only treatment before discharge to the distribution system, as in the chlorination of groundwater supplies and previously unchlorinated purchased supplies. Prechlorination is the application of chlorine to raw water before coagulation, sedimentation, or filtration. Postchlorination is the application of chlorine after filtration, but before the water leaves the treatment plant. Rechlorination is the application of chlorine into the distribution system or into a previously chlorinated purchased supply to maintain the chlorine residual.

The above applications are normally continuous. Very heavy chlorination for a limited period is sometimes applied at specific points in the distribution system to destroy localized contamination.

OTHER USES OF CHLORINE. - Chlorine is also used to control tastes and odors in water. It reacts with the substances causing taste and odor, such as hydrogen sulfide, minute organisms, algae, and organic compounds.

If the reaction is incomplete, the taste and odor of some substances may be intensified or become more objectionable. Chlorine is also used to a limited extent to oxidize iron and manganese and to remove color.

SAFETY. - Safety is important in the handling of chlorine. Some of these important factors are as follows:

Provide self-generating oxygen-breathing apparatus or self-contained oxygen-breathing apparatus designed to cope with chlorine.

Maintain only the supply of chlorine in any chlorinator room that will do for normal daily demands. Store the main supply in a detached noncombustible building or in a fireproof room that is vented only to the outside and separated from the main part of the building. Keep the chlorinator and chlorine storage building or rooms locked to prevent the entrance of unauthorized personnel and restrict these areas from any other use.

Allow only reliable and trained personnel to handle chlorine.

Handle containers carefully to avoid dropping or bumping them.

Avoid hoisting containers; if hoisting is necessary, use safe lifting clamps.

Store cylinders in a cool place, away from dampness, steam lines and fire, and in an upright position secured from tilting and falling.

Keep protective valve caps on containers when not in use; never tamper with safety devices on containers.

Never connect a full cylinder to a manifold with another cylinder unless the temperatures of both are nearly the same.

When not withdrawing chlorine or when cylinders are empty, keep the valves closed.

Disconnect the valves as soon as the containers are empty, and check for chlorine leaks at the valve outlets. Test for leaks by passing an opened bottle of strong ammonia solution around the valve. White fumes of ammonium chlorine will appear if there is any leakage. Leaks around fittings, connections, and line can be detected in the same way. Do not apply ammonia solution to plated metal parts, as it will remove the plating.

When chlorine is noticed, workers should avoid panic, refrain from coughing, keep mouth closed, avoid deep breathing, keep head high, and get out of the affected area. Only qualified personnel with suitable respiratory equipment should be assigned to investigate and correct the cause of chlorine leaks. If chlorine is being discharged, close the container valve immediately. If chlorine is escaping in liquid form, turn the containers so the chlorine escapes as gas. This will reduce leakage. Do not apply water to the leak; this dangerous practice causes corrosion that may increase the leakage. Electronic chlorine gas detector warning devices are widely used in plants and mechanical rooms that contain chlorine.

The handling of a persistent chlorine leak in a plant is best left to the chlorine supplier or local fire department.

Never apply a flame, blowtorch, or other direct heat to chlorine containers; discharge them in a room with a temperature of about 70F.

Never ship a defective or leaky cylinder unless it is completely empty. Paint "DEFECTIVE" plainly on all such cylinders.

Follow all regulations on shipping, storing, and using compressed gas cylinders.

Provide proper means of exit from areas where chlorine is stored or used.

Never use a chlorine cylinder except to hold chlorine gas.