Prairie Creek Wastewater Treatment Plant - Executive Summary |
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INTRODUCTION The Accidental Release Prevention Risk Management Program rule (40 CFR Part 68) is similar to the Occupational Safety and Health Administration's Process Safety Management Program, which is designed to protect workers from accidental releases of hazardous substances. The Risk Management Program rule addresses over 100 chemical substances, 77 of which are acutely toxic and 63 of which are flammable gases, and the accidental release of these substances. The United States Environmental Protection Agency (USEPA) estimates that over 100,000 sources are covered by the rule, including chemical manufacturers and wholesalers, certain retailers, potable water treatment systems, wastewater treatment plants, ammonia refrigeration systems, and federal facilities. The Prairie Creek Wastewater Treatment Plant (WWTP) falls under this regulation because of the on-site storage of chlorine and sulfur dioxide. The amounts stored for both chemicals are above the threshold limits specified by the USEPA, thereby making the facility subject to compliance with the regulation. The Prairie Creek WWTP personnel have complied with the USEPA Risk Management Program rule and have completed an Accidental Release Prevention Program (ARPP) Plan that contains the following required information: 7 A management system (40 CFR Part 68 Subpart A); 7 A hazard assessment that establishes the worst-case and alternate release scenarios and their impact on the population and the environment (40 CFR Part 68 Subpart B); 7 A prevention program that includes safety information, a hazard review, operating procedures, training, maintenance, compliance audits, and incident investigations. (40 CFR Part 68 Subpart C); and 7 An emergency response plan (40 CFR Part 68 Subpart E). The following subsections discuss details of the plan that has been implemented at the Prairie Creek WWTP. RELEASE PREVENTION AND EMERGENCY RESPONSE POLICIES The Prairie Creek WWTP facility in Lewisville, Texas has an excellent record in preventing and minimizing releases of chlorine and sulfur dioxide. The emergency response policies at this facility ensure that emergency response coverage is available 24 hours per day, 7 days per week. Adequate provisions have been implemented to coordinate response activities with outside agencies, such as the Lewisville Fire Department in the event of an emergency. The plant staff will receive training to enable an off-site response prior to the arrival of the City of Lewisville HAZMAT team. REGULATED SUBSTANCE The Prairie Creek WWTP uses chlorine as a disinfectant in the wastewater treatment process and uses sulfur dioxide to dechlorinate the treated wastewater prior to discharge back into the environment. The Prairie Creek WWTP regularly has a maximum storage quantity of 16 one-ton containers of chlorine at its facility that are stored outside, and has 12 one-ton containers of sulfur dioxide stored inside. These quantities are above the threshold limits set by the USEPA of 2,500 and 5,000 pounds, respectively. PROCESS DESCRIPTION The Prairie Creek WWTP is located at 897 Treatment Plant Road, Lewisville Texas. The wastewater is treated at the plant and, after being dechlorinated using sulfur dioxide, is subsequently discharged. The existing chlorine feed facility at the Prairie Creek WWTP consists of (1) 8 one-ton chlorine containers, (2) two vacuum regulators mounted to an automatic switchover module with gas piping and miscellaneous valves, drip lines, and filters to each, (3) three flow proportionate feed chlorinators, and (4) gas piping and ejectors. A release of chlorine gas could potentially occur at the chlorine storage area (items 1 and 2 above) Chlorine is delivered to the Prairie Creek WWTP by truck and chlorine containers are in the storage area located to the east of the Chemical Building. The storage area consists of a building with three walls and a roof. Chlorine is stored as a pressurized liquid and is removed as a gas from the individual containers. The majority of the chlorine feed is operated through a water injection induced vacuum which makes the system extremely safe and minimizes the potential of pressurized leaks along the feed lines. The existing sulfur dioxide feed facility at the Prairie Creek WWTP consists of (1) 4 one-ton sulfur dioxide containers, (2) two vacuum regulators mounted to an automatic switchover module with gas piping and miscellaneous valves, drip lines, and filters to each, (3) three flow proportionate feed sulfinators, and (4) gas piping and ejectors. A release of sulfur dioxide gas could potentially occur at the chlorine storage area (items 1 and 2 above) Sulfur Dioxide is delivered to the Prairie Creek WWTP by truck and sulfur dioxide containers are placed in the storage room of the dechlorination building. Sulfur dioxide is stored in the containers as a pressurized liquid and is removed as a gas from the containers. The majority of the sulfur dioxide feed is o perated through a water injection induced vacuum, which makes the system extremely safe and minimizes the potential of pressurized leaks along the feed lines. WORST-CASE AND ALTERNATIVE RELEASE SCENARIOS The storage and process areas for chlorine and sulfur dioxide have associated hazards that can potentially affect on-site employees and the