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Webster defines "ionisation" as "to convert wholly or partly into ions". Simply put, ionisation is the natural or artificially induced electrochemical process of converting electrically neutral (non-charge) atoms or molecules into electrically charged ions. The resulting charge is either positive (+) or negative (-).

  

Ionisation in the pool and spa industry is associated with devices (ionisers) that generate copper and silver ions which are claimed to provide disinfection efficacy for pool and spa water, while reducing or eliminating the need for conventional halogen disinfection (eg. chlorine or bromine). Under laboratory and field controlled conditions, copper (Cu2+) and silver (Ag+) ions have proven to be effective algaecides and bactericides, respectively, but their effectiveness in pool and spa water treatment appears questionable.


An ioniser electrochemically generates copper and silver ions from a solid elemental copper and silver electrode. The configuration of the electrode assembly varies from one manufacturer to the next, but the end result is the same: invisible (other than the potentially noticeable blue colour imparted by copper ions) copper and silver ions are generated. As previously indicated, ions are electrically charged species that carry a positive or negative charge. As Equations I and II illustrate, positively charged copper and silver ions are generated from their uncharged elemental form.

 

The effectiveness of copper and silver ions as an algaecide and bactericide is limited by their solubility and kill time. Copper sulfate (CuSO4), commonly called blue vitriol, is extremely effective as an algaecide. Its applications in reservoirs is well documented, but staining and kill time is not considered a problem in natural aquatic systems.

 

As was previously mentioned, the effectiveness of ionisation is determined by the solubility of copper and silver ions. If these ions do not remain in solution, then their ability to control algae and destroy potentially harmful bacteria is diminished. To convincingly illustrate the solubility problems associated with copper and silver, let's begin with an example. The solubility of copper, silver, and many other metallic ions is determined by their solubility product constant (Ksp) associated with precipitating counterions. The Ksp is nothing more than a numerical index of the solubility of a slightly soluble salt. The smaller the Ksp, the less soluble the salt; the larger the Ksp, the more soluble the salt. Below are listing some Ksp values of some potentially insoluble copper and silver salts found in pool and spa water.

 

 

As one can quickly see, the solubility of copper and silver sulfide salts is extremely low. The impact of this can be very visible and dangerous in areas of the country that experience sulfide water. For example, most ioniser manufacturers recommend continued maintenance of 0.25 ppm (250 ppb - parts per billion) copper and 0.05 ppm (50 ppb) silver ions. If a swimming pool is filled with water containing 0.30 ppm sulfide (S2-) ions in the absence of halogen oxidisers, then it is conceivable for all the copper to fall out as an unsightly, black precipitate. The same scenario can be illustrated with silver sulfide (Ag2S) and many other slightly soluble copper and silver salts.  The danger associated with deposition of insoluble copper and silver salts is twofold:

 

1. Staining of pool surfaces can obstruct clear view of all areas of the pool making it difficult to see swimmers in distress.

2. If recommended levels of copper and silver ions are not continuously maintained due to deposition of insoluble precipitates (precipitations of insoluble copper and silver salts is the major factor causing free ion depletion), then acceptable microbiological water quality will be impossible. The result: green, algaeladen water, swimmer's ear, skin rashes, and overall unacceptable swimming or soaking water.

 

As a biocide, silver is slow compared to chlorine. Chlorine is a bactericide and algaecide, while silver at the concentrations being used in ioniser treated pools (around 40 ppb), may be a bacteriostat. (See Table I) In addition, various items commonly present in swimming pools, such as chlorides ("spent" chlorine) and carbonates (from hardness and alkalinity), can interfere with the bacteriostatic action of silver. Under the right conditions, silver can be effective as a bacteriostat, but if the pool is balanced properly and being used regularly, silver alone, without residual chlorine, will not kill bacteria, nor prevent cross infection of bathers, as effectively as chlorine.

 

A study carried out at the University of Arizona in Tucson by researchers Landeen, Yahya, and Gerba and published in Applied and Environmental Microbiology (1) supports previous studies which have looked into silver as a bactericide and bacteriostat against Legionella bacteria. This study is also being used as support for the use of ionisers. This study stated: "Inactivation of L. pneumophilia by copper and silver is relatively slow compared with that of free chlorine..."Free chlorine (0.4mg/L) with or without the addition of 400 and 40ug of copper and silver per litre was significantly greater than all other [lower] free chlorine concentrations separately or combined with copper and silver. Although inactivation rates at all the tested levels of free chlorine appeared enhanced when 400 and 40ug per litre were added, this observation was relevant only when the chlorine concentration was increased from 03. to 0.4 mg/L."

 

The NASA studies state in a 1968 "NASA Tech Brief" (68-10555): "The sterilizer generates silver ions in concentrations of 50 ppb to 100 ppb in the water flow system...Laboratory tests of the sterilizer under simulated conditions have demonstrated essentially complete kill within 8 hours of Staphylococcus aureus and Escherichia coli bacteria present in initial concentrations of approximately 5 x 105 organisms per millilitre." What does this mean? The above studies, and others like it, have demonstrated 40 ppb silver alone takes many hours to achieve significant reductions in bacteria concentrations while chlorine alone (at 1 ppm) has been demonstrated to achieve similar reductions in less than 2 minutes. In addition, according to the Landeen et al study, many of the observed biocidal enhancements that silver may have when used with chlorine were not statistically significant when compared to small amounts of free chlorine used independently of silver. Remember also that these results were obtained under highly controlled laboratory conditions and the time expected for significant reduction rates in an actual pool will be different for any potential sanitiser.

