UV Solutions Address Growing Problem Of Water ScarcityUltraviolet is a proven technology for wastewater reuse that can protect against pathogenic microorganisms and chemical contaminants.by Adam Festger, Kenny Khoo, Wayne Lem, and Jennifer MullerWater scarcity is a growing issue in many regions around the globe. Many cities, large and small, are experiencing water stress (i.e., deterioration in water quality and growing shortage in water quantity). Reuse of treated municipal wastewater offers an attractive solution to the water stress problem. The treated wastewater can be reused for the purpose of irrigation, landscaping, toilet flushing, car washing, or industrial use. There is also a growing trend toward indirect potable reuse of indirect potable reuse (IPR) wastewater. Around the world, a number of IPR projects have been successfully implemented, and more are in progress to reuse wastewater to protect and augment potable supplies.In all these applications, wastewater reuse helps to relieve the burden on existing municipal potable supplies. Because people may be in contact with reused wastewater, its proper treatment is critical for ensuring public health protection. The use of ultraviolet (UV) technology has been successfully implemented for the purpose of providing disinfection and environmental contaminant treatment in water reuse applications. Disinfection For Water Reuse Chlorine is often used for disinfecting wastewater for reuse purposes, but there are two main risks associated with chlorine disinfection. First, it has been well established in literature that chlorine d i s i n f e c t i o n forms disinfection byproducts, such as trihalomethanes, haloacetic acids, and N-nitrosodimethylamine (NDMA). These byproducts can cause both acute and longterm health effects. Second, chlorine is ineffective at disinfecting the illnesscausing protozoa Cryptosporidium. In many parts of the world, Cryptosporidium is commonly found in municipal wastewater, even after conventional treatment. For decades, large-scale treatment plants in North America and around the world have been successfully applying UV disinfection of wastewater for reuse purposes. Reuse water is wastewater that has been treated to high standards to protect public health and to ensure environmental sustainability. Reuse water is typically treated to tertiary levels using sand filtration or membrane filtration prior to disinfection by a UV system. Both open channel (gravity flow) and closed vessel (pressurized) UV systems are currently used in reuse applications. Open channel reactors are often installed in existing chlorine contact chambers, thereby eliminating the need for major civil work, thus reducing installation costs. Furthermore, the open channel configuration provides a complete disinfection system with minimal head loss, so there is virtually no impact on the existing plant hydraulics (figure 1). With the rising use of membrane bioreactors to provide high-quality treated water for reuse applications, many plants are producing pressurized effluent which needs to be disinfected. Instead of “breaking head” and repumping, an alternative is to install a closed vessel UV system for disinfection (figure 2). Chemical Contaminant Treatment With UV Oxidation Beyond general water reuse for irrigation or nonpotable uses, there is a growing trend toward IPR or direct potable reuse (DPR) of wastewater. The term “indirect potable reuse” denotes injecting treated water into groundwater aquifers or releasing it into surface water reservoirs for future withdraw- 34 W Figure 1: This 16.7-million gallons per day (MGD) water reuse plant installed an open channel UV system that features highefficiency lamps to minimize power consumption. An automatic chemical/mechanical cleaning system prevents fouling on the lamp sleeves and reduces maintenance requirements. All images and photos courtesy of Trojan Technologies Figure 2: Four closed vessel reactors were installed postmembranes for this 20.4 MGD water reclamation plant. The reactors use energy-efficient, low-pressure, high-output lamps that allow this water reclamation plant to minimize total power consumption. Water Online The Magazine, Wastewater Edition wateronline.com Technique al, while “direct potable reuse” describes the direct use of highly purified wastewater directly for drinking. An obstacle to the potable use (both IPR and DPR) of this water has been the presence of chemical and biological contaminants not easily removed by conventional treatment processes. Worldwide, a number of plants that are performing IPR use a treatment train that consists of membranes (typically microfiltration [MF] and reverse osmosis [RO]) followed by UV oxidation. Within a UV system designed to perform UV oxidation, two processes occur to treat chemical contaminants found in reuse water: UV photolysis and UV oxidation. UV photolysis is the process by which chemical bonds of the contaminants are broken by the energy associated with UV light. UV oxidation systems rely on the generation of hydroxyl radicals by way of the UV photolysis