City of Toccoa Water Treatment Facility - Executive Summary
Executive Summary for City of Toccoa Water Treatment Facility |
1. Accidental Release Prevention and Emergency Response Policies
We at the City of Toccoa Water and Wastewater System are strongly committed to employee, public and environmental safety. This commitment is demonstrated by our accidental release prevention program that covers areas such as design, installation, operating procedures, maintenance, and employee training associated with the processes at our water treatment facility. It is our policy to implement appropriate controls to prevent possible releases of regulated substances.
2. The Stationary Source and the Regulated Substances Handled
Chlorine is the only regulated substance present at our facility. Chlorine is used for disinfection purposes in the treatment of potable water. Chlorine is stored as a liquified gas under pressure. The vapor pressure of stored chlorine is 6.8 atmospheres (99.89 psi - pounds per square inch). Chlorine gas is non-flammable, but it
is a strong oxidizer that can support combustion of other materials. Chlorine is a toxic gas that is considered by the National Institute of Occupational Safety and Health (NIOSH) to be immediately dangerous to life and health at concentrations exceeding 10 parts per million (ppm).
The maximum inventory of chlorine at our facility is 18,000 lbs. (pounds) or nine 2,000-lb. cylinders. The largest single vessel of chlorine at our facility is a 2,000-lb. cylinder. The chlorine cylinders are equipped with a fusible plug that is designed to fail if the temperature of the cylinder reaches approximately 158 ( F (Fahrenheit). The purpose of the fusible plug is to prevent cylinder failure in the event of an extended, hot fire by allowing a controlled release of the cylinder contents. All of the chlorine cylinders in our facility are located in the chlorine room, which is of fire-proof construction in accordance with the Chlorine Institute specification for chlorine storage facilities.
The Worst Case Release Scenario and the Alternative Release Scenarios, including administrative controls and mitigation measures to limit the distances for each reported scenario
The Risk Management Plan Rule requires Program Level 2 and 3 facilities to model the effects of a worst-case and one or more alternative release scenarios to predict the possible outcome of a chemical release from a facility. These models evaluate the Distance to Toxic Endpoint based on the Emergency Response Planning Guideline 2 (ERPG-2). These guidelines were developed by the American Industrial Hygiene Association to represent the maximum airborne concentration below which it is believed nearly all individuals could be exposed for up to one hour without experiencing or developing irreversible or other serious health effects or symptoms that could impair an individual's ability to take protective action. The ERPG-2 toxic endpoint for chlorine is 0.0087 mg/L. We performed the required offsite consequence
analysis for our facility using the EPA's RMP-Comp Model, which is an electronic version of the look-up tables and equations provided by the EPA in the RMP Offsite Consequence Analysis Guidance. The release scenario modeling was performed using the EPA RMP-Comp computer program. This program calculates the distance to the toxic endpoint for chlorine depending upon the effects of variable factors. The wind speed and air stability class, presence of partial enclosures and general topographic types are all factors that affect the model calculations. The type of release and release rates are also evaluated by the model.
Because the chlorine storage area/chlorination process equipment is located in a semi-enclosed area the RMP-Comp model modified the release rate to 55 percent of the release rate entered into the model. This takes into account the passive mitigation provided by the presence of the structure. As an example, a 10-minute release of 2,000 lbs. of chlorine (i.e., a linear
rate of 200 lbs. per minute) would be modeled as a 10-minute release at a rate of 110 lbs. per minute.
The population estimate data is produced using 1990 United States Census Bureau data in conjunction with a geographic information system (GIS) computer program. This data is designed to provide a general information about the population in a given area, but the data is provided by geographic block groups and the local accuracy of the data can be variable. The public and environmental receptor data represents a combination of United States Census Bureau data, information obtained from United States Geological Survey 7.5 minute topographic quadrangle maps and field observations.
The following paragraphs provide details of the chosen scenarios.
Worst-Case Release Scenario
The worst-case release scenario was estimated to involve the total release of one 2,000-lb. cylinder of chlorine. This is consistent with the regulation definition of a worst-case scenario as the release of th
e greatest quantity contained in the single largest vessel. The use of check valves and other release prevention mitigative measures can not be considered for the worst case scenario. The entire contents of the vessels were assumed to be released as a gas over a 10-minute period. This type of catastrophe is extremely unlikely and would possibly require the complete failure of the cylinder to occur. A simple puncture, valve failure or line failure would not be likely to result in a ten-minute release of the contents from the cylinder. The EPA's RMP-Comp model was used assuming an "Urban" topography setting due to the buildings and very hilly topography on and near the plant site (note - the RMP-Comp model uses a "rural" setting for flat, open topography and an "urban" setting for areas with many buildings, trees or rough and broken topography. In spite of the setting names, neither of these settings was designed to account for demographic variations). The wind speed was assumed to
be 3.4 miles/hour with a stability class of "F", according to EPA guidelines. This stability class assumes the least amount of potential for air mixing due to turbulence or other factors. The model determined the distance to the toxic endpoint to be 0.9 mile. The population within this area is approximately 290 people, according to the 1990 census data. Public receptors were identified within the radial distance to the worst case endpoint, including residences and recreational areas. Environmental receptors include the Chattahoochee National Forest.
Alternative-Case Release Scenarios
Two alternative-case scenarios were modeled to represent circumstances that were estimated to be more likely to occur at the facility. One alternative-case release scenario was estimated to involve the release of chlorine from a broken 3/4-inch valve-fitting over a ten-minute period (i.e., 120 pounds/minute). This scenario assumed that the release detection system/automatic shutoffs malfunctio
ned and the cylinders had to be closed manually by facility personnel. In this scenario 1,200 lbs. of chlorine was assumed to have been released as a gas over a ten-minute period. As with the worst-case scenario, the EPA's RMP-Comp model was used assuming an "Urban" topography setting. The wind speed was assumed to be 6.7 miles/hour with a stability class of "D", according to EPA guidelines. This stability class assumes a slight to moderate potential for air mixing due to turbulence or other factors. The model determined the distance to the toxic endpoint (i.e., 0.0087 mg/L based on ERPG-2) to be 0.4-mile. The population within this area is approximately 64 people, according to the 1990 census data. Public receptors were identified within the radial distance to the worst case endpoint, including residences and recreational areas. Environmental receptors include the Chattahoochee National Forest.
The second alternative-case release scenario was estimated to involve the total rel
ease of 83.9 lbs./minute of chlorine from a broken 1/4-inch feed line. This scenario assumed that the release detection system/automatic shutoffs malfunctioned and the cylinders had to be closed manually by facility personnel. In this scenario 839 lbs. of chlorine was assumed to have been released as a gas over a ten-minute period. As with the worst-case scenario, the EPA's RMP-Comp model was used assuming an "Urban" topography setting. The wind speed was assumed to be 6.7 miles/hour with a stability class of "D". The model determined the distance to the toxic endpoint (i.e., 0.0087 mg/L based on ERPG-2) to be 0.2-mile. The population within this area is approximately 16 people, according to the 1990 census data. Public receptors were identified within the radial distance to th