UV Disinfection for a Real-Word Effluent Making Sure UV Will Work for You

After the City of Longmont installed a new ultraviolet disinfection system at its recently-expanded wastewater treatment plant, the system was unable to meet the disinfection levels required by the discharge permit. Several factors were identified that contributed to the performance problems, including an inadequately sized system, project specifications that did not reflect actual conditions and variations in effluent quality that were larger than originally thought.The City decided to replace the system, but in order to be sure that the replacement would be sized properly and be able to meet permit limits, the City completed a series of collimated beam tests on the wastewater effluent in 2005 and 2006. During the course of the tests, several of the City's concerns about Ultraviolet (UV) disinfection were addressed, including the variability of UV transmittance in daily wastewater plant operations, possible UV inhibition in the wastewater, the effect of suspended solids and particle size on UV effectiveness, and the actual dose-response curves for the effluent. In addition, questions arose about the applicability of different coliform tests to UV-disinfected effluent.The extensive number of tests that were done allowed the City to develop more realistic performance requirements for a UV system. A new system based on these requirements was installed in late 2006 and has successfully completed validation testing.