Thornton Treatment Plant - Executive Summary |
|
According to EPA guidance the executive summary must include a brief descripton of the facilty's Risk Management Program. The following sections list each of the required executive summary data elements by rule citation. 68.155(a) Accidental Release and Emergency Response Policies Thornton Water Treatment Plant has used chlorine for disinfection without serious incident since its construction in 1954. This successful operating record is due to the City of Thornton's commitment to training programs, operating procedures, operator expertise, and maintenance activities. 68.155(b) Stationary Source and the Substance Handled The Thornton Plant is a 20-MGD municipal water treatment plant serving the communities of Thornton, Western Hill, Sherrel Wood and Federal Heights. The City of Thornton diverts water from Clear Creek near Golden and transports the water to Standley Lake, which is the supply for the Thornton Water Treatment Plant, through the Farmers High Line, Croke, and Church C anals. The plant treats the water by clarification, filtration, and disinfection. Once the treatment process is complete the water is suitable for human consumption. The chlorine gas is used for the disinfection process. Although the primary purpose of adding chlorine gas to drinking water is disinfection, the chlorine also provides taste and odor control for the drinking water. Chlorine for the chlorination process is stored in eight one ton cylinders. Usually two cylinders are manifolded delivering chlorine gas and remaining cylinders are held for stand by. Chlorine gas is pulled from the ton cylinders and feed to chlorinators that regulate the feed flow rate. Injectors mix the chlorine gas with a slipstream of the plant water. The chlorine storage building is secured to prevent unauthorized personnel from accessing the area. The treatment plant maintains chlorine leak detection devices in the cylinder storage area, chlorinator room and the injector area. Each of the ch lorine detecting probes is connected to a local and remote alarm system. The local system consists of a flashing red light and an audible horn. 68.155(c) Offsite Consequence Analysis One worst-case release scenario and one alternative release scenario was assessed for the chlorine process. To ensure a common basis for comparisons the EPA defined the worst-case scenario as the release of the largest quantity of a regulated substance from a single vessel or process line that results in the greatest distance to an endpoint. The alternative release rate is a release that is considered more reasonable or "more likely." Active mitigation measures can be used in determining the alternative release scenario while the worst-case scenario is only allowed consideration of passive mitigation measures. The toxic endpoint for chlorine has been defined by EPA to be 0.0087 mg/L (3 ppm). This airborne concentration that is used as the toxic endpoint is the maximum airborne concentration below which it is believed that nearly all individuals can be exposed for up to one hour without experiencing or developing irreversible or other serious health effects. The distance to the toxic endpoint becomes a radius for a circle around the covered process containing the regulated substance. Residential population within the circle is required to be determined based on available census information. The population number is reported as part of the EPA submittal. This method greatly over estimates the population potentially exposed to a single chlorine release, because the chlorine plume would seek lowest elevations in the direction of the wind during a release. Since wind direction cannot be anticipated for an accidental release, EPA mandated the circle estimation method. Release scenarios and distances to toxic endpoints are discussed in more detail in the subsequent paragraphs. Worst-Case Release Scenario Worst-Case Release Scenario is determined in accordance with the requirem ents provided in 40 CFR 88.22 and 40 CFR 68.25(b & c). As described in USEPA 40 CFR Part 68.25(b) the worst-case release shall be the greater of the following: 1. The greatest amount held in a single vessel or 2. The greatest amount in a pipe. For the chlorine process at the Thornton Water Treatment Plant the worst-case release is that of a 2,000 pound chlorine cylinder (maximum amount held in a single vessel) ruptures and its contents escape to the atmosphere in 10 minutes. In the event the cylinder would release its entire contents to the atmosphere in 10 minutes the release rate would be 200 pounds per minute. The distance to the toxic end point worst-case release distance was taken from Exhibit 4-4 of EPA's Risk Management Program Guidance for Wastewater Treatment Plants. The development in the area of the Thornton Water Treatment Plant is a mix of residential, open areas, and commercial buildings. Based on 40 CFR 68.22 (e) the Thornton Water Treatment Plant is in an area c onsidered to be urban. The residential population within the 1.3-mile radius of influence was determined to be approximately 16,736. The population was determined by block group proration method using LandView. III, Environmental Mapping Software developed by the U.S. Department of Commerce Economics and Statistics Administration Bureau of the Census. The software will estimate the residential population within a given area using 1990 census information. USGS Quadrangle maps were used to determine environmental and public receptors. The USGS Quadrangle maps used are listed below: ( Commerce City, CO., revised 1994 ( East Lake, CO., revised 1994 Alternative Release Scenario One alternative release scenario was evaluated for the chlorine process per USEPA 40 CFR 68.165(a)(2). The alternative release scenario considered is in accordance with the guidelines provided in 40 CFR 68.22 and 68.28. The scenario for the alternative release follows: The regulation dictates an alternati ve release scenario that results in an offsite impact. To achieve this end a gas release from a one ton cylinder was chosen for the alternative release scenario. In this scenario the flexible hosing on a one ton cylinder is dislodged while the valve is open. The dislodgment could be caused by a heavy object ramming against the valve/flexible connection, however the impact would have to be at a specific angle to result in the damage theorized. Due to the construction of the storage area and the cylinders, dislodging the connection is thought to be very unlikely. The cylinder valve opening is 9/32 inches according to the "Handbook of Chlorination and Alternative Disinfectants." Once dislodged, chlorine gas under pressure exits through the valve opening. According to EPA Guidance document for a choked release of vapor (i.e., emerging at the speed of sound from the hole), the release rate for chlorine is given by: QR = 190 x Ah Where: 190 = chemical-specific factor chlorin e at a tank pressure of 113 psia and temperature 250C Ah = hole area (in2) = pr2 = 1.18 in2 An opening of 9/32 equates to a release rate of approximately 12 pounds a minute. Based on Table exhibit 4-15 of The EPA's Risk Management Program Guidance for Wastewater Treatment Plants the distance to the toxic endpoint is 0.1 miles in a urban area. The development in the area of the Thornton Water Treatment Plant is a mix of residential, open areas, and commercial buildings. Based on 40 CFR 68.22 (e) the Thornton Water Treatment Plant is in an area considered to be urban. The residential population within the 0.1-mile radius of influence was determined to be approximately 99. The population was determined by block group proration method using LandView. III, Environmental Mapping Software developed by the U.S. Department of Commerce Economics and Statistics Administration Bureau of the Census. The software will estimate the residential population within a given area using 1990 census in formation. The identification of the environmental and public receptors was completed in the same manner as detailed for the worst-case release scenario. 68.155(d) Accidental Release Prevention Program Thornton Water Treatment Plant continuously evaluates and updates their safety and prevention procedures as needed. The existing prevention program was reevaluated during the development of the Risk Management Plan. 68.155(e) Five-year Accident History The chlorine has been used at the facility since its construction in 1954. No accidents, as described under 40 CFR 68.42(a), have occurred at this facility since it began operation. 68.155(f) Emergency Response Program The emergency response program established by the City of Thornton does not require personnel to act as the site incident commander. In the case of an incident the North Metro Fire Rescue Authority becomes the incident commander and assumes the role of directing response activities including any community emergency evac uation measures. Fire Rescue staff train at the Thornton facility on an annual basis. 68.155(g) Safety Improvements In response to the process hazard review, the City will be implementing additional training pograms for the operators. Conclusions The RMP*Submit satisfies the Thornton Water Treatment Plant's requirement to register and provide the EPA a summary of their Risk Management Program for the chlorine system. |