You will be given lots of ...

You will be given lots of conflicting advice as to how to control issues.  I would advise contacting a reputable specialist who can look at your specific process and how to overcome it.  Scale can occur for several reasons.  The obvious one is supply by a scale-forming water.  In this case, either scale retardents and or pre-softening can help overcome it.  If you have an existing scale then it will impact the process so either descaling using some form of acid (think carefully about handling issues) and passivation afterwards.  

The use of softening or RO water will mean that a corrosion inhibitor will be required.

It could also be caused by over-concentration of a cooling system.  In this case control using a conductivity controlled bleed and scale/corrosion inhibitors.

The third cause of scale is the presence of iron or manganese bacteria.  This could either be from the water source or just by development on the surface of iron or galvenised pipework.  This is harder to control.  Citric acid along with specialised dispersants can help break this down but you will need side-stream filtration to help remove the metal particulates.

This doesn't appear to answer the question about Legionella in the water.  This is because a cooling system needs a full package including scale or corrosion inhibitors to control conditions.  No single biocide provides a solution but general guide is the use of an oxidising biocide in conjunction with dispersant and/or non-oxidising biocide, which should be Legionella effective at the dose rates given and contact time.  Advise such as high levels of chlorine dioxide are not sustainable and chlorine dioxide becomes de-gassed under high agitation and by the scrubbing action of the air in a cooling tower.  Chlorine becomes less effective as the pH rises from 7-8 and is highly corrosive.  Bromine can extend the pH although it also starts dropping its activity above pH 8.  Ozone kills all but is highly corrosive. UV on its own does not work in re-cycled systems.  Non-oxidising biocides have to be selected carefully based on the pH of the system and its contact time within the system and how quickly it breaks down.  

To quickly remove Legionella firstly chlorinate the water with dispersant to a minimum of 50 ppm free chlorine for at least 1 hour but preferably for at least 5-6 hours, ensuring that all parts of the system including standby  pumps are run for 1 hour.  Neutralise the chlorine then run a de-scaling program based on acid whether it is sulphamic, citric or other.  Drain and flush the system then clean the tower (use a specialist service provider qualified to do this).  Re-fill then re-chlorinate the system.  Then make sure that the scale and corrosion dosing systems are working properly, that the automatic dosing and conductivity control system is operational and that your biocide regime is working properly.

You need to review your operation to see why you have developed both scale and Legionella growth and implement a full risk assessment for Legionairres Disease and carry out its monitoring program.