Pilot trials of ultraviolet (UV) irradiation, ozone addition, and hydrogen peroxide (peroxide) addition for the control of biofouling and microbial action in fresh water cooling towers in water-cooled air conditioning systems (WACS) were conducted at an industrial site in Tai Po, New Territories, Hong Kong. The pilot site had 4 separate independent cooling water circuits, each with 1 chiller unit. All the cooling towers were located on the rooftop of the building. The chiller units were located on various floors inside the building. Testing was conducted over a period of about 6 weeks to determine the effectiveness of the processes. In addition to the cooling circuits that were using either UV, ozone, or peroxide, a control test was setup in the smaller fourth cooling circuit. Includes table.