With development of the industry, water resources are increasingly polluted by the various organics. The traditional way of water system disinfecting by adding chlorine is found producing poisonous substances which are harmful to our body. Therefore it becomes a hot issue at home and abroad to research a new alternative sterilization method. Most of the present commercialized electrical disinfectant water processors work by adding electrolyte as sodium chloride, etc.        Through the electrolysis it can produce the microbicides such as chlorine dioxide and sodium hypochlorite. Xinxiang Future Hydrochemistry Co., Ltd (FHC) uses its core technology---a mixed metal oxide (MMO) ---to research & develop a new electrolytic sterilizer. The new product can sterilize bacteria and algae without any side effects on the water quality.

 Ⅰ.Conventional Disinfection Methods

    Conventional disinfection methods may be divided between chemical and physical processes. In chemical processes, disinfecting substances such as ozone, chlorine, sodium hypochlorite or chlorine dioxide are added to the water to be treated. These processes are reliable, and have proven their efficiency over many decades. They not only kill microorganisms, but also provide a disinfection reservoir which protects the water against recontamination for a certain time. A frequent drawback of the chemical processes is unwanted side reactions of the disinfectants with substances present in the water. And it is found that this way can produce three harmful substances (resulting in cancer, teratogenesis and mutations) in the drinking water.

In physical disinfection processes the microorganisms are removed or killed by means of irradiation with ultraviolet or ionising radiation, heating to elevated temperatures, ultrasound, or separation through membrane filtration. The main drawback of the physical disinfection methods is the lack of a reservoir effect. These processes are only effective in the immediate surroundings of their operating devices. But these methods are not widely used either for complex equipment or for high cost.

   The comparison of Several Disinfection Methods:

 Ⅱ.Electrochemical Water Disinfection

    Electrochemical water disinfection is a convenient and highly efficient way to produce germ-free water. The technique works without the addition of chemical compounds to the water to be treated, but is nevertheless based on the biocidal action of various chemical substances. Electrodes with platinum group metals or their oxides as active coatings are generally the best suited to electrochemical water disinfection.

 ⅰ. Background

    Electrochemical water disinfection can be defined as the eradication of microorganisms by using an electric current passed through the water under treatment by means of suitable electrodes. At the phase boundary between the electrodes and the water, the electric current leads to the electrochemical production of disinfecting species from the water itself (for example, ozone), or from species dissolved in the water (for example, chloride is oxidised to free chlorine).  Attempts to clean or disinfect water by direct electrolysis had been

reported as early as the nineteenth century.

Different terms are or have been in use to describe this type of water treatment process or the water produced by this process, such as ‘electrolytic disinfection’, ‘electrochemical disinfection’, ‘anodic oxidation’, ‘functional’ water and ‘electrochemically activated water’ among others.

As compared with other chemical disinfection methods, the advantages of electrochemical water disinfection are obvious: no transport, storage and dosage of disinfectants are required. The disinfecting effect can be adjusted according to the on-site demand.

    Electrochemical water disinfection shows a reservoir effect and is often more cost-effective and requires less maintenance than other disinfection methods. Photovoltaic power supply makes it possible to use electrochemical water disinfection far from the electrical supply grid. This may be important for its application to drinking water in developing countries. Electrochemical water disinfection can also be used in conjunction with other disinfection methods.

 ⅱ.The Working Principle of Electrochemical Water Disinfection

    As an effective treatment, the electrolytic sterilization is welcomed for its high practicability and water treatment technology without the secondary pollution.

Electrolytic sterilization includes indirect sterilization and direct sterilization.

Indirect Sterilization: this method uses ClO^-, HClO₃, H₂O₂, O₂ and OH^-, etc, which are produced by electrolysis, to combat microbes. Most of the present commercialized electrical disinfectant water processors work by adding electrolyte as sodium chloride, etc. Through the electrolysis it can produce the microbicides such as chlorine dioxide and sodium hypochlorite.

