Water chlorination has been one of the greatest innovations
of the last century. The technology not only has made our drinking water safe,
it has provided effective water purification for a multitude of other
Yet, while chlorination is a hero in many ways, as with any
chemical, there are considerations to be made regarding its application. This
article will focus on three popular forms of chlorine: gas, sodium hypochlorite
and dry calcium hypochlorite tablets. Each has advantages and disadvantages relative
to cost, convenience, effectiveness, storage and regulatory issues. A look at
the pros and cons of each can answer many questions about what's right for your
The good news about chlorine gas is that it is relatively
inexpensive and doesn't produce byproducts such as chlorite or chlorate ions.
For companies, small municipalities and small systems concerned about budgets,
chlorine gas is very attractive. Yet, that's where most of its "pros"
end. Two disadvantages of chlorine gas are toxicity and corrosiveness.
A more serious disadvantage of gaseous chlorine is the total
cost of handling and operating safety. Federal regulations limit the amount of
gas that can be stored at a single location without extensive provisions to contain
potential leaks. On-site storage of large quantities of gas must be enclosed in
and protected by systems called "scrubbers," which have the ability
to contain and neutralize gas in the event of a leak. Scrubbers are costly to
purchase and install and require regular maintenance. In addition, operators
must have an emergency response plan in place. Such plans usually are authored
by an engineering firm following a careful study of the operator's overall
operation. Costs can mount quickly.
Other maintenance costs include frequent overhauls of
regulators and the requirement to wear protective breathing gear when handling
containers or providing maintenance on the systems even with the smallest
Finally, a little-known cost associated with using chlorine
gas is the record-keeping and reporting required. Such regulations have been
imposed by the Occupational Safety and Health Administration (OSHA) and add to
the labor costs of using chlorine gas. In short, the overall related costs of
using chlorine gas can far outweigh the cost of the gas itself.
Sodium Hypochlorite (Bleach)
A second type of chlorine commonly used today is liquid
sodium hypochlorite or bleach. At first glance, bleach seems like the perfect
alternative to gas since it's relatively cheap, easy to apply and not nearly as
lethal as gas. After all, it's just household bleach, right? Most everybody has
that at home under their kitchen sink.
Liquid sodium hypochlorite also has several disadvantages.
First and foremost, the product breaks down over time, losing efficacy and
forming byproducts, which can add additional problems to solve. When you
consider that residual injection rates are sometimes less than 1 part per
million (ppm), continuous loss of chemical strength can be a real challenge in
maintaining consistent residuals. It's like shooting at a moving target with
bullets that are less than fully charged with gun powder. Constant corrections
must be made to the injection rate in order to compensate for the steadily
decreasing strength of the disinfectant. The breakdown of sodium hypochlorite
depends on storage temperature and the presence of impurities in the
concentrated product. A decomposition byproduct of sodium hypochlorite is
sodium chlorate. For every 1 percent of hypochlorite lossed, 0.8 percent sodium
chlorate is formed. Thus, if an 8 percent active hypochlorite product is used
to provide six ppm of available chlorine in water, it also will deliver 1.5 ppm
Metering pumps and other injection components are vulnerable
to corrosion. Since bleach is considered a hazardous chemical, containment at
the treatment site is a consideration. Metering pumps typically are placed on
top of containers, which requires lifting chemical to the pump. As bleach
off-gasses, pumps often lose their prime causing the system to either fail
entirely or pump inconsistently. Like chlorine gas, personnel working with this
form of chlorine should receive safety training and wear proper protective
clothing during handling. Employees working with the solution also take on the
risk of physical injury in just handling the heavy drum containers. Companies
using bleach also must pay careful attention to ambient temperatures and
proximity to equipment susceptible to corrosion when storing the chemical. In
addition, bleach never should be stored near acid products, since the reaction
between acid and sodium hypochlorite will produce a potentially fatal chlorine
Another consideration associated with the use of bleach is
its overall efficiency. Companies and small municipalities that use this form
of chlorine often report an uneven distribution of the solution throughout the
water supply. It appears that liquid chlorine provides "pockets" of
the substance near the distribution source and only trace amounts in outlying
Financially, bleach remains the lowest cost alternative to
gaseous chlorine relative to pure chemical cost. Similar to gas, the total cost
of disinfection includes many subtle factors beyond that of the basic chemical.
Dry Chlorine Tablets
A third type of chlorine becoming increasingly popular today
is calcium hypochlorite in dry tablet form. Calcium hypochlorite, particularly
in tablet form, is an effective alternative to gaseous chlorine and even liquid
sodium hypochlorite. It contains about 65 percent available chlorine, is safer
to handle and easily can be loaded into feed equipment.
Perhaps the greatest advantage of solid calcium hypochlorite
disinfection is the overall safety factor. Although solid tablets emit an odor
similar to that of swimming pool chemicals, other than a simple dust mask and
gloves, there is no special breathing protection or personal safety gear
required for general handling. Buckets can be stored in most dry facilities but
must be kept from coming into contact with organics such as acids or oils since
calcium hypochlorite is classified as an oxidizer. When stored properly,
calcium hypochlorite tablets are relatively stable over long periods of time,
losing only 5 percent of their strength in the first year.
Most manufacturers offer calcium hypochlorite tablets with a
sequestering agent blended into the tablets, which for the most part, keeps
calcium in the chemical from plating out on system components. Occasional
cleaning of sediment from solution tanks is considered easy and requires very
little on-site labor. It is important to choose a system that accurately meters
the solution evenly and consistently into the water stream. Calcium
hypochlorite systems previoulsy had a bad wrap due to some bad publicity
regarding some people who attempted to use little more than swimming pool
systems as a means to dissolve and distribute the chemical. The problem with
using such as system is that high volume centrifugal pumps cannot provide
fractional ppm consistency required for potable treatment. A well-designed
calcium hypochlorite system should have the option of manual or automatic
controls making it possible to receive pacing instructions from either a
chlorine analyzer or flow meter. For any calcium hypochlorite system to
function properly, it must provide even and consistent dissolution of the solid
tablets. From that point on, the system functions much like a bleach system,
only with far more stability.
The downside of calcium hypochlorite is chemical cost.
Depending on quantities purchased, the cost is in the range of 50 percent
greater than that of bleach. However, new developments may evolve that accept
granulated calcium hypochlorite. These new systems will have all the benefits
of tablets but compete heads up with bleach in cost.
A good case can be made for all three forms of chlorine.
However, it is important when choosing any long-term methodology to make more
than a superficial appraisal of the concept.
A difficult economy and an increased focus on efficiency
makes choosing disinfection equipment a difficult task. It involves far more
than simple cost of the chemical or equipment alone. A system that works for
you will be one that considers your organization's ability not only to purchase
and install, but to operate and maintain a program throughout its projected
life. Fail to consider that, and you will fail, indeed.
Tablet Chlorination Proves a Viable Alternative