75th Street Wastewater Treatment Plant - Executive Summary

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INTRODUCTION AND OVERVIEW OF THE 75th Street WWTP AND REGULATED SUBSTANCES HANDLED 
 
The United States Environmental Protection Agency (EPA) Risk Management Program (RMP) regulation, promulgated on June 20, 1996 under the Clean Air Act, requires facilities that have a regulated substance above the listed threshold quantities to develop a formal Risk Management Program (RMProgram) and to register and submit a Risk Management Plan (RMPlan).  The RMP rule regulates 140 chemicals for their toxic or flammable characteristics.  Of the chemicals regulated, one or more of the following typically triggers RMP requirements at wastewater treatment plants (threshold quantities noted in parentheses):  chlorine (2,500 pounds), anhydrous ammonia (10,000 pounds), anhydrous sulfur dioxide (5,000 pounds), aqueous ammonia (20,000 pounds) and methane (10,000 pounds).  If any of these chemicals are handled, stored, or used above the threshold quantity, the wastewater treatment plant (WWTP) is subject to the 
RMP rule.  The City of Boulder's 75th Street WWTP uses chlorine above the 2,500 lb. threshold quantity and sulfur dioxide above the 5,000 lb. threshold quantity and therefore must prepare a RMProgram. 
 
The chlorine room is equipped to store fifteen, one-ton containers.  Six chlorine cylinders are on scales (three per manifold) and hooked up to feed and the remainder are on the floor on permanent trunnions.  The maximum chlorine inventory is 30,000 lbs.  Each manifold is equipped with a regulator and connected to an automatic switch-over valve.  The storage room is equipped with an evaporator, however, it is not in use.   
 
The sulfur dioxide room is equipped to feed, two, one-ton containers.  Furthermore, up to 7 additional sulfur dioxide containers are stored outdoors.  Both of the sulfur dioxide cylinders in the feed room are on scales and hooked up to feed.  The maximum sulfur dioxide inventory is 18,000 lbs.  The two cylinders on the scales are each equipped with regulators and hea 
t traced trap legs and are connected to an automatic switch-over valve.  The storage room is equipped with an evaporator, however, it is off-line. 
 
Below is a brief summary of the 75th Street WWTP Risk Management Program. 
 
ACCIDENTAL RELEASE PREVENTION AND EMERGENCY RESPONSE POLICIES 
 
The purpose of the RMP rule is to lesson the number of serious chemical accidents that could affect public health and the environment, and also to improve the response to those accidents that do occur and thereby reduce the severity of the chemical accident.  Operators and management at the 75th Street WWTP have several measures already in place, and have initiated other measures as well,  to minimize the likelihood of an accidental chlorine or sulfur dioxide release. 
 
All operators at the WWTP are trained in operating the chlorine and sulfur dioxide processes, and follow all appropriate safety procedures.  Both management and staff are committed to ensuring safety at the WWTP.  Training and safety meetin 
gs are held on a regular basis. 
 
ACCIDENTAL RELEASE PREVENTION PROGRAM AND CHEMICAL SPECIFIC PREVENTION MEASURES 
 
The 75th Street WWTP facility follows all recommended safety, design, and operational and maintenance guidelines developed by the Chlorine Institute.  In addition, refresher training is routinely conducted. 
 
The chlorine containers are stored in an enclosed building.  Sulfur dioxide containers that are in-use or on stand-by (i.e. two containers) are housed in an enclosed building.  Spare sulfur dioxide containers are stored in a protected area outdoors.  Both the chlorine and sulfur dioxide rooms contain control panels for their respective chlorine and sulfur dioxide monitors, exhaust fan control, and lighting control. 
 
WORST-CASE AND ALTERNATIVE RELEASE SCENARIOS 
 
For either the chlorine or sulfur dioxide process, the worst-case release quantity is 2,000 lbs., i.e. the capacity of a one-ton container.  As defined by the RMP rule, the worst-case release scenario for toxic g 
ases (such as chlorine and sulfur dioxide) to be modeled is: 
 
"For regulated toxic substances that are normally gases at ambient temperature and handled as a gas or as a liquid under pressure, the owner or operator shall assume that the quantity in the vessel or pipe, is released as a gas over 10 minutes.  The release shall be assumed to be the total quantity divided by 10 unless passive mitigation systems are in place." 
 
