Butler Area Sewer Authority Wastewater Treatment - Executive Summary |
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EXECUTIVE SUMMARY 1. Purpose Chlorine is used as a disinfectant in the wastewater treatment. The raw wastewater contains microorganisms that are harmful to human health. Several treatment processes are utilized at the plant to treat the raw wastewater and a chlorination system provides disinfection of the final effluent prior to discharge. 2. Storage Chlorine is delivered to the treatment plant in one-ton containers. A maximum of 3 one-ton containers of liquid chlorine are stored on site in the chlorine storage room. 3. Overview of the Wastewater Treatment Plant The Butler Area Sewer Authority (BASA) treats approximately 5 million gallons per day (MGD) of raw domestic wastewater at the treatment plant. The chlorine used for disinfection is supplied from one-ton containers. A maximum of three one-ton containers of liquid chlorine are stored on site. This is the only chemical substance at this facility that is regulated under U. S. EPA's Accidental Release Prevention Pro gram. Chlorine gas is primarily a respiratory irritant. In sufficient concentrations, the gas irritates the mucous membranes, the respiratory tract and the eyes. In extreme cases difficulty in breathing may increase to the point where death can occur from respiratory collapse or lung failure. The characteristic, penetrating odor of chlorine gas usually gives warning of its presence in the air. At high concentrations, it is visible as a greenish yellow gas. Liquid chlorine in contact with the skin or eyes will cause chemical burns and/or frostbite. Chlorine containers are thoroughly inspected for mechanical integrity and leaking valves prior to leaving the supplier's premises. The containers are off-loaded from the delivery truck at the treatment plant by an overhead crane with a lifting beam that is specially designed for ton-containers. The off-loaded containers are also inspected by BASA personnel. There are two identical systems that can deliver chlorine gas, at the corre ct flow rate, to the wastewater. One system is normally active and the other system is on standby. Although both systems can be used simultaneously, one system has more than adequate capacity to supply the required amount of chlorine to the treated wastewater. The liquid chlorine leaves the ton container as a gas. There are two containers connected to a common pipe (manifold), however, only the chlorine gas valve on one container is normally open. This container is used until empty and then the other container is put in service. There are two chlorinators that are connected to the manifold and they regulate the flow of chlorine gas to the wastewater. The capacity of one chlorinator is more than adequate to supply the required amount of chlorine to the wastewater. The chlorine gas flows from the container to the manifold and then to the chlorinator. The chlorine gas is drawn from the chlorinator by inducing a vacuum in the line after the chlorinator. The flow of process wate r through an ejector creates a vacuum in the line and also mixes the chlorine gas with the process water. If the water supply to the ejector is stopped, or the operating vacuum is lost for any other reason, the spring-loaded gas inlet valve immediately closes to isolate the chlorinator from the chlorine gas supply. The average chlorine usage is 80 pounds per day. 4. Accidental Release Prevention Accidental releases of chlorine are prevented by routine training and adhering to the written operating procedures that cover the chlorination system from the ton-container unloading through the mixing of the chlorine gas with the process water. These procedures cover normal operations and emergency operations where personnel may have to respond to an accidental release of chlorine. The chlorine system is inspected at least once per shift and leak testing is performed on the valves and fittings when ton-containers are changed-out and at other times if a leak is suspected. Employees tha t are responsible for disconnecting empty containers and connecting filled containers to the chlorination system have been trained on the operating procedures, inspection of valves, fittings, and the connected piping, and leak testing with an aqueous ammonia solution. Aqueous ammonia reacts with chlorine gas to form a very visible white cloud. An outside contractor inspects the chlorination system and performs preventive maintenance on this system at least eight times per year. This insures that worn or damaged parts are replaced and routine maintenance is performed on the chlorinators and other equipment. These preventive measures have resulted in the absence of any accidental release of chlorine that resulted in personal injury or property damage since the treatment plant began operation. The treatment plant maintains four sets of Self-Contained Breathing Apparatus (SCBA) with extra air tanks and The Chlorine Institute's Emergency Kit "B" for ton containers. This equipment is st ored in a room that is adjacent to the room that houses the ton containers. SCBA's are also located in the laboratory and shop areas. BASA personnel receive annual training on the use of the breathing apparatus and the emergency kit. The kit contains the necessary tools and other equipment to contain valve leaks and repair small holes in a ton container, and capping devices for the fusible plugs in each end of the container. Ton containers are equipped with six fusible metal plugs, three in each end. The fusible metal is designed to melt between 158OF and 165OF to relieve pressure and prevent rupture of the container in case of fire or other exposure to high temperature. Chlorine gas is heavier than air and will settle to the lowest elevation when released. The chlorine storage area is equipped with an exhaust fan that is mounted near the floor on an outside wall. There are also air intake vents located near the floor so that the chlorine gas will be vented outside the buildi ng. The fan can be activated by a switch located outside the building. Two exit doors are provided in the chlorine storage room. 5. Emergency Response Plan The BASA and the Local Emergency Planning Committee for the Emergency Planning District of Butler County have developed an Off-Site Response Plan in the event of an accidental release of chlorine at the treatment plant. This plan was developed in accordance with the provisions set forth in Title III of the Superfund Amendments and Reauthorization Act of 1986. The plan assigns release response procedures to the emergency coordinator at the BASA, Butler County Emergency Control, South Butler and the Lyndora Fire