SCWA: Sonoma Valley CSDTP - Executive Summary
Sonoma County Water Agency (SCWA) operates the Sonoma Valley Treatment Plant, and this facility is covered by the California Accidental Release Prevention (Cal-ARP) program and the EPA's Risk Management Plan (RMP) regulations because it has chlorine and sulfur dioxide onsite in excess of the RMP threshold quantities. The Sonoma Valley Treatment Plant is a wastewater treatment plant located on 8th Street East in the city of Sonoma, Sonoma County, California. |
It is SCWA's policy to comply with the Cal-ARP and RMP regulatory requirements. SCWA employees, supervisors and managers are all responsible for complying with these regulations. Agency employees and managers are provided with training to familiarize them with their requirements and responsibilities.
SCWA has developed a process safety and risk management program that documents the policies pertaining to the prevention of accidental releases. In addition, SCWA has conducted technical studies such as the Process Hazards Analysis a
nd the Offsite Consequence Analysis to identify and evaluate potential hazards associated with the use of chlorine and sulfur dioxide at the Sonoma Valley Treatment Plant. SCWA has emergency procedures and an Emergency Operations Plan (EOP), and these are coordinated with Sonoma County Department of Emergency Services.
A management system is in place for implementing the RMP and Cal-ARP regulations. This system defines the scope and purpose for each program element, procedures for implementation, roles and responsibilities, and the record-keeping requirements. The SCWA Safety Officer has responsibility to coordinate the implementation of the risk management program. Delegated responsibilities are outlined in the SCWA's written programs.
At the Sonoma Valley Treatment Plant, chlorine is used in the wastewater treatment process for odor control and disinfection. Chlorine is supplied in one-ton containers, which are stored inside a chlorine building. The room has capacity to store ni
ne one-ton containers of chlorine (18,000 pounds). Administrative controls are in place to ensure that the maximum number of containers never exceeds this number.
In addition to the chlorine containers, the chlorination system includes vacuum regulators, chlorinators and chlorine injectors, all connected by industry-standard pvc piping. The chlorine flow rate is set at the chlorinator to dose the correct amount of chlorine at the point of application. The design is inherently safe with the vacuum regulator placed directly on the ton container. This means that if there is any damage to the piping system downstream there will be a break in the vacuum, and the chlorine flow will stop at the regulator.
Sulfur dioxide is used for dechlorination prior to effluent discharge. Sulfur dioxide is also supplied in one-ton containers stored inside the same building. At no time will there be more than nine one-ton containers of sulfur dioxide (18,000 pounds). The sulfur dioxide system design is si
milar to that for chlorine with vacuum regulators, sulfonators and injectors, with the same inherently safe design using vacuum regulators directly connected to the ton containers.
All chlorine and sulfur dioxide equipment is housed inside the chlorine building. Leak detectors have been installed to monitor for chlorine and sulfur dioxide leaks. If a leak is detected, it triggers a local alarm and relays an alarm signal to the SCWA's Operations Center in Santa Rosa.
In developing the Offsite Consequence Analysis (OCA), the approach taken for chlorine was to follow the overall guidance provided by EPA and the American Water Works Association in the Model RMP for Water Treatment Plants. This model plan was used to help select the worst-case and alternative release scenarios, and to evaluate the hazard distances for both scenarios. The AMSA Model RMP for Publicly Owned Treatment Works was used for sulfur dioxide.
For the worst-case scenarios (chlorine and sulfur dioxide), a one-ton c
ontainer was used to determine the release quantity (2,000 pounds). This is the single largest vessel in each process. For the worst-case release to occur, there would have to be a container failure, allowing the entire contents to become airborne within ten minutes. Using the Model RMP tables, the distance to the toxic endpoint for this scenario would be 3.0 miles for chlorine (AWWA Model RMP), and 3.14 miles for sulfur dioxide (AMSA Model RMP), assuming rural terrain. For both chemicals, the worst-case release is extremely unlikely to occur, as it takes no account of the many safety features inherent in the design, operation and maintenance of the system, described above.
For chlorine, the alternative release scenario was selected from the AWWA Model RMP. This was a release from a sheared line about a quarter of an inch in diameter, connected to the container. Using the Model RMP tables, the distance to the toxic endpoint for this scenario would be 0.56 miles, for assuming rural t
For sulfur dioxide, the same alternative release scenario was selected as for chlorine, i.e., a release from a sheared line about a quarter of an inch in diameter. Using the AMSA Model RMP, the distance to the toxic endpoint for this scenario would be 0.11 miles.
For the worst-case scenarios, the population within the vulnerable zone circle is about 9,000 for chlorine and 9,700 for sulfur dioxide. For the chlorine alternative release scenario the number is less than 190, and for the sulfur dioxide alternative scenario, the number is less than 10. The SCWA has identified the public and environmental receptors within these zones, and those within the alternative release zones are addressed in the emergency response plan.
During the last five years, there have been no accidents involving chlorine or sulfur dioxide that resulted in any offsite impacts, and there have been no serious injuries or fatalities onsite.
SCWA has implemented a comprehensive prevention program to mi
nimize the potential for a catastrophic release of chlorine or sulfur dioxide at the Sonoma Valley Treatment Plant. Process safety information is available to employees, and Process Hazards Analyses (PHAs) have been conducted. External events were considered in the PHAs, and a seismic assessment has also been conducted. Recommendations made by the PHA team to improve safety are being addressed. Following any system modifications, a pre-startup safety review is conducted. A management of change procedure has been implemented.
Standard Operating Procedures (SOPs) have been developed for normal and emergency operations. The SOPs provide written operating instructions for operating the chlorine and sulfur dioxide systems, and they include procedures for container deliveries and container change-outs. Equipment inspections, testing and maintenance activities are performed on a routine basis and documented in a computerized maintenance management system (MAXIMO). All operators are state-ce
rtified treatment operators, and all operators and mechanics are required to be trained in the SOPs for the chlorine and sulfur dioxide systems.
A compliance audit procedure has been developed for the risk management program, and this will be used every three years to verify compliance with the Cal-ARP and RMP regulations. The SCWA investigates any incident or accident that results in the release of a reportable quantity of chlorine or sulfur dioxide. Incident investigation procedures include root cause analysis.
SCWA operators and mechanics are trained as first responders under the HAZWOPER standard for emergency response. However, only members of the HazMat team are qualified to respond to an emergency. Guidance is provided to the operators and mechanics to help them distinguish between an incidental release and an emergency. In the event of an emergency, the procedures are to notify the response agencies, evacuate to a safe area, and assess the cause of the release, if this can b
e done safely. The emergency response plan has been coordinated with the local response agencies, and SCWA will cooperate with these agencies to coordinate any offsite response that may be required in an emergency.
Following the PHA conducted in February 1999, SCWA is implementing recommendations to improve safety. These improvements include equipment modifications, improvements to the detection and monitoring system, and revisions to the operating and maintenance procedures. Recommendations developed in any future MOC review will be included in the action tracking system to ensure they are addressed in a timely manner.
SCWA is continually looking at new technologies to improve safety and to reduce or eliminate the use of chlorine and sulfur dioxide. For example, a scrubber system will be installed at the Sonoma Valley Treatment Plant by the end of year 2000.
In summary, the design of the chlorine and sulfur dioxide systems at the SCWA Sonoma Valley Treatment Plant, and the imple
mentation of the safety and risk management programs provide a high level of assurance that the risks associated with the use of chlorine and sulfur dioxide are effectively managed.