Therefore, the worst-case release rate for chlorine and sulfur dioxide at the 75th Street WWTP is 200 lbs per minute.  Although the chlorine vessels are kept indoors, based on recommendations made in RMP guidance manuals, passive mitigation from the building was not included in the worst-case release scenario because the cylinders are unloaded outside the building upon delivery to the site and therefore the cylinders are some times outdoors.  Passive mitigation for the sulfur dioxide was not included either as those vessels are stored outdoors.  As required, the worst 
-case scenario for both the chlorine and sulfur dioxide release analysis was assumed to take place at ground level.  The toxic endpoint for chlorine for modeling the worst-case release is 0.0087 mg/L (3 ppm).   The toxic endpoint for sulfur dioxide is 0.0078 mg/L.    
 
A site-specific model, The Computer Assisted Protective Action Recommendation System (CAPERS), was used to estimate the chlorine and sulfur dioxide release distances to toxic endpoint.  The atmosphere in and around the mountainous terrain adjacent to Boulder, Colorado is complex and changeable.  Plume path and impact projections must reflect the non-linear flow and dispersion conditions in the region if they are to be useful in emergency planning and emergency response.  Recognizing the difficulty of this requirement, the Department of Energy (DOE) developed a specialized atmospheric dispersion model for use in areas-such as Boulder-with especially complex airflow patterns.  Called the Terrain-Responsive Atmospheric Code  
(TRAC), the computer program has been used for emergency plume predictions for more than nine years at the DOE's Rocky Flats Environmental Technology Site (RFETS), a nuclear weapons production facility near Denver, Colorado.  An updated version of the model, called CAPERS, suitable for application to chemical or radiological evaluations at any location has been applied to the City of Boulder's 75th Street WWTP to perform an off-site consequence analysis for chlorine gas.  The CAPARS model provides a variety of plume, weather, hazard, and related information with the accuracy and speed needed to support all levels of emergency management and response. 
 
The Risk Management Plan Rule requires an evaluation of chemical events for a minimum of two meteorological conditions:  (1) poor dispersion ("worst-case" release modeling), with class F air stability and a 1.5 m/s wind speed, and (2) typical dispersion ("alternative" release modeling) with class D air stability and a 3.0 m/s wind speed.  
 
 
Air flow and dispersion conditions are complex in areas of mountainous terrain such as Boulder.  Atmospheric stability, wind speed, and plume trajectories vary rapidly with location and time.  Thus, wind and stability conditions monitored at a single point generally will not be representative of the flow and dispersion of a toxic cloud over its lifetime. 
 
To address this complexity, historical meteorological records for multiple stations in and around Boulder were examined.  Six atmospheric flow/dispersion regimes were identified as dominant patterns for the area (corresponding initial wind speed used in the CAPERS model for each of these meteorological scenarios are shown in parentheses): 
 
    Stable nighttime downslope flow (3.0 m/s), 
    Unstable daytime upslope flow (2.0 m/s), 
    Stable, near-calm/variable flow (0.0 m/s), 
    High wind (Chinook) flow (18.0 m/s), 
    Morning transition from downslope to upslope flow (1.0 m/s), and 
    Evening transition from upslope to dow 
nslope flow (1.0 m/s). 
 
A representative example of each flow regime was chosen from historical meteorological records.  Then, the hypothetical release scenarios were modeled for each flow regime and the resulting concentration patterns evaluated.  Thus, the six flow regimes cover the spectrum of poor to good flow and dispersion conditions expected to occur in the Boulder area. 
 
Based on the results obtained from the CAPERS model, the worst-case release distance to toxic endpoint for chlorine and sulfur dioxide is estimated to be 7.6  and 7.8 miles, respectively (morning transition flow).   Residential population within the estimated distance to end point for the plume area was estimated by using recent population density data provided by the City of Boulder that is representative of a radius around the facility of approximately 5 miles.  The population density is estimated at 875 persons per square mile.  Using a worst-case plume area of 4.8 square miles, the estimated residential pop 
ulation within this area is 4,200 persons.  Public receptors located within the boundary of any of the plume areas modeled (i.e. the 6 meteorological conditions described above), include:  residences, recreational areas, and schools. 
 