 

Free chlorine is required to kill and control microorganisms in pools and spas using an ioniser. Ionisers have not been demonstrated to achieve biocidal activity comparable to chlorine. Some manufacturers are recommending a 0.4 ppm residual of free chlorine in addition to the ioniser in order to satisfy health department requirements and also increase the effectiveness of the system. Although the use of chlorine in these systems is essential, chlorine also may accelerate the deposit of residual copper onto pool surfaces and into light coloured hair (resulting in green hair). If an ioniser must be used, it is recommended that a suitable metal chelate be added to help prevent staining. The use of a chelate, however, does not guarantee that stains will not occur, since metals are being added on a continual basis with an ioniser. Also, even if silver ionisers are effective as a biocide, shocking with chlorine compounds will still be required for oxidation of organic wastes.

 

Other problems associated with copper/ silver ionisers are:

1. Complaints of green hair. In the presence of complimentary halogen residuals (a requirements of ionisation systems), dissolved copper ions can cause green hair and fingernails. This is especially true of blondes but can occur with anyone.

2. Toxicity concerns. Although many ioniser manufacturers have made claims that chlorine and bromine are highly toxic, they have failed to recognise that the EPA Interim Primary Drinking Water Standards has placed a 0.05 ppm (50 ppb) maximum allowable level on silver. If not properly controlled, it is conceivable for pool water levels to exceed this maximum level and then toxicity concerns certainly exist.

3. Environmental concerns. Copper is a necessary dietary trace mineral but its effect on the environment can be detrimental to aquatic ecosystems. Dissolved copper ions have been unequivocally identified as a pollutant to fish. Copper ions react with the oxygen-exchange sites on fish gills resulting in suffocation.

4. Lifeless water. Halogen compounds with significant oxidising potential (e.g., bromine and chlorine) can quickly inactivate algae and bacteria. In addition, the oxidative properties of bromine and chlorine can maintain or quickly re-establish pool sparkle and brilliance. The same is not true with non-oxidising properties of copper or silver. The result is usually lifeless, dull water.

 

Key Points to Remember

  • Ionisers deliver silver and copper into a swimming pool.
  • Silver may be a bacteriostat; copper is an algaecide.
  • Although silver has been demonstrated through many studies to kill and inhibit the growth of various microorganisms, it does so much more slowly than very small concentrations of chlorine.
  • Certain items common to any swimming pool, such as carbonates and chloride, can prevent silver from being effective as a bacteriostat.
  • Chlorine and all normal balancing is still essential to pools with ionisers. The use of chlorine may precipitate staining.
  • Chelating agents are essential, but cannot be guaranteed, to prevent staining from accumulated metals due to the continual introduction of metals into a pool using an ioniser.
  • Shocking with chlorine compounds is still required to oxidise organic matter and bather load factors.

 

TABLE: Inhibitory and Bactericidal Concentrations of Electrically Generated Silver for Some Microbial Species (2)

 

  • MIC = "Minimum Inhibitory (static) Concentration." A 0.5 mL aliquot (104 to 105 organisms) was added to the twofold serial dilution of silver treated broth. The MIC was interpreted as the lowest concentration of silver not associated with turbidity after 24 hours at 37°C.
  • MBC = "Minimum Bactericidal Concentration." A 0.5 mL aliquot was removed from the non-turbid tubes and mixed with a tube of agar (GIBCO). The MBC was defined as the lowest concentration of silver giving a count of less than 10 colonies/plate after 48 hours at 37°C.

 

Notes and Aids to Understanding the Table

The indicated MIC concentrations indicate how much silver was required to prevent the growth of these organisms. The indicated MBC concentrations indicate how much silver was required to kill these organisms. Boldfaced organisms are commonly associated with water environments. Ionisers deliver about 40 ppb silver into the pool water. Thus, to kill these organisms requires 6.5 to 251 times the delivered concentrations of silver. (260/40 and 10050/40).

 

References

1. Landeen, Yahya and Gerba, "Efficacy of Copper and Silver Ions and Reduced Levels of Free Chlorine in Inactivation of Legionella pneumophilia", Applied and Environmental Microbiology, Volume 55, December 1989, pp. 3045-3050.

2. Berger, T.J., Spadaro, J.A., Chapin, S.E., and Becker, R.O. 1976. Electrically Generated Silver Ions: Quantitative Effects on Bacterial and Mammalian Cells", Antimicrobial Agents Chemotherapy, Volume 9, 1976, pp. 357-358.

 

The above information is supplied by Bio-Lab and represents its best interpretation of available technical information at the time of preparation. The sole purpose is to supply factual information to Bio-Lab customers. It is not to be taken out of context nor used as support for any other claim not made herein.