Amesbury Water Treatment Plant - Executive Summary |
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Executive Summary The Amesbury Water Treatment Plant (WTP) accidental release prevention policy involves a consolidated approach that integrates technologies, procedures, and management practices. All applicable procedures of the U.S. Environmental Protection Agency (EPA) Prevention Program are followed. The WTP emergency response policy involves the preparation of response plans that are tailored to each facility and to the emergency response services available in the community, and meets the EPA Emergency Response Program requirements. The Amesbury WTP, located at Rings Corner, Newton Road, Amesbury, Massachusetts, uses chlorine to obtain proper disinfection for both pre-treatment and post-treatment practices. The WTP presently utilizes chlorine gas for disinfection to meet water quality standards. Ten 150-lb containers of pressurized liquid chlorine are connected to the treatment train, six containers connected to the pre-treatment system and four containers connected to the post-treatment system. The amount of 150-lb chlorine containers in storage peaks at thirty, therefore a maximum of forty 150-lb containers may be at the Amesbury Water Treatment Plant at one time. Any facility which handles, manufactures, uses or stores any of the toxic and flammable substances listed in 40 CFR Section 68.130 above the specified threshold quantities in a process is required to develop and implement a risk management plan. The 1990 Amendments to the Clean Air Act (CAA) require the EPA to issue a rule specifying the type of actions to be taken by facilities to prevent accidental releases of such hazardous chemicals into the atmosphere and reduce their potential impact on the public and the environment. Title 40 of the Code of Federal Regulations, Part 68 addresses the specifications and guidelines for development of risk management programs. If a facility is subject to part 68, it's risk management program must be in compliance no later than June 21, 1999. The C ity has contracted with Camp Dresser & McKee Inc. to assist in the completion of a comprehensive evaluation of the WTP including a risk management plan. The offsite consequence analysis includes consideration of two release scenarios, identified as "worst case release" and "alternative scenario." EPA defines the first scenario, which states that "the owner or operator shall assume that the . . . maximum quantity in the largest 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 the chlorine released before its concentration decreases to the "toxic endpoint" selected by EPA of 3 ppm (chlorine), which is the Emergency Response Planning Guideline Level 2 (ERPG-2). The American Industrial Hygiene Association (AIHA) defines this as the "maximum airborne concentrat ion 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 protective action." The residential population within a circle with a radius corresponding to the toxic endpoint distance has to be defined, "to estimate the population potentially affected." The worst-case release scenario involves the catastrophic failure of a 150-lb chlorine cylinder outside the chlorine storage room. The offsite consequence analysis for this scenario was predefined by the EPA. This analysis assumed that the vessel would completely release all of its contents in ten minutes. The toxic endpoint used was the one-hour average ERPG-2. The population residing within a full circle with a radius corresponding to the toxic endpoint distance was determined. For the worst-case modeling analysis, the EPA-mandated meteorological conditi ons were used. These correspond to Stability F, wind speed of 1.5 m/s, highest daily temperature of 77oF and an average humidity of 50%. From the modeling results, it was concluded that the radius which corresponded to the toxic endpoint distance was 0.8 miles. Within the 0.8 miles, approximately 1,100 people would be affected by this EPA-specified release and the corresponding conditions. The alternative release scenario involves the release of chlorine from a 3/8 inch diameter whip connected to a 150-lb cylinder. In this case, however, the release is inside a building. With the operating pressure of 120 psi, the release rate was calculated as 22.2 lb/min. This rate would release the entire contents of the 150-lb cylinder in approximately 7 minutes. Liquid chlorine released from the ruptured whip would flash immediately to a vapor. As required by EPA, the alternative release scenario was formatted as meteorological conditions of Stability D, wind speed of 3.0 m/s, average air temperature of 50oF, and average humidity of 50 percent. The fact that the release happens in an enclosed building allows enclosures to be an act of passive mitigation. Based on these conditions, the off-site impact extends 0.2 miles. The general WTP accidental release prevention program is based on the following critical elements: High level of training of 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 auditing and inspection program Chemical-specific preventions steps include availability of self-contained breathing apparatus (SCBA), worn by the operators during connection/disconnection of chlorine supply, awareness of the hazardous and toxic properties of chlorine, and presence of chlorine detectors. No accidental releases of c hlorine have occurred at this facility in the past five years. The facility has an emergency response program, which has been coordinated with the Amesbury Fire Department. This program includes an emergency coordination team, and a notification plan. |