Cooling towers with their combination of moisture, warmth, air and nutrients create an ideal environment for the growth of microorganisms. This biological activity results in a number of serious problems including:
- Growth of slime and bioﬁlm on heat transfer surfaces leading to reduced thermal efficiency
- Increased corrosion rates as a result of the formation of electrochemical cells within the bioﬁlm and microbiologically induced corrosion (MIC) due to the presence of anaerobic bacteria in the bioﬁlm
- Reduced efficiency of added corrosion inhibitors due to bioﬁlm blocking contact of the corrosion inhibitors with the metal surface
- Increased pumping energy to overcome the additional friction created by the presence of bioﬁlm
- Build up of objectionable odours
- Potential for pathogenic microorganisms to be harboured in the circulating water (e.g. Legionella)
CleanOxide 75 from Advance-Eco is the simple modern method of generating high purity chlorine dioxide as and when required by mixing two precursor solutions.
CleanOxide 75 is a 0.75% solution of chlorine dioxide in water containing only a small amount of sodium sulphate and sodium chloride as by-products of mixing. When dosed at the recommended rate, CleanOxide 75 will control bacteria, bacterial spores, fungi, protozoa, viruses, algae and bioﬁlm.
- CleanOxide 75 is eﬀective over a broad pH range (4-10).
- CleanOxide 75 controls bacteria, fungi, viruses, bioﬁlm, protozoa and algae.
- CleanOxide 75 does not react with ammonia and does not produce toxic compounds in contact with organic materials present in water.
- CleanOxide 75 is eﬀective at considerably lower dose rates than chlorine or bromine.
- CleanOxide 75 is not corrosive at the recommended dose rates.
- CleanOxide 75 rapidly degrades to harmless inorganic chemicals.
Conventional chlorine, bromine and non-oxidising biocides are satisfactory for maintaining low rates of bacterial counts in cooling water systems. However, these methods all have disadvantages and do not control bioﬁlm. Chlorine dioxide overcomes all these problems. It is effective against algae, bacteria, bacterial spore, fungi, protozoa and viruses and is the only method of disinfection that is effective at controlling biofilm.
Chlorine dioxide remains effective over a wide pH range from about 4 to 10. It does not react with ammonia or organic matter in the water and at the recommended dose rates is not corrosive. Residual chlorine dioxide rapidly breaks down to harmless inorganic compounds making discharge water less toxic to the environment than conventional biocides.
Although chlorine and bromine are regarded as broad specturm biocides, they are relatively ineffective against algae and generally do not control bioﬁ lm. In addition some pathogenic organisms (notably cryptosporidium) can develop resistance to chlorine. Non-oxidising biocides are not broad spectrum biocides and only control a narrow range of organisms depending on the biocide used. Chlorine diozide is a broad specturm biocide effectiveagainst all microorganisms.
In the UK, the Building Services Research and Information Association (BSRIA) has recommended chlorine dioxide as the best available technology for the control of Legionella in hot and cold water systems.
CleanOxide 75 controls bioﬁlm more effectively than chlorine, bromine or non-oxidising biocides even when dosed intermittently. The reason why CleanOxide 75 is so effective at controlling bioﬁlm is because it is a solution of pure chlorine dioxide in water. Chlorine dioxide does not hydrolyse in water and does not react with the polysaccharides holding the bioﬁlm together.
Consequently, it is able to diffuse deep into the bioﬁlm layers destroying the microorganisms that are present. Chlorine can provide bioﬁlm control in cooling systems if the system is relatively clean and continuous dosing is maintained. However, if bioﬁlm is already present or continuous dosing is not rigorously maintained, continuous dosing will not control bioﬁlm.
Chlorine is unable to penetrate bioﬁlm once it is established and only affects the surface layers. Bromine is comparable with or slightly better than chlorine for control of bioﬁlm unless the bacterial nutrient loading is high. In the latter case, bromine is a poor choice for bioﬁlm control. Non-oxidising biocides are generally less effective at controlling bioﬁlm than either chlorine or bromine.
CleanOxide 75 controls algae more effectively than chlorine, bromine or non-oxidising biocides even when dosed intermittently. If conditions for the growth of algae are present, chlorine is only effective if it is dosed continuously.
Even in otherwise clean cooling systems, chlorine often fails to control algae. Bromine is generally less effective than chlorine at controlling algae. Some non-oxidising biocides control algae more effectively than chlorine and bromine, others are ineffective.