Carlisle Region Water Pollution Control Facility - Executive Summary

| Accident History | Chemicals | Emergency Response | Registration | Source | Executive Summary |

a.        The Borough of Carlisle Sewer Authority owns the Carlisle Region Water Pollution Control Facility which is operated by the Borough of Carlisle.  Its service area includes Carlisle,  county seat for Cumberland County, Pennsylvania, and some surrounding communities.   The wastewater treatment plant,  located at 54 North Middlesex Road, north of U.S. 11 in Middlesex Township, processes up to 7 million gallons per day of wastewater and discharges under  NPDES permit PA 0026077 to the Conodoguinet Creek.   Wastewater treatment processes include grit and coarse screenings removal, preliminary sedimentation, secondary activated sludge treatment, clarification, and disinfection.  Solid material removed from this process is digested to inert material that is used as fertilizer in agricultural operations.  The plant is operated 24 hours per day, every day of the year. 
 
b.          The wastewater treatment plant includes a disinfection system.  Comprising it are a chlorine container roo 
m, which contains two (2) one-ton containers of chlorine liquified under pressure, and a chlorinator room containing evaporators, chlorinators,  various safety equipment, and instrumentation consisting of flow recorders, electrical panels, and a gas detector/alarm unit.  Up to four (4) one-ton containers may be on-site simultaneously.  The chlorinator system extracts about 50 to 120 pounds per day of chlorine gas from the storage containers through high pressure regulators and dissolves this gas in water to form a disinfection solution that is used to kill pathogenic bacteria in treated wastewater prior to its discharge into the creek. Treatment plant operators regularly check the room and its equipment, inventorying the rate of chlorine use and adjusting the rate as necessary.  The maximum intended inventory is 8,000 pounds. 
 
c.         The off-site consequence analysis includes consideration of two chlorine release scenarios, one "worst-case" and one "alternative-case."  The first sc 
enario is defined by the regulations as the "maximum quantity in the largest vessel" being released "as a gas over ten miutes," due to an unspecified failure.  The alternative-case scenario is defined as "more likely to occur than the worst-case scenario." 
 
           Atmospheric dispersion modeling was performed using the RMP*Comp program distributed by the Chemical Emergency Preparedness and Prevention Office of the EPA.  This model determed the distance traveled by the chlorine released until its concentration decreases to the"toxic endpoint" selected by EPA of 3 ppm, (0.0087 mg/L,) which is the Emergency Response Planning Guideline Level 2, defined by the American Industrial Hygiene Association 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 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 Carlisle Water Treatment Plant involves a failure of one of the two chlorine containers, or a 2,000 pound release.  The off-site consequence analysis was performed under the prescribed EPA conditions of all volume released as gas within a ten minute period, to an ERPG-2 endpoint radius of 1.3 miles according to the RMP*Comp model.  Within the 5.4 square-mile area bounded by this circle reside 1,200 people who may potentially be affected by the release.  The program Guidance suggested that passive mitigation such as "enclosure" not be considered in worst-case modeling because the nature of catastrophic incidents could compromise containment offered by the enclosure. 
 
         The alternative-case scenario chosen involves a gas leak from a rupture in a "pigtail" conn 
ection to one of the containers.  Such a rupture approximates a quarter-inch hole in the upper half of the vessel itself, a chlorine vapor release as defined in the program Guidance document.  The RMP*Comp model was run assuming a release of 9 pounds per minute for sixty minutes, the duration of the release, mitigated by enclosure, which yielded a distance to endpoint of less than 0.1 mile and an area of twenty acres surrounding the facility.  The meterological conditions used were taken from the Guidance to be stability class D and wind speed of 3.0 m/sec.  According to the Guidance, any endpoint of less than a tenth of a mile shall be reported as 0.1 mile.  Approximately 25 people would be affected by this release, of which eighty percent are employees of the wastewater treatment plant.  Actuation of a chlorine gas detector and alarm system is an active mitigation considered. 
 
d.         The Borough of Carlisle Sewer Authority's water treatment plant accidental release prevention pro 
gram is based upon the following elements: 
        1)  High level of training for the operators. 
        2)  Preventative maintenance program for equipment. 
        3)  Use of state-of-the-art process and safety equipment. 
        4)  Use of accurate and effective operating procedures, written with the participation of the  
             operators. 
        5)  Performance of a hazard review of the equipment and procedures. 
        6)  Implementation of an auditing and inspection program. 
Chemical-specific prevention steps include availability of self-contained breathing apparatus (SCBA,)  awareness of the hazards and toxic properties of chlorine, and the presence of a chlorine gas detector. 
 
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 reviewed and coordinated by the Cumberland County Local Emergency Planning Committee (LEPC) in Carlisle and by the Car 
lisle area fire companies and the Cumberland County HAZMAT Response Team.  This program includes an emergency response decision tree and a notification plan.  Emergency response drills have been conducted with the fire companies, and the operation and response procedures are reviewed and updated on a regular basis. 
 
g.         Changes to improve safety (recommended actions) were made following a February 4, 1999, inspection of the disinfection systems by the Borough's consulting engineer.  These recommendations included third-party training of all water plant employees in chlorine usage and safety, done on June 2, 1999.  Other considerations possible are the replacement of high-pressure gas lines on the chlorinator with remote vacuum-demand type regulators, similar to those employed at the Borough's water treatment plant, and the addition of a security system to prevent unauthorized entries.
Click to return to beginning