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 for many decades as a primary oxidizing agent for the control of microbiological growth. Free chlorine can also be introduced through the injection of sodium hypochlorite, chlorine dioxide and other chlorine compounds.
When chlorine is injected into waters with naturally occurring humic acids, fulvic acids or other naturally occurring material, trihalomethane (THM) compounds are formed. Approximately 90% of the total THMs formed are chloroform, with the remaining 10% consisting of bromodichloromethane (CHCl2Br), dibromochloromethane (CHBr2Cl) and bromoform (CHBr3). Since THMs have been shown to be cancer-causing to laboratory animals in relatively low concentrations, there is concern about limiting their prevalence. The United States Environmental Protection Agency (USEPA), for example, has set the maximum contaminant level in primary drinking water to be 100 parts per billion (ppb).
Although chlorine is widely used in industry, many processes cannot tolerate it because of contamination and unwanted chemical reactions. It can accelerate corrosion of process vessels and piping and also causes damage to delicate process equipment such as reverse osmosis (RO) membranes and deionization (DI) resin units. It can also affect the taste, flavour and smell of drinks and liquids. It therefore 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 bisulphate. Both of these methods have their advantages, but they also have a number of significant drawbacks.
Granular Activated Carbon (GAC)
Activated carbon is frequently used in industrial applications such as beverage and pharmaceutical manufacturing and in point-of-use units for residential and commercial applications. However, GAC filters, which are usually located upstream of the RO membranes, also can serve as an incubator of bacteria because of their porous structure and nutrient-rich environment. Additional problems encountered with the use of GAC filters are:
• Increased head loss
• Regeneration costs
• Unpredictable chlorine breakthrough
Sodium Metabisulphite or Sodium Bisulphate
This is either purchased in solution or bought as a dry powder and then mixed on site. It is commonly injected in front of RO membranes used in the pharmaceutical and semiconductor industries. One common problem with this approach is that the solution itself becomes an incubator of bacteria, causing biofouling of the membranes. It is also another chemical that has to be documented in use, handling and storage for regulators such as environmental protection or health and safety agencies. Additional problems encountered with the use of sodium metabisulphite are:
• Maintenance of dosing equipment
• Hazardous material to handle
• Scaling of RO membranes
• Sodium sulphate can be formed, acting as a stimulant to sulphate reducing bacteria
• Odour and taste implications also arise
The UV Alternative
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 nm to 400 nm UV light produces photochemical reactions which dissociate free chlorine to form hydrochloric acid. The peak wavelengths for dissociation of free chlorine range from 180 nm to 200 nm, while the peak wavelengths for dissociation of chloramines (mono-, di-, and tri-chloramine) range from 245 nm to 365 nm. Figure 1 shows the UV output of a high intensity Hanovia medium pressure UV lamp. Up to 5ppm of chloramines can be successfully destroyed in a single pass through a UV reactor and up to 15ppm 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. 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 neutralizing chemicals or GAC filters.
The UV dosage required for dechlorination depends on 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. Membranes therefore stay cleaner much longer because the dose for dechlorination is so much higher than the normal dose used if dechlorination was not the goal. Additional important benefits of using UV dechlorination are:
• High levels of UV disinfection
• TOC destruction
• Eliminate safety hazard associated with mixing bisulphate
• Eliminate risk of introducing micro-organisms into RO (via GAC or injection of neutralizing chemicals)
• 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
• Influent chlorine level and target effluent chlorine concentration level
Successful UV dechlorination applications range from pharmaceutical, food and beverage processing to semiconductor fabrication and power generation. In all these industries, dissatisfaction with conventional dechlorination methods has encouraged alternative methods to be found. The following are examples of some applications in which high-intensity, broad-spectrum output (medium-pressure) UV has been successfully used for dechlorination:
A Hanovia UV dechlorination unit was installed at a Procter & Gamble manufacturing plant in the Georgia. The unit was installed before two banks of RO membranes; prior to this dechlorination was achieved using sodium bisulphate. Trials run 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 annual savings of $70,000. The number of shutdowns for RO membrane maintenance has also been significantly reduced.
Many breweries, soft drinks manufactures and other processors use UV for general disinfection of product make-up and process water. UV kills all known spoilage microorganisms, including bacteria, viruses, yeasts and moulds (and their spores) and has many advantages over alternative methods. Unlike chemical biocides, UV does not introduce toxins or residue into process water and does not alter the chemical composition, taste, odour or pH of the fluid being disinfected.
One example is Shepherd Neame brewery in the United Kingdom, one of the oldest in the country. It uses a UV system to treat water drawn from a private well and used for deoxygenated beer cutting. The water passes through the UV treatment chamber before entering a storage tank, and from here it passes through a series of sterile filters before use. In addition to treating cutting water, the UV system also disinfects water used for bottle rinsing.
As can be seen from the above examples, the potential applications for high-intensity, medium-pressure UV for dechlorination and disinfection, 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.