City of Dunedin Wastewater Treatment Plant - Executive Summary
a. The City of Dunedin accidental release prevention policy combines sound operating procedures and management practices. The policy complies with all pertinent procedures of the U. S. Environmental Protection Agency (EPA) Accidental Release Prevention Program. The City of Dunedin emergency response policy involves the preparation of site specific response plans, taking into consideration the available emergency response services surrounding each facility. The emergency response policy is in accordance with the EPA Emergency Response Program requirements. |
b. The Dunedin Wastewater Treatment Plant (WWTP) is located at 1140 Highland Avenue in the City of Dunedin, Florida. The WWTP employs advanced secondary treatment processes to remove solids and contaminants from wastewater and produce a high quality treated effluent. The final treatment process involves the treatment of the effluent with chlorine and the dechlorination of the disinfected effluent with sulfur dioxide pri
or to discharge. The primary mode of discharge is into the City's reclaimed water system. If for any reason the effluent can not be discharged to the reclaimed system, the backup disposal method is via surface water discharge to St. Joseph's Sound. Disinfection and dechlorination of all water to be discharged to surface waters is mandated by Florida Department of Environmental Protection rules and is necessary to prevent waterborne diseases such as cholera, typhoid, and dysentery and to protect aquatic ecosystems. The WWTP contains a chlorination and a sulfonation room which houses four chlorinators, two sulfonators, and various process piping; an outdoor, open, roofed chemical storage area with up to nine ton chlorine containers, nine ton sulfur dioxide containers; and assorted safety equipment. The operations room containing flow and turbidity recorders, various electrical panels, and additional safety equipment is located several hundred feet from of the chlorination and sulfon
ation rooms. the facility is staffed 24 hours per day, seven days per week.
c. The offsite consequence analysis takes into consideration a "worst-case release" for chlorine and an "alternative release scenario" for both chlorine and sulfur dioxide. RMP regulations require that in a situation with more than one regulated toxic substance a worst-case release scenario for the regulated toxic substance with the greatest distance to the toxic endpoint and an alternative release scenario for each regulated substance be performed. The "worst-case release" is defined by the EPA as "the release of the largest quantity of a regulated substance (chlorine gas) from a vessel or process line failure that results in the greatest distance to a specified endpoint". The "alternative scenario" is defined as the "more probable" scenario of a failure, than is the "worst-case release".
Atmospheric models determine the distance from a point of release to the "toxic endpoint" where the chlorine or
sulfur dioxide concentration has decreased to three (3) ppm as selected by the EPA. The toxic endpoint selected by the EPA is based on Emergency Response Planning Guide 2 (ERPG-2) "toxic endpoint" which is 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 life-threatening health effects". The ERPG-2 for chlorine is 0.0087 mg/l (3 ppm) and for sulfur dioxide is 0.0078 mg/l (3ppm) (40 CFR 68.22, Appendix A). The residential population found within the circle defined by the radius to the toxic endpoint for each scenario has been determined using the 1990 census data projections for 1997 from geographic information systems (GIS) software produced by Environmental Systems Research Institute (ESRI). For both the worst-case and the alternative scenario the percentage of each census tract inside the area defined by the to
xic endpoint was estimated and multiplied by the total population for each affected tract to determine the residential population affected by an accidental release.
The worst-case release scenario at the Dunedin WWTP involves the release of one one-ton cylinder of chlorine (2,000 lbs.) outdoors. The offsite consequence analysis for this scenario is determined based on predefined conditions set by the EPA. The general conditions are as follows: the release of the entire contents from the cylinder as a gas in 10 minutes, an atmospheric stability class of F, wind speed of 1.5 m/s, ambient temperature of 25 degrees celsius, and a relative humidity of 50 percent. The conditions surrounding the Dunedin WWTP are defined as "urban" by the EPA. Based on exhibit 4-3 of the Risk Management Program Guidance for Wastewater Treatment Plants (EPA 550-B-98-010 October 1998), the distance to the toxic endpoint for the worst-case release scenario is 1.3 miles. The distance closely corresponds to
independent site specific modeling performed using the DEGADIS model.
The case study for the chlorine alternative release scenario involves the failure of a 5/16" gas valve to close. It is assumed that chlorine detectors alert the WWTP operators to the release. The amount of chlorine released is equal to 2,000 lbs., released at an average rate corresponding to the 5/16" hole, which equates to a release rate of 240 lb/min. The entire contents of the cylinder are released in 8.3 minutes. Exhibit 4-13 of the Risk Management Program Guidance for Wastewater Treatment Plants (EPA 550-B-98-010 October 1998), determines that the distance to the toxic endpoint is 0.3 miles.
The case study for the sulfur dioxide alternative release scenario involves the failure of a 5/16" fusible plug. It is assumed that mitigation efforts to repair the plug take one hour. The amount of sulfur dioxide released is equal to 420 lbs., released at an average rate corresponding to the 5/16" hole, which equate
s to a release rate of 7 lb/min. Exhibit 4-19 of the Risk Management Program Guidance for Wastewater Treatment Plants (EPA 550-B-98-010 October 1998), determines that the distance to the toxic endpoint is 0.1 miles.
d. The City of Dunedin accidental release prevention program consists of the following elements:
- The ongoing and continuous training of the operators.
- A preventative maintenance program that follows manufacturer's specifications and acceptable
- The Implementation of state-of-the-art process and safety equipment.
- Ongoing equipment and hazard reviews.
- The use of current operations and maintenance manuals.
- Continued equipment inspections.
The City of Dunedin has also included the following chemical specific prevention steps:
- The use of chlorine and sulfur dioxide dectectors.
- The use of self-contained breathing apparatuses (
SCBA), when handling chlorine and sulfur
- All operators have been trained to be aware of the hazardous effects and toxic properties of
chlorine and sulfur dioxide.
e. The Dunedin WWTP has experienced no accidental releases of chlorine or sulfur dioxide over the past five (5) years.
f. The Dunedin facility has an emergency response plan that has been coordinated with the Local Emergency Planning Committe (LEPC). This plan consists of a chain of command and decision tree for the response to an accidental release. The plan coordinates local and regional organizations to combat the effects of the release.