Direct Sterilization: direct sterilization uses the electrolytic electrode acting directly on the microbes and killing them.

The electrolysis sterilization works as follows:

_1.   Electrolyzing chloric water can produce ClO^- and a little higher valence chlorate. On the surface of anode the following reactions occur:

2Cl－2e →Cl₂, Cl₂+H₂O→HClO+H^-+Cl^-, The OH^- diffuse in the liquor around the anode and react with hypochlorous acid. This reaction produces ClO^-, which will react further in the water producing chloric acid：

12ClO^-＋6H₂O-12e^-→4HClO₃＋8HCl＋3O₂

The resultants HClO and HClO₃ are both strong oxidizer which can kill the microorganism.

_2.   Electrolyzing chlorine-free water also has an effective sterilization. It is proved that electrolysis producing other oxides except HClO. Through analysis the following reactions occur on the surface of the anode:

① H₂O → H⁺ ＋ OH^-;

② 2OH^- - 2e^- →〔O〕+ H₂O + H₂O_2;

③ 4OH^- - 4e^- → 2H₂O + O_2;

④〔O〕+ O₂ → O_3.

For that strong oxidizers as H₂O₂,O₃ and〔O〕are produced in the above reactions, the electrolyte containing these oxidizers has a strong sterilization effect.

3.   Electrode acts directly on the bacteria and kill it by destroying its cyton.

The modern view of the chlorine sterilization is that HClO can destroy the enzyme system of the bacteria and then kill the bacteria by entering into its inside. The final products of the oxidation are CO₂ and H₂O. So even if continuous use of the chlorine won’t make the bacteria resisting to the drugs.

In electrochemical water disinfection, electrodes (at least one cathode and one anode) are inserted either directly into the volume of water to be disinfected, or into a bypass pipe. A DC voltage is applied between the electrodes, leading to the electrolysis of the water. At the anode the main product is oxygen (Equation (i)):

2H₂O → O₂ + 4H⁺ + 4e⁻ (i)

Accompanied by an acidification of the water in the vicinity of the anode. At the cathode, hydrogen is formed (Equation (ii)):

2H₂O + 2e⁻ → H₂ + 2OH⁻ (ii)

and the water near the cathode becomes alkaline. Since the evolved hydrogen is generally unwanted, it must be separated from the water stream. Because only small amounts are formed at normal currents (about 0.4 litres of hydrogen is produced per amp-hour), this is possible without problems in most cases.

 ⅲ. Production of Free Chlorine from the Chloride Content of the Water

    If electrochemical disinfection is applied to drinking water, industrial water, seawater or other solute-containing water, its effect is mainly based on the electrochemical production of hypochlorite and/or hypochlorous acid from the chloride content of the water. The effectiveness of this method has always been accepted for water which contains higher concentrations of chloride ions , such as seawater with about 19 g/l chloride , or where large amounts of sodium chloride have been added, for instance to swimming pool water (chloride concentrations here are usually about 2–5 g/l). For the disinfection of drinking water and other waters with much lower chloride content, the effectiveness of the method was not clear for a long time. It was eventually demonstrated that even at very low chloride concentrations (less than 100 mg/l) sufficient free chlorine can be produced to efficiently disinfect water.

The disinfectant hypochlorous acid/hypochlorite is produced at the anode in a side reaction to oxygen evolution. The following simplified reaction mechanism is proposed. First, chlorine is produced electrochemically from chloride ions dissolved in the water (Equation (iv)):

2Cl⁻→ Cl₂ + 2e⁻ (iv)

Chlorine hydrolyses in water and hypochlorous acid (HClO) is formed

(Equation (v)):

Cl₂ + H₂O → HClO + HCl (v)

Hypochlorous acid and the hypochlorite anion form a pH-dependent equilibrium (Equation (vi)):

HClO → ClO⁻ + H⁺ (vi)

In the nomenclature of water disinfection, the sum of hypochlorous acid and hypochlorite concentrations is usually termed ‘free chlorine’ or ‘active chlorine’. The disinfecting effect of free chlorine is based on the release of atomic oxygen according to Equations (vii) and (viii):

HClO → O + Cl⁻ + H⁺ (vii)

ClO⁻ → O + Cl⁻ (viii)

During the disinfection, chloride ions which have been consumed by electrochemical free chlorine production are reformed. Thus there is, ,

no overall change in the chemical composition of the water during electrochemical water disinfection.

