Zacate Creek Wastewater Treatment Plant - Executive Summary

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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 Zacate Creek Wastewater Treatment Plant (WWTP) falls under this regulation because of the onsite storage of chlorine.  The amount of chlorine stored is well above the threshold limit specified by the USEPA, thereby makin 
g the facility subject to compliance with the regulation.  The Zacate 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 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 Zacate Creek WWTP. 
The Zacate Creek WWTP facility in Laredo, Texas has an excellent record in preventing and minimizing releases of chlorine.  This faci 
lity will be implementing a program for onsite emergency responders. 
The emergency response policies at this facility ensure that there will be emergency response coverage 24 hours per day, 7 days per week.  Adequate provisions have been implemented to coordinate activities with outside agencies, such as the Laredo Fire Department HAZMAT team, in the event of an emergency.   In the event of a release, plant staff will contact the Laredo Fire Department.  The staff will receive training to enable an onsite staff response prior to the arrival of the Laredo Fire Department HAZMAT team.  
The Zacate Creek WWTP uses chlorine as a disinfectant in the wastewater treatment process.  The Zacate Creek WWTP regularly maintains eight (8) 1-ton chlorine containers located onsite.  This amount translates into a quantity that exceeds the threshold limit of 2,500 pounds set by the USEPA.  
The Zacate Creek WWTP is located on the banks of the Rio Grande River in La 
redo, Texas.  Dual trickling filters and a contact stabilization process are used to treat wastewater.   
The existing chlorine facilities at the Zacate Creek WWTP consist of a storage area and process area.  One-ton chlorine containers are housed at the storage area.  The process area is comprised of chlorine feed equipment, including chlorinators and rotameters as well as piping, miscellaneous valves, and vacuum regulators.  On treatment plant grounds, the potential to generate a gaseous release of chlorine exists at both the storage area as well as the process area.  
The chlorine storage and process areas have associated hazards that can potentially affect onsite employees and the off-site population and environment.  The U.S. EPA requires that one worst-case scenario and one alternate release scenario be reported for each regulated chemical.  
Worst-Case Scenario 
The largest potential release of chlorine would occur with the rupture of a o 
ne-ton chlorine container, releasing all 2,000 pounds of 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 of a chlorine release at the Zacate Creek WWTP.  The distance to the toxic endpoint of 3 ppm was determined to be 1.43 miles.  The estimated affected residential population is 42,000 people.  Both commercial/industrial areas and residential areas would be affected in the worst-case release scenario.  
Alternate Scenario 
One alternate scenario was modeled for the Zacate Creek WWTP.  The release was established as a disconnected <-inch pigtail.  Given that maintenance rounds are conducted every two hours and chlorine alarms are present nearby, a release time of 10 minutes was used.  The distance to the USEPA defined toxic endpoint 
of 3 ppm was determined to be 0.97 miles.  The estimated affected residential population is 21,000 people.  
The Zacate Creek WWTP carries out consistent operation and maintenance of its chlorine equipment utilizing only fully trained personnel.  The management enforces consistent operation through discipline for operational deviations.  
The accident history for the Zacate Creek WWTP was reviewed for a period from June 1994 through June 1999.  During this period of time, no accidental releases of chlorine had occurred.   
As mentioned earlier, this facility has developed an Emergency Response Program in which plant employees will be trained in the initial response to a release.  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.  Once a leak is detected, the  
Laredo Fire Department will be contacted.   
The Emergency Response Plan includes: (1) procedures to follow in the event of a chlorine emergency, (2) information about the frequency of employee emergency response training, and (3) a detailed description of the emergency response training to be implemented. 
The Laredo Fire Department has been designated to provide back-up emergency responders and equipment, and will assume the role of Incident Onsite Command upon arrival at the plant.  
Based on the hazard review and prevention evaluation completed for chlorine, a list of action items was developed and is being considered by Zacate Creek WWTP.  The most notable planned changes include the following:  
7 Ensure that the delivery driver follows a set procedure in the delivery of chlorine. The staff will develop a procedure for the chlorine truck driver or vendor to follow while on plant grounds.  It is anticipated that the vendor can then be subjected to th 
is 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 Consider installing SCBAs closer to the chlorine areas.   
7 Consider the need for an automatic kill switch.  This switch would be useful in the case of an emergency when the chlorine supply has to be shut off immediately.  A remote switch would eliminate the danger of plant personnel having to shutdown the chlorine system.  Emergency shutdown procedures should be developed and implemented in the training sessions. 
7 Consider installing chlorine gas detection equipment near the storage area.  An audible and visual alarm should send a signal to a manned facility. 
7 When the plant facilities are expanded or upgraded in the future, consider the design and construction of a containment building to isolate the storage area from the process area.  Additional consideration should be given to the capability to  
chemically neutralize any accidental releases of chlorine.
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