Prairie 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 Prairie Creek 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 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 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 Prairie Creek WWTP. 
The Prairie Creek WWTP facility in Lewisville, Texas has an excellent record in preventing and minimizing releases of sulfur d 
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  Lewisville Fire Department in the event of an emergency.   In the event of a release, plant staff will contact the Lewisville 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.   
The Prairie Creek WWTP uses sulfur dioxide to dechlorinate the treated wastewater prior to discharge.  The Prairie Creek WWTP regularly has a storage maximum quantity of 12 one-ton containers of sulfur dioxide at its facility that is stored inside.  This is above the threshold limit (5,000 pounds) set by th 
e USEPA.  
The Prairie Creek WWTP is located at 897 Treatment Plant Road, Lewisville 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 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.   
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 Prairie Creek WWTP by truck and sulfur dioxide containers are stored in the dechlorination building in the storage room.  Sulfur dioxide is removed as a gas under pressure of the liquid in the individual containers; ho 
wever the majority of the sulfur dioxide feed is operated through a water injection induced vacuum, making the system extremely safe and minimizing potential pressurized leaks along the feed lines.   
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 Prairie Creek WWTP facility.  The distance to the toxic endpoint of 3 ppm was determined to be 1.37 miles.  The estimated affected residential population is 2,800 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 Prairie Creek 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 40.3 lb/min.  DEGADIS+ was also used to characterize the effects of the alternative case scenario at the Prairie Creek WWTP.  The distance to the USEPA defined to 
xic endpoint of 3 ppm was determined to be 1.03 miles.  The estimated affected residential population is 1,400 people.   
The Prairie Creek WWTP carries out consistent operation and maintenance of its sulfur dioxide equipment utilizing only fully trained personnel in this area.  Prairie Creek WWTP management enforces consistent operation through discipline for operational deviations. 
The Prairie Creek 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.   
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 mak 
e rounds for inspection and monitoring of the plant processes at least every three 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 Lewisville Fire Department has been designated to provide emergency responders and equipment, and will assume Incident Command upon arrival to the plant's emergency call.  
Based on the hazard review and prevention evaluation completed for sulfur dioxide, a list of action items was developed and is being considered by Prairie Creek WWTP management to determine if implementation is to be accomplished.  The most notable planned changes include the following: 
7 Ensure that the d 
elivery 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 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 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 system 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.
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