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
Activated carbon is frequently used in industrial applications such as beverage and pharmaceutical manufacturing and in point-of-use (POU) units for residential and commercial applications; however, GAC filters—which are usually located upstream of the RO membranes—can also serve as an incubator of bacteria because of their porous structure and nutrient-rich environment. Additional problems encountered with use of GAC filters are increased head loss, regeneration costs and unpredictable chlorine breakthrough.

Sodium metabisulfite or bisulfite
This is either purchased in solution or bought as a dry powder and then mixed on site. It's commonly injected in front of RO membranes used in the pharmaceutical and semiconductor industries. One common problem with this approach is the solution itself becomes an incubator of bacteria, causing biofouling of the membranes. It's also another chemical that has to be documented in use, handling and storage for regulators such as the USEPA and the Occupational Safety and Health Administration (OSHA). Additional problems encountered with the use of sodium metabisulfite are:

1. Maintenance of dosing equipment,
2. Hazardous materials to handle,
3. Sodium metabisulfite scales RO membranes, and
4. Sodium sulfate can be formed, acting as a stimulant to sulfate-reducing bacteria (SRBs); odor and taste implications also arise.

High-intensity UV systems
An increasingly popular dechlorination technology, with none of the above drawbacks, is ultraviolet (UV) treatment. High-intensity, broad spectrum UV systems (also known as medium-pressure UV) reduce both free chlorine and combined chlorine compounds (chloramines) into easily removed by-products.

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:
• High levels of UV disinfection,
• Total organic compound (TOC) destruction,
• Eliminate safety hazard associated with mixing bisulfite,
• Eliminate risk of introducing microorganisms into RO (via GAC or sodium bisulfite injection), and
• Overall improved water quality at point-of-use.

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.

UV applications
Successful applications of UV dechlorination range from pharmaceutical, food and beverage processing to semiconductor fabrication and power generation. In these industries, dissatisfaction with conventional dechlorination methods has encouraged alternative methods. The following are examples of some applications in which high intensity, broad-spectrum output (medium pressure) UV has been successfully used for dechlorination.

Pharmaceutical industry
A UV dechlorination unit was recently installed at Procter & Gamble’s Greensboro, N.C., manufacturing plant of pharmaceutical drugs. The unit was installed before two banks of RO membranes; prior to this, dechlorination was achieved using sodium bisulfite. Trials ran soon after the UV system’s installation showed a dramatic reduction in the RO membrane wash frequency—down from an average of eight cleanings per month to only two per month—amounting to an annual savings of $70,000. The number of shutdowns for RO membrane maintenance has also been significantly reduced.

“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.”

Semiconductor industry
A large West Coast semiconductor manufacturer used RO-treated water through an air scrubber to wash isopropyl alcohol (IPA) out of the exhaust air. After being saturated with IPA, the water was run through a RO system to remove the alcohol. The water is then sent back to the scrubber for reuse. Prior to this a powerful biocide was used but, due to hazardous conditions with application and handling and extremely high costs, injection of sodium hypochlorite (free chlorine) was substituted and UV was used to dechlorinate prior to the RO. The target was to reduce free chlorine levels of 1.0 ppm to less than 0.01 ppm with 500 ppm of IPA present. The flow rate was 80 gallons per minute (gpm). The actual concentration was 1.1 ppm free chlorine and 1,300 ppm of IPA with actual reduction down to 0.02 ppm.

Brewing industry
A mid-sized brewer on the West Coast used well water from a municipal source for plant makeup water. The municipality was forced to begin chlorinating this water due to federal regulation. Unfortunately, the chlorination altered the taste of the product.

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.

Conclusion
As can be seen from the above examples, the potential applications for high intensity, medium pressure UV for dechlorination and the benefits it brings cover a wide variety of industries and processes. UV dechlorination offers real opportunities for those willing to invest in this innovative technology.

About the author
With over 10 years UV experience, Brad Shipe is manager for the High Purity Water Department of Aquionics Inc., of Erlanger, Ky., near Cincinnati. A market leader in UV technology for disinfection, deozonation, dechlorination and TOC removal, it's part of Halma Plc., with affiliates in the United Kingdom and Holland. Shipe can be reached at (859) 341-0710, (859) 341-0350 (fax), email: sales@aquionics.com or website: www.aquionics.com