City of Gretna Water Treatment Plant - Executive Summary |
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LDEQ Facility ID Number: 5000 RISK MANAGEMENT PLAN FOR GRETNA WATER TREATMENT PLANT EXECUTIVE SUMMARY INTRODUCTION In 1990, the United States Congress amended the Clean Air Act to address Chemical Accident Prevention by requiring the USEPA to develop chemical accident prevention/community right-to-know regulations referred to as the Risk Management Program Rule. The Rule requires covered facilities, whether they be Public, Commercial, or Industrial entities, to create and implement a risk management/accident prevention program and to submit a structured Risk Management Plan (RMP) to the USEPA by not later than June 21, 1999. A covered facility is one that stores any of a long list of dangerous chemicals (e.g., chemicals that are either toxic, flammable, or explosive), in quantities greater than the threshold levels listed in the Rule. Listed toxic chemicals and their respective threshold quantities can be found in the Federal Regulations at 40 CFR 68. The stationary (potential) source that is addressed herein is the City of Gretna's Water Treatment Plant. The Plant's location is identified as follows: 1) Street Address: 100 Fifth Street Gretna, LA 70053 2) Geographical Location: 29o55'05" North Latitude 90o03'29" West Longitude Chlorine gas, an air toxic chemical, is listed. Chlorine is used by most water treatment and wastewater treatment plants as a disinfectant. The threshold quantity listed for chlorine is 2,500 pounds. The City of Gretna's water treatment plant stores chlorine gas in quantities that exceeds the threshold quantity and, as a result, is a covered facility. Accordingly, the City of Gretna has employed a consultant to conduct a Program 2 Hazard Review, a Compliance Audit, and to develop this RMP for the water treatme nt plant. The consultant, EPL, Inc., also is preparing written Safety, O&M, and Emergency Response Plans for the safe handling of the toxic gas, as required by the USEPA. It is pertinent to note, however, that the City of Gretna's water treatment plant has been in continuous operation for more than 50 years. During that long operational history, there has never been a release of chlorine gas of such a magnitude so as to cause injury to people, pets or property surrounding the water treatment plant. Pure chlorine gas (an air toxic), used to disinfect the City's drinking water supply, is received and stored at the above location in the form of a gas that is liquified under pressure. The liquified chlorine gas is supplied in steel pressure cylinders that contain one ton (2000 lbs) of chlorine when received. Typical inventory of chlorine at the Gretna water treatment plant is four ton cylinders. OFF-SITE CONSEQUENCE ANALYSIS "Worst-Case" Scenario The USEPA regula tions require an assessment of risk and emergency response planning for accidental release of any of the listed air toxics under two (or more) different scenarios. That is, the regulations call for analysis/planning for a "worst-case" scenario based on rigid scenario conditions for any air toxic. These "worst-case" scenario - required conditions are identified as follows: Release Duration 10 minutes Quantity of Air Toxic Released 2000 lbs Release Rate 200 lbs/min Endpoint Toxic Concentration (safe level) 0.0087 mg/l Windspeed 1.5 M/sec (3.36 mph) Meteorological Stabili ty Conditions "F" Ambient Relative Humidity 50 % (rare in New Orleans area) Ambient Air & Chemical's Temperature (before release) 77o (OK for air, not for chemical which drops temperature as released) Under the USEPA's regulations the above requirements must be plugged into any offsite consequence analysis (OCA) modeling, regardless if any of the factors are unrealistic or very unlikely to occur. The resulting risk area for this "worst-case" scenario was determined from the USEPA's Risk Management Program Guidance for Wastewater Treatment Plants for the Gretna water treatment plant. For an urban setting, such as is the condition at the Gretna water treatment plant, the distance to the endpoint (or safe exposure distance) is listed in Exhibit 4-12 in the Guida nce document as a 1.3 miles radius around the wastewater treatment plant. Additionally, the USEPA regulations provide for consideration of passive mitigation in "worst-case" assessments. Enclosure of air toxic containers/feed systems within a building is considered a valid, passive mitigation condition. Passive mitigation presence provides for a toxic gas release area reduction of 45 percent (i.e., by multiplying the model "worst-case" radius by a factor of 0.55). Thus, the USEPA's model endpoint for chlorine is reduced for the Gretna water treatment plant as follows: 1.3 miles x 0.55 = 0.72 miles ("worst-case" release area) Alternative Scenario The regulations also permit planners to provide one or more realistic, alternative scenarios involving accidental release of air toxics. The alternative scenario considered for the Gretna water treatment plant was based on a tubing failure, a bad connection or valve failure resu lting in the release of chlorine gas through a 5/16 inch-diameter whole or opening in a ton cylinder. This scenario was modeled by the USEPA study, (see Exhibit 4-15 in the Guidance document) and produced the following information. The release described above would last for 60 minutes with an average maximum gas flow rate of 15 lbs/min. Again, for an urban setting, the release impact area, i.e., the distance from the source out to the endpoint concentration for chlorine (0.0087 mg/l or 3 ppm) was found to be 0.1 miles, without consideration of the passive mitigation factor. When we adapted the model's result to include the containment of the gas inside the treatment plant, the endpoint distance is further reduced by 45 percent to 0.06 miles (317 feet). Further, while only passive mitigation can be considered for the "worst-case" scenario, the regulations permit inclusion of active mitigation factor(s) for the alternative scenario modeling that can further reduce the estim ated toxic gas spread area (i.e., distance to endpoint). The active mitigation system to be installed at the City of Gretna's water treatment plant is an air scrubbing system that will remove chlorine gas from the air. An alarm signal from the existing chlorine leak detector will be used to automatically trigger start-up of the air scrubbing system in the event of a leak. It is conservatively estimated, based on the effectiveness of the air scrubber system, that at least 90 percent of a chlorine release can be removed from the air by this system. Thus, as part of this alternative scenario, the endpoint distance is further reduced to about 0.006 miles (or about 32 feet). In this case, there would be no release to any public receptors off-site. The City has developed a toxic gas release Emergency Response Plan (ERP) that is applicable to both the City's Water and Wastewater Treatment Plants. The plan has been coordinated with the City's Fire and Police Departments. Durin g much more than the past 5 years there have been no toxic gas releases from either plant. The City plans to install air scrubbers at each plant to chemically neutralize chlorine before it can leave the plant. |