Dale Mabry Advanced Wastewater Treatment Plant - Executive Summary

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The Hillsborough County Water Department (HCWD) has an accidental release prevention policy integrating technology, established procedures and management practices into a unified approach to risk management.  The HCWD operates a number of water and wastewater treatment facilities within unincorporated Hillsborough County, Florida.  The HCWD adheres to all applicable policies and procedures of the United States Environmental Protection Agency, the Florida Department of Environmental Regulation and the Florida Department of Community Affairs.  The HCWD response plan involves the preparation of response plans for each facility.  Each facility is in compliance with the EPA emergency response program requirements. 
The Dale Mabry AWT is located at 13055 Delwood Road, Tampa, Fl.  In addition to the treatment facility, the complex includes wastewater transmission facilities, a reclaimed water system and surface water discharge facilities.  The complex also houses the Water Department Enviro 
nmental Laboratory and the Wastewater Operations Administration offices.  
The offisite consequence analysis includes consideration of two chlorine release scenarios, identified by the EPA as "worst case release" and "alternative scenario". The first, worst case release is defined by the 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 identified as "more likely to occur than the worst case scenario." 
The HCWD used  the EPA lookout tables to determine offsite consequences. 
The worst case scenario at the Dale Mabry AWT involves an unspecified failure of a single one-ton container of chlorine.  The EPA-specified conditions assume the release of the entire amount as a gas in ten (10) minutes, use of the one-hour ERPG-2 as the toxic endpoint and consideration of the population residing within a full circle with radius corres 
ponding to the toxic endpoint distance.  The Hillsborough County Water Department believes the EPA assumption to be unrealistic because: 
   A.     Only a fraction of the compressed liquified chlorine released to the atmosphere flashes to a vapor.  The remaining liquid forms droplet aerosols.  This results in the formation of a very dense chlorine cloud consisting of vapor and liquid droplets, with dispersion characteristics significantly different than for a cloud of only chlorine gas. 
    B.     Only the population within an elliptical plume extending downwind of the release point is potentially affected.  The plume area, or footprint, is approximately six (6) percent of the area of the full circle.  
Mitigation systems to be considered include the chlorine leak detection system and the vacuum piping. The Dale Mabry AWT chlorine system is a vacuum system.  This type of system is expected to significantly reduce the amount of chlorine released in the event of a pigtail or piping fa 
The HCWD's general release prevention program is based on several key elements: 
    High level of training of operations and maintenance staff. 
    Preventative Maintenance Program 
    State of the art process and safety equipment. 
    Accurate, effective operating procedures 
    Hazard review of equipment and procedures 
    An in-house inspection program 
Chemical specific prevention steps include availability of self-contained breathing apparatus (SCBA) worn by the operators during connection/disconnection of chlorine and/or sulfur dioxide supply, awareness of hazardous and toxic properties of chlorine and sulfur dioxide and the presence of chlorine and sulfur dioxide detection equipment.  The facility also maintains chlorine and sulfur dioxide repair kits on site.
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