Alternative release scenarios are intended to reflect more likely releases than the worst-case release scenario.  Program 2 facilities, such as the 75th Street WWTP, are required to evaluate and present at least one alternative release scenario. As a starting point, the alternative release scenario developed for the 75th Street WWTP was based on the failure of the 5/16-inch pigtail tubing connected to a single 1-ton chlorine or sulfur dioxide container.  The release rate for chlorine vapor and sulfur dioxide vapor from the 5/16-inch tubing is 15 lbs per minute and 7 lbs per minute, respectively (source:  Risk Management Program Guidance for Wastewater Treatment Plants, Exhibit 4-15).  It was assumed that the release would last a maximum of 60 minutes, giv 
ing a total chlorine and sulfur dioxide release of 900 lbs and 420 lbs, respectively. 
 
Based on site-specific modeling results from the CAPARS model, the estimated alternative release distance to toxic endpoint for chlorine and sulfur dioxide is 0.12 and 0.09 miles, respectively (using the prevailing meteorological condition of a downslope flow). 
 
For alternative release scenarios, both active and passive mitigation systems can be taken into consideration.  Including passive mitigation provided by the chlorine (sulfur dioxide) building will lower the release rate to atmosphere in the event of a chlorine or sulfur dioxide release.  Taking the same alternative release scenario, i.e. failure of the 5/16-inch pigtail tubing, a new release rate was developed by considering passive mitigation.  A simplified method for taking into account passive mitigation provided by the building, is to lower the rate of release.  Typically, the predicted rate of release is 55 to 70 percent for an indoor re 
lease compared to if the same accident should occur outdoors.  Taking 70 percent of the 900 and 420 lbs sited above for chlorine and sulfur dioxide, respectively, gives a total release of 630 lbs (10.5 lbs/minute) and 294 lbs (4.9 lbs/minute).   This release scenario was not run on the site-specific CAPARS model, however, it is estimated that the alternative release distance to toxic endpoint for chlorine and sulfur dioxide would decrease by at least 10 to 15 percent with the inclusion of passive mitigation. 
 
FIVE-YEAR ACCIDENT HISTORY 
 
The 75th Street WWTP has had no reportable accidental releases of chlorine or sulfur dioxide in the past five years. 
 
EMERGENCY RESPONSE PLAN 
 
The 75th Street WWTP has a written Emergency Response Plan that is routinely reviewed and amended.  The plant's Emergency Response Team, currently with 9 active members, meets three times per month.  In addition, the plant's Safety Committee, which currently has 7 members, meets one time per month.  Both the meet 
ings are "open" for anyone else to attend, if they so choose.  Plant-wide safety training is held once per month.  In addition, national or local safety conferences are attended by various personnel each year. 
 
Below is a brief summary of emergency response operations for the Emergency Response Team (ER Team).  When an alarm sounds (or the person is called in, or hears a P.A. announcement), each ER Team member is to report to the safety room, or alternate location if announced.  The senior member will become the Incident Commander (I.C.).  Two individuals will then verify the alarm condition.  These individuals will put on self-contained breathing apparatus (SCBAs) and will enter the chlorine or sulfur dioxide area to check the leak status and close all tank valves and start evacuating ejectors located in the feed rooms.  Subsequently, these individuals will return to the safety room and report the leak status to the I.C..  The I.C. will announce the status over the P.A. system, and de 
pending on the situation, will do one or more of the following:  (1) call for a plant-wide evacuation, if need be, (2) call 911, and/or (3) send ER Team members, or another operator, out to the plant entrance at 75th Street with a cell phone.  The individual at the plant entrance will be responsible for a head count, communications with the safety room I.C., and directing police, fire, and the HAZMAT Team to the safety room (or alternate location).  The I.C. will be in charge of direction of repairs and decontamination of personnel, equipment, and clothing that have been exposed.  After the Emergency Response, the I.C. or another designated individual, will record incident events.  The I.C. will also direct a debriefing.  The ER Team will convene to review activities and discuss any deficiencies, and if need be, revise the Emergency Response Plan. 
 
PLANNED CHANGES TO IMPROVE SAFETY 
 
The primary changes that have been made as a result of implementing an RMProgram are greater staff and m 
anagement awareness and commitment to safety.
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