance of a continuous minimum residual of 5mg/1 of free chlorine for a minimum period of 5 hours is considered the best practice, if the downtime to conduct such a lengthy operation is not available, some compromise may be necessary. Under such circumstances, it may be acceptable to shorten the pre-and post chlorination times to increase the free chlorine level, e.g. 50mg/1 for 1 hour or 25mg/1 for 2 hours. This should only be done of the operators are trained in this process because, at these levels, there is a greater risk of damaging the fabric of the system. The system should then be de-chlorinated, drained, flushed and refilled with fresh water and dosed with the appropriate start up level of treatment chemicals, including the biocides.
Before water containing high-residual free chlorine is discharged to drain, it should be de-chlorinated. The usual procedure is to add sodium thiosulphate, sodium sulphite or sodium bisulphate as a neutraliser. The level of free chlorine is determined by testing and the quantity of sodium salt then is calculated.