City of Attleboro Wastewater Treatment Facility - Executive Summary
Section 1 |
Risk Management Plan Elements
1.1 Executive Summary
The City of Attleboro Wastewater Treatment Plant (WWTP) 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 WWTP 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 City of Attleboro WWTP, located at 106 Pond Street, Attleboro, Massachusetts, is required to provide disinfection of the effluent water before it's discharge to the Ten Mile River. These requirements are outlined in the permit (NPDES) issued to the City by the U.S. Environmental
Protection Agency. The City presently utilizes chlorine gas for disinfection to meet the minimum standards of the permit. Four one-ton containers of pressurized liquid chlorine are connected to the disinfection system. Additional four cylinders of chlorine is stored in one-ton containers in the same building not manifolded to the chlorination system.
The City has contracted with Camp Dresser & McKee Inc. to perform a comprehensive evaluation of the WWTP including the existing disinfection process. Final recommendations will be for the City to eliminate the use of chlorine gas and change the system to sodium hypochlorite. This recommendation will be made because of the hazards involved with the use of chlorine gas.
The WWTFP evaluation is presently being conducted and is expected to be completed in August 1999. The anticipated schedule is for design of the recommended improvements to begin in January 2000 and construction to start in January 2001. Disinfection by sodium hy
pochlorite could be in operation at the Attleboro WWTF by January 2002.
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 concentration below which it is believed that nearly all individuals could be exposed for u
p 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 a failure of a one-ton storage tank containing either 2,000 pounds of chlorine. The offsite consequence analysis for these tanks was performed for two sets of conditions. The first set of conditions 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 conditions 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 3.0 miles. Within the 3.0 miles, approximately 79,000 people would be affected by this EPA-specified release and the corresponding conditions.
The alternative release scenario involves the release of a one-ton cylinder of chlorine. In this case, however, the release is inside a building. The release is assumed to be over a ten-minute time period. Liquid chlorine released from the ruptured whip would flash immediately to a vapor. The meteorological conditions used were Stability D, wind speed 3.0 m/s, average air temperature of 50oF, and 50 percent average humidity. Based on these conditions, the off-site impact extends 0.6 miles. The chlorine detector are the active mitigation measure considered.
The general WWTP 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 and sulfur dioxide supply, awareness of the hazardous and toxic properties of chlorine, and presence of chlorine detectors.
No accidental releases of chlorine have occurred at this facility in the past five years.
The facility has an emergency response program, which has been coordinated with the Attleboro Fire Department. This program includes an emergency coordination team, and a notification plan.