off-site population and environment. The USEPA requires that a single worst-case scenario for the facility and one alternate release scenario for each regulated chemical to be reported. Worst-Case Scenario The worst-case release scenario that creates the greatest distance in any direction to an endpoint off-site involves a chlorine release. The largest potential release of chlorine would occur through a valve failure on a one-ton containers located at the plant. This valve failure could potentially release all 2,000 pounds of the chlorine as a gas. Under Section 68.25(c)(1), the release time for a chemical such as chlorine is 10 minutes. Passive mitigation controls were not applicable to the worst-case release at this plant. The EPA-approved modeling program DEGADIS+ was used to characterize the effects of the worst-case scenario at the Prairie Creek WWTP facility. The distance to the toxic endpoint of 3 ppm was determined to be 1.67 miles. The estimated affected residential population is 5,400 people. Commercial, industrial, and residential areas would be affected in the worst-case release scenario. Alternate Scenarios One alternate scenario was modeled for the Prairie Creek WWTP chlorine system. The release was established as a leak that would potentially develop in the packing at the chlorine container valve. A release of chlorine through the 0.2 inch diameter hole in the valve packing would occur for about 10 minutes and is presumed to be observed or detected. The release rate of chlorine caused by leakage through this opening is calculated to be 73.8 pounds per minute. DEGADIS+ was also used to characte rize the effects of the alternative release scenario at the Prairie Creek WWTP. The distance to the USEPA defined toxic endpoint of 3 ppm was determined to be 1.34 miles. The estimated residential population affected is 2,600 people. One alternate scenario was modeled for the Prairie Creek WWTP sulfur dioxide system.. The release was established as a leak that would potentially develop in the packing at the sulfur dioxide container valve. A release of sulfur dioxide through the 0.2 inch diameter hole in the valve packing would occur for about 10 minutes, and is presumed to be observed or detected. The release rate of sulfur dioxide caused by leakage through this opening is calculated to be 40.3 pounds per minute. DEGADIS+ was also used to characterize the effects of the alternative release scenario at the Prairie Creek WWTP. The distance to the USEPA defined toxic endpoint of 3 ppm was determined to be 1.03 miles. The estimated residential population affected is 1,400 people . GENERAL ACCIDENTAL RELEASE PRECAUTION PROGRAM The Prairie Creek WWTP carries out consistent operation and maintenance of its chlorine and sulfur dioxide equipment, utilizing only fully trained personnel in these areas. Prairie Creek WWTP management ensures consistent operation through disciplinary measures for operational deviations. FIVE-YEAR ACCIDENT HISTORY The Prairie Creek WWTP's accident history was reviewed for the period from June, 1994, through June, 1999. During this period of time, no accidental releases of chlorine or sulfur dioxide had occurred. EMERGENCY RESPONSE PROGRAM As mentioned earlier, this facility has developed an Emergency Response Program involving immediate plant evacuation once the City's Fire Department is called to implement response and to repair the chlorine or sulfur dioxide leak. The plant is staffed 24 hours per day, 7 days per week. Plant operators are required to make rounds for inspection and monitoring of the plant processes at least e very three hours. Accordingly, plant staff will detect any releases of chlorine and the Fire Department is trained to respond to this situation. The Emergency Response Plan includes: (1) procedures to follow in the event of a chlorine emergency, (2) information about the plant evacuation plan, and (3) a detailed description of the emergency response training underway. The Lewisville Fire Department has been designated to provide emergency responders and equipment, and will assume Incident Command upon arrival at the plant. PLANNED CHANGES TO IMPROVE SAFETY Based on the hazard reviews and prevention evaluations completed for chlorine and sulfur dioxide, a list of action items was developed and is being considered by Prairie Creek WWTP management to implement, if feasible. The most notable planned changes include the following: 7 Ensure that the delivery driver follows a set procedure in the delivery of chlorine and sulfur dioxide. MW will coordinate with the staff to develop a procedure to govern the activities of the driver while on plant grounds. It is anticipated that the vendor can then be subjected to this procedure at the time of the plant's next bid for chemical purchase. However, the existing vendor will likely accept these requirements voluntarily to ensure customer satisfaction. 7 Provide backup power for the plant systems in case of a power outage. 7 In the future when the plant facilities are expanded or upgraded, consideration should be given to design and construction of a containment building to encompass the chlorine containers. The plant also needs to consider systems designed with the capability to chemically neutralize any accidental releases of chlorine and sulfur dioxide. This option might be compared with the conversion to nongaseous chlorine or sulfur dioxide compounds. |