Inherent Safety and Chlorine in Water Treatment

Abstract

Chlorine has been used for water treatment purposes for more than one hundred years. The simplicity and effectiveness of using chlorine and its derivatives for water treatment is one of the wonders of modern chemistry: it is cheap, it is safe, and it works. Chlorine has uses on water intake structures, for the removal of aquatic organisms, for pre-filtration, to kill bacteria and for water disinfection. The gas has a greenish-yellowish color and has a molecular weight that is two and a half times larger than that of air. In its gaseous form, chlorine is extremely toxic and dangerous. It also has a very high coefficient of expansion. For this reason, all chlorine containers’ volume must not be filled up past eighty five percent of their capacity. Chlorine gas is fed into the water treatment system under vacuum conditions. Chlorine tanks have an automated system of regulators, feed equipment and vacuum ejectors. Piping connections must be sealed with proper pipe thread compound and compression fittings must be sealed with a new lead washer. Also, chlorine gas scrubbers should be installed in any facility that uses chlorine gas. The Environmental Protection Agency (EPA) requires wastewater plants which store two-thousand five-hundred pounds or more of chlorine gas to conduct a risk management plan. Risk reduction begins with using the smallest cylinders possible of chlorine gas for the application. Water treatment plants can manifold as many ton containers as necessary while controlling for leaks at each individual container and throughout the entire system. In addition, the water plant should be located as far out of the city as possible, downwind of the prevailing winds. Booster systems at strategic locations can be placed. The Pasquill-Gifford model is a very good way to estimate the concentrations of a release at different distances from the source. However, a better Al-Obaidli, Anany, Hamad 3

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model to use would be the Britter and McQuaid model for dense gases. Risk assessment software such as PHAST provides planners and retrofitters with a tool to determine various levels of risk. The example used about Ras-Laffan was simulated using PHAST for the three cases involved. The companies at Ras-Laffan assume that the wind direction from that region will always be North-West. If that were true, then the results from PHAST show that there would be no risk of the leak reaching any of the surrounding cities. The rupture of a one- ton cylinder could potentially produce a cloud one mile high by a half- mile wide by one mile long of toxic mustard gas that will kill everything in its wake. A train in Ontario derailed and a tank car of chlorine gas ruptured; if there had not been a large propane fire funneling the heavier-than-air mustard gas upwards into the atmosphere, many thousands of people in the city of Mississauga might have died. Although chlorine is by far the cheapest chemical to use for water treatment, the most widely accepted, and has the fewest risks to public health, other chemicals should also be investigated