Electrochlorination is a chemical process which uses water, common salt and electricity to produce Sodium Hypochlorite(NaOCl). The brine solution (or sea water) is made to flow through an electrolyzer cell, where direct current is passed which leads to Electrolysis. This produces Sodium Hypochlorite instantaneously which is a strong disinfectant. This is then dosed in water in the required concentration to disinfect water, or to prevent Algae Formation and Bio Fouling.
In the Electrolyser, current is passed through the anode and cathode in the salt solution, which is a good conductor of electricity, thus electrolysing the sodium chloride solution.
This results in chlorine (Cl2) gas being produced at the anode, while sodium hydroxide (NaOH) and hydrogen (H2) gas is produced at the cathode.
The reactions that take place in the electrolytic cell is:
The reactions that take place in the electrolytic cell is:
The chlorine further reacts with the hydroxide to form sodium hypochlorite (NaOCl). This reaction can be simplified in the following manner:
The solution generated has a pH value between 8 and 8.5, and a maximum equivalent chlorine concentration of less than 8 g/l. It has a very long shelf life which makes it suitable for storage.
After dosing the solution into the water flow, no pH value correction is necessary, as is often required in sodium hypochlorite produced by the membrane method. The sodium hypochlorite solution reacts in a balance reaction, resulting in hypochlorous acid:
To produce 1kg equivalent of chlorine using on-site Sodium Hypochlorite generator, 4.5 kg of salt and 4 kilowatt hours of electricity is required. The final solution consists of approximately 0.8% (8 grams/liter) sodium hypochlorite.
(ON-SITE SODIUM HYPOCHLORITE GENERATOR)
- Simple: Only water, salt and electricity is required
- Non Toxic: Common salt which is the main substance is nontoxic and easy to store. Electro chlorinator provides the power of Chlorine without the danger of storing or handling hazardous materials.
- Low Cost: only water, common salt and electricity is needed for electrolysis. The total operating cost of an Electrochlorinator is less than the conventional Chlorination methods.
- Easy to dose to get a standard concentration: Sodium hypochlorite generated on-site does not degrade like commercial sodium hypochlorite. Therefore, the dosage need not be modified on a daily basis based on the strength of the hypo solution.
- Approved disinfection method complying with the drinking water regulations – an alternative with less safety requirements to chlorine-gas-based systems.
- Long service life, as compared with the membrane cell electrolysis
- On-site generation of sodium hypochlorite allows the operator to produce only what is needed and when it is needed.
- Safe for the Environment: As compared to 12.5% sodium hypochlorite, the use of salt and water reduces the carbon emission to 1/3rd. The hypo solution of less than 1% concentration produced by our system is benign and considered non hazardous. This translates to reduced safety trainings and improved worker safety.
Sodium Hypochlorite generated on-site with the help of synthetic brine or seawater is very efficient in protecting the equipment from the growth of micro organic fouling and control of algae and crustaceans. Compact Electrochlorinators manufactured by Pristine Water are ideal for disinfection of water during disasters like earthquakes, Floods or Epidemics. Electrochlorinators are designed for rural and village 'point -of- use' disinfection of drinking water.
Advantages of On-Site Generation of Sodium Hypochlorite
Although the economic consideration is the major advantage in using On- site generated Sodium Hypochlorite over the use of other forms of Chlorination, but the technical advantages are even greater. The following are some of the problems associated with using commercial grade liquid sodium hypochlorite. These have high concentration (10-12%) of active chlorine. These are produced by bubbling gas chlorine in Caustic soda (Sodium Hydroxide). They are also commonly called as Liquid Chlorine.
The corrosion due to Commercially produced hypochlorite is a concern because of its effect on equipment. A 10 to 15% hypochlorite solution is very aggressive due to its high pH and chlorine concentration. Because of its aggressive nature, the hypochlorite solution will exploit any weakened areas in the hypochlorite piping system and may cause leaks.
The formation of calcium carbonate scale is another concern when using commercial grade liquid hypochlorite for chlorination. Commercial grade liquid hypochlorite has a high pH. When the high pH hypochlorite solution is mixed with the dilution water, it raises the pH of the mixed water to above 9. The calcium in the water will react and precipitate out as calcium carbonate scale. Items such as pipes, valves and rotameters may scale up and no longer function properly. It is recommended that the commercial grade liquid hypochlorite not be diluted and that the smallest pipe lines, the flow rate will allow, should be used in the system.
Another concern with commercial grade hypochlorite is gas production. Hypochlorite loses strength over time and generates oxygen gas as it decomposes. The rate of decomposition increases with concentration, temperature, and metal catalysts.
A small leak in the hypochlorite feed lines would result in the evaporation of the water and in turn the release of chlorine gas.
The final area of concern is the possibility of chlorate ion formation. Sodium hypochlorite degrades over time to form the chlorate ion (ClO3-) and oxygen (O2). The degradation of the hypochlorite solution is dependent on the strength of the solution, temperature, and the presence of metal catalysts. Decomposition of Commercial Sodium Hypochlorite can be created by two major ways: a). The formation of Chlorates due to high pH, 3NaOCl= 2NaOCl+NaClO3. b). Chlorine evaporation loss due to temperature increase. Therefore, for any given strength and temperature, over a period of time, the higher strength product will eventually be lower in available chlorine strength than the lower strength product, since its decomposition rate is greater. The American Water Works Association Research Foundation's (AWWARF) concluded that the decomposition of concentrated bleach (NaOCl) is the most probable source of chlorate production. High concentration of Chlorate is not advisable in drinking water.
Chlorine Comparison Chart
|Product Form||PH Stability||Available Chlorine||Form|
|Calcium hypochlorite granular||11.5||20%||Dry|
Now which is the ideal disinfectant?
- a. Chlorine Gas—this is too dangerous to handle and not safe in residential areas. Most of the times, they are not available.
- b. Bleaching Powder—or Calcium Hypochlorite is effective, but the whole process of mixing, settling and disposing off of the sludge is very messy and cumbersome. This makes the whole area dirty. More over, the bleaching powder absorbs moisture during monsoon or in wet surroundings and emits chlorine gas, making the bleaching power lose its strength.
- c. Liquid Bleach—or Liquid Chlorine -or Sodium Hypochlorite is very effective. This is in liquid form so very easy to handle. But the commercially available Liquid Chlorine is not only expensive but looses its strength over a period of time and becomes water. The danger of spillage is a common problem.
- d. Electro Chlorinator—Very effective,economical, safe and easy to prepare and use. This is the latest technology being adopted in most nations.