South Central Regional Wastewater Treatment Plant - Executive Summary

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INTRODUCTION 
At the turn of this century water borne diseases were a leading cause of death in the United States.  Epidemics of typhoid, cholera, dysentery and other water-borne diseases occurred.  After chlorine's introduction into public water supplies, deaths from typhoid in the U.S. dropped dramatically from 25,000 in 1900 to less than 20 in 1960.  Water-borne disease is even still a leading cause of infant mortality in many countries throughout Asia, Africa, and Latin America where infant mortality rates are 10 to 20 times greater than in the U.S.  In the U.S., however, water-borne disease has been virtually eliminated due to an effective public health strategy of utilizing chlorine for drinking water and wastewater disinfection.  Chlorine disinfection is arguably one of the greatest achievements for public health worldwide in the last hundred years and is credited with increasing the life expectancy of Americans by more than 50 percent. 
 
The use of chlorine is not risk free, howe 
ver.  Historically it is clear that utilities have recognized the risks and have been successful in developing procedures to handle chlorine safely and to protect the off-site public and the environment from potential accidental releases. 
 
As an added layer of protection, the United States Environmental Protection Agency promulgated Hazardous Chemical Risk Management Program regulations to further ensure that facility owners understand the risks and take proactive efforts to reduce risk through comprehensive training programs, procedures, and risk mitigation measures.  Additionally, EPA wanted the public to be informed regarding these issues. 
 
The South Central Regional Wastewater Treatment and Disposal Board (SCRWT&DB) has always understood these risks and has maintained a well-trained staff and a safely operated and maintained facility.  The SCRWT&DB's track record is exemplary with respect to handling hazardous chemicals.  Furthermore, the SCRWT&DB has taken a comprehensive approach 
to be in full compliance with the RMP regulation by June 21, 1999. 
 
FACILITY BACKGROUND 
The SCRWT&DB owns and operates the South Central Regional Wastewater Treatment Plant (SCRWTP).  The facility's mission is to protect public health by treating and disposing of wastewater.  Approximately 2,000 pounds per day of chlorine gas is used to disinfect the treated wastewater effluent prior to disposal.  Chlorine is also used at the facility for odor control, to control sludge bulking, and to disinfect highly treated reuse water which is used for irrigation.  A maximum of 38,000 pounds of chlorine are stored on-site.  The chlorine is stored as a gas that is liquefied under pressure in containers that contain 2,000 pounds each.  
 
CHLORINE PROCESS AND SAFETY EQUIPMENT 
The chlorine system at the SCRWTP facility is a liquid feed system.  Typically, there are seven containers of chlorine connected to the liquid header.  A manually operated switchover system allows for switching from one bank of t 
hree containers to another bank of four containers. Liquid chlorine is drawn from a single one-ton chlorine container through the liquid header into the evaporator.  The liquid chlorine in the evaporator is contained in a pressure vessel.  A hot water bath surrounding the pressure vessel transfers heat to the chlorine, which in turn vaporizes into a gas. Chlorine gas flows under pressure from the evaporator to the vacuum regulator-check unit (VRCU).  At the VRCU, the gas pressure is reduced to a slight vacuum. 
 
As water flows through the injector, the vacuum created by the pressure differential draws the chlorine gas through the remaining part of the process.  Chlorine gas passes through a chlorinator that is used to control the chlorine flow rate.  The gas flow rate through the chlorinator is indicated on the chlorinator's rotameter.  At the injector, the chlorine gas is mixed with water to form a chlorine solution.  The chlorine solution is then distributed to the application points. 
 
 
The chlorine storage room is completely enclosed in a building.  A perimeter leak detector will activate an emergency chlorine scrubber, shutoff the building ventilation system, close the building louvers and overhead rollup doors, and activate a visible alarm and two audible alarms.  The scrubber is capable of neutralizing 2,100 pounds of chlorine (i.e., one full container of chlorine).  The chlorine scrubber system is tested on a weekly basis to ensure trouble free operation.  In addition, all preventive maintenance recommended by the manufacturer is performed by plant staff. 
 
RISK MANAGEMENT AND PROCESS SAFETY MANAGEMENT PROGRAM OVERVIEW 
Chlorine is subject to the Environmental Protection Agency's (EPA's) Risk Management Program (RMP) regulation which can be found in 40 CFR 68 and the Occupational Safety and Health Administration's (OSHA's) Process Safety Management Program (PSMP) regulation which can be found in 29 CFR 1910.119. 
 
The primary components of the RMP are as follows: 
 
 
*     A five-year accident history 
*     An off-site consequence analysis for a worst-case and alternative release scenario 
*     A comprehensive prevention program to minimize risks (i.e., minimize the potential for a release) 
*     An emergency response program to ensure that an accidental release is appropriately handled 
*     An overall management program to supervise the implementation of the RMP 
 
Following development of the RMP, the facility must submit a Risk Management Plan (Plan) to the EPA by June 21, 1999.  The Plan is a summary of the facility's Risk Management Program.  The RMP will be updated every five years, or whenever a process changes or a new process is added.  The OSHA PSM regulation has basically the same requirements as the prevention program element of the EPA RMP.  The SCRWT&DB's RMP meets the requirements of both regulations. 
 
The following sections briefly summarize the elements of the SCRWT&DB's RMP. 
 
