Central District Wastewater Treatment Plant - Executive Summary

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EXECUTIVE SUMMARY 
 
Chlorine is the most commonly used substance for disinfecting treated wastewater.  The Miami-Dade Water & Sewer Department (MDWASD) Central District Wastewater Treatment Plant (WWTP) also uses chlorine (for chlorination) for disinfecting treated wastewater to provide safe water discharges to the environment.  Storing and handling large quantities of chlorine can create hazardous situations.  The Central District WWTP takes safety obligations in storing and using chlorine as seriously as it takes care in treating watewater. The Central District WWTP also produces digester gas and stores it for on -site use as fuel. 
 
The Central District WWTP chlorine and digester gas handling processes are subject to the U.S. Environmental Protection Agency (EPA) Risk Management Program Rule.  This rule requires submission of a Risk Management Plan (RMP).  An integral part of the RMP is a summary of policies and procedures followed to safely operate the facility, including a descripti 
on of the possible consequences in case of an accident and the actions, which will be taken, by the facility in an event of an emergency. 
 
The following information is specifically required in the RMP Executive Summary:  
 
* Accidental release prevention and emergency response policies. 
 
* General facility and regulated substances information. 
 
* Offsite consequence analysis results. 
 
* Summary of the accidental release prevention program and chemical-specific prevention steps. 
 
* Five-year accident history summary. 
 
* Emergency response program summary. 
 
* Planned changes to improve safety. 
 
The above information for the Central District WWTP chlorine and digester gas systems are provided below. 
 
Accidental Release Prevention and Emergency Response Policies 
 
The MDWASD accidental release prevention policy involves a unified approach that integrates proven technology, trains staff in operation and maintenance practices, and uses tested and proven management system practices.  All applic 
able procedures of the EPA's Prevention Program are adhered to, including key elements such as training, systems management, and emergency response procedures.   
 
The MDWASD emergency response policy involves the preparation of emergency response plans for hazardous materials which are tailored to each facility and to the emergency response services available in the community, and is in compliance with the EPA Emergency Response Program Requirements.  Central District WWTP has prepared an Emergency Response Plan for the treatment plant to facilitate coordination and emergency planning with offsite response officials and facilities in the event of an emergency.  Central District WWTP has an excellent record in preventing accidents from occurring. 
 
General Facility and Regulated Substance Information 
 
The MDWASD Central District WWTP is located on Virginia Kay in Miami, Florida.  The Central District WWTP has two separate liquid treatment trains that are referred to as Plant 1 and Plant  
2 and are rated at 60 mgd and 83 mgd, respectively.  The facility stores chlorine and digester gas which are regulated toxic substances under RMP rule.  The two chlorine systems (for Plant 1 & 2) supply approximately 7,400 pounds of chlorine per day to the wastewater effluent and the non-potable water system. 
 
At Plant 1, chlorine is delivered in one-ton containers to the storage area of the Central District WWTP chlorine building.  The chlorine building is divided into a covered chlorine storage area and a chlorinator room.  The one-ton containers contain liquid chlorine under pressure.  The storage area in Plant 1 is equipped to store up to 32 (cylinders) tons of chlorine.  The maximum quantity of chlorine present at the Central District Plant 1 WWTP at any given time exceeds the listed threshold quantity in the EPA rule.  It also exceeds the listed threshold quantity in the federal Occupational Safety and Health Administration (OSHA) Process Safety Management (PSM) Standard. 
 
The pr 
essure from the container forces liquid chlorine through the tubing and into the manifold system.  Downstream of the manifold system, there is a pneumatically actuated automatic shut-off valve.  Pressing one of the automatic shut-off switches can manually close the shut-off valve.  The valve will also close automatically when the chlorine sensor senses a leak.  The liquid chlorine flows through the automatic shut-off valve to the evaporators. At the evaporators the liquid chlorine is heated and converted into chlorine gas.  The gas then flows from the evaporator to the chlorinator.  The chlorinators are used to control the feed rate of the chlorine gas.  The chlorinators are equipped with a gas rotameter and a v-notched orifice for controlling the feed rate.  Injection of the chlorine into the plant water produces the concentrated solution that flows through piping to various injections points in the treatment process. 
 
The Plant 1 chlorination process of the Central District WWTP is a 
lso provided with a number of safety features to protect the employees and the surrounding community.  The two important safety features are (1) automatic shut-off valves and (2) chlorine leak detection and alarm system.  The automatic shut-off valves are located immediately downstream of the chlorine containers.  Therefore, if a leak occurs anywhere in the system the chlorine source can be isolated and the flow of chlorine can be stopped.  The automatic shutoff valves are operated either automatically due to the detection of chlorine leak by the chlorine sensors or manually by pushing a switch.  Emergency shutoff switches are located inside the chlorine storage room and in the chlorinator room. 
 
