Testing Free Chlorine When Superchlorinating
In some applications, such as fresh-cut produce manufacturing, many regulations stipulate that free chlorine must be applied to the water at a concentration greater than 50 mg/L. Such levels are known as superchlorination.
What is superchlorination?
Superchlorination describes the process of adding chlorine in large enough quantities to overcome nitrogenous materials in water and exceed the ‘breakpoint’.
In order to understand what ‘breakpoint’ is, we need to be able to differentiate between free and combined chlorine. Free chlorine usually exists in water as hypochlorous acid (HOCl) or the hypochlorite ion (OCl–), with the proportion of each dependent on the pH of the sample. When nitrogenous material such as ammonia is present in solution, chloramines will form. There are three types of chloramines; monochloramine (NH2Cl), dichloramine (NHCl2) and trichloramine (NCl3). The concentration of free chlorine in a sample plus the concentration of chloramines in the sample is therefore described as the concentration of total chlorine.
Free Chlorine + Combined Chlorine (Chloramines) = Total Chlorine
Distinguishing between free and combined chlorine below the breakpoint is vital to ensure adequate disinfection.
Chloramines are weaker disinfectants than free chlorine. Trichloramine also known as Nitrogen Trichloride (NCl3) can be unpleasant and a respiratory irritant.
What is the breakpoint of chlorination?
Chlorine is added to the water sample and dissolves to form hypochlorous acid (HOCl)
Cl2 + H2O ↔ HOCl + HCl
Trace contaminants in the water plus the internal surfaces of the container inevitably consume some of the chlorine initially added to the solution. The amount it consumes is known as the ‘chlorine demand’ of the water.
After this initial period of consumption of chlorine by ammonia, the chlorine reacts with the free ammonia and forms monochloramine:
HOCl + NH3 ↔ NH2Cl + H2O
Once all the ammonia in solution has reacted to form monochloramine, that monchoramine will react further to form dichloramine and trichloramine:
HOCl + NH2Cl ↔ NHCl2 + H2O
HOCl + NHCl2↔ NCl3 + H2O
As more chlorine is added, the free chlorine begins to oxidise the chloramines. The point at which all the dichloramines and trichloramines are oxidised into nitrogen is the ‘breakpoint’ and is the point that superchlorination is normally seeking to achieve. Beyond this point any further chlorine added cannot react and exists as free chlorine with potent disinfectant properties.
Superchlorination of Fresh Produce
The approach in produce washing is not to use chlorine to sterilise the produce. Instead, the produce is being washed with water. The chlorine is present in the water to prevent pathogens that might be washed off on one item then being transferred to others. This is done by keeping the overall loading of viable microbes in the water to low levels.
Chlorine is excellent for this purpose and superchlorination is historically the predominant method used. It is a very effective way of reducing microbial load by 10 to 100 times, but this also depends on the contact time between the chlorine and the microbes. Further chlorine is routinely added to keep the level above the breakpoint, this keeps the required contact time to kill microbes down. The typical target is to keep the chlorine above 50 mg/L.
Palintest sensor technology has been independently verified as the best test method for testing produce wash water. A study by CEBAS investigated 5 test methods, and our chronoamperometric sensor was found to be the best test method offering precise, real-time monitoring of wash water.
Superchlorination of Pools and Spas
Pools and Spas are not normally superchlorinated as the high levels of chlorine required would not be conducive to bather comfort. Normally the ratio of free chlorine to total chlorine is continually monitored instead, at below breakpoint levels. When the free chlorine is less than half that of total chlorine value a superchlorination may then be carried out as a treatment process when the pool is closed to bathers. The chloramines will be oxidised by the high levels of chlorines and the nitrogen in them converted to nitrogen gas.