Des Moines Water Works - Executive Summary |
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The Des Moines Water Works accidental release prevention policy involves a unified approach that integrates technologies, procedures and management practices. All applicable procedures of the Iowa Department of Natural Resources and the U.S. Environmental Protection Agency (EPA) are adhered to. The Des Moines Water Works emergency response policy involves the preparation of a response plan tailored for our facility and to the emergency response services available from the community, and is in compliance with the EPA Emergency Response Plan. The Des Moines Water Works treatment facility has been in service for over 60 years. The hazardous substances stored and used on this site allow us to disinfect and purify the water we provide to our customers. The covered process involves a chlorine room, chlorinators, monitoring and detection equipment, instrument panels and additional safety equipment. The amount of chlorine handled inside the room is normally seven one-ton containers and f our one-ton containers are stored outside. The room is normally unmanned, however, it is within our treatment plant which has operators present 24 hours per day. The offsite consequence analysis includes consideration of two chlorine release scenarios, identified as "worst case scenario" and "alternate scenario". The first scenario is defined by the EPA, which states that the "owner or operator shall assume that the ... maximum quantity in the largest vessel... is released over 10 minutes," due to an unspecified failure. The alternate scenario is defined as "more likely to occur than the worst case release scenario". Atmospheric dispersion modeling has to be performed to determine the the distance traveled by the chlorine released before its concentration decreases to the "toxic endpoint" selected by the EPA of 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 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 radius corresponding to the toxic endpoint distance has to be defined,"to estimate the popluation potentially affected". The worst case scenario at the Des Moines Water Works treatment facility involves a failure of one one-ton container for a total of 2000 pounds of chlorine. The offsite consequence analysis for this scenario was performed using a model program developed jointly by the American Metropolitan Sewerage Association and the EPA. Using the EPA's criteria for a worst case scenario the toxic endpoint is 1.3 miles; however, the assumptions used may be unrealistic because: 1. Only a fraction (about 26% of the total) of the compressed liquified chlorine relea sed to the atmosphere flashes as a vapor. The remaining unflashed liquid forms liquid-droplet aerosols. This results in formation of a very dense chlorine cloud consisting of vapor and liquid droplets, with dispersion characteristics significantly different than for a cloud consisting only of chlorine gas. 2. The Des Moines Water Works treatment facility is surrounded by a 15 foot tall earthen berm (to keep flood waters out). This levee will contain some of the dense chlorine gas. 3. The treatment facility is also surrounded on three sides by the Raccoon River. The dense chlorine gas may settle into the river valley and travel its path until it disperses. 4. Only the population within an elliptical plume extending downwind of the release point is potentially affected. This plume area, or footprint, is approximately 6% (one-twentieth) of the area of the full circle. The alternative release scenario involves the failure of the flexible connections (pigtails) or a valve con nected to one container. The amount of chlorine released is 317 pounds over a 60 minute duration with a maximum average release rate of 10.5 pounds per minutes. The distance to the toxic endpoint is 0.48 miles. The chlorinators are of the vacuum type which reduces the amount of chlorine should a pigtail rupture. The Des Moines Water Works accidental release prevention program is based on the following: 1. High level of training of the operators. 2. Preventative maintenance program. 3. Use of state-of-the-art process and safety equipment. 4. Use of accurate and effective operating procedures, written with the participation of the operators. 5. Performance of a hazard review of equipment and procedures. 6. Implementation of an auditing and inspection program. Chemical specific prevention steps include availability of self-contained breathing apparatus, operators trained in release response, awareness of the hazardous and toxic properties of chlorine and the pre sence of chlorine detectors. No accidental releases of chlorine having offsite consequences have occurred at this facility in the past five years. The facility has an emergency response program with a response team capable of responding and controlling most releases. The facility has also coordinated with the Des Moines Fire Department HazMat team. The DMFD HazMat team has conducted drills at our facility to help better respond to a release. The Des Moines Water Works is a member of the Polk County Local Emergency Planning Committee, which has a public notification program. |