Straight from the Box

Nov. 14, 2003
Ensuring compliance for total chlorine residual at Alaska fish-processing plant proves daunting

About the author: Paul Hersch is a freelance writer based in Denver, Colo. He can be reached at [email protected].

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 plant product.

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 the detector.

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 cell.

Uniform reaction

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 increased.

"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.

Reinforced confidence

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 was needed.

For more information on Extech, please phone 781/890-7440.

About the Author

Paul Hersch

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