City of Westwego Water Treatment Plant - Executive Summary

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LDEQ Facility ID Number: 30415 
 
                 30415 LDEQ FACILITY ID NUMBER 
 
                   RISK MANAGEMENT PLAN FOR  
 
                 WESTWEGO 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 chemica 
ls 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 Westwego's Water 
Treatment Plant.  The Plant's location is identified as follows: 
 
                                  1) Street Address:       419 Avenue A 
                        Westwego, LA 70094 
 
                                       2) Geographical Location:     29o54'40" North Latitude 
                        90o08'36" 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 Westwego'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 Westwego has 
employed a consultant to conduct a Program 2 Hazard Review, a Complian 
ce Audit, and to develop 
this RMP for the water treatment 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 Westwego'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 Westwego water treatment plant is four ton cylinders.  
 
OFF-SITE CONSEQU 
ENCE ANALYSIS 
"Worst-Case" Scenario  
    The USEPA regulations 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 Stability 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 Plant for the Westwego 
water treatment plant.  For an urban setting, such as is the condition at the Westwego water treatment 
plant, the distance to the endpoint (or  
safe exposure distance) is listed in Exhibit 4-12 in the 
Guidance document as a 1.3 miles radius around the water 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 Westwego 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 Westwego 
water treatment plant was base 
d on a tubing failure, a bad connection or valve failure resulting 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 al 
ternative scenario modeling that 
can further reduce the estimated toxic gas spread area (i.e., distance to endpoint).  The active 
mitigation system to be installed at the City of Westwego'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 
coor 
dinated with the City's Fire and Police Departments. 
    During 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 either chlorine 
or sulfur dioxide before it can leave the plant.
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