Southwest Water Treatment Plant - Executive Summary

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INTRODUCTION 
The United States Environmental Protection Agency (USEPA) finalized the Accidental Release Prevention Program (ARPP) under Section 112(r) of the 1990 Clean Air Act Amendments and subsequently finalized the Accidental Release Prevention Risk Management Program rule (40 CFR Part 68).  This rule established the criteria of the Risk Management Plans (RMP's) required under the ARPP.  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 sy 
stems, 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 chlorine and ammonia.  The amounts stored for both chemicals exceed the threshold limits specified by the USEPA, thereby making the facility subject to compliance with the regulation.  The Southwest WTP personnel have complied with the USEPA Risk Management Program rule and have completed a RMP that contains the following required information: 
7 A management system (40 CFR Part 68 Subpart A); 
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); and 
7 An emergency response plan (40 CFR Part 
68 Subpart E). 
The following subsections discuss details of the plan that have 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 chlorine and ammonia.  The emergency response policies at this facility ensure that emergency response coverage is available 24 hours per day, 7 days per week.  Adequate provisions have been implemented to coordinate response activities with outside agencies, such as the  McAllen Fire Department in the event of an emergency.  The plant staff will receive training to enable an onsite staff response prior to the arrival of the City of McAllen HAZMAT team.  The City of McAllen Fire Department currently has contracted with Alamo Environmental to supply HAZMAT response services.  
 
REGULATED SUBSTANCE 
The Southwest WTP uses chlorine combined with ammonia to disinfect water.  The Southwest WTP regularly has a maximum  
storage quantity of 42 one-ton containers of chlorine at its facility that is stored in an outside storage area.  This is above the threshold limit of 2,500 pounds set by the USEPA. In addition to the chlorine stored onsite, the Southwest WTP regularly has a maximum storage quantity of 46,350 pounds of ammonia at its facility that is stored in two outside stationary tanks.  This is above the threshold limit of 10,000 pounds set by the USEPA.  
 
PROCESS DESCRIPTION 
The Southwest WTP receives water from the Rio Grande River as a raw water supply.  The treatment plant is 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 chlorine feed facility at the Southwest WTP consists of two process trains.  The first consists of up to 36 one-ton chlorine containers, four vacuum regulators mounted to two automatic switchover module with gas piping and miscella 
neous valves, five flow proportionate feed chlorinators, consisting of four at the expansion plant and one at the main plant, four flow proportionate rotameters at the main plant, three chlorine dioxide generators, and gas piping and ejectors.  A release of chlorine gas could potentially occur at the chlorine storage area or at the chlorine process area. 
The second train consists of up to 6 one-ton chlorine containers, tank mounted vacuum regulators, three flow proportionate chlorine dioxide generators, andgas piping and ejectors. A release of chlorine gas could potentially occur at the chlorine storage area or at the chlorine process area.  
Chlorine containers are delivered to the Southwest WTP by truck and the chlorine containers are stored outside on the chlorine storage dock located to the west of the Expansion Plant Chemical Building, and at a storage area located south of the main Chlorine Dioxide Building. Chlorine is removed as a pressurized gas; however the majority of the chl 
orine feed is operated through a water injection induced vacuum, making the system extremely safe and minimizing potential pressurized leaks along the feed lines.   
The existing ammonia feed facility at the Southwest WTP consists of  a 1,000 gallon and a 8,000 gallon stationary storage tank, three flow proportionate feed ammoniators at the expansion plant and two feeders for the main plant, and gas piping and ejectors. A release of ammonia gas could potentially occur at the ammonia storage area or at the ammonia process area.  
Ammonia is delivered to the Southwest WTP by truck and the stationary tanks are refilled. Ammonia is removed as a pressurized gas.    
WORST-CASE AND ALTERNATIVE RELEASE SCENARIOS 
The storage and process areas for both ammonia and chlorine have associated hazards that can potentially affect on-site employees and the off-site population and environment.  The USEPA requires that a single worst-case scenario for the facility and one alternate release scenario for eac 
h regulated chemical be reported.    
 
Worst-Case Scenario 
The worst-case release scenario is the one that creates the greatest dispersion of a regulated substance in any direction to an offsite endpoint. The largest potential release of chlorine 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 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 USEPA-approved modeling program DEGADIS+ was used to characterize the effects of the worst case scenario at the Southwest WTP.  The distance to the toxic endpoint of 3 ppm was determined to be 1.66 miles. Commercial, industrial and residential areas would be affected in the worst-case release scenario. The estimated affected residential population is 15,000 people. 
 
Alternate Scenar 
ios 
One alternate scenario for chlorine was modeled for the Southwest WTP. The alternative release scenario involved a leak that would potentially develop in the packing at the chlorine container valve.  A release of chlorine through the 1/5-inch diameter hole in the valve packing was presumed to occur for approximately 10 minutes, before being observed or detected and repaired.  The release rate of chlorine caused by leakage through this opening is calculated to be 73.7 pounds per minute.  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 3 ppm was determined to be 1.16 miles.  The estimated affected residential population is 9,900 people.   
 
One alternate scenario for ammonia was modeled for the Southwest WTP. The alternative release scenario involved 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 was presumed to occur for approximately 10 minutes, before being observed or detected, and repaired.  The release rate of ammonia caused by leakage through this opening is calculated to be 1,351 pounds per minute.  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 chlorine and ammonia equipment, utilizing trained personnel in these areas.  Southwest WTP management enforces consistent operation through disciplinary action for operational deviations. 
 
FIVE-YEAR ACCIDENT HISTORY 
The accident history for the Southwest WTP was reviewed for the period from June 1994 through June 1999.  During this period of time, no acc 
idental releases of chlorine or ammonia occurred.   
 
EMERGENCY RESPONSE PROGRAM 
This facility has developed an Emergency Response Program in which plant employees are divided into various management and response teams.  Back-up personnel have been designated in the event that the members of the management or response teams are unavailable. 
 
The Emergency Response Plan includes: (1) procedures to follow in the event of either a chlorine or ammonia emergency, (2) information about the frequency of employee emergency response training, and (3) a detailed description of the emergency response training underway.  
 
The McAllen Fire Department has been designated to provide emergency responders and equipment in emergency situations.  Currently, the City of McAllen Fire Department has a contract with Alamo Environmental to provide HAZMAT response services and along with the Fire Department, will assume Incident Command upon arrival to the plant's emergency call.  
 
PLANNED CHANGES TO IMPROVE SA 
FETY 
Based on the hazard review and prevention evaluation completed for chlorine and ammonia, a list of action items was developed and is being considered by Southwest WTP management to implement if feasible.  The most notable planned changes include the following: 
 
7 Ensure that the delivery driver follows a set procedure in the delivery of chlorine or ammonia.  Coordinate with the staff to develop a procedure to govern the activities of the truck driver or vendor 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 chlorine gas in the area of the outside storage of chlorine cylinders at the main chlorine dioxide building where there is no leak detection.  For ammonia gas, the consideration of leak detection in the area o 
f the storage tanks and along any portion of the feed line in which a potential pocket of gas could form beneath a containment or structure. 
 
7 In the future when the plant facilities are expanded or upgraded, consideration will be given to design and construction of containment buildings to encompass the chlorine cylinders and the ammonia tanks, designed in conjunction with the capability to chemically neutralize any accidental releases.  This option might be compared with the conversion to a nongaseous chlorine, or ammonia compound.
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