The topic of the “cooling tower study” carried out by the Bureau for Chemical Engineering TB-Klade on behalf of the City of Vienna is the use of biocides in open circuit cooling systems with direct material contact between the coolant water and the atmosphere. Chemicals are usually added to the cooling water, which prevent deposits and corrosion phenomena or are intended to  prevent or at least limit microbiological processes such as the formation of biofilms and the multiplication of pathogenic germs such as Legionella. In this “cooling water conditioning”, hardness stabilizers, dispersants, corrosion inhibitors and biocides are added. The latter  are the subject of the study. The aim is to evaluate and compare biocides in terms of environmental impact and occupational safety. With that relatively “low-risk” biocides become easier to detect. The study is closely related to the Legionella problem caused by the operation of cooling towers. Legionellae are bacteria and multiply at temperatures between 25 °C and 45 °C preferably in biofilms. They pose a problem with open cooling towers in that they can be discharged in the form of aerosols  and absorbed in the  vicinity of the cooling towers by those affected. Corresponding outbreaks with deaths are documented. However, the study does not evaluate the effectiveness of biocides in combating Legionella in cooling towers. Assuming  correct application and suitable operating conditions, this is taken for granted.The questions are:

  • What is the hazard potential of biocides used in open cooling circuits regarding the aquatic environment and human health?
  • Which active ingredients or offers appear to be (less) recommended when looking at hazard potential, maintenance, and operating conditions?

The selection of active substances is closely linked to the approval in product-type 11 of the European Biocidal Products Regulation. A total of 57 entries in product-type 11  are evaluated regarding the hazard potential. A detailed analysis also deals with the following active substances or groups of active substances regarding their application:

  • DBNPA (2,2-dibromo-2-cyanacetamide)
  • Active chlorine (from: sodium chloride by electrolysis; chlorine; sodium hypochlorite)
  • Benzalkonium chlorides (QAV)
  • BCDMH (Bromochlorohydantoin)
  • Chlorine dioxide
  • Glutaraldehyde
  • Hydrogen peroxide (i.a. with silver)
  • CMIT/MIT
  • Ozone
  • Peracetic acid
  • Active bromine from sodium bromide and sodium hypochlorite

The study attempts to integrate innovations for cooling water conditioning in the detailed analysis but remains within the spectrum of active ingredients specified by the EU Biocidal Products Regulation or the products and processes offered on the (European) market.

The results of the study will be published soon .