Volume 45 Number 1
The Case for UV in Dechlorination Applications
Summary: To eliminate free chlorine from potable water, activated carbon or addition of neutralizing chemicals are the most common solutions. Still, ultraviolet (UV) irradiation treatment, once an afterthought, is gaining wider acceptance as various industries are finding it reduces some more common drawbacks associated with dechlorination.
For many years, chemical disinfection techniques have been used to provide microbiologically pure water for industrial and domestic uses. Free chlorine, typically introduced by municipal water treatment plants in the gaseous form, has been employed for many decades as a primary oxidizing agent for the control of microbiological growth. Free chlorine can also be introduced through injection of sodium hypochlorite, chlorine dioxide and other chlorine compounds. When chlorine is injected into waters with naturally occurring humic acids, fulvic acids and other naturally occurring material (NOM), disinfection by-products (DBPs) such as trihalomethane (THM) compounds are formed. About 90 percent of the total trihalomethanes (TTHM) formed is chloroform and the remaining 10 percent are bromodichloromethane (CHCl2Br), dibromochloromethane (CHBr2Cl) and bromoform (CHBr3). Since THMs have demonstrated to be cancer causing to laboratory animals in relatively low concentrations, the U.S. Environmental Protection Agency (USEPA) set its maximum contaminant level (MCL) in primary drinking water at 100 parts per billion (ppb) in 1979.
Although widely used, many industrial processes cannot tolerate chlorine because of contamination and unwanted chemical reactions. It can accelerate corrosion of vessels, valves and piping, and also cause damage to delicate process equipment such as reverse osmosis (RO) membranes and deionization (DI) resin units. It can also affect the taste, flavor and smell of drinks and liquids. Therefore, it often must be removed once it has performed its disinfection function.
To date, the two most commonly used methods of chlorine removal have been granular activated carbon (GAC) filters or the addition of neutralizing chemicals such as sodium bisulfite. Both of these methods have their advantages, but they also have a number of significant drawbacks.
Granular activated carbon
Sodium metabisulfite or bisulfite
1. Maintenance of dosing equipment,
High-intensity UV systems
Between the wavelengths 180 nanometers (nm) to 400 nm UV light produces photochemical reactions that dissociate free chlorine to form hydrochloric acid. The peak wavelengths for dissociation of free chlorine range from 180 to 200 nm, while the peak wavelengths for dissociation of combined chlorine (mono-, di-, and tri-chloramine) range from 245 to 365 nm. Figure 1 shows the UV output of one high intensity, medium pressure UV lamp. Up to 5 parts per million (ppm) of chloramines can be successfully destroyed in a single pass through a UV reactor and up to 15 ppm of free chlorine can be removed.
Many water treatment systems include RO units, which commonly use thin-film composite membranes because of their greater efficiency. These membranes cannot tolerate much chlorine, however, so locating the UV unit upstream of the RO can effectively dechlorinate the water, eliminating or greatly reducing the need for chemical feed or carbon.
The UV dosage required for dechlorination depends on adsorption of UV in the water, total chlorine level, ratio of free vs. combined chlorine, background level of organics, and target reduction concentrations. The usual dose for removal of free chlorine is 15 to 30 times higher than the normal disinfection dose of 30,000 microWatt-seconds per square centimeter (µW-s/cm2). Membranes, therefore, stay cleaner much longer because the dose for dechlorination is so much higher than the normal dose used if dechlorination wasn't the goal.
Additional important benefits of using UV dechlorination are:
As with other dechlorination technologies, the UV dosage required at a given flow rate is dependent on several process parameters including process water transmittance level, background organics level, and influent chlorine and target effluent chlorine concentration levels.
“We are very pleased with the UV system,” said Kurt Loughlin, utilities process engineer. “Not only have we saved money since the UV system was installed, but the disruption caused by plant shutdowns as a result of RO membrane fouling has also been significantly reduced. UV provides a high standard of dechlorination without any of the drawbacks of using chemicals or GAC filters.”
The brewery chose to use carbon to remove the free chlorine, but was discouraged because of high capital costs, increased maintenance expenses, and difficulty sanitizing and cleaning the carbon. The chlorine levels were up to 1.0 ppm but, after a trial period using UV for dechlorination, the brewery reported results of 0.04 to 0.01 ppm levels. The company thus elected to eliminate its carbon entirely and use UV dechlorination instead.
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