There are but two U.S. EPA approved, in-field methods for
determining total chlorine residual (TCR). While both methods are
cost-effective, user-friendly and provide assured, accurate measurements on
digital readouts, the difference between the two is that one uses colorimetry
while the other incorporates iodometric electrode (IE) technology.
The IE instrument comes factory-calibrated and, purportedly,
is ready "out of the box."
Conversely, colorimetry requires more protocols; therefore
testing takes more time. Also, being light-based, it requires compensation for
raw-sample color and turbidity.
Of more concern, however, the colorimetric method may not be
reliable where one of the test protocols itself influences turbidity.
This last caveat became all too evident to Alan Ismond, a
Bellevue, Wash.-based consulting engineer. Ismond had the task of ensuring
compliance for TCR for an Alaska fish-processing plant at which the EPA issued
a discharge permit on April 1. The plant uses both fresh water--chlorinated to
1 ppm by the municipality--and seawater--chlorinated on site--for processing
Ismond's task was to keep the TCR of the combined-plant
discharge below 0.1mg/L. "The biggest challenge was trying to accurately
measure the TCR," Ismond told WWD. "The chlorine levels in the
incoming chlorinated seawater can be adjusted as necessary."
For this particular application, Ismond needed a reliable
method to test for TCR. Because the colorimetric method had an extensive
history, Ismond initially turned to it for testing. Besides, the plant has a
colorimeter and only lack reagent.
In the colorimetric method, the water to be tested for TCR
is added to the sample cell supplied with the colorimeter. Ismond found that
all ensuing procedures must then be done with care. Otherwise, the outcome
could have resulted in anomalous conditions that impair the passage of light to
Ismond also found the glass cell to be problematic if not
clean and free of scratches. The same cell must be used for zeroing and sample
testing because all cells are not identical. Finally, before sample testing, the
colorimeter needed zeroing and, possibly, calibrating. Only then could a
measured quantity of powder reagent be added to the unknown sample in the test
Following reagent addition, the cell's contents are shaken
for a short amount of time. This ensures uniform reaction of the reagent with
the water being tested. Subsequently, the cell must be rested for several
minutes, then checked for remaining suspended reagent and freedom from air
bubbles on the walls. Such air-bubble formation is possible during reagent
addition. If not, their presence may inappropriately affect the intensity of
the light beam.
Ismond found that test-to-test TCR results for the
wastewater using the colorimeter varied wildly for different plant operating
modes. These disparities occurred despite Ismond's best efforts and care to
maintain consistent testing protocols.
"The colorimetric method appeared to lower the pH of
the samples, which, in turn, increased the turbidity of the samples," said
Ismond. "The increase in turbidity affected the TCR measurement depending
on the initial turbidity of the sample. For higher turbidity samples, the
change in turbidity was masked. For lower turbidity samples, the change in
turbidity was detected as TCR.
Therefore, Ismond set up a series of experiments to isolate
the cause of the problem.
Eventually, it took Ismond 10 days to trace the problem to
variable turbidity and he surmised that adding reagent lowered the sample's pH.
This, in turn, reduced solvency of the proteins in the wastewater, increasing
turbidity. Ismond proved his supposition by making a series of raw wastewater
aliquots from a centrifuged and filtered sample. Following this, he
incrementally lowered the pH between successive aliquots by adding different
amounts of acid. The turbidity of the samples did, in fact, increase as acidity
"I filtered samples of wastewater to reduce the
turbidity," Ismond told WWD. "Then, I lowered the pH in step
increments in split samples. I then used the non-pH adjusted sample as the
blank, and read the apparent TCR of each sample without adding the reagent. The
colorimeter indicated increasing levels of TCR for decreasing pH."
This conclusion eliminated the colorimetric method from
Ismond's consideration for the Alaska job at hand.
Coincidentally, Ismond came across information on the
ion-selective-based Extech CL 200, which is marketed under the moniker ExStik.
According to the manufacturer, turbidity could have no effect on the ExStik's
ability to measure TCR accurately--even "straight from the box."
Also, the instrument measures TCR concentrations from less
than 0.01 ppm to 10 ppm. But, something else mattered more to Ismond--the
ExStik had recently gained EPA approval in July 2002 for testing chlorine from
water and wastewater treatment plants.
The EPA iodide method for determining chlorine requires
adding a small excess of iodide to a chlorine-containing sample adjusted to
approximately 4.5 pH. The iodide thus duly reacts with the chlorine to produce
iodine. The Extech sensor measures iodine produced and, based on the reaction
between iodine and chlorine, the instrument calculates and displays the
chlorine concentration in ppm.
The instrument's design--incorporating all electronics,
displays and sensors--is modeled after the well-established, successful
"stick" or "pen" concept. The sensor is resistant to
breakage with electrodes of platinum and a proprietary material. Additionally,
it has no reference electrode to refill and no conditioning requirement.
"The instrument was not affected by turbidity and was
easy to use," Ismond told WWD. "Also, I do not need to worry whether
or not the lab tech is properly washing out the glass sample cells that we were
using for the colorimeter."
To Ismond, the main features of the ExStik indicated that it
was suitable for the Alaska fish processing wastewater application for which it
For more information on Extech, please phone 781/890-7440.
Ensuring compliance for total chlorine residual at Alaska fish-processing plant proves daunting