Tolleson Wastewater Treatment Plant - Executive Summary |
|
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 Tolleson Wastewater Treatment Plant (WWTP) falls under this regulation because of the on-site storage of chlorine. The amount of chlorine stored is above the threshold limit specified by the USEPA thereby making the facili ty subject to compliance with the regulation. Additionally, in light of potential hazards, the City of Tolleson has completed a risk management plan for its onsite use of methane generated as a natural process in the biodegradation of wastewater treatment solids. The Tolleson 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 D) 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 To lleson WWTP. RELEASE PREVENTION AND EMERGENCY RESPONSE POLICIES The Tolleson WWTP facility in Tolleson, Arizona has an excellent record in preventing and minimizing releases of chlorine and 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 Tolleson Fire Department in the event of an emergency. In the event of a release, plant staff will contact the Tolleson Fire Department, relaying information regarding the release prior to implementing plant evacuation to a location outside the plant entrance, 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 Tolleson WWTP uses chlorine to disinfect treated wastewater. The plant routinely has up to seven one ton container s of chlorine onsite. This is above the threshold limit (2,500 pounds) set by the USEPA. PROCESS DESCRIPTION The Tolleson WWTP receives raw wastewater from a variety of residential and commercial users within the City of Tolleson. The wastewater is treated through a conventional treatment process and then disinfected with chlorine. Plant solids removed in the treatment process are anaerobically digested, producing as a byproduct, methane gas. Chlorine gas is delivered to the site in mutltiple one ton pressurized containers. The chlorine is then removed through a vacuum system, pulling the chlorine gas from the cylinders through the chlorine feeders which regulate the flow to the proportion of wastewater being treated. WORST-CASE AND ALTERNATIVE RELEASE SCENARIOS The chlorine 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 alter nate release scenario is reported for each regulated chemical. Worst-Case Scenario The largest potential release of chlorine would occur through a valve failure on one of the outside one ton containers located on the east side of the plant. This valve failure could potentially release all of the one ton contents 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 Tolleson WWTP facility. The distance to the toxic endpoint of 3 ppm was determined to be 1.7 miles. The estimated affected residential population is 1,900 people. Commercial/industrial areas and residential areas would be affected in the worst-case release scenario. Alternate Scenario Two alternate scenarios were modeled for the Tolleson WWTP. The release was establi shed as a leak that would potentially develop in the valving from the chlorine containers. A release of chlorine through the valve would occur for about 60 minutes, and is presumed to be observed or detected due to the pungent odor resulting from even a very small nuisance leak such as from a pipe joint. The release rate of chlorine caused by leakage through this opening is calculated to be 74 lb/min. DEGADIS+ was also used to characterize the effects of the alternative case scenario at the Tolleson WWTP. The distance to the USEPA defined toxic endpoint of 3 ppm was determined to be 1.4 miles. The estimated affected residential population is 1000 people. GENERAL ACCIDENTAL RELEASE PRECAUTION PROGRAM The Tolleson WWTP carries out consistent operation and maintenance of its chlorine equipment utilizing only fully trained personnel in this area. Tolleson WWTP management enforces consistent operation through discipline for operational deviations. FIVE-YEAR ACCIDENT HISTORY The T olleson WWTP'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 chlorine 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 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 responder's plan for handling such an emergency. The Tolleson Count y 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 Tolleson 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 chlorine. 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 satisfaction.. 7 The consideration of the need for an automatic kill switches on the chlorine system. This would be useful in the case of an emergency when the containers have to be shut off immediately. A remote switch would eliminate the danger of plant personnel having to shut the chlorine off manually. 7 The consideration of installing gas detection equipment to detect chlorine gas in the area of outside storage of one ton containers 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 storage tank designed in conjunction with the capability to chemically neutralize any accidental releases of chlorine. This option might be compared with the conversion to a nongaseous disinfection alternative. |