North 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 North Wastewater Treatment Plant (WWTP) falls under this regulation because of the on-site storage of sulfur dioxide. The amount of sulfur dioxide stored is above the threshold limit specified by the USEPA thereby making t he facility subject to compliance with the regulation. The North 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 Management System 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) 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 North WWTP. RELEASE PREVENTION AND EMERGENCY RESPONSE POLICIES The North WWTP facility in McAllen, Texas has an excellent record in preventing and minimizing releases of sulfur dioxide. The emergency response po licies 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 North WWTP uses sulfur dioxide to dechlorinate wastewater before discharge. The North WWTP regularly has a storage maximum quantity of 6 one-ton containers of sulfur dioxide at its facility that is stored outside. This is above the threshold limit (5,000 pounds) set by the USEPA. PROCESS DESCRIPTION The North WWTP is located at 2100 W. Spr ague Road, McAllen Texas. The wastewater is treated at the plant and subsequently discharged, after being dechlorinated using sulfur dioxide. The existing sulfur dioxide feed facility at the North 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, (3) two flow proportionate feed sulfinators, and (4) gas piping and ejectors. On treatment plant grounds, the only area in which sulfur dioxide has a potential to generate a gaseous release is the sulfur dioxide storage area (items 1 and 2 above). Sulfur dioxide is delivered to the North WWTP by truck and sulfur dioxide containers are stored in the outside storage area located to the northeast corner of the plant site. Sulfur dioxide is removed as a gas under pressure of the liquid in the individual containers; however the majority of the sulfur dioxide feed is operated through a water injection induced vacuum, making th e system extremely safe and minimizing potential pressurized leaks along the feed lines. WORST-CASE AND ALTERNATIVE RELEASE SCENARIOS The sulfur dioxide storage and process areas have associated hazards that can potentially affect on-site employees and 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 sulfur dioxide would occur through a valve failure on one of the one ton cylinders located at the plant. This valve failure could potentially release all 2,000 pounds of the sulfur dioxide as a gas. Under Section 68.25(c)(1), the release time for a chemical such as sulfur dioxide 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 North WWTP f acility. The distance to the toxic endpoint of 3 ppm was determined to be 1.61 miles. The estimated affected residential population is 4,000 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 North WWTP. 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 1/5-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 73.3 lb/min. DEGADIS+ was also used to characterize the effects of the alternative case scenario at the North WWTP. The distance to the USEPA defined toxic endpoint of 3 ppm was determined to be 1.40 miles. The estimated affected residential population is 3,000 people. GENERAL AC CIDENTAL RELEASE PRECAUTION PROGRAM The North WWTP carries out consistent operation and maintenance of its sulfur dioxide equipment utilizing only fully trained personnel in this area. North WWTP management enforces consistent operation through discipline for operational deviations. FIVE-YEAR ACCIDENT HISTORY The North WWTP's accident history was reviewed for a period from June, 1994, through June, 1999. During this period of time, no accidental releases of 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 repair to leaking sulfur dioxide 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 sulfur dioxide 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 sulfur dioxide emergency, (2) information about the plant evacuation plan, and (3) a detailed description of the emergency responder'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 sulfur dioxide, a list of action items was developed and is being considered by North WWTP management to determine if implementation is to be accomplished. The most notable planned changes include the following: 7 Ensure that the delivery driver follows a set procedure in the delivery of sulfur dioxide. MW will coordinate with the staff to develop a procedure for how the sulfur dioxide truck drive r 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 satisfaction. 7 The consideration of remote leak detection for sulfur dioxide gas in the sulfinator building, as a warning of vacuum regulator failure, and as a backup sensor. 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 sulfur dioxide cylinders designed in conjunction with the capability to chemically neutralize any accidental releases of sulfur dioxide. This option might be compared with the conversion to a nongaseous sulfur dioxide compound. |