UV solutions for water scarcity

By Jennifer Muller, Adam Festger, Kenny Khoo, Wayne Lem Trojan Technologies

Water scarcity is a growing issue in many regions around the globe. Many cities, large and small, are experiencing water stress, deterioration in water quality and growing shortage in water quantity. Reuse of treated municipal wastewater offers an attractive solution to water stress. 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 towards indirect potable reuse of wastewater. Around the world, a number of Indirect Potable Reuse (IPR) projects have been successfully implemented and more are in progress to re-use wastewater to protect and augment potable supplies. In all these applications, wastewater reuse helps to relieve the burden on existing municipal potable supplies. Since 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 (ECT) 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 disinfection forms disinfection byproducts, such as THMs, HAAs and NDMA.  These byproducts can cause both acute and long-term health effects.  Second, chlorine is ineffective at disinfecting the illness-causing protozoa, Cryptosporidium.  In many parts of the world, Cryptosporidium is commonly found in municipal wastewater even after conventional treatment.

UV disinfection of wastewater for reuse purposes has been successfully applied for decades in large scale treatment plants around the world. Reuse water is treated wastewater that has been disinfected to very stringent standards in order 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.

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 headloss, thus there is virtually no impact on the existing plant hydraulics (Figure 1).

Figure 1: This 732 L/s (16.7 MGD) water reuse plant installed an open channel UV system that features high-efficiency lamps to minimize power consumption. An automatic chemical/mechanical cleaning system prevents fouling on the lamp sleeves and reduces maintenance requirements.

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 re-pumping, an alternative is to install a closed vessel UV system for disinfection (Figure 2).

Figure 2: Four closed vessel reactors were installed post-membranes for this 894 L/s (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.

Chemical Contaminant Treatment with UV-Oxidation

Beyond general water reuse for irrigation or non-potable uses, there is a growing trend toward indirect (IPR) or direct potable reuse (DPR) of wastewater. The term "indirect potable reuse" describes injecting treated water into groundwater aquifers or by releasing it into surface water reservoirs for future withdrawal while "direct potable reuse" describes the direct use of highly purified wastewater directly for drinking. An obstacle to the potable use (both indirect and direct) 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 utilize a treatment train that consists of membranes (typically microfiltration and reverse osmosis) 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 to 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.

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.

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, California. In early 2009, OCWD placed into operation a facility that will purify a peak flow of 100 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 microfiltration (MF), reverse osmosis (RO) followed by a UV oxidation/disinfection system (Figure 4).

Figure 4: The Orange County Water District's 4381 L/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.

While MF and RO provide treatment for a variety of organic compounds, there are contaminants that, due to their small molecular size, can pass through even the most advanced RO membranes. Common in wastewater, a compound known as N-nitrosodimethylamine (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 by UV-photolysis (UV alone)
• Destruction of pharmaceuticals, personal care products and industrial chemicals by UV-oxidation (UV + hydrogen peroxide)
• Microbial disinfection to inactivate bacteria and viruses
• Additional protection - an additional barrier that helps build public confidence in treated water

A Sustainable Source of Water

Many arid and heavily populated urban centers around the world are experiencing decreasing ground water tables, land subsidence, saltwater intrusion and chemical pollution. Reuse of wastewater, now recognized as an ecological and economic necessity, is increasingly practiced globally in water scarce regions such as Australia, Italy, Spain, California and Florida. For the past two decades, and more so today, ultraviolet 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 by-products, is non-toxic 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 all drinking water standards.

All images and photos courtesy of Trojan Technologies (www.trojanuv.com). More information about UV disinfection and UV-oxidation is available on the Trojan Technologies website: www.trojanuv.com