Retrofitting Existing Wastewater Treatment Plants to Replace Gas Chlorination with U.V. Disinfection-Design Considerations

Increased security and safety concerns about gas chlorination at wastewater treatment plants (WWTP) have led to a growing trend among municipalities to replace existing gas chlorination systems with ultraviolet (U.V.) disinfection. Use of U.V. disinfection, a non-chemical process that utilizes ultraviolet radiation, eliminates the need for handling and storing large amounts of toxic and hazardous chlorine gas at the WWTP.The ultraviolet radiation, which is generated by an electric discharge through mercury vapor, destroys microorganisms in wastewater treatment effluent or disrupts their ability to reproduce by penetrating the genetic material (DNA).The author has participated in feasibility studies and design of U.V. disinfection systems for replacement of gas chlorination in several wastewater treatment plants in the Washington metropolitan area, capacities of which ranged between 20 and 60 million gallons per day (mgd). This paper discusses general feasibility criteria and design considerations for retrofitting existing wastewater treatment plants with a U.V. disinfection system in lieu of gas chlorination.