The leak detection system consists of chlorine gas sensors located in both the chlorinator room and the storage area.  The leak detection system is set to activate the alarms and safety processes when a chlorine concentration of 0.5 ppm or higher is detected.  When the leak detection system de 
tects a leak, audible and visual alarms (both local and in the control room) are activated and the automatic shut-off valves closed. 
 
At Plant 2, chlorine is delivered in one-ton containers to the storage area of Central District WWTP chlorine building.  The chlorine building is divided into a covered chlorine storage area and a chlorinator room.  The one-ton containers contain liquid chlorine under pressure. The chlorine storage area in Plant 2 is equipped to store up to 46 (cylinders) tons of chlorine. The maximum quantity of chlorine present at the Central District Plant 2 WWTP at any given time exceeds the listed threshold quantity in the EPA rule.  It also exceeds the listed threshold quantity in the federal Occupational Safety and Health Administration (OSHA) Process Safety Management (PSM) Standard. 
 
The pressure from the container forces liquid chlorine through the tubing and into the manifold system.  Downstream of the manifold system, there is a pneumatically actuated automat 
ic shut-off valve.  Pressing one of the automatic shut-off switches can manually close the shut-off valve.  The valve will also close automatically when the chlorine sensor senses a leak.  The liquid chlorine flows through the automatic shut-off valve to the evaporators. At the evaporators the liquid chlorine is heated and converted into chlorine gas.  The gas then flows from the evaporator chlorinator.  The chlorinators are used to control the feed rate of the chlorine gas.  The chlorinators are equipped with a gas rotameter and a v-notched orifice for controlling the feed rate.  Injection of the chlorine into the plant water produces the concentrated solution that flows through piping to various injections points in the treatment process. 
 
The Plant 2 chlorination process of the Central District WWTP is also provided with a number of safety features to protect the employees and the surrounding community.  The two important safety features are (1) automatic shut-off valves and (2) chl 
orine leak detection and alarm system.  The automatic shut-off valves are located immediately downstream of the chlorine containers.  Therefore, if a leak occurs anywhere in the system the chlorine source can be isolated and the flow of chlorine can be stopped.  The automatic shutoff valves are operated either automatically due to the detection of chlorine leak by the chlorine sensors or manually by pushing a switch.  Emergency shutoff switches are located inside the chlorine storage room and in the chlorinator room. 
 
The leak detection system consists of chlorine gas sensors located in both the chlorinator room and the storage area.  The leak detection system is set to activate the alarms and safety processes when a chlorine concentration of 0.5 ppm or higher is detected.  When the leak detection system detects a leak, audible and visual alarms (both local and in the control room) are activated and the automatic shut-off valves closed. 
 
The Central District WWTP also produces waste ac 
tivated sludge (WAS) which requires additional treatment prior to disposal.  In addition, the Central District WWTP also receives a mixture of primary sludge/WAS from the North District WWTP.  The sludge treatment process is divided into two trains from secondary clarifiers through anaerobic digestion.   One train is associated with Plant 1 at the Central District WWTP and receives WAS from the Plant 1 secondary clarifiers.  The Plant 2 train receives WAS from the Plant 2 secondary clarifiers in addition to the North District WWTP sludge.  After anaerobic digestion, the sludge treatment processes are combined into a single process train.  The sludge streams are first conditioned with polymer and fed to gravity thickeners.  The sludge, from the thickeners, is then pumped to two-stage anaerobic digesters for stabilization.  The gas produced by the digestion process is compressed, scrubbed to remove hydrogen sulfide and stored on-site in compressed gas spheres.  There are two 80 feet diam 
eter spheres.  The digester gas is used to power engine driven blowers and electric generators.  Electricity generated by burning the digester gas is used to power some of the plant equipment.  Waste heat from the gas-powered engines is used to heat the primary digesters.  The maximum quantity of digester gas present at the Central District WWTP at any given time exceeds the listed threshold quantity in the EPA rule.  It also exceeds the listed threshold quantity in the federal Occupational Safety and Health Administration (OSHA) Process Safety Management (PSM) Standard. 
 
A perimeter fence and security gates surround the Central District WWTP.  The plant operates three shifts with employees onsite 24 hours per day, 365 days per year. 
 
The Central District WWTP is provided with a backup emergency power generator that supplies power to the entire facility. 
 
