By: Shin-ichi Tokuno
Previous research on the boosting of chlorine residual1
included how to increase low levels of chlorine disinfectants (free and
combined chlorine) in the distribution system. Simple bench tests using a
pocket photometer showed that there are no problems in boosting the low level
of chlorine residual when boosting the same disinfectant to the water (e.g., free
chlorine to free chlorine, or chloramine to chloramine). In the boosted
chlorine residual, there is no significant instability in decay or dissipation
during the time needed (72 hours) after boosting for the small utility
distributors.
Some utilities (Ingleside, Port Aransas, Texas) inject
ammonia and chlorine to the Corpus Christi (C.C.) water to boost the residual.
However, most utilities (30-40 miles from the plant) boost the low chlorine
residual only by chlorine gas. This means that the free chlorine interacts with
the preexisting chloramine of C.C. water to form the breakpoint (BP)
chlorination. As a result, the mode of chlorine changes from the combined
chlorine to the free chlorine or, if the total ammonia (free and combined) is
completely depleted prior to boosting, the free chlorine develops without BP
formation after chlorination. Because this boosting forms free chlorine, it
produces more disinfectant action against bacteria and viruses than
chloramines. However, a few stability studies have made it clear that the newly
formed free chlorine becomes significantly unstable in the boosted water,
probably with an increase in the disinfection byproducts (DBPs).
This type of boosting is similar to a situation where
chlorinated water is mixing with chloraminated water somewhere in the
distribution system from two separate plants that are discharging the treated
water with two different disinfectants.
BP Chlorination Results in the Lab and Field
Several different distribution waters within the normal range
of chloramines (3.0-3.5 mg/L) changed into low levels (0.3-1.6) after being
stored in the refrigerator (5° C) for four days. However, the water still
maintained the same level of total ammonia (ammonia value after thiosulfate
dechlorination). These samples then were boosted with BP chlorination all to
the same level (3.0-4.0 mg/L). However, a rapid decline occurred (approximately
50 percent reduction at 25° C in 24 hours) with the all-boosted
distribution water after BP chlorination.1 Such a reduction was not detected
when BP chlorination was made with a phosphate buffer (Table 1).
Over the last several years, C.C. wholesale purchasers had
occasional difficulties in maintaining the chlorine residual at the utilities
or in the field after applying the chlorine gas for boosting. In order to
prevent this situation, a suggestion was made to remove the biofilm layers
inside the storage tank. This removal minimizes problems in nitrification or
other biofilms involved in the dissipation during the storage of boosted water.
Another suggestion was to measure the nitrite and nitrate increase due to the
nitrification (C.C. discharge water contains nitrate 0.2-0.7 mg/L, and nitrite
<0.02). While this might be a partial solution, it was found that
significant dissipation occurred even without nitrification after the
breakpoint chlorination.
A rapid decline of free chlorine after boosting was observed
occurring in the C.C. distribution water in the laboratory. However, it may be
developing on a large scale in the storage tank. The same mechanisms might
cause this rapid decline in both the laboratory and storage tank. This
phenomenon shows the difficulty of chlorine dosing and residual maintenance by
utilities after boosting with chlorine gas.
BP chlorination tests were conducted not only with the C.C.
distribution water, but also with the plant discharge and field waters at the
utility's intake. All water originally was processed through the C.C. treatment
plant.
Instability of BP Free Chlorine
When the same disinfectant is used to boost the C.C. water,
there is no problem with the dissipation of the chlorine residual after
boosting. However, a rapid decline of the chlorine residual occurs during the
first 24 hours (at a rate between 50 and 75 percent). This same decline occurs
in the sampled water from the plant discharge, the water from utility intakes
(30-40 miles from the plant) or the distribution water inside the city (Table
2a, b, c and d). This rapid decline after BP chlorination also was observed in
the neighboring town of Robstown's plant discharge water (monochloramine) that
shares the same source water (Nueces River).
Most likely, some of the stable, unreacted free chlorine
demand from the source water had not been neutralized at the plant due to the
concurrent injection with chlorine and ammonia to form the chloramine. In
addition, the residual demand moves toward the distant sites to react with the
newly formed free chlorine. This type of demand is so stable that it is active
even after three weeks in the refrigerator (Table 2b2). The monochloramine is
formed in the C.C. plant by injecting free chlorine and ammonia sulfate
concurrently at two different sites, at the front (before rapid mixing) and in
the clear well to form the monochloramine.
Therefore, some of the free chlorine demand would not react
with the free chlorine in the plant to slip away into the distribution system.
The TOX (total organic halide), formed by chloramination is only 10 to 20
percent of the TOX formed by chlorination with source water.2
The residual free chlorine demand can survive the long haul
from the plant to the intake to affect the chlorine disinfectant even in the
summer. Studies were made on chlorine boosting by breakpoint chlorination using
two utilities' waters both receiving very low chloramines (0.1-0.5 mg/L) and
needing boosting, as well as distribution water with a normal chloramine level
(2.5-3.5 mg/L) inside the city. A comparison also was made in order to find out
differences between the laboratory and the field, especially in respect to the
stability of residual chlorine after boosting chlorination.
A low residual (total <0.5 mg/L) is common when sampled
30 to 40 miles from the plant. The dissipation of monochloramine1 is caused by
pH, temperature, longer retention time in the biofilmed pipes (e.g.,
nitrification) to interact with chloramines and the operator's setting of the
ratio of Cl:NH3.
Stability of Chloramine and Cl:NH3
Studies were made with a phosphate buffer (Table 1) in which
the monochloramone is formed using different ratios of ammonia to free chlorine
without any chlorine demand. The results were found to be different depending
on the ratio of Cl:NH3-N. A monochloramine formed at a low ratio, (e.g., 1:1,
2:1 or 3:1) is very stable at 25° C for several days, but at a higher ratio
(4:1), the residual is stable for the first 24 hours, but slowly becomes
unstable (Table 1a and b).
A ratio as high as 10:1 to 14:1 enters into breakpoint
chlorination. Free chlorine formed after BP is stable except for the first 24
hours (20 to 35 percent reduction, Table 1b). This probably is due to residual
chlorine-chloramine interaction and the resultant reduction of chlorine
residual. The formed free chlorine is very stable after 24 hours because of the
absence of any chlorine demand in the phosphate buffer.
Part two of this article covers chloramine levels in the
water as well as the THM rule and boosting safety.
About The Author: Shin-ichi Tokuno retired as a microbiologist in December 2002 from the Water Utilities Laboratory at the City of Corpus Christi, Texas.
