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Chlorine and chloramines are two common chemicals added by municipalities to provide residual bacterial disinfection. However, the addition of these chemicals necessitates their removal from a water stream if it affects a downstream product or equipment.
Typically, chlorine and chloramines are removed from water utilizing a carbon bed or sodium metabisulfite injection. A third method, ultraviolet light (UV), may be preferred by some operations, including bottled water plants.
Removal via carbon
A 5 to 10 micron filter is usually installed downstream to collect particles that are shed by the carbon during normal operation. Organics that tend to concentrate in the bed are trapped within the bed and provide an excellent food source for bacteria.
In addition, the removal of chlorine or chloramine occurs through the process of adsorption, and there is no way to determine exactly when a bed will be exhausted, resulting in a chemical breakthrough.
Steam regenerable carbon systems are available, but the installed cost is much higher than the operating cost of replacement beds, and higher than UV.
Removal via sodium metabisulfite
The addition of sulfur to the water stream can provide a food source for sulfur reducing bacteria that can lead to microbiological induced corrosion, as well as the potential scaling of reverse osmosis (RO) membranes.
However, on a cost basis, this technology may be the least expensive chlorine reduction solution.
Removal via ultraviolet
Specific lamp patterns, electropolished interior surfaces and hydraulic baffling systems within a UV reactor enhance the ability of UV to destroy both chlorine and chloramines.
The reaction scheme for the UV radiation of water containing residual free chlorine can be depicted as follows:
Cl¯ + H2O + hv -> HCl + OH (free radical)
The use of UV for chlorine or chloramine removal is an established technology that has been widely accepted in pharmaceutical, beverage and dialysis applications.
One major application of UV is for the removal of chlorine and chloramines before a thin film composite RO membrane.
While cellulosic RO membranes are chlorine tolerant, thin film composite membranes are not, and both chlorine and chloramines readily oxidize these membranes.
The UV application in the beverage industry is similar, usually protecting an RO membrane system. The other major benefit of putting a UV in front of an RO is that membrane bio-fouling is virtually eliminated, which translates into significant cost savings over time.
The main drawback is the larger capital costs of dechlorination systems compared to carbon, and particularly sodium metabisulfite systems. Additional research is required to optimize the reactor design to help reduce these costs.
This non-chemical method offers significant benefits compared to conventional dechlorination technologies utilized thus far by the industry.