Offsite Consequence Analysis Results 
 
The offsite consequence analysis includes consideration of two chlorine and two digester gas r 
elease scenarios, identified as "worst-case release scenario" and "alternative release 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 over 10-minutes," due to an unspecified failure. The alternative scenario is defined as "more likely to occur than the worst-case release scenario". 
 
Chlorine is the only regulated toxic substance stored and handled at the Central District WWTP, which is subject to the RMP rule.  Thus, chlorine was selected for the worst-case release scenario.  Only passive or administrative controls are allowed under this scenario to reduce off-site impacts.  The scenario used for the Central District WWTP is the rupture of a single one-ton chlorine container resulting in the release of 2,000 pounds of chlorine over a 10-minute duration.  There are no passive or administrative controls at the Central District WWTP for the chlorine containers; thus, the chl 
orine release rate will be 200 lbs./min. 
 
The released liquid chlorine is assumed to form a denser-than-air cloud consisting of chlorine vapor and liquid droplets (aerosols) and then disperse in the atmosphere.  The distance to the toxic endpoint was estimated using the EPA's RMP*Comp software (version 1.06).  The toxic endpoint selected by EPA rules for chlorine is 3 ppm, which is the Emergency Response Planning Guideline Level 2 (ERPG-2).  The toxic endpoint was conservatively set by EPA to ensure public notification and that local emergency response planning takes into account the greatest possible impacted area surrounding the release point. EPA-mandated meteorological conditions, namely atmospheric Stability Class F, wind speed of 1.5 meter per second, highest daily maximum temperature (77 deg F), and average relative humidity (50%) were used for the worst-case release scenario analysis.  The results of the dispersion analysis indicated that the worst-case release scenario has off 
site impacts. 
 
Digester gas containing methane is the only regulated flammable substance stored and handled at the Central District WWTP, which is subject to the RMP rule.  Thus, digester gas was selected for the worst-case release scenario. Only passive or administrative controls are allowed under this scenario to reduce off-site impacts.  The scenario used for the Central District WWTP is the rupture of one digester gas sphere of 80 feet in diameter resulting in the release of 25,468 pounds of digester gas.  There are no passive or administrative controls at the Central District WWTP for the digester gas storage spheres. 
 
The released digester gas is assumed to form a vapor cloud and a detonation occurs.  A yield factor of 10 percent of TNT-equivalency model is used to determine the distance to the endpoint.  The distance to the endpoint is defined as the distance over which a minimum pressure of 1 psi occurs from the pressure wave formed by the detonation.  The distance to the endpo 
int was estimated using the EPA's RMP*Comp software (version 1.06). The results of the analysis indicated that the worst-case release scenario has offsite impacts. 
 
RMP rules require that a scenario which results in offsite toxic endpoint distance and is more likely to occur than the worst-case scenario should be selected as the alternative release scenario for each regulated toxic and flammable substance, unless no such scenario exists. Unlike the worst-case scenario, the alternative release scenario may consider "active" mitigation such as automatic shutoff valves, excess flow valves, and containment with scrubbers.  Active mitigation is defined as requiring mechanical, electrical, or human input. 
 
The alternative release scenarios must consider the facility accident history and/or failure scenarios identified in the process hazard analysis.  A review of the past five-year accident history data for the chlorination and digester gas facilities pursuant to these rules indicated that th 
ere were no chlorine or digester gas releases, which could have resulted in offsite (outside the Central District WWTP boundary) toxic or flammable endpoint distances.  Similarly, no credible accident scenario was identified from the process hazard analysis, which would reach offsite.  Thus, an acceptable/credible alternative release scenario had to be selected based on expert judgement.  
 
The alternative release scenarios selected for the chlorine and digester gas processes are summarized below: 
 
The chlorine alternative release scenario selected involves the release of chlorine from a pigtail (1/4" diameter) rupture that occurs during the connection of the chlorine container to the chlorination process.  The normal response of the operator is to shut-off the container valve.  However, the operator may become incapacitated, leaving the valve open.  It is assumed that approximately 15 to 30 minutes would be required for the emergency response team to respond to the accident.  In the me 
antime, all of the chlorine in the container would have been released.  The chlorine release rate from the one-ton container was estimated at 150 pounds per minute; thus, the whole container would be emptied in about 13 minutes. No passive mitigation was considered for the estimation of chlorine release rate.  However, human input in closing the shut-off valve was taken into account as an active mitigation, which will reduce the quantity of chlorine release to the atmosphere. 
 
