North Wastewater Treatment Plant - Executive Summary

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INTRODUCTION 
The United Sates 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 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 systems,  
wastewater treatment plants, ammonia refrigeration systems, and federal facilities. 
The North Wastewater Treatment Plant (WWTP) falls under this regulation because of the onsite storage of chlorine and sulfur dioxide.  The amount stored for both chemicals exceed the threshold limits specified by the USEPA, thereby making the 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 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); a 
nd 
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 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 chlorine and sulfur dioxide.  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 response prior to the arrival of the City of McAllen HAZMAT. The City of McAllen Fire Department currently has contracted with Alamo Environmental to supply HAZMAT response services. 
REGULATED SUBSTANCE 
The North WWTP uses chlorine as a disinfectant in the wastewater treatment process.   
The North WWTP regularly has a maximum storage quantity of 6 one-ton containers of chlorine at its facility that is stored outside.  This is above the threshold limit of 2,500 pounds set by the USEPA.  In addition to the chlorine, the North WWTP uses sulfur dioxide to dechlorinate treated wastewater before discharge.  The North WWTP regularly has a maximum storage quantity of 6 one-ton containers of sulfur dioxide at its facility that is stored outside.  This is above the threshold limit of 5,000 pounds set by the USEPA.  
 
PROCESS DESCRIPTION 
The North WWTP is located at 2100 W. Sprague Road, McAllen Texas.  The wastewater is treated at the plant and after being disinfected using chlorine and dechlorinated using sulfur dioxide, is subsequently discharged back into the receiving stream.   
The existing chlorine feed facility at the North WWTP consists of 6 one-ton chlorine containers, two vacuum regulators mounted to an automatic switchover module with gas piping and miscellaneous valves 
, four flow proportionate feed chlorinators, and gas piping and ejectors.  A release of chlorine gas could potentially occur at the chlorine storage area or at the chlorine process area.  
Chlorine is delivered to the North WWTP by truck and chlorine containers are stored in the outside storage area located to the north of the Chemical Building. Chlorine is removed as a pressurized gas, however; the majority of the chlorine feed is operated through an induced vacuum, which makes the system extremely safe and minimizes potential pressurized leaks along the feed lines.   
The existing sulfur dioxide feed facility at the North WWTP consists of  4 one-ton sulfur dioxide containers on-line, two vacuum regulators mounted to an automatic switchover module with gas piping and miscellaneous valves, two flow proportionate feed sulfunators, and gas piping and ejectors.  A release of sulfur dioxide could potentially occur at the sulfur dioxide storage area or the sulfur dioxide process area.  
Sulfur 
dioxide is delivered to the North WWTP by truck, and stored in the outside storage area located to the northeast corner of the plant site. Sulfur dioxide is removed as a pressurized gas, however; the majority of the sulfur dioxide feed is operated through a water induced vacuum, which makes the system extremely safe and minimizes the potential for pressurized leaks along the feed lines.   
WORST CASE AND ALTERNATIVE RELEASE SCENARIOS 
The storage and process areas for both sulfur dioxide and chlorine have associated hazards that can potentially affect onsite 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 each 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.  For the North WWTP, chlorine poses the greatest risk.  The larges 
t 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 under worst-case conditions 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 North WWTP facility.  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 4,800 people. 
 
Alternate Release Scenarios 
One alternate scenario for chlorine was modeled for the North WWTP. The release was established as a leak that would potentially develop in the packing at the chlorine  
container valve.  A release of chlorine through the 0.2-inch diameter hole in the valve packing is 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 North WWTP.  The distance to the USEPA defined toxic endpoint of 3 ppm was determined to be 1.33 miles.  The estimated affected residential population is 2,700 people.   
 
One alternate scenario for sulfur dioxide 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 is presumed to occur for approximately 10 minutes, before being observed or detected, and repaired.  The release rate of sulfur dioxide caused by  
leakage through this opening is calculated to be 73.3 pounds per minute.  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 1.33 miles.  The estimated affected residential population is 3,000 people.   
 
GENERAL ACCIDENTAL RELEASE PRECAUTION PROGRAM 
The North WWTP carries out consistent operation and maintenance of its chlorine and sulfur dioxide equipment utilizing trained personnel in these areas.  North WWTP management ensures consistent operation through disciplinary actions for operational deviations. 
 
FIVE-YEAR ACCIDENT HISTORY 
The accident history for the North WWTP's was reviewed for the period from June 1994 through June 1999.  During this period of time, no accidental releases of chlorine or sulfur dioxide had occurred.   
 
EMERGENCY RESPONSE PROGRAM 
As mentioned earlier, this facility has developed an Emergency Response Program in which plant emp 
loyees are divided into various management and response teams.  Back-up personnel have been designated in the event that employees designated are unavailable. 
 
The Emergency Response Plan includes: (1) procedures to follow in the event of a chlorine or sulfur dioxide 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.  Alamo Environmental along with the Fire Department will assume Incident Command upon arrival to the plant.  
 
PLANNED CHANGES TO IMPROVE SAFETY 
Based on the hazard review and prevention evaluation completed for chlorine and sulfur dioxide, a list of action items was developed and is being considered by North WWTP manage 
ment 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 or sulfur dioxide.  MW will coordinate with the staff to develop a procedure to govern the activities of the 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 chlorinator building, as a warning of vacuum regulator failure, and as a backup sensor; 
 
7 The consideration of remote leak detection for sulfur dioxide gas in the sulfunator 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; and 
 
 
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 sulfur dioxide containers 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 sulfur dioxide compound.
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