An Overview of California's Experience with Ultraviolet Disinfection in Water Recycling Applications
Abstract:Since the National Water Research Institute's (NWRI) UV Disinfection Guidelines were published in the 1993 for low-pressure, low-intensity horizontal lamp UV systems, the UV industry has developed a number of “nonconforming” systems. Under the NWRI guidelines these nonconforming systems must be tested and their performance verified before they can be used to disinfect municipal wastewater for reuse or recycling applications. Several studies to verify the performance of these nonconforming systems have been conducted at wastewater facilities located in California. Observations from early testing of the nonconforming systems showed that performance similar to UV systems accepted under the NWRI guidelines could be achieved. However, a review of historical data suggests that a great deal of variability in performance (as measured by pathogen inactivation) exists between static testing (collimated beam) and continuous flow pilots.
Based on collimated beam test data, some of the variability may be attributed to the differences in the protocols used to propagate and harvest the pathogen seed used in the testing. Even though the pathogen seed used in these studies was an MS2 bacteriophage, traceable to an ATCC stock culture, there were subtle differences in growing, harvesting, and concentrating the seed stock between the studies.
The lack of standardized protocols for seed handling and collimated beam test conditions may not be the only source of variability in UV system testing. UV system performance at various California reclamation facilities has indicated variable disinfection abilities with variable flow rates, indicating sub par system hydraulics at low flow rates. UV studies are underway to evaluate system hydraulics and disinfection capabilities over variable flow rates.
Without adequate controls in place to aid in the interpretation of results, the observed variations between collimated beam and continuous-flow pilots may be leading to inappropriate conclusions regarding the performance of these units. Because subtle variations can be magnified in full-scale facilities (resulting in under or inadequately designed facilities), the need for developing a standard testing protocol has become obvious.
This paper will discuss the problems and pitfalls of the current verification process using examples to illustrate how these observations and hypotheses are being used to lead toward standardization of some aspects of the testing protocol. Properly developed, a standardized testing protocol will produce results that better reflect the performance of the UV systems. In turn, this will ensure more consistent performance of UV systems through proper design and construction.
Document Type: Research Article
Publication date: 2000-01-01
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