WOONSOCKET REGIONAL WASTEWATER COMMISSION - Executive Summary |
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The Woonsocket Regional Wastewater Commission's (WRWC) Accidental Release Prevention Policy involves a unified approach that integrates state of the art technologies, procedures and management practices. For process ID 1 (Chlorination), the WRWC utilizes chlorine in its clarification process in order to meet its Permit limit of fecal coliform. The Chlorination System consists of two (2) banks with three (3) one toncylinders of chlorine each. There is an automatic switch valve to switch over when onebank is empty. Each chlorine cylinder is equipped with a rapid shut off valve. This system provides a rapid closure (3 second response) of chlorine cylinders upon contact closure from gas detectors and fire alarm. To make the system safer, a second chlorine gas detector was installed and connected to the cylinders rapid shut off valves controller as a back up in the event of failure of the main chlorine gas detector. In addition a motorized shut off valve was installed on each chlo rine line to the vacuum regulators. Those valves will be activated upon receiving a signal from the PLC in the event that the chlorine gas detectors or the fire alarm are activated. Total response time from the time of release to complete shut down of system is seventeen (17) seconds. The offsite consequence analysis includes consideration of two chlorine release scenarios, identified as "worst-case release" and "alternative 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 as a gas over 10 minutes," due to an unspecified failure. The alternative scenario is defined as "more likely to occur than the worst-case release scenario". Atmospheric dispersion modeling has to be performed to determine the distance traveled by the chlorine released before its concentration decreases to the "toxic endpoint" selected by EPA of 3 ppm, which is the Emergency Response Plann ing Guideline Level 2 (ERPG-2). This is defined by the American Industrial Hygiene Association (AIHA) as the "maximum airborne concentration below which it is believed that nearly all individuals could be exposed for up to one hour without experiencing or developing irreversible or other serious health effects or symptoms which could impair an individual's ability to take protective action." The residential population within a circle with a radius corresponding to the toxic endpoint distance has to be defined, "to estimate the population potentially affected". The worst-case release scenario at the Chlorination System involves a failure of the one ton- container The offsite consequence analysis for this scenario was performed by RMP*Comp ver 1.06 . This conditions was pre-defined by EPA, namely release of the entire amount as a gas in 10 minutes, use of the one-hour average ERPG-2 as the toxic endpoint and consideration of the population residing within a full circle with radius c orresponding to the toxic endpoint distance. EPA set these conditions to facilitate the performance of the offsite consequence analysis; however, the assumptions used may be unrealistic because: When atmospheric dispersion modeling for the worst-case scenario was performed using the EPA assumptions, a distance to toxic endpoint of 0.9 miles and an estimate of residential population potentially affected of 28,000 was obtained. The alternative release scenario involves the rupture of the flexible connections (pigtails) connected to three ton-containers, possibly due to an earthquake. The amount of chlorine released is 1 lbs., at an average rate over one minute (the duration of the release) of 1 (one) lb/min. The estimated toxic endpoint was 0.10 mile with 30 persons effected. Actuation of the chlorine detector is an active mitigation measure considered. An additional mitigation system installed of the automatic shut off valves on the chlorine containers. For process ID 2 (Dechlori nation) which is utilizing sulfur dioxide for the removal of residual chlorine from the final effluent in order to meet plant permit of 0.056 mg/L of chlorine residual. The dechlorination system is consisting of two (2) banks with two (2) one ton cylinders of sulfur dioxide each. The same as in process ID 1, the dechlorination system has an automatic switch over when one bank is empty. There are two (2) gas detectors and both detectors are connected to automatic shut off valves which are installed on each sulfur dioxide cylinders. Using RMP*Comp ver 1.06 to determine the toxic endpoint for worst case release scenario of one ton sulfur dioxide which estimated at 0.90 miles, the same as in the chlorination process. As for alternative release of which was based on one pound of sulfur dioxide, similar to the same analysis for process ID1, the estimated toxic endpoint found to be 0.10 mile with 30 persons were effected. The general Woonsocket Regional Wastewater Commission (WRWC) accide ntal release prevention program is based on the following key elements: The Woonsocket Fire Department will be the first responder in case of a leak Preventive maintenance program. Use of state-of-the-art process and safety equipment. Use of accurate and effective operating procedures, written with the participation of the operators. Performance of a hazard review of equipment and procedures. Implementation of an auditing and inspection program.. No accidental releases of chlorine have occurred at this facility in the past five years. The facility has an emergency response program, which has been coordinated (reviewed) by the City of Woonsocket Fire Department, which is a member of the Local Emergency Response Planning Committee (LEPC). This program includes an emergency response decision tree and a notification plan. |