Monterey Regional Treatment Plant - Executive Summary
It is the policy of the Monterey Regional Water Pollution Control Agency (MRWPCA) to comply with the regulatory requirements of the California Accidental Release Prevention (Cal-ARP) program and the EPA's Risk Management Plan. The RTP has developed a Process Safety Management (PSM) program and an Emergency Response Plan (ERP) that documents MRWPCA policies pertaining to the prevention of accidental releases and emergency response. In addition to implementing the PSM and ERP programs, the MRWPCA has conducted technical studies such as the Process Hazards Analysis (PHA) and an Offsite Consequence Analysis (OCA) to identify and evaluate potential hazards. |
The Monterey Regional Water Pollution Control Agency (MRWPCA) Regional Treatment Plant (RTP) is located northeast of the City of Marina, in Monterey County, California. Major plant construction took place in the 1980s, with the Salinas Valley Reclamation Project (SVRP) was completed in 1998.
Chlorine is used in the wastewater treatmen
t process for odor control and disinfection, and sulfur dioxide is used for dechlorination. Chlorine and sulfur dioxide are supplied in one-ton containers. Containers, evaporators and chlorinators/sulfonators are contained inside a building equipped with a scrubber system. The building has capacity to store 32 ton containers of chlorine, and eight tons of sulfur dioxide. Methane from the wastewater digesters is also stored onsite, but the quantity stored is less than the RMP threshold quantity for flammable materials. The maximum quantity of chlorine delivered to the RTP should not exceed 11 tons per week.
The main purpose of the sulfur dioxide system is to dechlorinate the plant effluent prior to discharge from the ocean outfall. The quantity of sulfur dioxide used is very low under normal operations. Only when there is discharge to the ocean is sulfur dioxide used to dechlorinate the effluent. No sulfur dioxide is stored onsite when dechlorination is not required.
From the chlorin
e building, pvc piping delivers chlorine and sulfur dioxide under vacuum or in a water solution to the application points around the plant. The scrubber is installed on a separate pad outside the main building. The scrubber is interlocked with the chlorine/sulfur dioxide detectors in the building and in the relief vent. Exhaust gases from the chlorine building can be routed to the scrubber during an emergency.
The quantity of chlorine supplied by each chlorinator can be controlled manually or automatically using a flow control loop controlled by the Programmable Logic Control (PLC) system. The correct chlorine flow rate is calculated automatically depending on process parameters such as reclamation plant process flowrate.
Both the chlorine and sulfur dioxide systems are equipped with alarms to provide warning of system malfunctions or out of range operation. Some of the alarms are interlocked to the systems providing important safety functions. Leak detectors are located throughou
t the chlorine building to monitor ambient air for potential leaks. All ambient detectors trigger a leak alarm and activate the scrubber automatically.
In developing the OCA, the approach taken was to follow the overall guidance provided by EPA in the RMP OCA Guidance (EPA Guidance) document (June 1996). The EPA look-up tables were used to evaluate the worst-case release scenario, and computer models (SuperChemsT and SLAB) were used for the alternative release scenario.
For chlorine, the worst-case scenario was chosen to be a five-ton release over ten minutes with a passive mitigation factor of 0.55 because the facilities are contained inside the chlorine building. This gave a worst-case release rate of 550 pounds per minute and a hazard distance of 9.9 miles. There are approximately 218,100 people within the worst-case hazard zone circle. The alternative release scenario was chosen as a half-inch release from a single one-ton container during offloading. This would result in an av
erage release rate of 18 pounds per minute over 20 minutes, and a hazard distance of 2,840 feet (866 meters). With this scenario, public receptors would be limited to the industries located at the adjacent Monterey Waste Management Marina Disposal Site.
For sulfur dioxide, the worst-case scenario was chosen to be a one-ton release over ten minutes with no mitigation. This gave rise to a worst-case release rate of 200 pounds per minute and a hazard distance of 6.6 miles. There are approximately 99,400 people within the worst-case hazard zone circle. The alternative release scenario was chosen as a half-inch release from a single one-ton container during offloading. The release would result in an average release rate of 10 pounds per minute over 20 minutes, and a hazard distance of 2,090 feet (604 m).
The results included in this analysis indicate that the worst-case release for the chlorine and sulfur dioxide processes can extend for several miles from the Regional Treatment Plant bou
ndary. However, it is important to emphasize that such a worst-case release is extremely unlikely to occur, and this analysis takes no account of the many safety features inherent in the design and operation of the plant that prevent such a release from happening.
For the worst-case release of chlorine ever to occur, the following sequence of events would have to take place:
- There would be a complete rupture of a liquid chlorine line or a fusible plug or valve would have to be completely sheared from a container.
- All five online chlorine containers would have to be full at the time of release.
- The entire contents of the online containers (5 tons) would have to be released within a time period of ten minutes.
- The scrubber system would have to fail allowing all the chlorine to be released, and the chlorine building would have to fail allowing chlorine to be released directly to the atmosphere.
