Point Loma Wastewater Treatment Plant - Executive Summary |
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The City of San Diego Metropolitan Wastewater Department (MWWD) has developed a Risk Management Plan (RMP) for management and prevention of risks associated with the storage and processing of methane, a regulated flammable substance, at the Point Loma Wastewater Treatment Plant (PLWWTP). This RMP was prepared in compliance with 40 CFR Part 68, CCR Title 19 Sections 2735 through 2785, and Clean Air Act Section 112 (r)(1). Process Safety Management (PSM) elements of the RMP were prepared in compliance with 29 CFR Part 1910.119 and CCR Title 8 Section 5189. Accidental Release Prevention and Emergency Response Policies It is the MWWD's policy to comply with applicable Federal and California rules. The anaerobic digestion process at the PLWWTP produces methane which is considered a hazardous flammable substance by EPA and OSHA. Due to the flammable characteristics of methane, the MWWD observes various safety precautions in the storage, conveyance and combustion of methane. The safety precautions are necessary to protect employees, contractors, and visitors at the PLWWTP and the immediately surrounding areas. Safety precautions include written procedures and safety equipment to minimize the potential for accidental release, as well as a written emergency response program to respond to fires or other emergencies. Safety equipment (such as vacuum and pressure relief valves, flame arrestors, flame trap assemblies, pressure switches, and flares) are used within the digester gas system to minimize potential for uncontrolled release of digester gas. Fire extinguishers and fire hydrants are available for response to potential fires. Combustible gas detectors are used to detect digester gas leaks. The Stationary Source and Regulated Substances Handled The PLWWTP provides treatment for up to 240 million gallons per day of wastewater collected from sewers throughout the City of San Diego and surrounding communities. Organic waste, or sludge, that is a byproduct of the wastewater treatment process is treated in large covered tanks (anaerobic digesters). In the anaerobic digesters, bacteria break down the organic waste producing digester gas. Digester gas is a mixture of methane (about 65%) and carbon dioxide (about 35%). Gas piping collects the digester gas and distributes it to boilers and engines, where it is used as fuel to produce hot water and electricity for the plant. Digester gas is also compressed and recirculated through the anaerobic digesters to provide mixing of the organic waste. Excess digester gas is burned in enclosed flares. The regulated substance handled at the PLWWTP is methane. The total inventory of methane may be 22,905 pounds at any given time. Worst-case and Alternative Release Scenarios The worst-case release scenario was determined following the EPA's "Risk Management Program Guidance for Wastewater Treatment Plants (40 CFR Part 68)." The worst-case release scenario is a vapor cloud explosion of the methane con tained in Digester 8, the largest digester at the PLWWTP. This maximum distance from the center of Digester 8 to the explosion endpoint (an overpressure of 1 psi) is 0.153 miles (806 feet or 246 meters). The following are known to be located within 0.153 miles of Digester 8: Public parking at the south entrance to PLWWTP (located within Cabrillo National Monument); and Navy complex located uphill and east of PLWWTP. A berm located south of Digester 8 and the terrain (bluff) located east of Digester 8 act as passive mitigation to limit the impacts of a potential vapor cloud explosion in the southern (public parking) and eastern (Navy complex) directions. There are no residences within 0.153 miles of Digester 8. The alternative release scenario was determined following the EPA's "Risk Management Program Guidance for Wastewater Treatment Plants (40 CFR Part 68)." The alternative release scenario is a vapor cloud explosion of the methane contained in Digester N1, the second largest digester at the PLWWTP. The maximum distance from the center of Digester N1 to the explosion endpoint (an overpressure of 1 psi) is 0.141 miles (742 feet or 226 meters). The following are known to be located within 0.141 miles of Digester N1: Fort Rosecrans National Cemetery located uphill and northeast of PLWWTP; and Navy complex located uphill and east of PLWWTP. The terrain (bluff) located north of Digester 8 acts as passive mitigation to limit the impacts of a potential vapor cloud explosion in the northern direction. There are no residences within 0.141 miles of Digester N1. General Accidental Release Prevention Program and Chemical-specific Prevention Steps The PLWWTP complies with EPA's Accidental Release Prevention rule and OSHA's Process Safety Management rule, as well as related California codes and regulations. Operators, maintenance staff and contractors are informed of digester gas hazards. Standard operating and maintenance procedures have been developed, and employees who work with the digester gas process are provided training. Emergency Response Program The PLWWTP has an emergency response program that has been approved by the County of San Diego Hazardous Materials Division as part of the PLWWTP business plan. The emergency response program is coordinated with the Fire Department and the City of San Diego Public Works Station 38. Five-year Accident History Within the past five years there have been no accidental releases of digester gas resulting in deaths, injuries, or significant property damage on site; or known offsite deaths, injuries, evacuations, sheltering in place, property damage, or environmental damage. Planned Changes to Improve Safety MWWD has used a Process Hazard Assessment (PHA) to identify and recommend changes to ensure a high level of safety. Issues identified through the PHA are addressed in the PLWWTP's Prevention Program. Recommendations from the most recent PHA include improvements to leak detection, im provements to traffic control, and improvements to fire prevention planning. An audit of the RMP and PSM programs will be conducted within three years to verify the effectiveness of the programs. |