Hanover Regional Wastewater Treatment Facility - Executive Summary
a. The Borough of Hanover operates a wastewater treatment facility whose service area includes Hanover and McSherrystown boroughs, Conewego Township, and a portion of Penn Township, all in Adams County, Pennsylvania. The wastewater treatment plant, located at the end of O'Brien Lane in Conewego Township, processes up to 4.5 million gallons per day of wastewater and discharges under NPDES permit PA 0026875 to the South Branch Conewego Creek. Wastewater treatment processes include grit and coarse screenings removal, preliminary sedimentation, secondary activated sludge treatment, clarification, filtration, disinfection, and dechlorination. 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 room, which contains two (2) one-t
on containers of chlorine liquified under pressure, a chlorinator, various safety equipment, and instrumentation consisting of flow recorders, electrical panels, and a gas detector/alarm unit. Up to six (6) 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 12,000 pounds.
In addition, to improve the quality of water being added to the receiving stream, the wastewater treatment facility also uses sulfur dioxide gas, another regulated substance, to dechlorinate its effluent. The process is cal
led "sulfonation." The sulfonator is similar to the chlorinator and injects sulfur dioxide gas into water to remove excess chlorine.
c. The off-site consequence analysis includes consideration of a worst-case scenario for chlorine release and one "alternative-case" scenario each for chlorine gas and for sulfur dioxide gas. The first scenario is defined by the regulations as the "maximum quantity in the largest vessel" being released "as a gas over ten minutes," 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 determined the distance traveled by the chlorine released until its concentration decreases to the"toxic endpoint" selected by EPA of 0.0087 mg/L in air, which is the Emergency Response Planning Guideline Lev
el 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 Hanover Wastewater Treatment Plant involves a failure of one of the 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 4,400 people who may potenti
ally 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.
Interestingly, both chemicals produce the same off-site consequences, having the same or very similar distances to endpoint. The AIHA Emergency Response Planning Guideline Level 2 concentration for sulfur dioxide is 0.0078 mg/L in air.
The alternative-case scenario chosen involves a gas leak from quarter-inch rusthole in one of the containers, in the liquid portion of the container, while it is outside of the enclosure. This results in a flashing liquid release. The RMP*Comp model was run assuming a release of 150 pounds per minute for 13.33 minutes, the duration of the release, which yielded a distance to endpoint of 0.2 mile and an area of eighty acres surrounding the facility. The meteorological 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 44 residents off site would be affected by this release.
d. The Hanover wastewater treatment facility's accidental release prevention program 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 chl
orine, 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 Adams County Local Emergency Planning Committee (LEPC) in Gettysburg and by the Hanover area fire companies and the Adams 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 May, 1999, inspection of the disinfection and dechlorination systems by the Borough's consulting engineer. These recommendations included third-party training of all water plant employees in chlorine usage and safety and increased documentation of routine functi
ons related to the performance of the two regulated processes. Other considerations possible the addition of a security system to prevent unauthorized entry to the chlorinator and sulfonator buildings.