Disinfection Part 2: Is Chlorine Still the Answer?
As everyone knows, the practice of using chlorine in
drinking water treatment has been acclaimed as one of the most significant
public health advances of the 20th century, if not the entire millennium. More
than 98 percent of water treatment plants use some form of chlorine to treat
their water because of several benefits: germicidal potency, sustained residual
disinfection properties, taste and odor control, as well as cost-efficiency.
Chlorine-based disinfecting agents also are recognized as an effective defense
against many biological warfare agents. In the past few years and, more
recently, with the signing of the Bioterrorism Act by President Bush, the
federal government has increased the effort to keep this nation's water supply
safe. Disinfection of the public water supply should not be compromised. Title
IV of the Bioterrorism Act indicates that over the next two years, water
utilities will be required to perform vulnerability assessments of their water
systems in regard to such issues as water collection and treatment; the use,
storage or handling of various chemicals; and the operation and maintenance of
their water treatment systems.
Water suppliers continually are being challenged to prevent
the presence of disease-causing microorganisms in their water systems, and
methods of treatment vary depending on site-specific factors as well as the
quality of the raw water supply. The importance of water disinfection is
evidenced by the fact that most past cases of outbreaks of waterborne diseases
were due to inadequate disinfection or no disinfection at all. Alternatives to
the use of chlorine have received increased interest since concerns over the
formation of disinfection byproducts (DBPs) have emerged. However, most of these
alternatives (i.e., chloramine, chlorine dioxide and ozone) also produce DBPs.
Less information is known about the DBPs formed by some of the alternatives,
and the risks using these technologies may be equivalent or higher. Chlorine
still is the most common drinking water disinfectant used today and the one we
have the most information about. On balance, the health risks of not
chlorinating water appear to be greater than risks associated with DBPs.
Emerging national security issues along with complying with
other federal regulations such as EPA's Risk Management Plan and OSHA's Process
Safety Management have pushed the water and wastewater treatment industry into
looking for alternatives. Alternatives for water treatment such as ozone, UV
irradiation and chlorine dioxide have been used. Although these other processes
do provide efficient disinfection capabilities, each alternative has associated
disadvantages. Ozone and UV irradiation do not provide a persistent residual
disinfection capability, require high capital investments and have relatively
high operating and maintenance costs associated with them. Chlorine dioxide
forms organic byproducts and requires on-site generation equipment and the
handling of several chemicals.
As mentioned before, chlorine has many benefits. First, the
use of chlorine has been demonstrated to reduce the level of microorganisms
that cause waterborne diseases.
It is easy to apply, and small amounts stay in the water
from the treatment plant through the distribution system to the consumer's tap.
Chlorine also controls biological growth by eliminating bacteria and algae as
well as other organisms. Since chlorine oxidizes natural substances such as
decaying vegetation, reduction in odors and tastes occurs. For these reasons, chlorine
still is a good choice of drinking water experts. However, in looking for
alternatives, one need not go far from the traditional forms of chlorine to
find one. Water treatment facilities have been turning to another form of
chlorine--calcium hypochlorite--as their system for water chlorination.
Typically two well-recognized forms of chlorine have been
used in water treatment: the use of chlorine gas in cylinders or sodium
hypochlorite (bleach) solutions. Due to the physical nature of these chemicals,
both of these technologies present specific safety concerns about potential
releases and spills and both typically require special buildings and spill
containment designs. These chemicals also present handling issues. For example,
chlorine gas requires personnel training and use of personal protective
equipment when changing cylinders. Similarly, handling drums of bleach is
difficult and presents safety issues. Maintenance costs are another factor.
Keeping chlorine gas eductors and bleach addition pumps operating efficiently
is a chore because the equipment's small orifices are prone to clogging. Bleach
loses strength and efficacy over time that can result in increased material
costs to keep residual disinfection capability in the system.
Calcium hypochlorite is an alternative to chlorine gas or
sodium hypochlorite (bleach) solutions because it is a dry form of chlorine
that offers several handling advantages Calcium hypochlorite contains
approximately 65 percent available chlorine as compared to the 12 percent in
bleach and does not require operator certification or containment areas. Many
facilities have opted for a technology using calcium hypochlorite tablet
systems as the preferred method of introducing chlorine disinfectant. This
technology is selected because of its lower capital costs, accuracy,
reliability, safety and maintenance benefits. It has opened new horizons in
water chlorination for applications of many types and sizes. Calcium
hypochlorite systems have been used for years and currently are being used for
drinking water applications in more than 40 states as the primary disinfection
treatment or as remote booster chlorination stations. They also have been
successful and proven in food processing including fresh vegetables and
poultry, pool and spa applications, cooling towers, grain milling and
Technology combining calcium hypochlorite in a three-inch
tablet form along with a specifically designed patented erosion feeder is
becoming a standard in the industry. Tablets are eroded by incoming water from
a side stream contacting only those tablets at the bottom of the feeder.
The erosion rate is a predictable rate because it is
dependent upon water flow to the unit; therefore, chlorine dosage can be
achieved by controlling the water flow rate. The chlorinator effluent then is
returned to the unchlorinated main system flow, providing the desired level of
available chlorine to meet operational requirements. Water plants as large as
14 million gallons/day with chlorine demands exceeding 400 lbs./day have been
chlorinated by these systems for years. Smaller units have turndown ability to
supply the 35 gpm well water user without overchlorination.
As regulatory requirements and safety issues provide
increasing incentive for water treatment plants to reconsider their water
treatment systems, it is important to recognize that calcium hypochlorite--the
solid form of chlorine--offers safety and low maintenance benefits together
with small capital investments. It is becoming the preferred alternative.