Ozone can be used but has a ...

Ozone can be used but has a disasterous effect on any metal in the system and can cause corrosion of pipework, heat exchangers and cooling towers but will kill Legionella.  The treatment of cooling systems to control Legionella is relatively simple but requires an assessment of risk generation, a management system to ensure that all systems put in place are actually carried out and that routine monitoring is carried out.  

One of the common sources of Legionella bacteria is areas of poor or low flow.  The number of times I have been assured that there are no dead legs, yet when tracing the pipework I found old pieces of equipment out of use with no circulation, or pipework with caps on the end several meters away from the main circulation.  Is there an area where flow is very slow that deposits fall out of suspension.  It is in these areas where Legionella bacteria can develop.

Another question should be is there an automatic bleed system.  Does this limit the cycles of concentration?  If the cycles of concentration becomes too high, then sediments start depositing within the system.  If the cycles of concentration are too low, then either chemicals being dosed become too dilute to be effective or the cost of dosing becomes too high and very costly.

I advise that you employ a professional water treatment company, who look at the whole system not just the cooling tower.  There are questions, which need to be asked, is there ingress of dirt, product or dust getting into the cooling system?  I worked with a cooling system, which was always clear of Legionella until building work caused an ingress of building dust into the environment, the same occurred with powder in the environment.  As air is drawn through the water in a cooling tower, materials are scrubbed from the air and then become part of the water circulating.

As a result the program of treatment needs to be complete.  It will always need some form of corrosion inhibitor to stop iron from becoming available for Legionella growth.  It will need some form of scale control, because either scale will form causing inefficiency in running the plant, or it will generate suspended solids, which will settle and allow bacteria to grow.  It will need some form of dispersant to keep these suspended solids in suspension until they leave the plant through the bleed system.  Often these components are formulated into a single product.  However, if there is dust ingress additional dispersant may be required.

Biological control will be required.  If non-oxidising bacteria are being used, they are dosed on an intermittent basis, which allows for bacteria to grow in between doses and if just a single biocide is used, then resistance frequently occurs.  If materials such as oxidising biocides are used, they are dosed on a continuous basis.  They are able to kill bacteria in the water but may not kill bacteria, which generate biofilms.   Chlorine is pH dependant.  As the pH rises above 7.2, its effectiveness starts dropping over the next pH to 8.2 and severely deteriorates above pH 8.  Bromine changes the pH range where effectiveness starts dropping to 8.2 to 9.2.  Because bromine is a bigger molecule than chlorine, it is less volatile and less is lost due to losses through the tower.  Chlorine dioxide is effective but can be totally absent within the cooling tower because it is a gas dissolved in water and it easily de-gasses.

The tower pack will need to be removed for cleaning.  The process should involve a pre-chlorination of a very high level of 50 ppm for at least 1 hour and ensuring that all parts of the system are circulated.  The tower will need to be shut down and cleaned removing the pack and cleaning it.  After it is put back and the tower is re-filled, the system will again need to be re-chlorinated to a minimum of 50 ppm for 1 hour and allowed to decline. (Any chlorinated water will need to be neutralised prior to sending it to any waste treatment plant, also if there is hardness scale then acid will need to be used in the cleaning process and this will need to be neutralised).

If a system is already fouled with slime, then a combination of biocides will be required.  I once took over a plant where dust was scrubbed into the cooling system, and slime had colonised all the internal surfaces.  It took over 6 months of treatment to bring the slime under control.  The biofilm declined over that period and did not become re-established.  But it required heavy dosing of chemical and increased cleaning.  During this period Legionella did appear until it was brought under control.

Routine monitoring is required, weekly microbial counts, pH, conductivity and chlorine or bromine.  Monthly analyses should be carried out, which consists of more detailed tests.

If your current water treatment supplier is unable to provide a solution, then look for an alternative, reputable supplier.  The clue will be that the cheapest solution is unlikely to provide the result you want.