Chlorine and Chloramine Removal with Activated Carbon

http://www.carbonresources.com/pdf/0906Potwora.pdf

Municipalities routinely began
using chlorine to treat drinking
water starting in 1908 with
Jersey City, NJ. Its use has helped to virtually eliminate diseases
like typhoid fever, cholera and dysentery in the US and other
developed countries. Globally the World Health Organization
(WHO) estimates that 3.4 million people in underdeveloped
countries die every year from water-related diseases.
Use of chlorine in water can produce an undesirable taste;
therefore, many consumers prefer to remove it. Disinfection
byproducts (DBPs) may also unintentionally form when chlorine
and other disinfectants react with natural organic matter that is
in the water. To reduce DBP formation, many municipalities are
switching to monochloramine.
Monochloramine treatment was first used in Ottawa,
Ontario, Canada in 1916 and in Denver, CO in 1917. Use of
monochloramine took a downturn during World War II due to
ammonia shortages. Currently the US EPA estimates more than
30 percent of larger US municipalities use monochloramine.
It’s a common misperception that activated carbon removes
chlorine and monochloramine from water by adsorption.
Understanding how activated carbon
removes chlorine and monochloramine
from water is critical to the design and
operation of such systems.
Chlorine formation and
reactions
Use of chlorine is the most common
method to disinfect public water
supplies. Chlorine is a powerful germicide,
killing many disease-causing
microorganisms in drinking water, reducing
them to almost non-detectable
levels. Chlorine also eliminates bacteria,
molds and algae that may grow in
water supply systems.
US EPA’s maximum residual disinfection levels (MRDLs) are
four mg/L for chlorine; however, chlorine may cause problems
that activated carbon can help resolve. The addition of chlorine
to disinfect water is accomplished by one of three forms: chlorine
gas (Cl2), sodium hypochlorite solution (NaOCl) or dry calcium
hypochlorite, Ca (OCl)2.
The addition of any of these to water will produce
hypochlorous acid (HOCl). This disassociates into hypochlorite
ions (OCl-) to some degree. (The reaction is summarized
below).
Cl2 + H2O → HOCl + H+ + Cl–
HOCl – → H+ + OCl+
The ratio of hypochlorous acid and hypochlorite ion in water
is dependent upon pH level and, to a much lesser degree, water
temperature. The ratio of hypochlorous acid and hypochlorite ion
at various water pH and temperature is shown in Table 1.
It is important to understand the ratio of hypochlorous
acid and hypochlorite ion in water. First,
it has been estimated that hypochlorous
acid is almost 100 times more effective
for disinfection than hypochlorite ion. Secondly, activated
carbons more readily remove hypochlorous acid compared to
the hypochlorite ion.
Chlorine concentrations greater than 0.3 ppm in water can be
tasted. Activated carbon is very effective in removing free chlorine
from water. The removal mechanism employed by activated
carbon for dechlorination is not the adsorption phenomena that
occur for organic compound removal.
Dechlorination involves a chemical reaction of the activated
carbon’s surface being oxidized by chlorine. There are reactions
when hypochlorous acid and hypochlorite ion react with activated
carbon (shown below).
Carbon + HOCl → C*O + H+ + Cl–
Carbon + OCl– → C*O + Cl–
C*O represents the oxidized site of activated carbon after
reacting with chlorine; the chlorine has been reduced to chloride
ion (Cl-). These reactions occur very quickly.
Factors impacting
dechlorination
When designing an activated
carbon dechlorination system, several
process factors must be considered. If
the system is being designed for organic
removal and dechlorination, design
criteria for organic removal will override
design criteria for dechlorination.
Since organic adsorption onto activated
carbon is a slower process than
dechlorination, a system that has been
properly designed for organic removal
will work well for dechlorination.
When the design is strictly for dechlorination, consideration
must be given to any dissolved organics that may be present
in the water. These organics can reduce the capacity of carbon
for dechlorination by occupying the available sites used for
dechlorination.
Particle size of activated carbon is the most important factor
impacting effective dechlorination. The smaller the activated
carbon particles, the faster the dechlorination rate. A disadvantage
of smaller particles is greater pressure drop within the media bed
and, therefore, must be given careful consideration in the overall
system design.
A 20×50 mesh size granular activated carbon (GAC) will be
more effective than a 12×30 or 8×30 mesh GAC. Carbon block
filters are made with fine mesh powder activated carbon with
particle sizes predominantly between 50 and 325 mesh.
Carbon block filters, therefore, are very effective for
dechlorination because of their very small activated carbon
particle size. If a GAC dechlorination system was designed for
20×50 mesh GAC and it was replaced with 12×40 mesh GAC, it
Chlorine and Chloramine Removal
with Activated Carbon
By Robert Potwora
Table 1. Percentages of HOCl and OCl –
% HOCl % OCl – % HOCl % OCl –
pH 32°F 32°F 68°F 68°F
4 100.0 0.0 100.0 0.0
5 100.0 0.0 97.7 2.3
6 98.2 1.8 96.8 3.2
7 83.3 16.7 75.2 24.8
8 32.2 67.8 23.2 76.8
9 4.5 95.5 2.9 97.1
10 0.5 99.5 0.3 99.7
11 0.05 99.95 0.03 99.97
J U

