Rahway Valley Sewerage Authority - Executive Summary |
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The Rahway Valley Sewerage Authority (RVSA), a secondary treatment Publicly Owned Treatment Works (POTW), is located on the border of Union County and Middlesex County in the Town of Rahway. Wastewater treatment operations include primary settling, aerated activated treatment, secondary clarifying, and contact chlorination. Sludge treatment includes thickening, dewatering, anaerobic digestion, final dewatering, and solids concentrating. Anaerobic digestion of sewage sludge produces methane gas that is used as fuel for stationary engines and a boiler at the facility. Methane gas is produced as a result of microbial degradation of organic material in the separated sludge. This process takes place in three large digester tanks with expandable covers. The covers float on the collected gas above the sludge volume. The digesters are equipped with sludge and gas recirculation systems, as well as a heat circulation system to maintain maximum microbial activity. This methane gas (or di gester gas) is utilized in much the same way as natural gas would be and poses the same potential fire and explosive hazards. Low-pressure gas flows out of the digesters through pipes and valves to a separate building containing filters to remove sulfur compounds. After filtration, the low-pressure gas moves on to the compressor room where it is pressurized to approximately 40 pounds per square inch (psi). The high-pressure gas is transported via pipeline to another building (Pump and Blower Building) where it is burned in two stationary internal combustion engines and a boiler. Another pipeline used to move low-pressure gas to boilers and engines in another building (Primary Building) has been shut-off and is no longer used. The RVSA employs flammable gas detectors to monitor the digester gas system. The digesters are also equipped with self-actuated pressure relief valves and gas flares to prevent over-pressurization and vacuum relief valves to prevent tank implosion. In add ition, facility personnel monitor the components of the digester gas system daily. Digester gas not only poses a fire and explosion hazard, but also the potential for asphyxiation. All workers are cautioned in their activities around the digester gas system and their response to monitor alarms. The RVSA takes worker safety seriously and all new workers are trained and tested on facility equipment and procedures before they are allowed to perform tasks without oversight. For the worst-case accident scenario, it is assumed that one of the digester tanks ( the one closest to the property boundary) explodes with the maximum amount of gas contained within it, as recommended in the United States Environmental Protection Agency (USEPA) Guidance. Explosions of this type are highly unlikely, as the tanks are operated void of oxygen and are at positive pressure so all leaks would push gas out of the tanks rather than allow oxygen in. The worst-case scenario results in 1 psi over-pressure out to a distance of 156 meters or just under 0.1 miles. After trying several low probability but realistic alternative case scenarios without generating an off-site impact, an extremely unlikely scenario was selected. This scenario consists of a major break in one of the low pressure pipes which results in the Digester Gas Building being filled with digester gas. (Since no major machinery can fit into the building and multiple safeguards exist to keep the lines from being worked on while gas is in that segment, this type of release was ruled as unrealistic during the hazardous review performed in conjunction with this RMP.) The alternate scenario also assumes that just at the time the methane concentration reaches the upper explosive limit a static spark ignites a vapor cloud explosion. For this explosion, the 1 psi over-pressure wave reaches 118.5 meters which constitutes an off-site impact to the immediate West. However, for this over-pressurization to actually reach any fen celine within this distance, it must blow out the walls of the Digester Gas building and then level at least one of the three digester tanks while retaining enough explosive force to still generate a 1 psi off-site pressurization. The RVSA currently is not subject to federal Occupational Safety and Health Administration (OSHA) worker safety regulations. Oversight is provided by the New Jersey Department of Labor Office of Employee Safety (PEOSHA). To date federal OSHA and PEOSHA have not required POTWs to comply with OSHA Process Safety Management (PSM) regulations in regards to the digester gas operations. However, the program elements of the RMP will accomplish a majority of the goals of the PSM Program. The RVSA has not had an accident associated with the digester gas system in over five years. The facility now has in place a formal Accident Reporting and Investigation Program that will provide appropriate review and follow-up to future incidents at the facility. The RVSA und ergoes annual safety inspections by the Rahway Fire Department (most recently on June 8, 1999) and their fire insurance carrier, NewJersey Manufacturers (most recently in March, 1999). Additional inspections that have occurred include: 1) PEOSHA - April 24, 1992 2) NJDEP Right-To-Know - August 1, 1995 3) Workers Compensation (Insurance) - July 13, 1998 Should a fire or explosion occur at the facility that is associated with the digester gas system, RVSA will not respond with on-site personnel. The RVSA has in place a mechanism to contact the Rahway Fire Department for response. RVSA personnel will be evacuated, if necessary, or moved away from the accident area. RVSA currently has generic facility programs in place for the elements of the RMP program. Over the course of the following year, these elements will be updated and revised to be specific to the digester gas system. |