on systems that operate on a 24 hour basis effective control of legionella was able to be achieved with conventional levels of biocides by heating the cooling water in a cooling water tower for 5 minutes at 70 u00b0 c once in every 24 hours temperature monitoring and control were able to be achieved easily and this included the capability of remote monitoring of the operation of the system referring to fig2 in this embodiment the heat rejection equipment 14 is not isolated from the recirculating cooling water the recirculating cooling water is heated by a bypass heater and held in a holding tank 19 and preheater heat exchanger 20 the preheater 20 is optional but is preferred as it increases the efficiency of the operation of the system the heating process may also be achieved by utilising waste heat heat from desuperheaters or other sources this system is suitable for intermittent and continuous use the processes of the invention would also usually involve automated controls to action and control the low and high temperature cycles monitoring equipment to demonstrate that the cycles had been completed satisfactorily would be included as part of a quality assurance requirement the present invention also includes the operation of cooling water system such that the cooling and heating cycles are sequentially combined while the present invention includes the control of legionella without reliance on biocides these materials may be used to augment the efficacy of the cooling water and or heating system and these are our preferred systems although the present invention has been described with reference to cooling towers for air conditioning it is also applicable to other community commercial and industrial situations where there is potential for aerosols with legionella infected water suppliers since modifications within the spirit and scope of.