of hydrogen peroxide and the subsequent oxidation of chemical contaminants by those hydroxyl radicals. Hydroxyl radicals have a higher oxidation potential compared with other oxidants like ozone and chlorine. However, unlike those oxidants, the hydroxyl radicals are extremely reactive and short lived, and do not exist beyond the boundaries of the UV reactor. The combination of UV photolysis and UV oxidation allows for the destruction of chemical contaminants such as nitrosamines, pharmaceuticals, and other industrial contaminants that can be present in reuse water. The UV oxidation reaction is illustrated in figure 3. A large UV oxidation application is the Groundwater Replenishment System (GWR), an IPR project located at the Orange County Water District (OCWD) in Fountain Valley, CA. In early 2009, OCWD placed into operation a facility that will purify a peak flow of 100 million gallons per day (MGD) of wastewater for distribution into area-spreading basins and injection into local aquifers (to prevent seawater intrusion). This project is the largest IPR project in the world and consists of MF and RO followed by a UV oxidation/disinfection system (figure 4). While MF and RO provide treatment for a variety of organic compounds, there are a number of contaminants that, due to their small molecular size, can pass through even the most advanced RO membranes. Common in wastewater, the compound NDMA is present at the GWR system as a byproduct formed during upstream wastewater treatment processes. The treatment objectives accomplished by the UV oxidation system include: ● destruction of nitrosamines and other contaminants treated by UV photolysis (UV alone) ● destruction of pharmaceuticals, personal care products, and industrial chemicals treated by UV oxidation (UV + hydrogen peroxide) ● microbial disinfection ● additional protection — an additional barrier that helps build public confidence in treated water. A Sustainable Source Of Water In North America, especially in the Southern and Western states, urban centers are experiencing decreasing groundwater tables, land subsidence, saltwater intrusion, and chemical pollution. Reuse of wastewater, now recognized as an ecological and economic necessity, is increasingly practiced not only in the United States but globally as well in water-scarce regions such as Australia, Italy, Spain, and Portugal. For the past two decades, UV treatment has been successfully used to disinfect reuse effluents and provide contaminant destruction. UV is a proven technology for wastewater reuse that can protect the public against pathogenic microorganisms and chemical contaminants. As an alternative to other methods of treatment, UV in water reuse applications does not produce harmful byproducts, is nontoxic to the environment, and is energy efficient. The use of UV oxidation in IPR and DPR applications destroys chemical contaminants that may pass through MF and RO membranes and ensures recycled water meets or exceeds drinking water standards. 36 Adam Festger is the Market Manager for Trojan Technologies’ Drinking Water and Environmental Contaminant Treatment (ECT) Division. He is responsible for directing Trojan’s global activities concerning the use of UV light for drinking water disinfection and for the removal of trace contaminants from water. He holds a Master’s Degree in Hydrology and a Bachelor of Science in Mechanical Engineering from the University of Arizona. Adam joined Trojan Technologies in 2001 as Communications Manager and UV-Oxidation System Designer for Trojan’s ECT Division. Jennifer Muller is a Professional Engineer and expert in the field of water and wastewater UV disinfection. As Municipal UV Marketing Director for Trojan Technologies, she commercializes technology and product innovations to help municipalities install sustainable and cost-effective water treatment solutions. Kenny Khoo is a Biochemical and Environmental Engineering graduate from the University of Western Ontario and is currently the Municipal Market Specialist at Trojan Technologies. Wayne Lem is a registered Professional Engineer and has been working in the field of UV disinfection for over 17 years. He has authored many technical papers and has spoken at various conferences to educate and promote knowledge on UV disinfection for municipal applications. Wayne currently holds the position of Municipal Wastewater Market Manager for Trojan Technologies. Figure 3: The UV oxidation process requires the combination of UV and hydrogen peroxide, which forms hydroxyl radicals (red and white). The hydroxyl radicals rapidly react with chemical contaminants (in yellow) within the UV reactor. Figure 4: The OCWD’s 100 MGD Groundwater Replenishment System employs UV oxidation as a final barrier for pathogens, nitrosamines, pharmaceuticals, industrial chemicals, and other contaminants. The resulting treated water exceeds drinking water standards and is injected into local aquifers for eventual potable reuse. Water Online The Magazine, Wastewater Edition wateronline.com |
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