The American Water Works Assn. (AWWA) announced the launch of its new ...
If you live in a city such as New York, San Francisco or
Chicago that is on a large body of water, would you be able to tell if your
drinking water was disinfected using chlorine, chloramines or a completely
different halogen--bromine? All three have been effectively used as
disinfectants in thousands of drinking water applications, but your answer
would depend on whether you're on land or in the ocean. Even though bromine is
used to disinfect drinking water systems, it lacks visibility in many water
treatment circles because The U.S. Environmental Protection Agency (EPA) has
limited the use of bromine as a disinfection agent to marine drinking water
applications. In fact,
disinfection systems using bromine are as common on board U.S. warships,
commercial vessels and offshore oil drilling platforms as chlorine and
chloramines are in municipal water treatment facilities. You may not be aware of bromine because currently it is not used in municipal
drinking water applications.
While water is drawn from many sources in offshore and
marine environments, a reliable supply of high-quality safe water is critical
for the well-being, productivity and morale of the crew stationed onboard.
Often the water is produced by the ship's desalinization system, which could be
reverse osmosis or a distillation system. Other common water sources are shore
water, barged water, another ship or a mixture of sources.
At present, two halogen compounds--chlorine and bromine--are
the only EPA-approved methods for disinfecting shipboard potable water. While
chlorine is the preferred disinfectant for land-based water treatment
facilities, applying it in offshore applications poses some significant disadvantages.
Chlorine in a gas, liquid or solid form is very dangerous on ships or offshore
oil platforms. Chlorine is very reactive and corrosive, which makes it an
extremely hazardous chemical to handle. Use of chlorine gas is impractical on
ships due to handling problems and the confined spaces where it would be
Powdered calcium hypochlorite has been used as a source of
chlorine because it provides 70 percent available chlorine. However, it is a
very strong oxidizing agent and has been responsible for causing several
shipboard fires.1 Fires on a ship or offshore oil platform are especially
dangerous because the only place to escape a fire that can't be extinguished is
overboard. There are other disadvantages to powdered calcium hypochlorite. It must
be used in the dissolved form and any calcium hypochlorite particles that
remain undissolved will interfere with the operation of the feeder releasing it
into the water supply.
Before 1957 bromine wasn't a good alternative for offshore
applications. As with chlorine, elemental bromine (a liquid) is too hazardous
to use in offshore applications. Elemental bromine is extremely corrosive and
becomes even more so when combined with even trace amounts of water, which
makes the liquid form of bromine incompatible with most materials. However, in
1957 The Dow Chemical Corp. addressed the negatives of using bromine by
creating a brominated ion exchange resin, which opened up the use of bromine in
offshore water treatment applications.2 Today, polybromide resin is used in
many offshore water treatment systems. It is a strong base anion exchange resin
impregnated with elemental bromine to the level of 30 percent by weight.
Polybromide resin is a dry solid that is safer to handle than calcium
hypochlorite because it is nontoxic and will not start fires if combined with
petroleum distillates such as paint or fuel. Polybromide resin is available in
sealed cartridges for use in brominating systems, making it easy to contain,
handle and monitor as the bromine needed for disinfection is released into the
water supply at a predictable rate. The fact that bromine can be added to water
that previously has been chlorinated makes polybromide resin a practical
solution to accommodate the multiple sources of water in offshore applications.
One of the benefits of the dry brominated resin is that it
can be stored under the proper conditions, for up to two years. When it is
needed, the cartridge is loaded into a proportioning or recirculating feeder,
and a controlled amount of bromine is released into the water. Once the
cartridge is spent, it can be safely disposed of in the regular trash without
the concern of causing a fire or releasing a dangerous gas.
Bromine Advantages Over Chlorine
Although the general properties of bromine are similar to
chlorine, there are distinct differences between the two halogens. Among its
advantages, polybromide resin is safer to use, it is affected only negligibly
by ammonia, it has a long shelf life and it Is an effective disinfectant over a
wider pH range than chlorine.
Bromine and Ammonia
The most common source of ammonia in a marine application
would be from water that was received from a municipal supply that contained
chloramines or naturally occurring ammonia. When the ammonia is recombined with
chlorine or bromine in an offshore holding tank, it is important to know the
effect of this ammonia on the halogen that is used to disinfect the water. A
study by Johanneson has demonstrated that the bactericidal activity of bromine
is not significantly affected by the presence of ammonia.3 However, the
bactericidal properties of chlorine are appreciably affected by the presence of
even trace amounts of ammonia, requiring much longer times for 99 percent inactivation
of E. coli organisms. (See Table 1.)