FIVE-YEAR ACCIDENT HISTORY 
The SCRWT&DB has used chlori 
ne to disinfect water for 20 years. In the last five years, the facility has had one accidental release, which was required to be reported under the RMP regulation.  The accident occurred on December 22, 1997, at 12:00 p.m. during maintenance of the in-line liquid chlorine strainers.  The liquid chlorine lines where purged of chlorine by closing the chlorine container supply valve and running the system until the pressure gauge on the header read zero psig.  A small amount (approximately one-pound or less) of chlorine remaining in the line evaporated when the line was opened.  The mechanic performing the maintenance was not wearing a self-contained breathing apparatus (SCBA) when the line was opened and was overcome by the chlorine fumes.  The mechanic had difficulty breathing and vomited.  Emergency services were notified and the mechanic was taken to Bethesda hospital by the paramedics for evaluation.  The mechanic was released from the hospital on December 22, 1997, with instruction 
s to rest.  The mechanic returned to work on December 24, 1997.  As a result of this accident, plant staff received training by the chlorine supplier on the hazards associated with chlorine with an emphasis on using SCBA when performing a line opening during system maintenance. 
 
WORST-CASE RELEASE SCENARIO 
The worst case release scenario for a toxic gas has been defined by the EPA to be an accidental release in which the largest on-site vessel containing chlorine releases its contents as a gas over ten minutes.  Since the largest container stored at the facility holds 2,000 pounds of chlorine, the worst case release scenario is a release of 2,000 pounds of chlorine over 10 minutes.  This scenario was modeled using RMP*Comp software to estimate the distance to an endpoint of 3 ppm.  It should be noted that this concentration has been found to typically cause minor eye and nose irritation.  It was assumed that the worst-case release occurred inside the storage building.  The enclosure of 
the building provides passive mitigation to reduce the release rate of chlorine gas to the atmosphere.  Active mitigation, such as the emergency chlorine scrubber is not allowed to be considered for the worst-case scenario. 
 
ALTERNATIVE RELEASE SCENARIO 
The alternative release is a "more likely" incident than the worst-case.  The RMP regulation allows the owner to define the alternative release scenario based on historical experience or operations staff knowledge of their system.   The alternative release scenario for the facility was assumed to be a disconnection of the pigtail (i.e., transfer hose) from the container valve.  This scenario was modeled using RMP*Comp software to estimate the distance to an endpoint of 3 ppm.  It should be noted that this concentration has been found to typically cause minor eye and nose irritation. The passive mitigation properties of the building were considered.  However, the effects of the emergency chlorine scrubber were not considered.  If the mi 
tigating affects of the scrubber were considered, there would be a minimal release with no significant impact on facility staff or off-site residents. 
 
PREVENTION PROGRAM 
There are always inherent risks associated with handling and using chlorine.  These risks include the potential inhalation of chlorine gas if it is accidentally released.  The prevention program is a key component to reducing the risk associated with a potential chlorine gas release.  Key elements of the prevention program include: 
 
*     Employee participation 
*     Process safety information 
*     Process hazard analysis 
*     Incident investigation 
*     Standard operating procedures 
*     Mechanical integrity 
*     Management of change 
*     Pre-startup review 
*     Training 
*     Contractors 
*     Compliance audits 
*     Hot work permits 
*     Trade secrets 
 
The following briefly states the benefits of the following prevention program elements: standard operating procedures, mechanical integrity program, employee 
training and the process hazard analysis. 
 
The facility staff has developed up-to-date and accurate written standard operating procedures (SOPs)  to ensure that operators have clear instructions for safe operation of the chlorine system.  Effective SOPs, when combined with operator training, are instrumental in ensuring safe operation of the system and in preventing accidental releases. 
 
The purpose of the mechanical integrity program is to ensure the continued integrity of the process equipment.  An effective mechanical integrity program is integral to preventing accidental chlorine releases that may result from mechanical failure of improperly maintained equipment.  The SCRWT&DB mechanical integrity program includes maintenance, inspection, and testing procedures and schedules along with maintenance personnel training. 
 
Knowledgeable well-trained personnel are essential to preventing and mitigating the effects of accidental chemical releases. The SCRWT&DB training program ensures th 
at personnel working on or near the chlorine system are adequately trained in operation and maintenance procedures and the appropriate response actions to an accidental chlorine release. 
 
The process hazard analysis (PHA) is a valuable risk reduction tool that outlines deficiencies in equipment and procedures, identifies potential system failure modes, and provides recommendations for system and operational improvements.  As a result of the PHA, several safety improvements were recommended for further investigation.  These recommendations are discussed in a latter section. 
 
EMERGENCY RESPONSE PROGRAM 
A comprehensive emergency response program has been prepared which outlines the procedures and lines of communication that are necessary to effectively respond to and mitigate a potential chlorine gas release. 
 
In the event of a chlorine gas release, a detector located in the storage area, will detect the presence of chlorine gas.  The detector will activate audible and visual alarms throu 
ghout the facility to give warning to plant staff.  In addition, the detectors well activate an emergency chlorine scrubber system that has the capacity to neutralize the release of 2,100 pounds of chlorine.  The facility emergency response program includes procedures for notifying the local hazardous materials (hazmat) teams of the incident and procedures for evacuating the facility.  Facility staff have coordinated with the local hazmat team and/or fire department to ensure that they are fully trained and equipped to quickly respond to an incident. 
 
PLANNED CHANGES TO IMPROVE SAFETY 
The process hazard analysis (PHA) identified 48 recommendations to improve the safety of the chlorine system.  The majority of the recommendations dealt with revisions to standard operating procedures and adding equipment to the facility's preventive maintenance program.  Key equipment upgrades planned include the addition of a motor operated gate and security code system to control unauthorized entry int 
o the facility.  In addition, facility staff plan to evaluate the need for installing automatic shut-off valves at each chlorine container.
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