City of Pullman Wastewater Treatment Plant - Executive Summary

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   The City of Pullman Wastewater Treatment Plant uses chlorine and sulfur dioxide gasses in our effluent disinfection process.  These gasses are stored primarily in one-ton pressurized cylinders.  We may have up to eight cylinders on hand.  Vacuum-operated regulators are attached directly to the cylinders in use so potential leaks are limited mostly to the cylinders.  In the fifteen-year history of this process we have had no reportable incidents.  Personnel are trained in the use of air packs and tank repair kits.Two people are required to change cylinders and use procedures to minimize potential problems.  We have an ongoing maintenance program to keep our gas equipment in good shape.  We reevaluate our safety procedures at least on an annual basis.  For this risk management plan I have described two hazardous gas leak scenarios.  In the "worst case scenario" a one-ton tank completely empties to atmosphere within a ten-minute period.  The toxic endpoint (the distance at which gas l 
evels could harm a person if breathed for an hour or more) is calculated at 1.3 miles.  If this situation were to occur, plant personnel would dial "911", appropriate emergency units would be notified and the area would be evacuated.  In the "alternative release scenario" which is still much worse than we have experienced, a one-ton tank empties to atmosphere in two hundred minutes.  The toxic endpoint is calculated at one-tenth of a mile and impacts only the treatment plant.  Backup personnel will be called in, and, if plant operators determine it is reasonable, they will follow safety procedures and work to stop the leak.  Otherwise, the city fire department will be called in to monitor the perimeter, and the plant will be evacuated.
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