Departments, Local Police, Emergency Medical Services, and the Butler County Emergency Management Coordinator. The plan is updated as necessary, including the list of emergency telephone numbers. The South Butler Fire Department has visited the treatment plant on several occasions and the City of Butler Bureau of Fire has trained BASA personnel on the proper use of the SCBA equipment. If a chlorine leak develops in a ton-container or in the connected piping, valves, or other equipment, a chlorine monitor in the chlorine storage area will activate an audible alarm in both this area and in the remotely located control room building. The control room is manned 24-hours per day, seven days per week. The treatment plant emergency operating procedures will be followed to determine the severity of the chlorine release and whether or not the gas may migrate offsite. If the chlorine release may travel offsite, then the Offsite-Response Plan will be activated. If the chlorine release does not pose a threat to the general public, BASA personnel will repair the leak. The BASA have installed windsocks in two locations near the chlorine storage building to determine the wind direction and the area that is likely to be affected by a chlorine release. 6. Worst-Case and Alternate-Case Accidental Release Scenarios Worst-Case Release Scenario The worst-case release scenario as postulated in the regulation (40 CFR 68.25) is the release of the greatest quantity (of chlorine) in a single vessel. The entire contents of this vessel are assumed to be released as a gas over a 10-minute period. The largest single vessel at the treatment plant is a one-ton container; therefore, 2,000 pounds of liquid chlorine will be released as a gas over a 10-minute period. A release of this magnitude would only be possible if the shell of the container failed and the liquid chlorine formed a pool and vaporized over a 10-minute period. The likelihood of this type of catastrophic release is very minimal. The off site consequence of this release was determined by use of U. S. EPA's RMP*Comp software. The RMP*Comp results indicate that the chlorine concentration will exceed 3 parts per million (ppm) out to a radial distance of 1.3 miles from the chlorine storage room. This is the distance to the to xic endpoint. Beyond 1.3 miles the chlorine concentration will be less than 3 ppm. Short-term exposure to chlorine concentrations less than 3 ppm is not considered to be a health hazard. The chlorine storage room is located on a hillside that slopes downward to the west and south. Based on the guidance provided in the regulation and U. S. EPA's Risk Management Program Guidance for Wastewater Treatment Plants, the area surrounding the treatment plant was classified as urban due to the presence of nearby buildings and other structures, trees, and hilly terrain. The meteorological conditions associated with the worst-case release scenario, as prescribed in the regulation, is a wind speed of 1.5 meters per second (3.36 miles per hour) and very stable atmospheric conditions. These stable atmospheric conditions will limit the mixing of the chlorine gas with the ambient air as the gas travels downwind from the point of release. The cloud formed by the chlorine release will grow in size and decrease in concentration as it travels downwind. Alternate-Release Scenario The alternate release scenario is one that is more likely to occur than the worst case release scenario. For this scenario, it was assumed that a 3/8-inch, type K copper transfer hose that connects the ton container to the manifold is sheared off and mitigation occurs in one hour. This assumption is based on the alternate release scenarios listed in Edition 3 of The Chlorine Institute Pamphlet No. 74 - Estimating the Area Affected by a Chlorine Release, April 1998. More than half of the contents of the ton-container (1,260 pounds) is released over a one-hour period. For this scenario, the meteorological conditions prescribed in the regulation are neutral atmospheric stability conditions and a wind speed of 3 meters per second (6.7 miles per hour). RMP*Comp was also used to determine the radial distance at which the chlorine concentration falls off to less than 3 ppm. This distance to the toxic endp oint, as determined by RMP*Comp, is 0.10 miles (528 feet). There are no residences, schools or hospitals within this radial distance. It is unlikely that the entire contents of a ton-container would be released prior to BASA personnel closing the shutoff valve on the ton container. Actual response time to shut off the valve is estimated to be 15 minutes. The worst-case and alternate release scenarios are summarized in Table 1 along with some additional information regarding population estimates, schools, hospitals and other building or recreation areas within the radial distance to the toxic endpoint. Census data and the location of schools, hospitals, etc. were obtained from the LandView III mapping system that includes database extracts from U. S. EPA, the Bureau of Census, the U. S. Geological Survey, the Nuclear Regulatory Commission, the Department of Transportation, and the Federal Emergency Management Agency. 7. Planned Changes to Increase Safety * Increase the freq uency of practice drills to respond to an accidental release of chlorine to three times per year. * Automate the chlorination system controls so that when a ton container is empty, the second chlorinator activates and the active chlorinator switches to the off position. This would include the installation of vacuum regulators on the ton container gas outlet valve. * Investigate the feasibility of constructing a containment area for the chlorine containers * Investigate the feasibility of installing a scrubber or similar control device to capture and control an accidental release of chlorine in the container storage room. Table 1 SUMMARY OF THE WORST-CASE AND ALTERNATE RELEASE SCENARIOS Worst-Case Release Alternate Release Type of Accidental Release Rupture of ton-container Shearing of 3/8" tubing Quantity of Chlorine Released 2,000 pounds 1,260 pounds Duration of Chlorine Release 10 minutes 60 minutes Rate of Release of Chlorine Gas 200 lb per minute 21 lb per minute Wind Speed 1.5 m/second (3.36 mph) 3.0 m/second (6.7 mph) Atmospheric Stability Class F (very stable) D (neutral) Method used to determine toxic endpoint RMP*Comp RMP*Comp Chlorine concentration at the endpoint 3 ppm 3 ppm Radial distance to the toxic endpoint 1.3 miles (6,900 feet) 0.1 miles (528 feet) Estimated population within radial distance 5,416 20 Number of residences 2141 8 Number of hospitals 0 0 Number of schools 1 0 Other aensitive areas State Hwy. Rte. 8 State Hwy. Rte. 8 |