South Wastewater Treatment Plant - Executive Summary
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, w
astewater treatment plants, ammonia refrigeration systems, and federal facilities.
The South Wastewater Treatment Plant (WWTP) falls under this regulation because of the on-site storage of chlorine and sulfur dioxide. The amount of both chemicals stored is above the threshold limit specified by the USEPA thereby making the facility subject to compliance with the regulation. The South 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); an
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 South WWTP.
RELEASE PREVENTION AND EMERGENCY RESPONSE POLICIES
The South 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 on-site 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.
The South WWTP uses chlorine as a disinfectant in the wastewater treatment proces
s. The South WWTP regularly has a maximum storage quantity of 8 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. The South WWTP uses sulfur dioxide to dechlorinate wastewater before discharge. The South WWTP regularly has a maximum storage quantity of 8 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.
The South WWTP is located at 4100-A Idela, McAllen Texas. The wastewater is treated at the plant and subsequently discharged, after being disinfected using chlorine and dechlorinated using sulfur dioxide.
The existing chlorine feed facility at the South WWTP consists of four one-ton chlorine containers, two vacuum regulators mounted to an automatic switchover module with gas piping and miscellaneous valves, pressure check relief valves, three flow proportionate feed chlorinat
ors, miscellaneous gas piping and ejectors. A release of chlorine gas could potentially occur in the chlorine storage area and the chlorine process areas listed above.
Chlorine is delivered to the South WWTP by truck and containers are stored outside, to the east of the Chemical Building. Chlorine is removed as pressurized gas from the individual containers, however; the majority of the feed system 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 South WWTP consists of 4 one-ton sulfur dioxide containers, two vacuum regulators mounted to an automatic switchover module with gas piping and miscellaneous valves, pressure check relief valves, three flow proportionate feed sulfurnators, and gas piping and ejectors. A release of sulfur dioxide could potentially occur in the sulfur dioxide storage area and the sulfur dioxide process area.
oxide is delivered to the South WWTP by truck and containers are stored outside, to the east of the Chemical Building. Sulfur dioxide is removed as a pressurized gas, however, the majority of the feed system is operated through an induced vacuum, which makes the system extremely safe and minimizes potential pressurized leaks along the feed lines.
WORST-CASE AND ALTERNATIVE RELEASE SCENARIOS
The storage and process areas for both chlorine and sulfur dioxide 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 each regulated chemical be reported.
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 South WWTP, chlorine poses the greatest threat. 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 EPA-approved modeling program DEGADIS+ was used to characterize the effects of the worst case scenario at the South WWTP facility. The distance to the toxic endpoint of 3 ppm was determined to be 1.64 miles. Commercial, industrial and residential areas would be affected in the worst-case release scenario. The estimated affected residential population is 6,600 people.
Alternate Release Scenarios
One alternate scenario for chlorine was modeled for the South 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 1/5-inch diamete
r hole in the valve packing presumably would occur for about 10 minutes, before being observed or detected, or 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 South WWTP. The distance to the USEPA defined toxic endpoint of 3 ppm was determined to be 1.32 miles. The estimated affected residential population is 4,300 people.
One alternate scenario for sulfur dioxide was modeled for the South 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 presumably would occur for approximately 10 minutes before being observed or detected and shut-off. The release rate of sulfur dioxide caused by leakage through this opening is calculated to be 73.3 pounds per minute. D
EGADIS+ was also used to characterize the effects of the alternative case scenario at the South WWTP. The distance to the USEPA defined toxic endpoint of 3 ppm was determined to be 1.39 miles. The estimated affected residential population is 4,300 people.
GENERAL ACCIDENTAL RELEASE PRECAUTION PROGRAM
The South WWTP carries out consistent operation and maintenance of its chlorine and sulfur dioxide equipment utilizing trained personnel in these areas. South WWTP management enforces consistent operation through disciplinary action for operational deviations.
FIVE-YEAR ACCIDENT HISTORY
The accident history for the South WWTP's was reviewed for the period from June 1994 through June 1999. During this period of time, no accidental releases of chlorine had occurred.
EMERGENCY RESPONSE PROGRAM
As mentioned earlier, this facility has developed an Emergency Response Program in which plant employees are divided into various management and response teams. Back-up personnel have also b
een indicated in the event that members of the teams 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 to the plant in emergency situations. Currently the 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.
PLANNED CHANGES TO IMPROVE SAFETY
Based on the hazard review and prevention evaluation completed for chlorine, a list of action items was developed and is being considered by South WWTP management to determine if implementation is to be accomplished. The most notable planned changes include the following:
ure 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 chlorine storage area, and in the chlorinator building, as a warning of vacuum regulator failure, and as a backup sensor. For sulfur dioxide, consider remote leak detection in the storage area, and in the sulfunator building, as a warning of vacuum failure.
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 will be given to design and constru
ction of containment buildings to encompass the chlorine cylinders and the sulfur dioxide cylinders 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.