City of Longmont Wastewater Treatment Plant - Executive Summary |
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Section 1 Risk Management Plan Elements 1.1 Executive Summary The City of Longmont 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 Longmont WWTP, located at 501 East 1st Avenue, Longmont, Colorado, disinfects wastewater by adding chlorine and sulfur dioxide. Nine one-ton cylinders of pressurized liquid chlorine and four one-ton cylinders of sulfur dioxide are stored in the Chlorine/Sulfur Dioxide Building. Only six cylinders of chlorine and two cylinders of sulfur dioxide are connected to the disinf ection system at any time. The offsite consequence analysis includes consideration of two release scenarios, identified as Aworst case release, and "alternative scenario." EPA defines the first scenario, which states that Athe 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 or sulfur dioxide released before its concentration decreases to the "toxic endpoint" selected by EPA as 3 ppm for both chlorine and sulfur dioxide, 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 or 2,000 pounds of sulfur dioxide. 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 meteorolog ical conditions were used. These correspond to Stability F, wind speed of 1.5 m/s, highest daily temperature of 99oF and an average humidity of 57%. From the modeling results, it was concluded that a one-ton release of either chlorine or sulfur dioxide would generate a similar impact area. For a one-ton release of chlorine or sulfur dioxide, the radius which corresponded to the ERPG-2 of 3 ppm was 3 miles. Within the 3 miles, approximately 41,000 people could be affected by this EPA-specified release and the corresponding conditions. More realistically, the number of people affected by an accidental release can be estimated by creating a "footprint" based on the data obtained from the model runs. Only the population within this footprint extending downwind of the release point is potentially affected. Within this plume, approximately only 400 people would be affected. The alternative release scenario involves the release of a one-ton cylinder of chlorine or sulfur dioxide. In this case, however, the release is inside a building and the emergency scrubber is activated. The scrubber is designed to have a maximum emission concentration of 5 parts per million (ppm) of chlorine or 20 ppm of sulfur dioxide. Since the scrubber is 10.5 feet tall, this was assumed to be the release height. The surface area of the release was assumed to be the stack on the scrubber. The release is assumed to be over a ten-minute time period. Toxic endpoint distances to ERPG-2 levels for chlorine and sulfur dioxide were assumed. The meteorological conditions used were Stability D, wind speed 3.0 m/s, average air temperature of 50oF, and 57% average humidity. Based on these conditions, there are no off-site impacts. The emergency scrubber and chlorine detector are the active mitigation measures considered. The general WWTP accidental release prevention program is based on the following critical elements: # High level of training of operators # Preventive maintenance progra m # 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 prevention 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 sulfur dioxide, and presence of chlorine detectors. No accidental releases of chlorine or sulfur dioxide have occurred at this facility in the past five years. The facility has an emergency response program, which has been coordinated with (reviewed by) the Mountain View Fire Protection District. |