Valley 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 Valley Creek WWTP. It is JCESD's policy to adhere to all applicable federal, state, and local laws applicable to the Valley Creek WWTP. Although not required by regulation, the JCESD has installed a chlorine/sulfur dioxide scrubber to reduce the risk to the surrounding community, personnel, and environment at the Valley Creek WWTP in the event of a release. Description of Facility and the Regulate d Substance Handled Valley 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 that can be stored at the facility is 60,000 pounds (30 one-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 Valley Creek WWTP. The maximum quantity of sulfur dioxide that can be stored at the facility is 40,000 pounds (20 one-ton containers). The dechlorination system consists of vacuum regulators, evaporator, automatic switchover sy stem, sulfur dioxide gas dispensers, sulfur dioxide leak detectors, alarms, a chlorine/sulfur dioxide scrubber, associated piping, valves, and other miscellaneous equipment. The Valley Creek WWTP has a 54-foot-diameter digester gas storage sphere; two new floating roof primary digesters of 90-foot diameter; two old fixed roof primary digesters with a diameter of 90 ft; and one old fixed roof secondary digester with a diameter of 90 ft. The total volume of digester gas that can be stored in the storage sphere and all digesters is estimated at 225,936 cubic feet (ft3), and the corresponding weight is estimated at 30,100 pounds. The digester gas generated by the facility is used as a fuel for engines that produce hot compressed air that is used for sludge mixing in the digesters. Water is pumped in a loop through the engines to capture the heat generated by the engine. Some of the hot water is used to heat the sludge in the digester and to heat the building. The digester gas system consists of hot water supply and return headers, gas engine blowers, digester gas purifier, 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 c ircle is estimated to be 7,800 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 21,444 pounds. No passive mitigation 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 0.18 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 90 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 meters per second 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 100 persons, based on 1990 Census Data. The sulfur dioxide ARS is the release of liquid sulfur dioxide from a <-inch-diameter pipe connecting the sulfur dioxide 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 1.10 kg/s. On the basis of a 99 percent scrubber efficiency, active mitigation by the scrubber reduces the sulfur dioxide release rate to 0.011 kg/s. Under realistic weather conditions of 3.0 meters per second wind speed and a wind stability class of "D," the DEGADIS 2.1 estimated sulfur dioxide endpoint distance is 0.21 mile. The total population within the alternative release toxic endpoint circle is estimated to be 80 persons, based on 1990 Census data. The digester gas ARS is the release of all gas from a secondary digester and associated piping because of the failure of the pressure relief valve. The estimated quantity of digester gas released is 3,489 pounds in a 5-minute duration. For a release rate of 698 pounds per minute, the flammable endpoint distance to the LFL 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 Chemica l-specific Prevention Steps The Valley 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 Valley 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 Valley 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 Sa fety It was observed during the hazard review that the Valley 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 were made for improving the safety of the digester gas system: 1. Provide and document annual refresher training regarding digester gas system SOPs. 2. Increase the visual inspection frequency of the digester gas burner pilot. 3. Review and update the preventive maintenance and preventive maintenance schedule for digester gas system. |