Biocides in evaporative cooling systems

The Vienna Environmental Ombudsman’s Office commissioned the TB Klade to conduct a study investigating the use of biocides in evaporative cooling systems.

The use of biocides in open circuit cooling systems is carried out to prevent or at least limit the formation of biofilms and the proliferation of pathogenic germs such as legionella. In addition, chemicals are added to the cooling water to prevent deposits and corrosion.

The study is closely related to the legionella problem caused by the operation of cooling towers. Legionellae are bacteria and prefer to multiply in biofilms at temperatures between 25 °C and 45 °C. They pose a problem with open cooling towers in that they can be discharged in the form of aerosols and absorbed by those affected in the vicinity of the cooling towers. Corresponding outbreaks with deaths have been documented.

However, the study does not assess the effectiveness of biocides in combating legionella in cooling towers (such a situation is assumed to be given for all biocides examined). The aim of the study was to identify “low risk” biocides by comparison, the questions were: What is the hazard potential of biocides used in open cooling circuits regarding the aquatic environment and human health? Which active ingredients or technologies appear to be (less) recommendable in a synopsis of hazard potential, maintenance and operating conditions?

In addition to the elimination of structural defects and the regular, chemical and technical maintenance of these systems, oxidizing processes with low AOX formation are recommended. These include the production of active chlorine by electrolysis from sodium chloride on site or the use of chlorine dioxide that is as pure as possible. Unfortunately, there are still few references for processes that use the environmentally friendly hydrogen peroxide or that work with monochloramine or ozone, at least in Austria. However, with suitable maintenance and operational management, the use of wastewater-relevant biocides as well as environmentally harmful corrosion inhibitors could be dispensed with. The study advises against the use of the biocidal active ingredients CMIT/MIT, DBNPA and glutaraldehyde, not least for reasons of employee protection. The analysis deals in detail with the environmental behaviour and application criteria of the following active substances or groups of active ingredients:

• Monochloramine (produced from…)
• Chlorine dioxide (produced from…)
• Active chlorine (produced or released from…)
• Active bromine (produced from…)
• BCDMH (bromochlorohydantoin)
• DBNPA (2,2-dibromo-2-cyanoacetamide)
• Bronopol (2-bromo-2-nitropropane-1,3-diol)
• CMIT/MIT
• Hydrogen peroxide (including the addition of silver)
• Ozone
• Peracetic acid
• Benzalkonium chloride (QAV)
• Glutaraldehyde
• Other biocidal active substances (THPS, peroxomonosulphate etc.)