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OXIDATION AND DISINFECTION 10.25

tem (aqueous ammonia and anhydrous ammonia feed systems). The vapors above the so-
lution in an aqueous ammonia tank are extremely potent. Therefore provision should be
made for disposing of the displaced vapors in the tank in a safe manner, such as through
transfer back to the delivery vehicle when the storage tank is being refilled. Pressure re-
lief valves on these tanks should pass through a water-type scrubber before going to the
atmosphere.
Delivery of aqueous ammonia to the feed point is typically performed with diaphragm
metering pumps. The ideal control system would permit a feed rate proportional to the
product of the flow and the chlorine residual such that the appropriate ammonia-to-
chlorine ratio for chloramine formation can be maintained.
Aqueous ammonia is often fed through stainless steel end-type diffusers with corpo-
ration stops for ease of cleaning. If carrier water is used, the water should be softened and
the diffuser should have 3A6-in.-or-larger (4.5-mm) orifices designed with a significant
backpressure such that any scale formation is knocked off the diffuser.
When one is ordering aqueous ammonia, debris-free chemical should be specified, be-
cause aqueous ammonia is sometimes delivered with considerable debris present. Alter-
natively, facilities may be installed to strain undesirable debris from the product before it
is used.

Ammonium Sulfate Systems
Solid ammonium sulfate is usually fed into a simple mixing tank using a gravity or vol-
umetric feeder. Once mixed, the solution can be transported using the same methods de-
scribed earlier for aqueous ammonia. If the local water is hard, scaling problems may
occur, and softening of this carrier water should be considered.

Sodium Hypochlorite Systems

Sodium hypochlorite is used by many small and medium-size water systems to feed chlo-
rine because of the greater ease of handling and much greater safety as compared to chlo-
rine gas. In recent years, some larger water systems that are located in metropolitan ar-
eas have also changed from chlorine gas to the use of sodium hypochlorite because of an
increased emphasis on safety.

Purchasing Hypochlorite
Sodium hypochlorite (liquid bleach) is formed by combining chlorine solution and so-
dium hydroxide. In other instances, it is manufactured from chlorine and sodium hydroxide
that have been separately shipped to the manufacturing site. The reaction that proceeds
in this instance is as follows:
C12 + H20 ~ HOC1 + HC1 (gas chlorine to chlorine solution)
HOC1 + HC1 + 2NaOH ~ NaOC1 + NaC1 + H20

A slight excess of sodium hydroxide is often added to increase the stability of the so-
dium hypochlorite product.
When the sodium hypochlorite is added to water, it hydrolyzes to form hypochlorous
acid (HOC1), the same active ingredient that occurs when chlorine gas is used. The
hypochlorite reaction slightly increases the hydroxyl ions (pH increase) by the forma-