- There would be no operator response to close off valves or to activate t
- The release would have to occur during the night or early morning at a time when the weather conditions would favor maximum downwind dispersion (Stability Class F and wind speed of 1.5 m/s).
The alternative release scenarios are based on credible assumptions. The alternative scenario was selected from a number of credible scenarios, based on their potential to reach offsite. In reality, any release that does occur would probably be less than that evaluated in the alternative scenario analysis. For example, it is more likely that the release would be a small leak from an orifice - such as a valve or fusible plug - rather than a half-inch diameter hole as assumed in the analysis, and the release could be terminated in less than 20 minutes if the operators respond quickly.
Implementation of the RTP Process Safety Management Program is another important consideration that helps to prevent a release or to minimize the impacts of a release that does occur. Some of the key
elements that are incorporated into the RTP PSM program are summarized below.
A comprehensive set of process safety information has been compiled for the new chlorine and sulfur dioxide systems. HAZOP studies were conducted prior to the startup of the chlorine and sulfur dioxide systems to evaluate potential hazards with the systems. External events (including earthquakes) were considered in the Process Hazards Analysis. Each external event with the potential to initiate a release was investigated in detail. All recommendations made by the HAZOP team to improve the safety of the systems were addressed before the systems were started up in 1998. A Management of Change procedure is used for all changes made to the chlorine and sulfur dioxide systems.
An Operations and Maintenance Manual provides detailed written operating procedures for both the chlorine and sulfur dioxide systems. Additional procedures are written for offloading containers and changing over full/empty containers. All
chlorine and sulfur dioxide systems are included in the RTP maintenance program. Equipment inspections, testing and preventive maintenance activities are performed following a structured schedule. Training is provided to each operator before they are certified to operate the system and to each maintenance person before they conduct maintenance.
A pre-startup safety review was conducted for the new chlorine and sulfur dioxide systems and the associated scrubber (1998). A checklist was used to ensure that essential elements of the prevention program (i.e., engineered systems and documentation) were in place prior to startup. A compliance audit procedure has been developed for the risk management program, and this will be used every three years to verify compliance with Cal-ARP, PSM and RMP regulations.
The MRWPCA routinely investigates any incident or accident that occurs at the RTP. Prior to startup of the chlorine and sulfur dioxide systems, the Incident Investigation procedures w
ere revised and improved to meet the expectations of the Cal-ARP regulations.
Employees are involved in many aspects of the Risk Management Program. Having them involved - in HAZOP studies, pre-startup reviews, development of procedures, etc. - is an excellent way for employees to retain ownership of the risk management programs. Contractor safety procedures are also in place at the RTP. They include reviews of contractor safety performance and the inclusion of safety performance as a criterion for contractor selection.
The chlorine and sulfur dioxide systems were new in 1998, and there has been no accident since that time.
The ERP for chlorine and sulfur dioxide uses a systematic approach depending on the type of incident:
Level 1 - This would be a minor leak that occurs during normal operation and maintenance activities. Such leaks can occur from a faulty gasket setting, a container valve packing gland problem, or a valve that is not seated properly. With level of incident an
operator or maintenance technician would be on hand to attend to the leak. Level 1 leaks are those that can be stopped quickly with routine operating and maintenance actions such as closing a valve or tightening a connection. Level 1 releases are controlled releases, and as such they are not emergencies.
Level 2 - This would be an uncontrolled leak that occurs within the chlorine building.
This may occur when no-one is in the building, or it may occur when work is bing performed on the system. Such a release would set off the ambient detectors and trigger the scrubber system, and this would be the primary mitigation. If an operator is present, the appropriate action would be to leave the building and close all doors. Precautionary evacuations may be taken.
Level 3 - This would be any release that occurs outside the chlorine building. Such a release would not be contained and treated by the scrubber, so an alternative response is required. A release outside could occur in three wa
ys: (1) during the off-loading of containers from a delivery truck; (2) when a delivery truck brings a leaking container onsite or a container falls off a truck and leaks; or (3) if a release occurs inside the building and the scrubber fails to work. The most likely cause of a Level 3 release would be a release that occurs during off-loading as this is the time when a container could be mishandled or dropped.
The ERP includes procedures that defined response actions for Level 2 and Level 3 incidents. These include evacuations, notifying response agencies and adjacent businesses, emergency equipment list with maintenance schedule, and for medical and first aid. The ERP is coordinated with the local response agencies.
No safety-related recommendations were left open following the HAZOP study. As such, there are no pending changes to improve safety. However, it is RTP policy to review operating, maintenance and emergency response procedures on a routine basis to ensure applicability. T
his continuous review process will generate further recommendations for improved safety during the course of operating the chlorine/sulfur dioxide systems.
In summary, the design of the RTP chlorine and sulfur dioxide facilities and the implementation of the safety and risk management programs provide a high level of assurance that the hazards and risks associated with the use of chlorine and sulfur dioxide are managed in an appropriate manner. These programs reduce the likelihood of a release and minimize the severity if a release were to occur.