Bristol Borogh Wastewater Treatment Plant - Executive Summary |
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The Bristol Borough Water & Sewer Authority (BBW&SA) owns, operates and maintains a Wastewater Treatment Plant (WWTP). The BBW&SA's accidental release prevention policy involves a unified approach that integrates technologies, procedures and management practices. All applicable procedures of the U.S. Environmental Protection Agency (EPA) Prevention Program are adhered to. The BBW&SA's emergency response policy plans are tailored to each facility and to the emergency response services available in the community, and is in compliance with the EPA Emergency Response Program requirements. The chlorination system has been installed at the BBW&SA's WWTP to disinfect WWTP effluent prior to its discharge into the Delaware River via Otter creek, as per the BBW&SA's National Pollution Discharge Elimination System (NPDES) permit. The BBW&SA's Wastewater Treatment Plan serves Bristol Borough and portions of Bristol Township. The chlorination facility is located within the Bristol Borough Wastewater Treatment Plant. The chlorination facility includes a chlorine cylinder weighing room, which contains liquid chlorine cylinders (four each one-ton cylinders), and a chlorine feeding room, which contains two wall-mounted vacuum regulator. The vacuum regulator, which is of the spring opposed, diaphragm type serves, via its vacuum operated inlet valve, to reduce the gas from a varying supply pressure to a constant regulated vacuum. At any given time, two each one ton chlorine cylinders are turned on and used to feed chlorine. The chlorine cylinders are connected to a manifold which transport chlorine to the chlorinators. Chlorine gas flow to the manifold is controlled by two valves: one at the each cylinder and one at the manifold. At any given time, only two chlorine cylinder are open. Generally, there are three (3) cylinders which are stored in a chlorine cylinder storage area. The chlorination facility is normally unmanned. The Wastewater Treatment Plant operator s visit the facility daily and respond to any trouble alarms which may occur. The offsite consequence analysis includes consideration of two chlorine release scenarios, identified as "worst case release scenario" and "Alternative scenario". The worst-case release 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". Reference tables contained in RMP offsite consequence guidance were used to determine the distance traveled by the chlorine released before its concentration decreases to the "toxic endpoint" selected by EPA of 0.0087 mg/l or 3 ppm, which is the Emergency Response Planning Guideline Level 2 (ERPG-2). This is defined by the American Industrial Hygiene Association (AIHA) as the "maximum airborne concentration below which it i s 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 radius corresponding to the toxic endpoint distance has to be defined, "to estimate the population potentially affected". The worst-case release scenario at the Bristol Borough WWTP Chlorination Facility involves a failure of the two each one ton-containers which could be connected concurrently (a total of 4,000 lb of chlorine). The offsite consequence analysis for this scenario was performed for the set of conditions pre-defined by EPA, namely release of the entire amount of 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 corresponding to the toxic endpoint distance. EPA set these conditions to fac ilitate the performance of the offsite consequence analysis. EPA-mandated meteorological conditions, namely Stability F, wind speed of 1.5 m/sec, temperate of 25:C (77:F), and humidity of 50% were used. When analysis of the worst case scenario was performed using the reference table and using the EPA assumptions and considering release of chlorine gas in enclosed space a distance to toxic endpoint of 4.3 miles and an estimate of residential population potentially affected of 144,513 was obtained. The alternative release scenario involves the rupture of the flexible connections (pigtails) or valves connected to two (2) one (1) ton-containers, possibly due to corrosion. The amount of chlorine released is 4,000 lb, at an average rate over one hour (the duration of the release) of 67 lb/min. Toxic endpoint distance to ERPG-2 levels was obtained. The typical meteorological conditions used were Stability D, wind speed 3 m/s, average air temperature of 25:C, and 50% average humidity. The estimated distance traveled to the toxic endpoints was determined to be 1.2 miles for the ERPG-2 and an estimated residential population potentially affected of 15,200 was obtained. Since, both chlorine container weighing scale and chlorine feeding system/chlorinators are located within the rooms, enclosure is considered as a passive mitigation system. Actuation of the chorine detector is an active mitigation measure considered. As an additional mitigation system, the chlorination system is a vacuum type, with vacuum piping. This will reduce significantly the amount of chlorine released in case of pigtail rupture/break or manifold pipe break. The general Bristol Borough WWTP accidental release prevention program is based on the following key elements: 7 Training of the operators and mechanics 7 Preventive maintenance program 7 Use of process and safety equipment 7 Use of accurate and effective operating procedures 7 Performance of a hazard review of equipment and procedu res 7 Implementation of an auditing and inspection program Chemical-specific prevention steps include availability of self-contained breathing apparatus (SCBA), worn by the mechanics and operators during connection/disconnection of chlorine supply, awareness of the hazardous and toxic properties of chlorine, and presence of chlorine detectors. The facility has an emergency response program, which has been coordinated (reviewed) by Bristol Borough Fire Department and the Bucks County Emergency Planning Committee. The Bucks County Emergency Planning Committee is designated as the lead Local Emergency Response Planning Committee (LEPC). This program includes an emergency response decision tree and a notification plan. |