City of Columbus Wastewater Treatment Facility - Executive Summary |
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Accidental Release and Emergency Response Policies The City of Columbus WWTF has operated their disinfection and dechlorination processes without incident within the last five years. This successful operating record is due to the City's commitment to ongoing training programs, operating procedures, and computer-generated maintenance activities. Stationary Source and the Substance Handled The City of Columbus operates a 4.5-MGD (by design) municipal wastewater treatment facility at 865 14th Avenue South in Columbus, Nebraska. Discharge from the treatment facility enters the Loup River. The facility uses chlorine to reduce the pathogenic (disease causing) bacteria and viruses in the wastewater discharge to levels below those which can be achieved through ordinary secondary treatment. The use of chlorine reduces the threat to the quality of recreational waters. The facility uses sulfur dioxide to remove residual chlorine remaining after the disinfection process and prior to d ischarging to the receiving waters. This is done because it has been shown that residual chlorine is harmful to some species of fish and carcinogenic to other wildlife. Chlorine and sulfur dioxide utilized in the disinfection and dechlorination processes arrive at the facility in one ton containers that are stored inside a ventilated building equipped with leak detection devices. The maximum quantities of chorine and sulfur dioxide stored at the facility are 10,000 pounds and 8,000 pounds, respectively. Off-Site Consequence Analysis One worst-case release scenario and one alternative release scenario was assessed for the chlorine and sulfur dioxide processes. To ensure a common basis for comparisons, EPA defined the worst-case scenario as the release of the largest quantity of a regulated substance from a single vessel or process line that results in the greatest distance to an endpoint. The worst-case scenario allows consideration of passive mitigation measures. The worst-case release scenarios for the chlorine and sulfur dioxide processes were determined in accordance with the requirements provided in 40 CFR 68.22 and 40 CFR 68.25(b,c). The alternative release rate is a release that is considered more reasonable or "more likely." The alternative release scenarios for the chlorine and sulfur dioxide processes were determined in accordance with the guidelines in 40 CFR 68.22 and 68.28. Active mitigation measures, as well as passive mitigation measures, can be used in determining the alternative release scenario. According to EPA, the toxic endpoint for chlorine and sulfur dioxide have been defined to be 0.0087 mg/L (3 ppm) 0.0078 mg/L (3 ppm), respectively. An airborne concentration that is used as the toxic endpoints is the maximum airborne concentrations below which it is believed that nearly all individuals can be exposed for up to one hour without experiencing or developing irreversible or other serious health effects. The distance to the toxic end point becomes a radius for a circle around the covered process. Residential population within the circle is required to be determined based on available census information. The population number is reported as part of the EPA submittal. This method greatly overestimates the population potentially exposed to a chlorine or sulfur dioxide, because the chlorine or sulfur dioxide plume would seek lowest elevations in the direction of the predominant winds. Since wind direction cannot be anticipated for an accidental release, EPA mandated the circle estimation method. Release scenarios and distances to toxic endpoints are discussed in more detail in the subsequent paragraphs. Chlorine Worst-Case Release Scenario For the chlorine process at the Columbus WWTF, the worst-case release results from a one-ton chlorine cylinder (maximum amount held in a single vessel) rupturing when unloaded from the delivery truck and its contents escaping to the atmosphere in 10 minutes. In the event the cylinder would release its entire contents to the atmosphere in 10 minutes, the release rate would be 200 pounds per minute. The distance to the toxic endpoint worst-case release distance, 1.3 miles, was taken from Exhibit 4-3 of the EPA's Risk Management Program Guidance for Wastewater Treatment Plants, for an urban setting. The residential population within the 1.3-mile radius of influence was determined to be approximately 2600 people. The population was calculated by utilizing the U.S. Department of Commerce software package LandViewTM III, which sums 1990 population information from the Census Bureau for areas within the 1.3-mile radius impact zone. Alternative Release Scenario The alternative release scenario, or scenario more likely to result in a release, involves a chlorine storage cylinder or other equipment such as ladders or tools dropping on or hitting the flexible connector used to release chlorine gas into a small stream of treated wastewater effluent. The flexib le connector ruptures and the pressure regulators at both the point of injection and the storage cylinder shut down the flow of chlorine gas. This results in a release of less than 1 pound of chlorine gas for 0.1 minutes at a rate of 15 pounds per minute assuming a 5/16-inch hole (Exhibit 4-15 of the EPA's Risk Management Program Guidance for Wastewater Treatment Plants). The distance to the toxic endpoint alternative release distance, 0.10 miles was taken from Exhibit 4-15 of the EPA's Risk Management Program Guidance for Wastewater Treatment Plants, for an urban setting. The residential population within the 0.10-mile radius of influence was determined to be 0 people. The population was calculated by utilizing the U.S. Department of Commerce software package LandViewTM III, which sums 1990 population information from the Census Bureau for areas within the 0.10-mile radius impact zone. Sulfur Dioxide Worst-Case Release Scenario For the sulfur dioxide process at the Columbus WW TF, the worst-case release results from a one-ton sulfur dioxide cylinder (maximum amount held in a single vessel) rupturing when unloaded from the delivery truck and its contents escaping to the atmosphere in 10 minutes. In the event the cylinder would release its entire contents to the atmosphere in 10 minutes, the release rate would be 200 pounds per minute. The distance to the toxic endpoint worst-case release distance, 1.3 miles, was taken from Exhibit 4-5 of the EPA's Risk Management Program Guidance for Wastewater Treatment Plants, for an urban setting. The residential population within the 1.3-mile radius of influence was determined to be approximately 2600 people. The population was calculated by utilizing the U.S. Department of Commerce software package LandViewTM III, which sums 1990 population information from the Census Bureau for areas within the 1.3-mile radius impact zone. Alternative Release Scenario The alternative release scenario, or scenario more likely to r esult in a release, involves a sulfur dioxide storage cylinder or other equipment such as ladders or tools dropping on or hitting the flexible connector used to release sulfur dioxide into a small stream of treated wastewater effluent. The flexible connector ruptures and the pressure regulators at both the point of injection and the storage cylinder shut down the flow of sulfur dioxide gas. This results in a release of less than 1 pound of sulfur dioxide for 0.10 minutes and a release rate of 7 pounds per minute assuming a 5/16-inch opening (Exhibit 4-17 of the EPA's Risk Management Program Guidance for Wastewater Treatment Plants) The distance to the toxic endpoint alternative release distance, 0.10 miles was taken from Exhibit 4-17 of the EPA's Risk Management Program Guidance for Wastewater Treatment Plants, for an urban setting. The residential population within the 0.10-mile radius of influence was determined to be 0 people. The population was calculated by utilizing the U.S. Department of Commerce software package LandViewTM III, which sums 1990 population information from the Census Bureau for areas within the 0.10-mile radius impact zone. Accidental Release Prevention Program The City of Columbus has developed a Prevention Program in accordance with the requirements of 40 CFR 68 for Program Level 2 Facilities. The City is implementing procedures that require frequent evaluation and updating of their safety and prevention procedures as needed. The City conducted a process hazard review for the processes determined to be "covered", the chlorine and sulfur dioxide processes, as part of the development of the Risk Management Program Plan. Five-Year Accident History No accidents, as described under 40 CFR 68.42(a), have occurred at the Columbus WWTF in the previous five years. Emergency Response Program The emergency response program established by the City of Columbus for their WWTF does not require personnel to respond to accidental releases of re gulated substances. The local emergency response agencies will be relied upon for coordinating all response actions resulting from the release and for informing the public about the release. The facility has developed an Emergency Action Plan outlining actions taken in the event of an accidental release of regulated substances as well as in the event of natural disasters. Safety Improvements The facility is currently in the process of evaluating the recommendations from the process hazard review completed in April 1999. Improvements made in response to the recommendations will be documented. |