TM 5-814-3/AFM 88-11, Volume III
(7) Precautions in the use of chlorine. The presence of chlorine gas in the atmosphere can pose
immediate and serious hazards to the health of any person breathing the air. Gas masks approved by the
National Institute of Occupational Safety and Health (NIOSH) will be provided outside any area where an
individual would be exposed in the event of chlorine leaks, spills, etc. All rooms in which chlorine is to be
stored or handled should be adequately ventilated to the outside. A fan which automatically turns on prior
to entry into the chlorination or storage facility will be provided. Since chlorine gas is heavier than air, vent
outlets will be placed at floor level. Chlorine detectors of the liquid reagent, electrode cell, indicating meter
type, sensitive to one part per million O)y volume) chlorine in the air, will be used to continuously monitor
the air for chlorine leaks. Alternatively, the enclosed space should be entered only if the worker is under
observation by a co-worker and if the worker has in his possession a respirator suitable for escape. The
applicable safety recommendations, as given in the American Water Works Association (AWWA)
publication, Safety Practice for Water Utilities, and Water Pollution Control Federation (WPFC) Manual
of Practice No.1, Safety in Wastewater Works, should be followed. Information on the properties of
chlorine and its safe handling are also available from the Chlorine Institute. When hypochlorite compounds
and generators are used, the above requirements do not apply.
h. Chlorine generators. Units are commercially available for generating sodium hypochlorite. These may
be used for the source of chlorine at facilities where an ample supply of brine solution or salt water is readily
available or can be readily mixed. Such systems should be evaluated in accordance with requirements of
chapter 5 of this manual when design analysis confirms that, technically, they are equal to or better than
alternate chlorine sources. Design must include adequate facilities for safely handling the hydrogen gas
released in the generating process.
i. Chlorine contact chambers. A chlorine mixing contact chamber will be designed to provide a
minimum of 30 minutes detention time at the peak design flow. Consideration will be given to two flow-
through units with common-wall construction so that each side satisfies the detention requirements. The
ability to segregate each side for cleaning purposes will be included. Short circuiting will be minimized with
inlet baffles and end-around baffling within the tank. A minimum length-to-width ratio of 40 to 1 should be
utilized. The chlorine feed rate will be proportioned in accordance with the flow and the chlorine demand of
the wastewater. Adequate mixing during the chlorine contact period will be insured by the installation of
adequate baffling, or by either mechanical or air mixing.
j. Chlorine control equipment. Residual chlorine in the plant effluent must meet the NPDES permit
requirements. Residual chlorine analyzers are required for all plants using chlorine gas. Maintaining a consis-
tent chlorine residual in the related wastewater is the best control for adequate disinfection. Control will be
accomplished by the practice of closed-loop chlorination whereby the chlorine metering equipment is
controlled by a chlorine residual analyzer downstream from the point of application. The requirement for
continuous or intermittent, automated or manual sampling, analyzing and recording will be directed by
Federal permit requirements. Chlorinator alarms will be provided to signal either an abnormally high or an
abnormally low vacuum in the line between the chlorinator and the injector.
17-4. Dechlorination.
Dechlorination is the partial or complete reduction of residual chlorine in water by chemical or physical treat-
ment. Disinfection by chlorination will have to be followed by dechlorination to meet NPDES requirements
stipulating bacteriological effluent limitations as well as extremely low or zero chlorine concentrations in the
final effluent. The theoretical chemical quantities required for dechlorination are given in table 17-2.
17-4
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