The chlorine alternative release scenario toxic endpoint distance was also estimated using the RMP*Comp (version 1.06) software.  Toxic endpoint for chlorine is 3 ppm.  EPA suggested typical meteorological conditions used were Stability D, wind speed of 3.0 meter per second, average air temperature of 77 deg F, and average relative humidity of 50 percent. The results of the dispersion analysis indicated that the chlorine alternative release scenario has offsite impacts. 
 
The digester gas alternative scenario sele 
cted represents an event, which actually occurred at the Central District WWTP.  The scenario involves the release of digester gas (consisting of 65% methane) from a pressure relief valve installed on the 80 feet diameter storage sphere. It is assumed that the valve vents to the atmosphere when the pressure in the sphere is 10% above the design value of 40 pound per square inch gauge (psig), at a pressure of 44 psig. The release rate of digester gas was estimated at 1560 pounds per minute (lb/min).  It is assumed that the tank vents for 15 minutes with a methane release rate of 1012 lb/min for a total release of 15,180 lbs of methane.  The alternative release scenario also assumes that a vapor cloud fire (flash fire) may result from the dispersion of the cloud of flammable vapor and ignition of the cloud following dispersion.  Such a fire could flash back and could represent a severe heat radiation hazard to anyone in the area of the cloud.  The flammable endpoint is characterized by t 
he lower flammability limit (LFL) of the substance.  No passive or active mitigation systems were considered.  The distance to the endpoint was estimated using the EPA's Offsite Consequence Analysis Guidance document.  The results of the analysis indicated that the digester gas alternative release scenario has no offsite impacts (flammable endpoint does not extend beyond the facility boundary). 
 
Finally, no chlorine or digester gas releases that could have caused safety or health hazard (no deaths, injuries, property or environmental damage, evacuations, or sheltering in place) occurred at the Central District WWTP during the last five years.   
 
Summary of the Accidental Release Prevention Program and Chemical-Specific Prevention Steps 
 
Central District WWTP is in compliance with Federal and State Process Safety Management requirements.  Central District WWTP accidental release prevention program is based on the following key elements: 
 
* Detailed management system.  
 
* Comprehensive p 
rocess safety information that is readily available to staff, emergency responders, and contractors. 
 
* Comprehensive preventive maintenance program. 
 
* Performance of process hazard analysis of equipment and procedures with operation and maintenance staff participation and review. 
 
* Use of state-of-the-art process and safety equipment. 
 
* Use of accurate and effective operating procedures, written with the participation of the operators. 
 
* Training of the operators and maintenance staff. 
 
* Implementation of an incident investigation program. 
 
Chemical-specific prevention steps include availability of self-contained breathing apparatus (SCBA) for chlorine, personnel protective equipment for digester gas, awareness of the hazardous and toxic properties of digester gas and chlorine, presence of chlorine and digester gas detectors and alarms, and automatic shutoff device on the chlorine storage. 
 
Process and Chemical Safety Information 
 
Comprehensive chemical data have been assembled t 
o include regulatory reporting and action thresholds, health hazard, and chemical exposure limitations, as well as detailed physical properties of chlorine and digester gas.  This information includes chlorine and digester gas background information and MSDS sheets. 
 
Equipment safety information was meticulously compiled on the chlorine and digester gas processes.  Specifications for chlorine and digester gas processes are collected and provided in one place for easy reference.  Details such as maximum intended inventory; safe upper and lower temperatures; safe upper and lower pressures; and codes and standards used to design, build, and operate the processes are on file at the facility. 
 
Process Hazard Analysis  
 
In 1997 and 1999, detailed Process Hazard Analysis (PHA) was conducted for the chlorine and digester gas systems equipment and procedures, respectively.  To further assess the integrity of the chlorine and digester gas systems for the preparation of this RMP, checklists were  
used to assess the overall general condition of the chlorine and digester gas systems operation and maintenance, including human factors that affect personnel performance and system integrity.  The PHA(s) will be updated again within a five-year period or whenever there is major change in the processes.  A list of recommended actions were developed to further improve the chlorine and digester gas safety and staff is currently evaluating these recommendations.  Staff will document the completion of recommended actions. 
 
Operating Procedures 
 
MDWASD Central District WWTP has prepared written operating procedures that provide instructions or steps for safely conducting activities relating to the chlorine and digester gas processes.  They are consistent with the chlorine and digester gas Process Safety Information.  Written operating procedures include : initial startup, normal operations, emergency shutdown, normal shutdown, and start up after emergency shutdown.  In addition, Central Dis 
trict WWTP developed Standard Operating Procedures (SOPs) for the following steps in the chlorination process: receipt of chlorine delivery, chlorine cylinder change, and chlorine release/spill response.  Operating procedures will be developed and put in place prior to any new process equipment coming on line or changes made in the handling of chlorine and digester gas equipment, and reconfiguration of the facilities. 
 
