Shamokin-Coal Township Joint Sewer Authority - Executive Summary
a. The Shamokin-Coal Township Joint Sewer Authority Chlorination Station has been installed to disinfect the (Authority treated wastewater) flow discharged from the Authority as treatment facility. The chlorination station is located along the Shamokin Creek within the facility, fenced area. The station includes a chlorination room, which contains liquid chlorine containers, a chlorinator, and various safety equipment, and a utility water pump room. The amount of chlorine handled is two one-ton containers. Additional one-ton cylinders are stored in reserve. The facility is normally unmanned. Wastewater treatment operators visit the facility daily and respond to any trouble alarms which may occur. |
b. The off-site 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 3 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 up to one hour without experiencing or developing irreversible or other serious health effects or symptoms which could impair an individuals ability to take protective action." The residential population within a circle with a radius corr
esponding to the toxic endpoint distance has to be defined, "to estimate the population potentially affected".
The worst-case release scenario at the Authoritys Chlorination Station involves a failure of the two ton containers which could be connected concurrently (a total of 4,000 lb. Of chlorine). The off-site 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 off-site consequences analysis; however, the assumptions used may be unrealistic because:
(1) Only about 26% of the compressed liquefied chlorine released to the atmosphere flashes as a vapor. The remaining unflashed liquid forms liquid-dr
oplet aerosols. This results in the formation of a very dense chlorine cloud consisting of vapor and liquid droplets.
(2) Only the population within an elliptical plume extending downwind of the release point is potentially affected. This plume area, or footprint, is approximately6% (one-twentieth) of the area of the full circle.
(3) However, the actual lowest mean wind speed is 6.0 mph in August, based upon Williamsport NWS data. (See p.1 of the Hazardous Material Off-Site Response Plan, June 1997.) Therefore, the actual condition is not 1.5 meters per second as described by the EPA Worst-Case Scenario information, but rather is approximately 3.0 meters per seconds.
(4) The topography of the geographic area to the south, southwest and southeast of the chlorination station rises 820-840 feet above the ground elevation at the Shamokin-Coal Township Joint Sewer Authority facility and effectively blocks the dense chlorine gas.
The topography of the geographic area to the northe
ast, in the direction into which a chlorine cloud would most probably be directed by the prevailing winds, is less severe, with various rises of 100-280 feet above the chlorination station group surface for a distance of approximately 14,000 linear feet (2.65 miles). Beyond this distance, the topography again becomes more tortuous, with elevations rising to 520-580 feet above surface elevations at the chlorination station. This topographic condition would again function as a barrier to a dense chlorine cloud.
EPA mandated meteorological conditions, namely Stability F, wind speed of 1.5 m/sec, highest daily maximum temperature (1160F), and average humidity (65%) 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 5.4 miles and an estimate of residential population potentially affected of 25,200 was obtained. When the same modeling was performed with the second set of condi
tions, namely using the actual low mean wind speed of 3.0 meters per second, and consideration of the plume footprint only, a distance to toxic endpoint of 2.0 miles and an estimate of population potentially affected of 1,200 resulted.
The alternative release scenario involves the failure of a single one tone container containing 2,000 pounds of chlorine. The amount of chlorine released is 2,000 pounds, at an average rate over 10 minutes, the release would be inside a structure, and would therefore be mitigated by the regulatory factor of .55. The actual release would be 110 pounds per minute. Using the actual weather conditions, which is 3.0 meters per seconds low annual mean speed in August, the estimated distance traveled to the toxic endpoint is 1.2 miles.
d. The general Authority accidental release prevention program includes:
Operators trained to perform cylinder convections and cylinder handling.
Maintenance and replacement of cylinder appurtenances.
ion of Chlorination building.
Availability of self-contained breathing apparatus (SCBA)
Audible and visible alarm system is in place.
e. There have been no accidental releases of chlorine at the Authority facility in the past five (5) years.
f. The facility has an emergency response program. This response program has been prepared by the Northumberland County Emergency Management Agency, and was issued June 1997. The Hazardous Material Off-Site Response Plan is specifically addressed to chlorine, CAS #7782505.