Cooling towers   [Download this application case as PDF file]

In industrial cooling towers, biofilm growth can cause serious problems, both contributing to the deterioration of materials and, most important, increasing the risk of harmful microbiological contaminations (e.g. Legionella). For these reasons, large amounts of chemical substances (biocides) are used to limit microbiological growth in such systems.

The manager of this cooling tower planned to automatically dose chlorine continuously (24/7) in the water; later on, he decided to install an ALVIM probe in the plant, to check the effectiveness of the treatment.

Taking into account that ALVIM probe signalizes:

• the injection of oxidizing agents through a sharp and large increase of the signal, and
• biofilm growth through a relatively slow increase from about 500 mV (in this case; marked in figure as "Signal baseline") to about 1200 mV,

data plotted in the figure indicate that:

• the chlorination really applied in the tower was irregularly intermittent, and not continuous as planned by the plant manager (this conclusion was confirmed by ORP measurement, made by plant technical staff after ALVIM installation, see the figure below);
• when chlorine concentration dropped for enough time, biofilm started to grow; this bacterial layer was disrupted later on by the following chlorination.

Looking at the figure below, it can be easily noted that ALVIM probe can detect both biofilm growth and changes in chlorine concentration. On the contrary, an ORP sensor can detect only changes in the concentration of oxidizing agents, while it cannot monitor biofilm growth.