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Chlorination

by Mark Heyda

Chlorine is commonly used as a disinfectant or an oxidizer. That is to say, chlorine combines with other chemical compounds to make the resulting mixture safer. In the proper form, it kills pathogenic organisms making it the universal disinfectant. Chlorine is typically used in the form of bleach or sodium hypochlorite (NaOCl) and water. The aqueous solution of bleach consists of H-OCl and Na-OH, where HOCl is referred to as hypochlorous acid.

Hypochlorous acid is the fast acting lethal form of free chlorine. It is a very weak acid which dissociates as the pH increases to form OCl- (hypochlorite ion) and H+ (hydrogen ion). Unfortunately the hypochlorite ion is a very slow acting form of free chlorine. Its efficacy is 80-300 times less than that of HOCl. So it is important that the level of pH be controlled to achieve disinfection (i.e. prevent the formation of hypochlorite ion).

Another factor to consider is the formation of combined forms of chlorine. When added to water containing ammonia (NH3), hypochlorous acid reacts to form monochloramine (NH2Cl), dichloramine (NHCl2) and trichloramine (NCl3). All of these forms have less than 1% of the disinfecting power of hypochlorous acid. The formation of these combined forms is concentration and pH dependent.

If enough chlorine is present most of the chloramines are "broken" as well as the chloro-organics. This is referred to as breakpoint chlorination. After this point, additional chlorine will create free residual chlorine for disinfection.

Swimming Pools

The optimum pH would be the same as that of the human eye or a value of 7.2 pH. A slightly higher pH value up to 7.6 is acceptable. However, increasing pH past 7.2 decreases the ratio of HOCl in the water. Reducing the pH below 7.2 is not recommended as it causes a breakdown of steel and copper parts, will dry out the pool liner materials and have other undesirable chemical effects.

Concentration at various pH levels
(20ºC)      
pH HOCl OCL-  
7.0 80% 20%  
7.2 70% 30%  
7.4 60% 40%  
7.6 50% 50%  
7.8 40% 60%  
8.0 30% 70%  

Some state requirements show residual chlorine levels to be between 0.6 and 5.0 ppm at a pH of 7.8 or less. ORP however is a much better indicator of the efficacy of the killing power of chlorine. Typically ORP values in excess of 650 will achieve disinfection. At 750mV ORP, E-coli bacteria are killed in less than one second! High values of ORP indicate the presence of HOCl in sufficient amounts to achieve disinfection. Using chlorine concentration (ppm) rather than ORP represents a danger because of the equilibrium between hypochlorous acid and other forms of chlorine in water which may not translate into proper disinfection. Continuous monitoring of the concentration of free chlorine can be done by using a monitor such as the PCA Chlorine Analyzer. These analyzers also now feature ORP and pH measurement to give operators the full picture.

Wastewater Treatment Plants

Like in swimming pools chlorine is used to destroy pathogens and make harmless certain chemical compounds.  This is more complex because the presence of many more bacterial and chemical species.  And after doing its job, de-chlorination is required.  The concentration of pathogens is also much higher in wastewater than in pool water.

Potable (Drinking) Water

It is essential for all municipal and private water supply facilities to monitor and control the chlorine levels as supplied to the distribution system.  Residual chlorine levels between 0.8 and 1.2 are monitored to insure healthful drinking water.  The new PCA series Analyzers do this very well with EPA approved methods.  The units are built so that results are reliable with a minimum of maintenance.  The display and controls are simple to use and easy to operate.  The unit will not only allow for monitoring via an external chart recorder, but can log data and be connected to a computer.  Alarming capabilities include programmable relays and the ability to send text messages to a cell phone with an optional GSM module.  The unit is housed in a NEMA 4X waterproof enclosure with a transparent Lexan® window.

                   

 

                 

     

 

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Last modified: 29-Oct-2008