Westside Wastewater Treatment Plant - Executive Summary
a. The Sanitary Board of Bluefield (BSB) accidental release prevention policy involves a unified approach that integrates technologies, procedures, and management practices. All applicable procedures of the U.S. Environmental Protection Agency (EPA) Prevention Program are adhered to. The BSB emergency response policy involves the preparation of response plans which are tailored to each facility and to the emergency response services available in the community, and is in compliance with the EPA Emergency Response Program requirements. |
b. The Westside Wastewater Treatment Plant serves the Bluefield, VA, Bluefield, WV the Tazewell County, VA and Mercer County, WV area. The plant is located on State Route 102 one mile north of the town of Bluefield, VA. The plant includes a sealed chlorination room, which contains liquid chlorine containers, chlorinators, alarms and safety equipment. The maximum amount of chlorine stored at the facility is 20,000 pounds. The amount of chlorine in use is
two one-ton cylinders (4,000 pounds). The facility is manned 24 hours a day.
c. The offsite consequence analysis includes consideration of two chlorine release scenarios, identified as "worst case release" and "alternative scenario". The first scenario is defined by EPA, which states that "the owner or operator shall assume that the maximum quantity in the largest vessel is released as a gas over 10 minutes," due to an unspecified failure. The alternative scenario is defined as "more likely to occur than the worst-case release scenario".
Atmospheric dispersion modeling has to be performed to determine the distance traveled by the chlorine released before its concentration decreases to the "toxic endpoint" selected by EPA of " PPM, which is the Emergency Response Planning Guideline Level 2 (ERPG-2). This is defined by the American Industrial Hygiene Association (AIHA) as the "maximum airborne concentration below which it is believed that nearly all individuals could be exposed for u
p to one hour without experiencing or developing irreversible or other serious health effects or symptoms which could impair an individual's ability to take protective action." The residential population within a circle with a radius corresponding to the toxic endpoint distance has to be defined "to estimate the population potentially affected".
The worst-case release scenario at the Westside Wastewater Treatment Plant involves a failure of the two ton-containers which could be connected concurrently (a total of 4,000 lbs. of chlorine). The offsite consequence analysis for this scenario was performed for two sets of conditions. The first set followed conditions pre-defined by EPA, namely release of the entire amount as a gas in 10 minutes, use of the one-hour average ERPG-2 as the toxic endpoint, and consideration of the population residing within a full circle with radius corresponding to the toxic endpoint distance. EPA set these conditions to facilitate the performance of the offsi
te consequence analysis; however, the assumptions used may be unrealistic because:
(1) Only a fraction (about 26% of the, total) of the compressed liquefied chlorine released to the atmosphere flashes as a vapor. The remaining un-flashed liquid forms liquid-droplet aerosols. This results in the formation of a very dense chlorine cloud consisting of vapor and liquid droplets, with dispersion characteristics significantly different than for a cloud consisting only of chlorine gas.
(2) It is not appropriate to compare a 10-minute release to a one-hour average standard. The 3-PPM one-hour EPPG value can be modified using available time of exposure/concentration relationships to match the ten-minute release time; the value obtained is 7.3 PPM.
(3) Only the population within an elliptical plume extending downwind of the release point is potentially affected. This plume area, or footprint, is approximately 6% (one-twentieth) of the area of the full
circle. The topology of land also great
ly reduces the dispersion of the chlorine. The facility sets in a valley boarded on four sides by mountains with an elevation of twenty-five feet or more.
EPA-mandated meteorological conditions, namely Stability F, wind speed of 1.5 m/sec, highest daily maximum temperature (980F), and average humidity (75%) were used for both sets.
When atmospheric dispersion modeling for the worst case scenario was performed using the EPA assumptions, a distance to toxic endpoint of 3.0 miles and an estimate of residential population potentially affected of 15,678 was obtained. When the same modeling was performed with the second set of conditions, namely using a vapor/aerosol release, a 10-minute average ERPG-2 of 7 PPM, and consideration of the plume footprint only, a distance to toxic endpoint of 1.5 miles and an estimate of population potentially affected of 6,104 resulted. The affected population data was obtained by using ESRI Arc View software in conjunction with 1990 census database inform
The alternative release scenario involves the rupture of the flexible connections (pigtails) connected to three ton-containers, possibly due to an earthquake. The amount of chlorine released is 581 lb., at an average rate over one hour (the duration of the release) of 9.7 lb./min. Toxic endpoint distances to ERPG-2 and -3 levels were obtained. The latter is defined by AIHA as "the maximum airborne concentration below which it is believed that nearly all individuals could be exposed for up to one hour without experiencing or developing life-threatening health effects". The typical meteorological conditions used were Stability E, wind speed 2.0 m/s, average air temperature of 62'F, and 63% average humidity. The estimated distances traveled to the toxic endpoints are 4,300 ft for the ERPG-2 and 1,400-ft for the ERPG-3.
Actuation of the chlorine detector is an active mitigation measure considered.
d. The general BSB accidental release prevention program is based on the following
High level of training of the operators
Preventive maintenance program
Use of state-of-the-art process and safety equipment
Use of accurate and effective operating procedures, written with the participation of the operators
Performance of a hazard review of equipment and procedures
Implementation of an auditing and inspection program.
Chemical specific prevention steps include availability of self-contained breathing apparatus (SCBA), worn by the operators during connection/disconnection of chlorine supply, awareness of the hazardous and toxic properties of chlorine, and presence of chlorine detectors.
e. No accidental releases of chlorine have occurred at this facility in the past five years.
f. The facility has an emergency response program, which has been coordinated (reviewed) by the City Bluefield Fire Department, which is a member of the Local Emergency Response Planning Committee (LEPC). This program includes an emergency response decisi
on tree and a notification plan.
g. The chlorination equipment was evaluated and replaced in 1995 during a plant upgrade and expansion.
To the best of my knowledge, information, and belief formed after reasonable inquiry, the information submitted is true, accurate and complete.
Ronald L. Dodson
Asst. Executive Director
Sanitary Board of Bluefield