Using free chlorine to disinfect, however,
can cause problems. Free chlorine can
react with naturally occurring organics in
the water, like humic and fulvic acids, to
form total trihalomethanes (TTHMs) and
haloacetic acids (HAAs).
Trihalomethanes in water are generally
composed of chloroform and, to a
lesser extent, bromodichloromethane, dibromochloromethane
and bromoform. To minimize TTHM
and HAA formation, many municipalities have
switched to alternate disinfection methods, the most
common being monochloramine.
Chloramines are formed by adding ammonia to chlorinated
water. The reactions are:
HOCl + NH3 → NH2Cl + H2O (monochloramine)
HOCl + NH2Cl → NHCl2 + H2O (dichloramine)
HOCl + NHCl2 → NCl3 + H2O (trichloramine)
The chloramine formed is dependent upon water pH. At
pH less than 4.4 trichloramine is formed. Between pH 4.4 – 6.0,
dichloramine is formed. At pH above 7, monochloramine is the
most prevalent.
Since most municipalities have a pH greater than 7, monochloramine
is the only chlormaine to be concerned about. Monochloramine
may impact taste and odor, but to a lesser extent
than chlorine. It is toxic to tropical fish and may cause anemia in
patients being treated with kidney dialysis.
Removal by activated carbon, therefore, is becoming more
common. How monochloramine is removed by activated carbon
is summarized in these reactions.
GAC + NH2Cl + H2O → NH3 + H+ + Cl- + CO*
CO* + 2NH2Cl → N2 + H2O + 2H+ + 2Cl + C
CO* represents a surface oxide on the GAC
The preferred reaction is the second one because nitrogen
and chloride are the end products. With a new bed of traditional
GAC, the first reaction occurs to some degree with ammonia
being formed. Over time with traditional GAC, the second reaction
will occur.
GAC systems designed for free-chlorine removal may need to
be retrofitted for monochloramine removal. The reaction rate for
monochloramine removal is considerably slower than removing
free chlorine using traditional GAC.
At least two to four times more EBCT will be required for
monochloramine removal with traditional GAC. Regulatory
authorities and some standards may require 10 minutes EBCT
when removing monochloramine from water for kidney dialysis.
100
90
80
70
60
50
40
30
20
10
0
Percent reduction
Carbon contact elapsed time (minutes)
0 1 2 3 4 5 6 7 8 9 10
Competitive surface-enhanced
Spartan coconut surface-enhanced
Standard coconut AC
Figure 3.
Removal of monochloramine
Information on treating water for hemodialysis may be found in
ANSI/AAMI Standard RD 62:2006, “Water Treatment Equipment
for Hemodialysis Applications.”
Surface enhanced activated carbons
To compensate for poor performance of traditional GAC
for monochloramine removal, manufacturers have developed
surfaced-enhanced activated carbons. These activated carbons
have surface reaction sites enhanced during the manufacturing
process. They are superior for monochloramine removal compared
to traditional GAC. For surface-enhanced GAC, an EBCT of three
minutes will be sufficient to remove monochloramine from water.
A coconut shell-based, surface-enhanced GAC can be compared to
a bituminous coal-based, surface-enhanced
activated carbon (Table 2).
In addition to excellent monochloramine
removal with surface-enhanced coconut
shell-based GAC, its higher iodine
number means it has superior volatile
organic chemical (VOC) capacity. It also
has lower ash content and higher hardness,
resulting in less dust.
A quick bench-scale test is used to
evaluate how well different types of activated
carbons perform for monochloramine
removal. In a beaker containing 400 mL
water and four-ppm monochloramine,
0.2 grams of pulverized activated carbon
is added.
With constant stirring, reduction in
monochloramine is monitored over time. Different types of
surface-enhanced activated carbons can be compared with a
traditional activated carbon (Figure 3). The surface-enhanced,
coconut shell-based activated carbon proved superior.
Based upon field studies for surface enhanced GAC, a
minimum EBCT of three minutes is recommended. For traditional
GAC, a minimum EBCT of 10 minutes is recommended. Using the
recommended EBCT for each type of GAC, the volume of GAC
required for various flow rates may be compared (Table 3). Surfaceenhanced
GAC costs more, but based upon the lower volume
requirements, it is cost effective compared to traditional GAC.
About the author
 Robert Potwora is Technical Director for Carbon Resources, LLC. He
has 30 years experience in the activated carbon industry and is currently
Vice Chairman of the ASTM D28 Committee on Activated Carbon.
Potwora may be reached by phone at (760) 630-5724 or by email at
robert@carbonresources.com.
About the company
 Carbon Resources, based in Oceanside, CA, is a quality supplier of
activated carbon products and services that is backed by technical support
and individualized customer service. The Carbon Resources management
team has over 85 years of experience in the activated carbon industry and
offers an unmatched line of the most diverse activated carbon products
on the market. The Sabre-series®, Spartan-series® (the surface-enhanced
coconut shell-based activated carbon used in Table 2), Guardian Adsorberseries
® and newly introduced Sentry-series® activated carbon products
are widely recognized in the industry. For more information, please visit

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: