Village Creek Wastewater Treatment Plant - Executive Summary |
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Policy Statement It is the Jefferson County Environmental Services Department's (JCESD) policy to operate safe wastewater treatment plants (WWTPs), reducing to the greatest extent possible any hazards associated with the necessary treatment processes and reducing any subsequent risk to the surrounding community, personnel, and environment. The policy includes working with the surrounding community and local emergency response agencies to promote a spirit of cooperation and teamwork, to orchestrate an effective contingency plan in the unlikely event of a process incident occurring at the Village Creek WWTP. It is JCESD's policy to adhere to all applicable federal, state, and local laws applicable to the Village Creek WWTP. Although not required by regulation, the JCESD has installed a chlorine/sulfur dioxide scrubber to reduce risk to the surrounding community, personnel, and environment at the Village Creek WWTP in the event of a release. Description of Facility and the Regulated Substance Handled The Village Creek WWTP is a publicly owned and operated wastewater treatment plant. Chlorine and sulfur dioxide are used primarily for disinfection and dechlorination, respectively. Other possible uses of chlorine are for odor control and to assist in settling biosolids in the clarification process. The maximum quantity of chlorine stored at the facility is 20,000 pounds (ten 1-ton containers). The chlorination system consists of vacuum regulators, evaporator, automatic switchover system, chlorine gas dispensers, chlorine leak detectors, alarms, a chlorine/sulfur dioxide scrubber, associated piping, valves, and other miscellaneous equipment. Sulfur dioxide is used for dechlorinating the effluent before it is discharged into the Village Creek. The maximum quantity of sulfur dioxide stored at the facility is 12,000 pounds (six 1-ton containers). The dechlorination system consists of vacuum regulators, automatic switchover system, sulfur dioxide gas dispensers, s ulfur dioxide leak detectors, alarms, a chlorine/sulfur dioxide scrubber, associated piping, valves, and other miscellaneous equipment. The Village Creek WWTP has a 50-foot-diameter digester gas storage sphere; three fixed roof primary digesters of 100-foot diameter, and three floating roof secondary digester of 100-foot diameter. The total volume of digester gas that can be stored in the storage sphere and all digesters is estimated at 324,981 ft3, and the corresponding weight is estimated at 33,315 pounds. The digester gas generated by the facility is combusted in boilers to produce hot water for use in heating the sludge in the digesters. The digester gas system consists of water headers, boilers, digester gas scrubber, recirculation compressors, digester gas sediment and drip traps, combination pressure/vacuum relief valve and flame arresters, flame traps, associated piping, valves, and other miscellaneous equipment. Worst-case and Alternative Release Scenarios The worst-case scenario for toxics is the failure of one liquid sulfur dioxide container resulting in a release of 2,000 pounds of sulfur dioxide in 10 minutes. Because the containers are stored in an enclosed building, the actual release rate is 55 percent of the total sulfur dioxide released due to passive mitigation . The release rate of 0.83 kg/s is modeled using DEGADIS 2.1. Under the worst-case weather conditions prescribed by the RMP Rule, the distance to a toxic endpoint of 3 ppm is 1.68 miles, beyond which there will be enough dispersion to reduce the concentration below the toxic endpoint. The total population within the toxic endpoint circle is estimated to be 15,800 persons, based on 1990 Census data. The worst-case scenario for flammable digester gas is the release and subsequent vapor cloud expansion of the contents of the gas storage sphere (single largest vessel) and associated piping. The estimated worst-case release quantity of digester gas is 17,832 pounds. No passive mitig ation is considered because the storage sphere is not enclosed. On the basis of the EPA's Offsite Consequence Analysis Guidance, the distance to the flammable endpoint distance of 1.0 psi overpressure is estimated to be 0.17 mile, beyond which there will be no overpressure to cause damage to buildings and population. The total population within the flammable endpoint circle is estimated to be 160 persons, based on 1990 Census Data. The chlorine alternative release scenario (ARS) is the release of liquid chlorine from a <-inch-diameter pipe connecting the chlorine container to the feed manifold. This scenario is more likely to happen when the evaporator in the chlorination system is in service. For a release duration of 22 minutes, the release rate is 1.56 kg/s. On the basis of a 99 percent scrubber efficiency, active mitigation by the scrubber reduces the chlorine release rate to 0.0156 kg/s. Under realistic weather conditions of 3.0 m/s wind speed and a wind stability class of "D," the DEGADIS 2.1 estimated chlorine endpoint distance is 0.24 mile. The total population within the alternative release toxic endpoint circle is estimated to be 110 persons, based on 1990 Census data. The sulfur dioxide ARS is the release of sulfur dioxide gas from a <-inch-diameter pipe connecting the sulfur dioxide container to the feed manifold. For a release duration of 22 minutes, the release rate is 0.020 kg/s. On the basis of a 99 percent scrubber efficiency, active mitigation by the scrubber reduces the sulfur dioxide release rate to 0.002 kg/s. Under realistic weather conditions of 3.0 m/s wind speed and a wind stability class of "D," the DEGADIS 2.1 estimated sulfur dioxide endpoint distance is 0.058 mile. The total population within the alternative release toxic endpoint circle is estimated to be 10 persons based on 1990 Census data. The digester gas ARS is the release of all gas from a secondary digester and associated piping, due to failure of the pressure relie f valve. The estimated quantity of digester gas released is 4,478 pounds in a 5-minute duration. For a release rate of 895.6 pounds per minute, the flammable endpoint distance to 1.0 psi overpressure is estimated to be 0.07 mile. The estimation of the endpoint distance is based on EPA's Offsite Consequence Analysis Guidance. The total population within the alternative release flammable endpoint circle is estimated to be 10 persons, based on 1990 Census data. General Accidental Release Prevention Program and Chemical-specific Prevention Steps The Village Creek WWTP is in compliance with the Risk Management Program (RMP) Rule, 40 CFR Part 68. The facility has conducted a comprehensive review of all systems, as well as administrative, technical, and operating and maintenance procedures, in addition to the other required program elements of the RMP Rule. A hazard review was conducted at this facility using a "What-If" analysis. Five-year Accident History The Village Creek WWTP has never had a release of chlorine, sulfur dioxide, or digester gas resulting in injuries, deaths, property or environmental damage, evacuations, or shelterings in place. Emergency Response Program The JCESD has an Emergency Response Plan for the Village Creek WWTP, which coordinates response efforts with the fire department's HAZMAT team, the police department, and the hospital. Response activities also have been coordinated with the Jefferson County Local Emergency Planning Committee (LEPC). Planned Changes to Improve Safety It was observed during the hazard review that the Village Creek WWTP has the necessary equipment and the operating and training procedures required for the safe operation of the chlorination, dechlorination, and digester gas systems. However, the following recommendations for improving the safety of the digester gas system were made: 1. Provide and document annual refresher training on digester gas system standard operating procedures. 2. Increase the visual ins pection frequency of digester gas burner pilot. 3. Review and update preventive maintenance and the preventive maintenance schedule for the digester gas system. |