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The combination of metal ions with an oxidising agent is a process combination that was researched in the 1990’s, but not in too much depth (Landeen, Yahya et al. 1989, Yahya, Landeen et al. 1990, Pedahzur, Lev et al. 1995, Fewtrell 2014, Cassells, Yahya et al. 1995). Lately, it seems little additional research has been done, but it has been implemented more often (Carefree Clearwater 2015, Aquaking SA 2016, Fewtrell 2014). Research that mentioned such combinations, proposed it as an alternative to chlorine treatment with a direct decrease in chlorine used for disinfection (Pedahzur, Lev et al. 1995, Pyle, Broadaway et al. 1992, Yahya, Landeen et al. 1990). A decrease in chlorine usage will decrease the dangers of chlorine treatment, but probably also some of the advantages. The addition of the metal ions should fill the vacancy of the removed chlorine. Such a combination should be effective in deactivating a wider range of pathogens because of the variety of disinfectant mechanisms involved (Sambhy, MacBride et al. 2006). Other advantages could also include financial benefits, lower environmental impact, and a lower risk disinfection procedure (Pedahzur, Lev et al. 1995).

The disinfection mechanisms of a combined metal ion and oxidising agent treatment procedure are complex and has not been studied (Yahya, Landeen et al. 1990). The metal ions could theoretically react with the oxidants and form other complexes that react differently to the normal disinfecting species present. Theoretically HOCl and HOBr both react with the cell membrane and with the interior of cells, metallic ions also react with the exterior and interior of cells, but the interior of a pathogen is more vulnerable than the exterior (OSU 2011). The oxidation species can therefore weaken the cell membrane or the cell wall to allow metal ions through, which makes the cell interior open to disinfectant actions. The same could be happening other way round (Yahya, Landeen et al. 1990, Lin, Vidic et al. 1996). Therefore, a pathogen might have a resistant membrane to HOCl, but not to silver ions, the silver ions then weaken the membrane causing it to allow other substances through, including HOCl, which then react with the cell interior and cause cell destruction. The contact time requirements for combined treatment is not well documented, but should be thought-provoking, since metal ions require at least double the contact time that oxidising agents require. Referrals to silver-chlorine for point-of-use treatment make it seem that the metal ions enhance the oxidising agent (Fewtrell 2014).

The combination of metal ions and oxidising agents should have several strengths other than the strengths of the individual processes. Firstly, a reduced amount of the oxidising agent can be used if the process is synergistic (Yahya, Landeen et al. 1990). The process should have a low toxicity because silver and copper have a low toxicity and less toxic oxidising agents need to be used. The process should have a long-lasting disinfecting residual, because metal disinfection is known to have a long

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residual and some oxidising agents as well (Pedahzur, Lev et al. 1995). A larger variety of pathogens should be susceptible to disinfection because of the different disinfecting mechanisms of the metal ions and oxidising agents (Landeen, Yahya et al. 1989). A final advantage would be a small number of disinfecting by-products, since fewer oxidising agents are used (Pedahzur, Lev et al. 1995).

Metal ions and oxidising agents function on different disinfecting mechanisms, which is a strength, but could be a weakness if these disinfectants react with each other. If the disinfectants react with each other the total efficiency will drop. This should not be the case, however, since metals are oxidised to ionic form at low pH levels and in the presence of oxidising agents, additionally it is the ionic form that becomes the efficient biocide (Dowling, Betts et al. 2003). The precipitation of metal oxides and other uncontrollable reactions would be some of the dangers. The oxidising agent and metal ions are often implemented in series, one after the other, which give some contact time for the disinfectants to react with contaminants before reacting with each other. Controlling pH and maintaining it could also proof challenging with the complex reactions taking place.

There have been different studies on metal ions with oxidising agents, but the results are not comparable, since different pathogens were investigated. Pedahzur, Lev et al. (1995) combined silver treatment with hydrogen peroxide in a ratio of 1:1000 (w). They found that the combined process showed more efficient disinfection than the individual processes, with cases of a synergistic effect. The inactivation rate was slow and the disinfection action seemed similar to chloramines, therefore, it showed promise as a secondary disinfectant with a long-lasting residual for biofilm control (Pedahzur, Lev et al. 1995). Pyle, Broadway et al. (1992) investigated the use of metallic ions with iodine as disinfectant. They combined 100 ppb Cu and 11 ppb Ag and 200 ppb iodine and found these low concentrations efficient for disinfection. The combined treatment was more effective against

Pseudomonas cepacian than any of the treatments separately. The combined treatment also

prevented regrowth, which did occur when only iodine was used (Pyle, Broadaway et al. 1992). Yahya, Landeen et al. (1990) combined 400 ppb copper, 40 ppb silver and 300 ppb free chlorine and found the combined treatment had a synergistic log reduction when compared to the individual treatment processes. They recommended that copper-silver ionisation should always be implemented with a small amount of free chlorine in swimming pool treatment (Yahya, Landeen et al. 1990). Yahya, Landeen et al. (1989) also showed that the difference in bacterial inactivation between chlorine treatment and copper-silver-chlorine treatment increased with larger chlorine concentrations, Figure 10 (Landeen, Yahya et al. 1989). Martı ́nez, Gallegos et al. (2004) investigated 0.2 ppm silver, 1.2 ppm copper and 0.3 ppm chlorine, and found it controlled bacteria concentrations sufficiently to limit biofilm growth and corrosion due to bacterial action (Martı ́nez, Gallegos et al. 2004). In South Africa,

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Aquaking SA (Pty.) has patented technology that combine copper, silver, and zinc ions with the halogen releasing BCDMH (Aquaking SA 2016).

Figure 10: Inactivation rates of chlorine treatment compared to copper-silver-chorine treatment redrawn from Landeen, Yahya et al. (1989)

Abad, Pinto et al. (1994) investigated viral inactivation through a copper-silver-chlorine treatment. They combined 700 µg/L copper and 70 µg/L silver with 0.5 or 0.2 mg free chlorine, and found that viral inactivation was comparable to inactivation found with higher free chlorine dosages, but the addition of metal ions did not enhance the viral inactivation rates. The research did not promote copper-silver ionisation as an alternative to chlorine treatment, although it did mention the residual presence of copper and silver (Abad, Pinto et al. 1994).