   Where there is a low chloride concentration in the water to be treated (as in drinking water) the current efficiency of the electrode material for the production of free chlorine is crucial; it should be as high as possible.

This is optimised for the cyclic operation of hot water recirculating systems in larger residential and commercial buildings. Potable water, especially in such systems, is especially prone to microbial contamination. This is due to high water temperatures which favour the growth of microorganisms such as legionella. From time to time the media report on legionellosis outbreaks due to contaminated drinking water systems in hotels, hospitals etc. However, Legionella is only the most notorious genus of germs. Others are also dangerous, such as Pseudomonas aeruginosa, Escherichia coli and Staphylococcus aureus, to name but a few.

   In many industrial areas, large amounts of water are used for cooling purposes, as evidenced by the cooling towers dominating the landscape. Microbial contamination of cooling water, whether in cooling towers or air conditioning systems, currently poses a major problem. Large concentrations of disinfectant are usually added to the cooling water. The two commonest biocides for cooling towers are free chlorine and quaternary ammonium compounds. New biocides continue to be developed, but electrochemical water disinfection is a

promising alternative for this application.

 ⅳ. Disinfection or Germ Minimisation by Electrochemically Produced Oxygen

    In some applications, electrolytically produced oxygen, the main anodic reaction product, shows some germicidal activity. This is especially true if anaerobic bacteria are the disinfection target.

An example of this type of application is the wash water cycle of car wash stations. Here, the formation of anaerobic digestion products often leads to bad odors. Anaerobic conditions are eliminated via the fine dispersion of bubbles of electrolytically produced oxygen in the water. This is a highly effective mode of dissolution.

 ⅴ. Disinfection by Cathodically Produced Hydrogen Peroxide

    While most of the possible disinfectants in electrochemical water treatment are produced at the anode, hydrogen peroxide may also be produced at the cathode (Equation (x)):

O₂+ 2H₂O + 2e⁻ → H₂O₂ + 2OH⁻ (x)

Oxygen dissolved in the water may serve as the reactant in Equation(x).  The maximum concentration of oxygen in water which is in equilibrium with air at 25°C is about 10 mg l⁻¹ (0.3 mmol l⁻¹). The oxygen produced by the anodic half reaction according to Equation (i) can also be used for the cathodic production of hydrogen peroxide. In this case, higher concentrations of dissolved oxygen are possible, because the water is in contact with pure oxygen, and not merely with air. It is also possible to use a gas diffusion cathode on which the oxygen from the surrounding air is reduced to H₂O₂.

Because of its lower oxidation potential, H₂O₂ is a less effective disinfectant than free chlorine or ozone. Therefore, higher concentrations and/or longer disinfection times are necessary, limiting its applicability. Hydrogen peroxide has the advantage that its disinfectant action produces neither byproducts nor residues.

 ⅶ. Conclusions.

    Electrolytic sterilizer can apply to waste water treatment of hospital, potable water sterilization, swimming pool disinfection, freshwater aquaculture sterilization, rejection of oil flied affluent, recirculating cooling water treatment, fire fighting pool treatment and marine shipping biofouling removal, etc. Through 3~30min electrolytic sterilization, the rate of biocide is up to 99.9%. With electrolysis time increasing, the rate will rise. Increasing current intensity can also lead to a rise of biocide rate. Electrochemical water disinfection has also been used or tested for the reduction of bacterial contamination in dental water supplies, and for the disinfection of contact lenses and ion exchange resins, etc.