Cooling Water Treatment : NaOCl Vs ClO2

For the cooling water treatment in cooling tower chlorine is required and that can be obtained in many forms.

	ClO2 (Chlorine Dioxide)	NaOCl(Sodium Hypochlorite)
Production	The first is the most common method of generating ClO2 is through the reaction of chlorine gas with a solution of sodium chlorite. 2NaClO2 + Cl2    2ClO2 + 2NaCl sodium chlorite chlorine dioxide chlorine sodium chloride Theoretically, 1 lb of chlorine gas is required for each 2.6 lb of sodium chlorite. However, an excess of chlorine is often used to lower the pH to the required minimum of 3.5 and to drive the reaction to completion. Sodium hypochlorite can be used in place of the gaseous chlorine to generate chlorine dioxide. This process requires the addition of sulfuric or hydrochloric acid for pH control. If allowed to dry Liquid NaClO2, this powerful oxidizing agent forms a powdered residue that can ignite or explode if contacted by oxidizable materials.	- By dissolving salt in softened water, which results in a concentrated brine solution. The solution is electrolyzed and forms a sodium hypochlorite solution in water. This solution contains 150 g active chlorine (Cl2) per liter. During this reaction the explosive hydrogen gas is also formed.
	The second is the possibility to order Chlorine dioxide in its stabilised form (SCD). SCD is activated on-site whenever its usage is desirable. It can be dosed into an existing or new process where disinfection is required. This makes it an easy-to-use, safe and versatile disinfectant.	- By adding chlorine gas (Cl2) to caustic soda (NaOH). When this is done, sodium hypochlorite, water (H2O) and salt (NaCl) are produced according to the following reaction: Cl2 +2NaOH → NaOCl+ NaCl+ H2O
Process	Chlorine dioxide is generally applied directly into the suction of the cooling system's recirculating pump. Alternatively, it may be applied beneath the water in the sump as close to the pump intake as possible.	Sodium hypochlorite solution is diluted with water in a mixing/holding tank. The diluted solution is injected by a chemical pump into the water supply pipe at a controlled rate. Adequate mixing and contact time must be provided
Advantages	It is a very powerful disinfectant and biocide; Unlike chlorine, Chlorine dioxide is effective at pH between 4 and 10. No dumping and filling with fresh water required; The corrosive effects of chlorine dioxide are minimal compared to the corrosive effects of plain tap water; The bactericidal efficiency is relatively unaffected by pH values between 4 and 10. Acidisation, therefore is not required; Chlorine dioxide can be used as a spray. All parts therefore, can easily be reached; And last but not least: less environmental impact. Chlorine dioxide an excellent choice for cooling water that has a high pH, or that has high levels of organic or ammoniacal contamination.	It can easily and be stored and transported when it is produced on-site. Dosage is simple. Transport and storage of sodium hypochlorite are safe. Sodium hypochlorite is as effective as chlorine gas for disinfection. Sodium hypochlorite produces residual disinfectant.
Disadvantages	Rather than hydrolyzing in water as chlorine does, chlorine dioxide forms a true solution in water under typical cooling tower conditions. For this reason, chlorine dioxide is volatile (700 times more volatile than HOCl) and may be easily lost from treated water systems, especially over cooling towers	Due to the presence of caustic soda in sodium hypo chlorite, the pH of the water is increased. The stability of hypochlorite solutions is adversely affected by heat, light, pH, and metal contamination. The rate of decomposition of 10% and 15% solutions nearly doubles with every 10°F rise in the storage temperature. Sunlight reduces the half-life of a 10%-15% hypochlorite solution by a factor of 3 to 5. If the pH of a stored solution drops below 11, decomposition is more rapid. As little as 0.5 ppm of iron causes rapid deterioration of 10-15% solutions. The addition of concentrated ferric chloride to a tank of sodium hypochlorite causes the rapid generation of chlorine gas. Normal industrial grades of sodium hypochlorite may be fed neat or diluted with low-hardness water. The use of high-hardness waters for dilution can cause precipitation of calcium salts due to the high pH of the hypochlorite solution. Hypochlorite solutions decompose and should not be stored for more than one month. It must be stored in a cool, dark, dry area.
Safety Issues	Chlorine dioxide is explosive under pressure. It is difficult to transport and is usually manufactured on site. Chlorine dioxide is usually produced as a watery solution or gas. As a gas, chlorine dioxide is more irritating and toxic. Chlorine dioxide in air is detectable by odor at 14-17 ppm, irritating at 45 ppm, fatal in 44 min at 150 ppm, and rapidly fatal at 350 ppm. Concentrations greater than 14% in air can sustain a decomposition wave set off by an electric spark.	Sodium hypochlorite is a dangerous and corrosive substance. While working with sodium hypochlorite, safety measures have to be taken to protect workers and the environment. Sodium hypochlorite should not come in contact with air, because that will cause it to disintegrate. Sodium hypochlorite should be stored with care and kept away from equipment that can be damaged by corrosion.
Equipment		A basic liquid chlorination system, or hypochlorinator, includes two metering pumps (one serving as a standby), a solution tank, a diffuser (to inject the solution into the water), and tubing.
Quantity		Approximately 1 gal of industrial strength sodium hypochlorite is required to replace 1 lb of gaseous chlorine.