Training  
 
Central District WWTP employees presently involved in operating or maintaining the chlorine and digester gas processes are trained in an overview of the processes and the applicable operating and maintenance procedures.  Central District WWTP ensures that each employee newly assigned to the processes, is trained and tested to be competent in the operating procedures listed pertaining to their duties.  Each employee (presently involved in operating the chlorine and digester gas process) has been trained to receive the required knowledge, skills, and abilities t 
o safely carry out the duties and responsibilities, including chlorine and digester gas emergency response, as provided in the operating procedures.   
 
Refresher training is provided every three years or less to each employee operating the covered process to ensure that the employee understands and adheres to the current operating procedures.  In addition, the Central District WWTP ensures that operators are trained in any updated or new procedures prior to startup of a process after a major change as indicated in their Management of Change procedures. 
 
The Central District WWTP prepares and retains records of initial and refresher training, provides certification of the records, which includes the identity of the employee, the date of training, and the signature of the person (s) administering the training. 
 
Contractors 
 
MDWASD Central District WWTP has procedures and policies in place that specify the information required to be provided to the contractors performing work on the chlor 
ine and digester gas processes, the training requirements for contractor employees, and mechanism to obtain assurance from contractors that they have informed their employees of the appropriate safety rules.  MDWASD Central District WWTP is required to provide information and explanations concerning the hazards and processes and obtain and evaluate information regarding the contractor's safety program.  
 
Pre-Startup Safety Review and Mechanical Integrity Program 
 
MDWASD Central District WWTP has procedures in place to ensure that a pre-startup safety review is conducted prior to starting up a new covered process or after shutdown event, or prior to starting up modifications to the chlorine and digester gas processes that require a MOC procedure implementation. 
 
Hot Work Permits and Management of Change  
 
The Central District WWTP requires employees and contractors to employ safe work practices when performing "hot work" in, on, or around the covered process.  The Central District WWTP  
uses a permitting program to ensure that hot work is conducted safely on or near a process involving chlorine and digester gas.  
 
Internal Compliance Audits 
 
Internal compliance audits will be conducted every 3 years to verify compliance with the programs and procedures contained in the RMP.  The Central District WWTP will assemble an audit team that will include personnel knowledgeable in the Risk Management Program rule and in the process.  This team will evaluate whether the prevention program satisfies the requirements of the Risk Management Program rule and whether the prevention program is sufficient to help ensure safe operation of the process.  The results of the audit will be documented, recommendations resolved, and appropriate enhancements to the prevention program will be implemented. 
 
Incident Investigation 
 
The Central District WWTP investigates all incidents that could reasonably have resulted in a catastrophic release (serious injury to personnel, the public, or the env 
ironment) so that similar accidents can be prevented.  An investigation team is assembled and the investigation is initiated within 48 hours of the incident.  The results of the investigation are documented, recommendations are resolved, and appropriate process enhancements are implemented.  Information found during the investigation is reviewed by affected staff and added or used to revise operating and maintenance procedures. 
 
Five-year Accident History Summary 
 
No chlorine or digester gas releases that could have caused safety or health hazard (deaths, injuries, property or environmental damage, evacuations, or sheltering in place) occurred at the Central District WWTP during the last five years. 
 
Emergency Response Program Summary 
 
The Central District WWTP is a first responder, plant employees respond to chlorine and digester gas accidental releases.  Depending on the severity of the accidental release, external resources such as the City of Miami Fire Department may be solicited  
to aid in handling chlorine or digester gas release.  As part of the emergency response program, the Central District WWTP has developed and implemented an emergency action plan for the purpose of protecting public health and the environment. 
 
The emergency response plan has been coordinated with local response agencies (City of Miami Fire Department).  The main elements of the emergency response plan are: (1) chlorine and digester gas response flow chart, responsibilities of various personnel at the facility, duties of on-scene incident commander, site response team, and site safety representative, (2) details of emergency recognition and prevention at the facility, (3) procedures for planning and coordination with off-site emergency response organizations, and (4) details of the training program for all employees involved with the chlorine and digester gas processes.  
 
Planned Changes to Improve Safety 
 
Numerous changes to improve safety (recommended actions) were previously identifi 
ed for the chlorine and digester gas process in 1997 and 1999, respectively, for chlorination and digester gas systems equipment and procedures when Process Safety Management (PSM) Plans were prepared. All of these recommended actions have been evaluated for implementation as required.
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