Moccasin Bend Wastewater Treatment Plant - Executive Summary |
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The City of Chattanooga runs and operates MBWWTP in Chattanooga, TN.. The plant uses chlorine, which is classified as an extremely hazardous substance, for disinfection. Liquid chlorine flows into the bottom of the hot water bath vaporizer and is converted to gaseous chlorine through a series of pipes and a chlorinator. The vacuum draws the gaseous chlorine into water passing through the injector. The chlorine gas then goes into solution and is piped to chlorine diffusers, where it is mixed with clarified effluent from secondary clarifiers before entering the chlorine contact tanks. In these tanks, the chlorine solution is mixed with the clarified effluent to achieve adequate disinfection. Two release scenarios were modeled using the Aerial Location of Hazardous Atmospheres (ALOHA) computer program developed by the National Safety Council for emergency planning and training. The worst-case scenario (i.e., catastrophic falure of a 2000-pound tank over a 10-minute period due to cor rosion, impact, or construction defect, a windspeed of 1.5 meters per second, and an atmospheric stability class of F) resulted in a toxic endpoint of 2.6 miles. Approximately 33,630 people in Marion and Hamilton counties are estimated to reside in the 2.6 mile radius surrounding the facility. The alternate-case scenario (i.e., a release of liquid chlorine from a horizontal cylindrical 1-ton container through a short pipe or valve in the bottom of the tank, resulting in a two-phase liquid/gas release, a windspeed of 2.24 meters per second and an atmospheric stability class of C) resulted in a toxic endpoint of 1.2 miles. Approximately 3,230 people in Marion and Hamilton counties are estimated to reside in the 1.2 mile radius surrounding the faciltiy. The plant is subject to federal requirements, including the preparation of PSM and RMP programs (29 CFR 1910.119 and 40 CFR Part 68). Additionally, MBWWTP is subject to and in compliance with OSHA rules. MBWWTP has (1) in-house proce dures for safe operation of the clorination process; (2) a training program for workers (including contractors and subcontractors) on or near the chlorination process of process hazards and appropriate response actions; (3) a mechanical integrity program to ensure that regular preventative maintenance is performed; (4) an incident investigation program to identify the underlying causes of incidents and implementing steps to prevent sinilar events from occuring; (5) compliance audits every 3 years to ensure compliance with OSHA and EPA; and (6) an emergency response and action plan. The City of Chattanooga has not had an accident involving chlorine that caused deaths, injuries, property or environmental damage, or sheltering in place at this facility in the last 5 years. |