Seeing the light: UV for dechlorination

Mike Shaw, sales director for Hanovia, discusses the use of ultraviolet treatment for dechlorination in the pharma sector

For many years chemical disinfection techniques have been used to provide microbiologically pure water for industrial and domestic use. Free chlorine, typically introduced by municipal water treatment plants in gaseous form, has been employed as the primary oxidising agent for the control of microbiological growth; however it can also be introduced through the injection of sodium hypochlorite, chlorine dioxide and other similar compounds. When chlorine is injected into waters with naturally occurring humic acids, fulvic acids or other natural materials, trihalomethane (THM) compounds are formed. Approximately 90% of the total THMs formed consists of trichloromethane, with the remaining 10% comprised of bromo dichloro methane (CHCl₂Br), dibromo chloro methane (CHBr₂Cl) and tribromo methane (CHBr₃). Since THMs have been shown to be carcinogenic in laboratory animals in relatively low concentrations, there is concern about limiting their prevalence. The EPA, for example, has set the maximum contaminant level in primary drinking water at 100 parts per billion (ppb).

Chlorine is widely used for water disinfection in many different process industries. Because of its properties, however, it can damage delicate process equipment like RO membranes and deionisation (DI) resin units and 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 neutralising chemicals such as sodium bisulfite.

broad spectrum

Both these methods have their advantages, but they also have a number of significant drawbacks. Because of their porous structure and nutrient-rich environment, GAC filters can become a breeding ground for bacteria; and dechlorination chemicals such as sodium bisulfite, which are usually injected just in front of RO membranes, can also act as incubators for bacteria, causing biofouling of the membranes. Furthermore, these chemicals are hazardous to handle and there is a danger of over- or under-dosing due to human error.

An increasingly popular dechlorination technology, with none of the above drawbacks, is UV treatment. High intensity, broad-spectrum UV systems dissociate both free chlorine and mono, di- and tri-chloramine compounds into easily removed byproducts. UV has the added benefit of providing both high levels of microbial disinfection and also total organic carbon (TOC) removal.

From 180-400nm, UV light produces photochemical reactions which dissociate free chlorine to form hydrochloric acid. The peak wavelengths for dissociation of free chlorine range from 180-200nm, while the peak wavelengths for dissociation of the chloramines range from 245 to 365nm. Up to 5ppm of chloramines can be successfully destroyed in a single pass through a UV reactor with up to 15ppm of free chlorine being removed.

Many water treatment systems include RO units, which commonly use thin-film composite membranes because of their greater efficiency. However, these membranes cannot tolerate much chlorine, so locating the UV unit upstream of the RO can effectively dechlorinate the water, eliminating or greatly reducing the need for neutralising chemicals or GAC filters.

reduced concentrations

The UV dosage required for dechlorination depends on total chlorine level, the ratio of free to combined chlorine, the 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.

Membranes therefore stay cleaner much longer because the dose for dechlorination is so much higher than the normal dose used if dechlorination were not the goal.

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

• influent chlorine level and target effluent chlorine concentration level

With the potential applications, and the benefits it brings, UV dechlorination offers real opportunities for those willing to invest in this innovative technology.