Today’s water treatment facility managers face numerous challenges in the quest to produce pure, safe drinking water. Each environmental region is unique, and can potentially encounter a wide variety of groundwater and surface water contaminants – including organic, inorganic, dissolved solids, algae, bacteria, viruses, cysts, and more – that require treatment. The typical water treatment process involves a multiple-barrier approach, where a combination of filtration and disinfection technologies are used to effectively process feed water, rendering it suitable and safe for distribution as drinking water to the public.
Thanks to technological advances over the years, water treatment facility managers have a myriad of disinfection technologies to choose from. But like anything, each technology has its strengths and weaknesses. Most municipalities choose water disinfection technologies based on factors such as safety, handling, ease of operation, the longevity of the equipment, the amount of waste generated, the size of the actual system, capital investment, and more. A concise evaluation of all these factors affects how a specific disinfection technology is integrated into water and wastewater treatment facilities.
So which disinfection technology is right for your water treatment facility? Take a look at the top four technologies on the market to see which best suits your needs:
Chlorine Gas Disinfection
For more than a century, chlorine gas has been successfully used to disinfect drinking water, eliminating diseases such as typhoid and dysentery. When added to water in the appropriate amounts, chlorine forms hypochlorous acid (HOC1), an active disinfectant. This disinfectant is used by more than 90 percent of U.S. drinking water plants, and is heavily used throughout the Middle East, Asia, and the Pacific Rim.
Chlorine gas is either produced at chlor-alkali plants and shipped to water treatment facilities as a liquefied gas in pressurized bulk containers or, thanks to new technology, generated on site at the treatment facility. The main strengths of traditional chlorine gas disinfection include:
- Destroys a broad range of microorganisms, including viruses, bacteria, and some protozoa;
- Controls many taste, odor, and color problems in raw water through the oxidation of the constituents that cause these problems; and
- With proper dosages, remains as chlorine residual in water distribution systems to protect against the regrowth of algae or microorganisms.
This broad range of capabilities makes chlorine gas disinfection very cost-effective. However, many municipalities have become concerned about the hazards it presents in transportation and storage, the possible creation of harmful disinfection byproducts (DBPs), and its weakness in inactivating Cryptosporidium. Despite the safety concerns, however, gas chlorination still maintains the highest documented safety record when compared with the alternative methods of chlorine disinfection.
Sodium Hypochlorite Disinfection
Whether generated on site or shipped in bulk form, sodium hypochlorite (NaOC1) is an excellent alternative to gaseous chlorine disinfection. Often referred to as liquid bleach, sodium hypochlorite is widely considered the second most affordable disinfectant after bulk liquid chlorine gas. It is commercially available at varying solution strengths, ranging from 15 percent to .6 percent solutions, and offers most of the benefits of chlorine gas as a disinfectant, oxidizing agent, and residual disinfectant, but without the risk of transporting or storing hazardous chemicals.
Unlike gas chlorine disinfection, however, bulk sodium hypochlorite tends to decompose in storage depending on temperature, age, and concentration. Also, the sodium chloride ingredient may contain bromide impurities that lead to the creation of EPA-regulated bromates. However, when employing an on-site sodium hypochlorite generation system, three simple components – salt, water, and electricity – are combined to produce sodium hypochlorite, eliminating the common degradation problems of bulk sodium hypochlorite since the disinfectant is produced on site, as needed.
Chloramination Disinfection
This technology involves the separate introduction of chlorine compounds and ammonia into a water-treatment system at a five-to-one ratio. The two ingredients react to form chloramines. Chloramines react more slowly than chlorine, staying active longer and forming smaller concentrations of trihaloamines (THMs), a disinfection byproduct, when combined with organic matter present in water.
Although chloramines are not as potent as chlorine, their increased residual effect enables them to keep bacteria from growing in the distribution system. It also eliminates the need for additional chlorine at booster sites throughout the distribution system.
Ultraviolet Disinfection
Traditionally, ultraviolet disinfection (UV) has been successfully used in municipal wastewater treatment across Europe and the United States. However, it’s a relatively new disinfection treatment technology for drinking water applications and has recently gained acceptance in groundwater applications where water quality factors such as total suspended solids (TSS), transmittance, and temperature are conducive to optimal performance.
Ultraviolet disinfection works by exposing waterborne microorganisms to UV light in the germicidal range of 250 to 270 nm, at a specified intensity for a specified period of time. This exposure renders the microorganism "microbiologically dead" by penetrating the cell wall and affecting the DNA in such a way that it can no longer reproduce. UV disinfection has effectively treated certain bacteria unaffected by traditional chlorine disinfection. However, UV disinfection does not produce a residual; when used in a treatment system, it may require the addition of a terminal disinfectant, such as chlorine, to provide a residual in the distribution system.
The EPA recently released the Ultraviolet Disinfection Guidance Manual in draft form for review. Over recent years, UV disinfection has progressed from an emerging technology to a best-available technology (BAT) into one that will soon have specific regulations for use and implementation attached to it by the US regulatory agency.
Weighing In
When all is said and done, municipalities and water facility managers must weigh the pros and cons of each technology and consider their unique environmental needs before choosing the technology that’s right for them. For more information about disinfection technologies and available equipment, contact Nadia Abbott at nabbott@severntrentservices.com or 215.997.3733.