With today’s technology, it is impossible to monitor the total biofilm level in open recirculating cooling water systems. Consequently, biocides are typically dosed to reach certain concentrations and not in response or proportion to microbial activity. This paper presents a new method to measure actual system microbiological growth by adding a fluorescent bio-sensing molecule into the recirculating water. This bio-sensor interacts with both planktonic and sessile microbes, with the change in fluorescence as detected by an on-line fluorometer directly proportional to living microbial populations. An algorithm uses this information to optimally time and dose oxidizing biocide. Field results from the application of this technology at two HVAC cooling water applications are presented. In each case, an improved level of microbial and operational performance was obtained with less human intervention and less chemical consumption. Advance notification of problems, rapid response to unit upsets, and 24/7 remote control are demonstrated.

Units: SI