Upper Blackstone Reg. Wastewater Treatment Plant - Executive Summary

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Source and Process Description/Summary of Major Hazards - The Upper Blackstone Water Pollution Abatement District (UBWPAD) is committed to safe management of chlorine used at its chlorination facility.  Chlorine is the only substance regulated under the Environmental Protection Agency's (EPA's) Chemical Accident Prevention Provisions (40 CFR Part 68) stored on site in quantities exceeding the chemical-specific regulatory threshold of 2,500 pounds. 
 
UBWPAD is an operational municipal wastewater treatment plant with a design capacity of 56 million gallon per day (MGD).  The plant was constructed in the mid-1970's and started operation in 1976.  The District serves the wastewater treatment needs of its member communities, Worcester, Auburn, Millbury, Sutton, Holden, Rutland, and West Boylston.  
 
As part of the wastewater treatment operation, treated effluent is chlorinated during the summer months prior to discharge to the Upper Blackstone River.  Occasionally, during both winter and sum 
mer, return sludge is also chlorinated.  The chlorination facility is housed in a dedicated building that has been designed for the purpose of protecting the chlorination equipment and the one ton cylinders of chlorine used in the operation. 
 
The EPA has regulated chlorine stored in excess of 2,500 pounds under its Chemical Accident Prevention Provisions because there is a possibility of off-site consequences due to accidental release of this compound.  UBWPAD has not experienced any accidental releases of chlorine at the facility during the 23-year history of its operation.  However, in order to adequately prepare for the possibility of an accidental release and to communicate with emergency responders, the facility has identified possible consequences of such events using models and accident scenarios prescribed by the EPA. 
 
Consequences of Failure to Control the Hazards - The offsite 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 by EPA as one that is "more likely to occur than the worst-case release scenario". 
 
Atmospheric dispersion modeling was performed for both scenarios to determine the distance traveled by the released chlorine before its concentration decreases to the "toxic endpoint" selected by EPA of 3 parts per million (ppm), which is the Emergency Response Planning Guideline Level 2 (EPRG-2).  This level 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 a 
n individual's ability to take protective action".   
 
 
The worst-case chlorine release scenario and modeling follow conditions predefined by the EPA: 
 
(1)    the entire contents of a one ton chlorine cylinder are released as a gas within a ten minute period; and 
 
(2) the meteorological conditions during this release are atmospheric stability class F and a wind speed of 1.5 meters per second (m/s). 
 
The result of this release scenario and modeling is that off-site impacts (chlorine concentrations in excess of 3 ppm) are predicted. 
 
The alternative chlorine release scenario, considered by UBWPAD more likely than the worst-case scenario (but still highly unlikely), was modeled as follows: 
 
(1)    the entire contents of three one-ton chlorine cylinders are released through two <" tubing openings as a compressed liquid, which then vaporizes, within a 150 minute period; and 
 
(2) the meteorological conditions during this release are atmospheric stability class D and a wind speed of 3 m/s. 
 
The resu 
lt of this release scenario and modeling is that, while off-site impacts were predicted, the distance to the 3 ppm toxic endpoint was significantly reduced.  For both the worst-case and alternative case scenarios, the chlorination building was assumed in the modeling to provide some mitigation of the release by delaying the liberation of chlorine to the outdoors. 
 
The most important differences between the worst-case and the alternative scenarios are the meteorological conditions.  Regarding meteorological conditions, atmospheric stability classes range from A to F, with A being the least stable and F being the most stable.  The F stability class condition occurs only at night when there is little atmospheric mixing to help disperse the chlorine.  The D stability class condition is the most stable daytime condition possible, the daytime condition when the least atmospheric mixing occurs. 
 
This is significant for two reasons.  First, handling of chlorine cylinders and startup of the ch 
lorination system only take place during the day.  This means that if an accidental release were to take place, it would be much more likely to take place during the daytime than at night.  This is the time of day when atmospheric conditions are more favorable to mixing, dilution and dispersion.  Second, the D stability class condition and 3 m/s wind speed conditions are conservative.  It is more likely that a less stable condition (A, B or C) with a higher wind speed will occur during the daytime, with even greater dispersion.  With even greater dispersion, in the event of an accidental release, chlorine concentrations would likely be below EPA toxic endpoint levels before a release left the UBWPAD property. 
 
Explanation of How Releases are Prevented - In order to diminish the likelihood of accidental chlorine release, UBWPAD maintains a prevention program.  This program consists of periodic review of safety information and hazards, maintenance of standard operating procedures, an emp 
loyee training program, regular equipment inspection, and ongoing equipment maintenance. 
 
Response Action in the Event of a Release - UBWPAD has developed an emergency response plan that includes notification of the Millbury Fire Department for emergency management assistance in the event of elevated levels of chlorine in the chlorination building.  If any chlorine related incidents do occur, they will be investigated in accordance with EPA guidelines, and recommendations for preventing a repeat of the incident will be developed and implemented. 
 
Steps Taken to Address Hazards - The most recent safety review has resulted in several planned changes that will improve the safety of the chlorination facility and emergency preparedness.  These include installation of a new chlorine alarm system, installation of a wind sock, and upgrade of the chlorination building ventilation system.
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