91st Avenue WWTP - Executive Summary |
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Executive Summary 91st Avenue Wastewater Treatment Plant (WWTP) is one of the many facilities owned and operated by the City of Phoenix. 91st Avenue WWTP has prepared a Risk Management Plan (RMP) in accordance with the EPA and OSHA regulatory requirements. A brief overview of the RMP and the associated policies at 91st Avenue WWTP is described below. Prevention and Response Policies The City takes an active role in preventing accidental releases by ensuring that its employees are properly trained in the safe operation of covered processes and the safe handling of treatment chemicals. The City has also established and maintains procedures for emergency notification and response. These are reviewed with employees on a periodic basis and revised to accommodate changes in staffing when they occur. To increase employee safety and awareness, the City: 7 maintains up-to-date safety information and operating procedures, 7 performs regular preventative maintenance, 7 provides periodic refresher training on safe handling of chemicals, and 7 conducts monthly safety drills. Facility Description and Regulated Substances 91st Avenue WWTP disinfects wastewater from residential and business sources through a series of physical and chemical treatment operations that include grit removal, sedimentation, aeration, clarification, disinfection, and dechlorination. The regulated substances handled at 91st Avenue WWTP are chlorine and methane. The chlorine is stored in four 30-ton tanks and two 22-ton tanker trucks, for a total of 164 tons (328,000 lbs). The methane generated from sludge digestion is contained in fifteen digesters and associated piping; the largest digester contains 3,200 lbs of methane. There is a maximum of 13 tons (26,000 lbs) of methane at the facility. Release Scenarios Worst-case and alternative release scenarios have been determined after careful review of the regulation and consideration of the storage vessel configuration at t he facility. The EPA's software program RMP*Comp was used to model both release scenarios and determine the radius of impact. In the worst-case release scenario, the regulation clearly states the release quantity shall be the contents of the single largest vessel of the covered process chemical. Therefore, the scenario assumed a complete release of chlorine from one 30-ton tank (60,000 lbs) inside the chlorine storage building. Per the regulation, no active mitigation measures, such as a scrubber, could be considered. The passive mitigation effect of the chlorine storage building reduces the amount of chlorine released to the atmosphere by 55% (from 60,000 lbs to 33,000 lbs). The worst-case release scenario for methane assumed a complete release methane gas from the largest digester (3,200 lbs). From this release, 10% of the released methane resulted in a vapor cloud explosion. For the alternative release scenario, the assumptions were that (a) a failure occurred in the feed line, (b) 7,000 lbs of chlorine were released over a 60-minute period, (c) the release occurred in the chlorine storage building, and (d) the scrubber did not work. The passive mitigation effect of the chlorine storage building reduces the amount of chlorine released to the atmosphere by 55% (from 7,000 lbs to 3,800 lbs). The alternative release scenario for methane assumed a complete release methane gas from the largest digester (3,200 lbs). From this release, 3% of the released methane resulted in a vapor cloud explosion. Prevention Steps for Chlorine and Methane The prevention program fulfills the requirements of the OSHA Process Safety Management (PSM) rule and the EPA RMP rule, and includes: 7 formal and on-the-job training, 7 written operating procedures, and 7 a process equipment preventive maintenance program. The primary means of mitigating an accidental chlorine release at 91st Avenue WWTP is the installation of a chemical scrubber for the chlorine storage building. The scrubber is maintained on a regular basis to ensure proper operation if needed during a chlorine release. The digesters are equipped with pressure/vacuum relief valves and flame arrestors to prevent accumulation of methane gas. Accident History There has been no accidental release of the covered chemicals in the past five years. Emergency Response Program This facility has established and maintains an emergency response program that is coordinated with local response agencies. The goals of the program are to protect on-site employees from the hazardous effects of the releases and to minimize the effects of releases on the general public. The program is routinely reviewed and updated to reflect personnel and regulatory changes. Planned Changes for Improved Safety Ideas for changes to improve safety are actively sought from employees. Employee meetings that focus on safety issues are held regularly at the facility. Employees are encoura ged and trained to recognize hazards and present ideas to eliminate them or to minimize the potential consequences of those hazards. During the development of this RMP document, a process hazard analysis of the chlorine system was conducted with key employees to fulfill the prevention program requirements. During these sessions, recommendations were made for the purpose of improving safety and preventing accidental chemical releases. Each recommendation will be or has been considered for implementation. Though not all recommendations may be implemented, the exercise provided all affected employees with a heightened awareness of safety issues related to the covered processes. |