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 neutralising 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
High Intensity Broad-Spectrum 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 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. 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 neutralising 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 neutralising 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 USA. 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 Euro 65,000 (US$70,000). The number of shutdowns for RO membrane maintenance has also been significantly reduced.
“We are very pleased with the UV system,” said Utilities Process Engineer Kurt Loughlin. “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.”
A large semiconductor manufacturer in the USA uses RO-treated water through an air scrubber to wash isopropyl alcohol (IPA) out of the exhaust air. After being saturated with IPA, the water is run through RO 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, in addition to extremely high cost, 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 from 1.0 part per million (ppm) to less than 0.01 ppm with 500 ppm of IPA present. The flow rate was 300 litres/minute. The actual concentration was 1.1 ppm free chlorine and 1,300 ppm of IPA with actual reduction down to 0.02 ppm.
A mid-sized brewer in the USA uses 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 were discouraged because of high capital costs, increased maintenance expenses and difficulty sanitising 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 ppm to 0.01 ppm. The company therefore elected to eliminate their carbon entirely and use UV dechlorination instead.
Many breweries, soft drinks manufactures and other processors also 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 Kent, 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.
Another example is Harveys Brewery in Lewes, East Sussex, which uses UV to treat all water drawn from an artesian well which subsequently comes into contact with product. The head brewer commented, 'Although the quality of the water from our well is sound, we wanted to ensure there was no subsequent microbial contamination in-line. We chose UV as it is the simplest system to use with the least operator risk associated with it.'
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.