Interest in ultraviolet (UV) light disinfection is seeing unprecedented growth in the U.S. and Canadian drinking water industries as many communities consider alternative disinfection processes that will inactivate Cryptosporidium and reduce disinfectant byproduct (DBP) concentrations in the distribution system. Both requirements are covered in the Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR) and the Stage 2 Disinfectants and Disinfection Byproducts Rule, respectively, which were promulgated by the U.S. Environmental Protection Agency (EPA) in January 2006.
UV disinfection is an established photochemical technology that exposes contaminated water to a spectrum of electromagnetic radiation in the germicidal UV light range (200 to 300 nanometers). Research in the late 1990s proved that UV disinfection was very effective for inactivating Cryptosporidium—a chlorine-resistant pathogen. Based on this research, EPA developed dosage tables allowing disinfection credit for Cryptosporidium at low UV doses (e.g., 12 milliJoules per square centimeter for 3-log inactivation).
For drinking water applications, closed-vessel UV reactors with lamps placed in a particular configuration (either parallel or perpendicular to water flow) are designed to provide efficient and cost-effective dose delivery. Furthermore, UV light does not form harmful DBPs, compared to chemical disinfectants like chlorine, resolving the risky trade-off between microbial disinfection and chemical byproducts.
However, since this chemical-free approach does not leave post-treatment residuals, there is no direct-measurement method to ensure reliable dose delivery. Therefore, public water systems planning to use UV disinfection are required by LT2ESWTR to use UV reactors that have undergone validation testing.
At its most basic level, a validation test involves introducing a test microorganism into a UV reactor and taking sample counts before and after the reactor. Commonly referred to as a bioassay, this test proves the system will be accepted for the target UV dose under a certain set of operating conditions, such as flow rate, UV transmittance (UVT), UV intensity measurements, and lamp power setting. Validation tests, which are administered by an independent and recognized third party under rigorous methodology outlined in the EPA UV Disinfection Guidance Manual for LT2ESWTR, are currently being conducted in test centers in Johnstown, New York, and Portland, Oregon, for flows up to 40 million gallons per day (mgd) and UVTs from 70 to 98 percent.
UV equipment manufacturers serving the North America drinking water market have responded to the new opportunities for drinking water applications with pre-validated UV reactors, and at least one vendor is in the process of validating large-scale reactors larger than 40 mgd. Tested at the manufacturer's expense, these UV reactors can be purchased and installed by public water systems without the need to perform their own expensive and time-consuming onsite validation testing.
The benefits? Pre-validated reactors help ensure a system is optimally designed and constructed to meet EPA disinfection requirements, minimizing the need to resize the system after an untested reactor is installed and validated. This approach also helps to reduce future operations and maintenance costs. As UV disinfection becomes a viable disinfection alternative, pre-validated UV reactors will further spur interest among all sizes of water utilities to begin installing UV disinfection systems for regulatory compliance and improved public health protection.