The removal of this free chlorine (ie dechlorination) is one of the oldest process applications of activated carbon. Residual free chlorine is often present in drinkable water supplies and can cause undesirable taste and odour issues with drinking water at concentrations starting at 0.3 parts per million. Many water cooler filtration systems utilising activated carbon are designed with free chlorine removal in mind.
Activated carbon doesn’t remove reactive free chlorine by physical adsorption. Instead, activated carbon reacts with the free chlorine and converts it to the less reactive and tasteless chloride (Cl-) ion. These reactions occur quite rapidly and so carbon beds designed to work even at relatively low contact times can be very effective at free chlorine removal, with reduction to below detection limits easily achievable.
In order to reduce the formation of disinfection byproducts (DBP) from the reaction of free chlorine with residual organics in potable water, the application of alternative disinfectants has become increasingly widespread. Although ~200 times less effective than chlorine as a sterilant, monochloramine has emerged as one of the leading alternative disinfectants for municipal water supplies.
Monochloramine offers two advantages over free chlorine as a disinfectant. Firstly, it’s less reactive, avoiding the creation of DBPs. Secondly, it’s a more persistent disinfectant remaining in the public water supply throughout the distribution system up to the faucet. However, its persistence in the supply and its tendency to form breakdown products below a pH of 7.5 causes taste and odour issues for consumers.
To comply with new regulations on DBPs, many municipal water sources have switched their disinfection method to chloramination. This switch can have an impact on carbon filters used in water coolers and other water treatment devices.
The stoichiometry of the dechloramination reactions on carbon are well known. Two parallel reactions occur, one via reduction which converts the monochloramine to ammonia and a catalytic decomposition which converts it to nitrogen.
One challenge with chloramine removal: compared to free chlorine, the reactions occur much more slowly. Longer contact times are required in carbon beds for proper reduction of chloramine, thus, a water cooler system employing a carbon bed designed for free chlorine removal may be undersized if the incoming water it’s treating is taken from a source that has switched to chloramination.
Fortunately, to tackle the problem of chloramine removal, a new class of activated carbons has been developed and commercialised over the past 20 years. These types of carbons are known as ‘catalytic’ carbons and are manufactured via proprietary processes that modify the surface chemistry of the activated carbon to enhance the carbon’s ability to facilitate catalytic reactions, such as monochloramine removal.
The resulting product remains as an activated carbon, with adsorptive capacity and the ability to remove free chlorine, but with the added benefit of monochloramine removal at similar design contact times as those used for free chlorine reduction.
The chart (above) shows the relative dechloramination performance of a standard coal- and coconut-based activated carbon, catalytic coal-based carbon, and Jacobi Carbons’ catalytic coconut-based activated carbon AquaSorb CX-MCA. Comparison of the activated carbons shows that Jacobi’s product is superior to all activated carbons for the removal of chloramine.
The conversion from free chlorine to chloramine by water providers can pose a significant challenge to carbon filters used in water cooler systems and other water treatment devices. Fortunately, by switching to a catalytic activated carbon, users can maintain their desired water quality without adding additional carbon online, when they find that their inlet water source has switched to chloramine.
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