City of Mercedes Sewage Treatment Plant - Executive Summary |
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EXECUTIVE SUMMARY The City of Mercedes Sewage Treatment Plant accidental release prevention policy involves a unified approach that integrates technologies, procedures, and management practices. The Mercedes emergency response policy involves the preparation of a response plan that is tailored to each facility and to the emergency response services available in the community. The policy adheres to EPA Prevention Program procedures where applicable, and is in compliance with EPA Emergency Response Program requirements. The sewage operations include one facility of concern: The chlorination building located at the Sewage Treatment Plant, where the chemical of concern is chlorine. The City of Mercedes Sewage Treatment Plant is located at Mile 8 North and Mile 1/2 East, in Mercedes, Hidalgo County, Texas. The chlorination building at the Sewage Treatment Plant consists of a two walled canopy structure facing south that is located approximately in the middle of the facility. Two 1- ton cylinders of chlorine are stored at the facility. Only one 1-ton container is connected to the system. The 1-ton container is located inside the walled canopy structure. The chlorine container has a manual shutoff valve in case of an emergency and a chlorine alarm will be triggered if a release occurs. Emergency air supply and power are available. The facility is manned 8-hours a day, and operators check the chlorination building daily. Three Self-Contained-Breathing- Apparatus (SCBA) are stored in the office and workroom located approximately 300 yards away from the chlorination building, respectively. The offsite consequence analysis for the Sewage Treatment Plant includes consideration of two different release scenarios for each of the parameters of concern (chlorine): 1) A "Worst Case Release"; and 2) an "Alternative Scenario". The Worst Case Release is defined by EPA guidance which states that "the owner or operator shall assume that the ... maximum quantity in the la rgest vessel... is released as a gas over 10 minutes," due to an unspecified failure. The Alternative Scenario is defined as "more likely to occur than the worst-case release scenario." Atmospheric dispersion modeling has to be performed to determine the distance traveled by a parameter of concern before its concentration decreases to the "toxic endpoint". The toxic endpoints are defined by the EPA as 0.0087 mg/L for chlorine. These values correspond to the Emergency Response Planning Guidelines Level 2 (ERPG-2) concentrations which are defined by the American Industrial Hygiene Association (AIHA) as: " The maximum airborne concentration below which it is believed that nearly all individuals could be exposed for up to one-hour without experiencing or developing irreversible or other serious health effects or symptoms which could impair an individual's ability to take protection action." The estimated population potentially affected by the release is defined as the residential p opulation within a circle with a radius corresponding to the toxic endpoint distance. The Worst Case Release Scenario at the Mercedes Sewage Treatment Plant was defined by the following conditions: Failure of one 1-ton container resulting in the total release of 2,000 pounds of chlorine; Release of the entire amount as a gas in 10 minutes; Use of the one-hour average ERPG-2 as the toxic endpoint; Consideration of the population residing within a full circle with radius corresponding to the toxic endpoint distance; and EPA mandated meteorological conditions, specifically an F atmospheric stability class, wind speed of 1.5 m/sec, and air temperature of 77:F. Atmospheric dispersion modeling for the Worst Case Release scenario resulted in a chlorine toxic endpoint distance of 1.3 miles and an estimated residential population potentially affected of 6,352. The calculations were performed with the CAMEO and RMPComp32 models as provided by USEPA and NOAA. The affecte d population was estimated through Tiger and Landview III information. The Alternative Release Scenario at the Mercedes Sewage Treatment Plant was defined by the following conditions: Failure of valve mounted automatic shutoff valve connected to one 1-ton container resulting in the total release of 2,000 pound of chlorine; Release of the entire amount as a gas in 30 minutes; Use of the one-hour average ERPG-2 as the toxic endpoint; Consideration of the population residing within a full circle with radius corresponding to the toxic endpoint distance; and EPA mandated meteorological conditions, specifically a D atmospheric stability class, wind speed of 3 m/sec, and an air temperature of 77:F. Atmospheric dispersion modeling for the Alternative Release scenario resulted in a chlorine toxic endpoint distance of 0.4 miles and an estimated residential population potentially affected of 2040. The calculations were performed with the CAMEO and RMPComp32 models as provided by USEPA and NOAA. The affected population was estimated through Tiger and Landview III information. The general Mercedes Sewage Treatment Plant accidental release prevention program is based on the following key elements. High level of training of the operators; Preventive maintenance program; Use of state of the art process and safety equipment; Use of accurate and effective operating procedures, written with the participation of the operators; Performance of a hazard review of equipment and procedures; Implementation of an audit and inspection program. Chemical specific prevention steps include availability of Self-Contained-Breathing-Apparatus (SCBA) worn by the operator during connection/disconnection of each chemical supply, awareness of the hazardous and toxic properties of chlorine, and the presence of a chemical detector. No accidental releases of chlorine have occurred in the past five years. The facility has an emergency response program which has been coordinated (re viewed) by the City of Mercedes's Fire and Police Departments. Both of these entities are members of the Hidalgo County Local Emergency Planning Committee (LEPC). The program includes an emergency response decision tree and a notification plan. Emergency response drills and drill evaluations, and emergency operation and response procedures are reviewed annually. |