Southwest Water 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 Southwest Water Treatment Plant (WTP) falls under this regulation because of the on-site storage of ammonia. The amount of ammonia stored is above the threshold limit specified by the USEPA thereby making the facility subj ect to compliance with the regulation. The Southwest WTP 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: ? Management System ? 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). ? 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) ? An emergency response plan (40 CFR Part 68 Subpart E) The following subsections discuss details of the plan that has been implemented at the Southwest WTP. RELEASE PREVENTION AND EMERGENCY RESPONSE POLICIES The Southwest WTP facility in McAllen, Texas has an excellent record in preventing and minimizing releases of ammonia. The emergency response policies at this facility ensure that there is emergency response coverage 24 hours per day, 7 days per week. There are also adequate provisions for coordination with outside agencies, such as the McAllen Fire Department in the event of an emergency. In the event of a release, plant staff will contact the McAllen Fire Department, relaying information regarding the release prior to implementing plant evacuation to a location outside the plant gates, awaiting the arrival of the fire department to inform its personnel of all information about the release and to provide assurance that the plant has been totally evacuated. REGULATED SUBSTANCE The Southwest WTP uses ammonia as a component of a chloramine disinfectant in the water treatment process. The Southwest WTP regularly has a storage maximum quantity of 46,350 pounds of ammonia at its facility that is stored in an outside stationary tank. This is above the threshold limit (10,000 pounds) set by the USEPA. PROCESS DESCRIPTION The Southwest WTP receives raw water from the Rio Grande River as a raw water supply which is pumped into the treatment plant located at 2501 W. Expressway 83, McAllen Texas. Water is treated at the plant and subsequently pumped to the distribution network of pipelines, pump stations, and storage tanks. The existing ammonia feed facility at the Southwest WTP consists of (1) a 1,000 gallon and a 8,000 gallon stationary storage tank, (2) three flow proportionate feed ammoniators at the expansion plant and 2 feeders for the main plant, and (3) gas piping and ejectors. On treatment plant grounds, ammonia has a potential to generate a gaseous release at all three sections. Ammonia is delivered to the Southwest WTP by truck and the stationary tanks are refilled. Ammonia is removed as a gas under pressure of the liquid in the stationary tanks. WORST-CASE AND ALTERNATIVE RELEASE SCENARIOS The ammonia storage and process areas have associated hazards that can potentially affect on-site employees an d the off-site population and environment. The U.S. EPA requires that one worst-case scenario and one alternate release scenario is reported for each regulated chemical. Worst-Case Scenario The largest potential release of ammonia would occur through a valve failure on one of the stationary tanks located at the plant. This valve failure could potentially release all 41,000 pounds of the ammonia as a gas from the 8,000 gallon tank. Under Section 68.25(c)(1), the release time for a chemical such as ammonia 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 Southwest WTP facility. The distance to the toxic endpoint of 200 ppm was determined to be 1.35 miles. The estimated affected residential population is 14,000 people. Commercial/industrial areas and residential areas would be affected in the worst-case release scenario. Alternate Scenario One alternate scenario was modeled for the Southwest WTP. The release was established as a leak that would potentially develop in the gaseous piping between the tank and the ammoniator on a 1-inch line. A release of ammonia through a 1/5-inch diameter hole would occur for about 10 minutes, and is presumed to be observed or detected. The release rate of ammonia caused by leakage through this opening is calculated to be 1,351 lb/min. DEGADIS+ was also used to characterize the effects of the alternative case scenario at the Southwest WTP. The distance to the USEPA defined toxic endpoint of 200 ppm was determined to be 0.63 miles. The estimated affected residential population is 4,000 people. GENERAL ACCIDENTAL RELEASE PRECAUTION PROGRAM The Southwest WTP carries out consistent operation and maintenance of its ammonia equipment utilizing only fully trained personnel in this area. Southwest WTP management enforces consistent operation through discip line for operational deviations. FIVE-YEAR ACCIDENT HISTORY The Southwest WTP's accident history was reviewed for a period from June, 1994, through June, 1999. During this period of time, no accidental releases of ammonia 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 repair to leaking ammonia gas. The plant is staffed 24 hours per day and 7 days per week. Plant operators are required to make rounds for inspection and monitoring of the plant processes at least every two hours. Accordingly, plant staff will detect any releases of ammonia 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 ammonia emergency, (2) information about the plant evacuation plan, and (3) a detailed description of the emergency respon der's plan for handling such an emergency. The McAllen Fire Department has been designated to provide emergency responders and equipment, and will assume Incident Command upon arrival to the plant's emergency call. PLANNED CHANGES TO IMPROVE SAFETY Based on the hazard review and prevention evaluation completed for ammonia, a list of action items was developed and is being considered by Southwest WTP management to determine if implementation is to be accomplished. The most notable planned changes include the following: ? Ensure that the delivery driver follows a set procedure in the delivery of ammonia. MW will coordinate with the staff to develop a procedure for how the ammonia truck driver or vendor should perform 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 satisfactio n.. ? The consideration of remote leak detection with an audible and visual alarm for ammonia gas in the area of the storage tanks of ammonia and along the ammonia feed path. ? Need to make certain that SCBA's are located so that plant personnel can get to the packs quickly in case of an ammonia release. ? 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 ammonia tanks designed in conjunction with the capability to chemically neutralize any accidental releases of ammonia. This option might be compared with the conversion to a nongaseous ammonia compound. |