NIRAS - Aurélie

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Alternatives to chlorination


Some of the alternatives that can be adapted to a spa are the following ones: ozonation, ultra-violet system, copper-silver ionization, salt water chlorination or a hydro physical dynamic flow water treatment. There are also other treatments on the market like physical treatment with magnets, disinfection with hydrogen peroxide or disinfection with PHMB (polyhexamethylene biguanide). They are not proposed for this project for several reasons. Concerning the treatment with magnets, it is claimed to be a help for the pool’s normal disinfection. They are maybe a help but they do not disinfect the water so a disinfection system is required. Besides some scientific are sceptical with this technique: there is no clear theoretical basis for why it should work. A disinfection system with hydrogen peroxide is not allowed for swimming pool disinfection unless it is used in combination with other disinfectant (UV, ozone, silver salts…). It improves the disinfection ability of other disinfectants. But hydrogen peroxide is not a powerful disinfectant compared with chlorine, bromine or ozone. Finally, the PHMB is a powerful disinfectant but it needs to be used with an algaecide (like hydrogen peroxide). However, its cost is quite high; it needs to be maintained once a week and it is incompatible with chlorine, bromine or copper. It required a complete draining of the pool every three years. As it is a dangerous product, caution is required for use and storage.



Ozonation (with corona discharge):


 Ozone is produced where and when it is needed from oxygen-containing gases in ozone generators. A high voltage is applied between two concentrically arranged electrodes and some of the oxygen molecules in the input gas break down in the electric field. Then they immediately attach themselves to free oxygen molecules, forming ozone. Ozone reacts rapidly with organic matter, viruses, bacteria… and is converted back into oxygen dioxide. This treatment takes place in a technical room. A carbon bed is often used after that to avoid ozone residual to enter the pool. Indeed, in a lot of national legislations ozone residual is not allowed in the pool. As a disinfectant residual should remain in the pool (legislation), a chlorine or bromine dosage is added. Typically the treatment is made with 95% of ozone and 5% of chlorine. Ozone technology for swimming pools has been in regular use for over 50 years in Europe especially in Germany and France.



Ultra-violet system:


It is a physical treatment with UV radiation. The light necessary for UV disinfection is generated in special UV lamps. One of the most effective wavelengths and the most often used for disinfection is at 254 nm; it belongs to UV-C. This intensive UV light reaches the micro-organisms in the water and impacts directly on their DNA. By changing the DNA, the cell division of the micro-organism is interrupted: it can no longer reproduce itself and thus loses its pathogenic effect.


Copper-silver ionization:

 It is brought about by electrolysis. Electrodes are placed close together and the water flows past the electrodes. An electric current is created causing positively charged copper (Cu+ and Cu2+) and silver (Ag+) ions to form. The positively charged copper ions form electrostatic compounds with negatively charged cell walls of micro-organisms. The copper ions then penetrate the cell wall and as a result they will create an entrance for silver ions. These latter penetrate the core of the micro-organisms: they bond to various parts of the cell (DNA, RNA, cellular proteins and respiratory enzymes), causing life support systems into the cell to be immobilized. There is no more growth or cell division, causing bacteria to no longer multiply and eventually die out. The ions remain active until they are absorbed by a micro-organism. Globally, silver inhibits bacterial growth, rather slowly, and copper is an algaecide.



Salt water disinfection:


 With this method, chlorine is generated by electrolysis. The electrolysis is achieved by passing a mild saline solution through an electrolytic cell. First sodium chloride (NaCl à salt) is added to the pool water. The anode of the cell makes hypochlorous acid (HOCl) and hypochloric acid (HCl). The cathode of the cell makes sodium hydroxide (NaOH) and hydrogen gas (H2). Then these reactions take place:


HOCl (anode) + NaOH (cathode) à NaOCl (sodium hypochlorite) that kills micro-organisms


HCl (anode) + NaOH (cathode) à NaCl (salt) reused for the electrolysis


The pool actually produces its own chlorine to sanitise itself. It is a closed loop system because the salt is used over and over again and only lost through splash out, carry out, backwashing and rainfall.



Hydro physical dynamic flow water treatment:



This is a physical treatment. In a reactor chamber there is a cyclone process. The water is supplied by a high pressure pump to the reactor chamber. Consequently, very high flow and rotation velocities are created. The flow volume inside the reactor is diverted into several water layers. The vacuum centre cleaning (about -1,0 bar) permits to add natural oxygen mixture from the outside air directly to the reactor chamber process. The additional oxygen is dissolved in the water, forming free radicals, which act as an oxidation cold combustion cleaning grade process.


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Started by NIRAS supervisor Sergio Fox on 27th March 2006.