Tupelo Water Treatment Plant - Executive Summary |
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EXECUTIVE SUMMARY 1. Purpose Chlorine is used as a disinfectant in water treatment. The water from the Tombigbee River contains microorganisms that are harmful to human health. Several treatment processes are utilized at the plant to treat the raw water and a chlorination system provides disinfection of the treated water prior to transfer into the drinking water distribution system. 2. Storage Chlorine is delivered to the treatment plant in one-ton containers and 150 lb cylinders. A maximum of 12 one-ton containers and three 150 lb cylinders of liquid chlorine are stored on site in the chlorine storage room. 3. Overview of the Wastewater Treatment Plant The Northeast Regional Water Supply District (NRWSD) treats approximately 8 million gallons per day (MGD) of raw water from the Tombigbee River. The chlorine used for disinfection at the treatment plant is supplied from one-ton containers. A maximum of 12 one-ton containers of liquid chlorine are stored on site. The 150 lb cylinders are used at remote booster pumping stations to inject additional chlorine into the water, if required. This is the only chemical substance at this facility that is regulated under U. S. EPA's Accidental Release Prevention Program. 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 trea tment plant by an overhead crane with a lifting beam that is specially designed for ton-containers. The off-loaded containers are also inspected by NWMD personnel. The liquid chlorine leaves the ton container as a gas. There are two containers connected to a common pipe (manifold) and the chlorine gas valves on both containers are normally open. One container is used until empty and then an automatic switchover system shuts off the flow from the empty container and starts the flow of chlorine gas from the second container. There are three chlorinators that are connected to the manifold and they regulate the flow of chlorine gas to the treated water. The capacity of one chlorinator is more than adequate to supply the required amount of chlorine to the treated water. There are two chlorinators that are normally in service and one is on standby. The chlorine gas flows from the container to the manifold and then to the chlorinators. The chlorine gas is drawn from the chlorinators by inducing a vacuum in the line after the chlorinators. The flow of wastewater through an ejector creates a vacuum in the line and also mixes the chlorine gas with the wastewater. If the water supply to the ejector is stopped, or the operating vacuum is lost for any other reason, a vacuum regulator shuts off the flow of chlorine gas from the manifold. The average chlorine usage is approximately 250 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 chlorine system from the ton-container and 150-lb cylinder unloading through the mixing of the chlorine gas with the wastewater. 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 daily 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 that 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. All maintenance on the chlorine system is performed by NWMD personnel. The plant uses USFilters Maintenance Management System and this insures that worn or damaged parts are replaced and routine maintenance is performed on the chlorinators and other equipment according to the manufacturer's recommendations. These preventive measures have resulted in the absence of any accidental release of chlorine that resulted in personal injury or property damage. The treatment plant maintains four sets of Self-Contained Breathing Apparatus (SCBA) and The Chlorine Institute's Emergency Kit "B" for ton conta iners. NWMD 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 has is equipped with an exhaust fan that comes on automatically when the chlorine monitor detects a leak. There is also a switch inside and outside of the storage room to manually operate the exhaust fan. A duct with a pickup near the floor exhausts any leaking chlorine through the roof. Two exit doors are provided in th e chlorine storage room. 5. Emergency Response Plan The NRWSD has coordinated their Emergency Response Plan with the Dorsey-Friendship Volunteer Fire Department and they have an informal agreement with the City of Tupelo Local Emergency Planning Committee to respond to an accidental release of chlorine at the treatment plant. The Itawamba County Local Emergency Planning Commission does not have the capability to respond to an accidental release of chlorine. 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 and in the chlorine feed room will activate an audible alarm and a light in both these areas. The facility 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 th e Offsite-Response Plan will be activated. If the chlorine release does not pose a threat to the general public, NRWSD personnel will repair the leak. There is a windsock located near the chlorine storage building so that the area that is likely to be affected by a chlorine release can be determined. 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 likelih ood 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 toxic 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. 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 a re 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 endpoint, 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 frequency of practice drills to respond to an accidental release of chlorine to three times per year. * Work with the Itawamba County LEPC so that this agency can respond to an accidental release of chlorine. 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-inch copper 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 lbs per minute 21 lbs per minute Wind Speed 1.5 m/sec. (3.36 mph) 3.0 m/sec. (6.7 mph) Atmospheric Stability Class F (very stable) D (neutral) Method used to determine RMP*Comp RMP*Comp toxic endpoint Chlorine concentration 3 ppm 3 ppm at the endpoint Radial distance to the 1.3 miles (6,900 ft) 0.1 miles (528 ft) toxic endpoint Estimated population within 253 1 radial distance Number of residences 90 0 Number of hospitals 0 0 Number of schools 0 0 |