Biocide is a chemical added to the make-up water for evaporative applications such as condensers and cooling towers.

The purpose is to kill any living organisms in the make-up water and reduce bacterial growth the can harm humans with for example with Legionnaires disease.

Biocides are commonly used in medicine, agriculture, forestry, and in industry to prevent the fouling of water pipelines.
Biocides are often added once a week as a preventive measure. Newer technology is phasing out chemicals and reduces the number of living organisms with ultrasound. It is a less effective method but still applicable for the majority of all applications.