Polybromide resin is an effective disinfectant over a wider
pH range than chlorine. Chlorine is 50 percent active at a pH of 7.6, whereas
bromine is 50 percent active at a pH of 8.7. Bromine also is less affected than
chlorine by the presence of organics that contribute to halogen demand.
Adjustments for Temperature Variations
Water temperature and total dissolved solids (TDS) do affect
the amount of bromine that is released from the cartridge. The lower the water
temperature, the less bromine released; therefore, an insufficient quantity of
bromine may be released from the resin. To increase the bromine residual level
to a proper amount, operation of a recirculating bromine feeder is required.
The recirculating bromine feeder can treat up to four water storage tanks,
depending on the type of feeder used, to maintain the proper residual. (See
Figure 1.) The higher the temperature and/or TDS, the greater the amount of
bromine released. (See Table 2.) If a test reveals an increase in source water
temperature, an operator adjustment can be made to deliver a lower amount of
Types of Bromination Systems
Bromination of a potable water system requires two types of
brominators. A proportioning bromine feeder is used to treat potable water by
adding a predetermined quantity of bromine to the desalinization system
discharge line. The quantity of bromine added is contingent upon the seawater
intake to the ship's water maker. If the seawater intake is considered noncontaminated,
the low feed rate of 0.7 ppm bromine at 100ºF is acceptable. However, if
the seawater intake is considered contaminated, the high feed rate of 2.7 ppm
bromine at 100ºF is required. An operator can change the feed rate by
adjusting the feed valve handle from either the parallel (high feed) to
perpendicular (low feed) position or vice versa. (See Figure 2.) Monitoring the
bromine residual will ensure the potable water is bacteriologically safe for
cooking and drinking purposes.
The other type of brominator is the recirculating bromine
feeder system, which adds bromine to water after it is placed in a storage
tank. This allows the convenience of multiple tank treatment at one location.
This system recirculates the water in each individual tank and while
recirculating, it adds bromine to the water, dispersing it evenly throughout
the storage tank(s). The system utilizes a multiple position tank selector
valve, which is designed to easily select the tank to be tested and/or treated.
This system permits regular sampling of the treated water to insure that the
desired disinfectant level in the tank can be reached. With the aid of a graph
that displays water temperature versus tank volume, the feed time is determined
(Bromine Feed Chart). This programmed time will increase by approximately 0.5
ppm the bromine content of potable halogen demand-free water.
Maintaining Halogen Residual for Safe Potable Water
Halogen-based disinfectants such as chlorine and bromine
react with oxidizable substances in water. The initial utilization (usually
within 30 minutes) of the disinfectant in a water supply is referred to as
"halogen demand." The halogen demand in water will vary with respect
to the amount of interference from the oxidizable substances that are present.
This will reduce the initial supply of bromine added to the water. If the
proper amount of halogen residual cannot be maintained in the tanks or potable
water system, it is an indication that some substance has used or neutralized
the disinfectant in the system. Further inspection of the water system should
be done to determine the source of the contamination.
A measurable trace halogen residual should be maintained
within the distribution system to ensure safe potable water. Water without a
halogen residual should be brominated to provide 0.2 parts per million (ppm)
halogen residual at the end of a 30-minute contact period. However, if the
source water is of poor quality or
unknown quality, it is best to maintain a residual of 2.0 ppm after a 30-minute contact time. Poor quality water often is the case
when a ship is in port or within
several hundred miles of a large city or port while making its own water. If water from a municipal supply cannot
be trusted, it also would be considered poor quality.4 This is because water in
harbors or along the shore line as
well as some municipal water around the world may contain harmful waterborne bacteria and viruses, which need to be destroyed to make the water safe to drink and use.
As you can see, making and purifying water in a marine
application is quite different than in most land-based systems. Much of what
has been learned about purification of water in standard municipal applications is applied differently away from shore. Through the use of polybromide resins, water
treatment can be conveniently and safely accomplished over a wide range of
water chemistries and conditions without compromising the safety of the crew.
The next time you go onboard a ship, ask how they make and purify their water.
You are bound to discover something interesting and learn more about this field of water treatment.