City of Westwego's Wastewater Treatment Plant - Executive Summary
LDEQ Facility ID Number: 27841 |
27841 LDEQ FACILITY ID NUMBER
RISK MANAGEMENT PLAN FOR
WESTWEGO WASTEWATER TREATMENT PLANT
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 chem
icals 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
Wastewater Treatment Plant. The Plant's location is identified as follows:
1) Street Address: 8700 Lapalco Blvd.
Westwego, LA 70094
2) Geographical Location: 29o53'47" North Latitude
90o09'14" West Longitude
Chlorine gas and sulfur dioxide gas, both air toxic chemicals, are listed. Chlorine is used by
most water treatment and wastewater treatment plants as a disinfectant. Sulfur dioxide is used to de-
chlorinate the treated wastewater following disinfection. The threshold quantity listed for chlorine
is 2,500 pounds. The threshold quantity for sulfur dioxide is 5,000 pounds. The City of Westwego's
wastewater treatment plant stores chlorine gas in quan
tities 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 Compliance Audit, and to develop this RMP for the
wastewater treatment plant. The consultant, EPL, Inc., also is preparing written Safety, O&M, and
Emergency Response Plans for the safe handling of both toxic gases, as required by the USEPA.
It is pertinent to note, however, that the City of Westwego's wastewater treatment plant has
been in continuous operation for 34 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 wastewater treatment plant. Similarly, sulfur dioxide has been used for only the last
few years without incident.
Pure chlorine gas (an air toxic), used to disinfect the City's treated wastewater supply, is
received and stored at the above location i
n 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. Another air toxic, sulfur dioxide, also is handled as a pressure-liquified gas
in one-ton cylinders. Typical inventory of chlorine at the Westwego wastewater treatment plant is
four ton cylinders. Typical inventory of sulfur dioxide is two ton cylinders.
OFF-SITE CONSEQUENCE ANALYSIS
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 mu
st 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 Westwego
wastewater treatment plant. For an urban setting, such as is the condition at the Westwego
wastewater 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 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 Westwego
wastewater treatment plant as follows:
1.3 miles x 0.55 = 0.72 miles ("worst-case" release area)
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
wastewater treatment plant was based 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 alternative 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 wastewater treatment plant is an air
scrubbing system that will remove either chlorine or sulfur dioxide gases from the air. An alarm
signal from the existing chlorine leak detector (also responds to sulfur dioxide) will be used to
automatically trigger start-up of the air scrubbing system in the event of a leak. It is con
estimated, based on the effectiveness of the air scrubber system, that at least 90 percent of a chlorine
or sulfur dioxide 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.
During much more than the past 5 years there have been no toxic gas releases from either
The City plans to install air scrubbers at each plant to chemically neutralize either chlorine
or sulfur dioxide before it can leave the plant.