Chlorine and Nitrate Progress Control: Testing

U.S. Water Services Corporation - Chlorine and Nitrate Progress Control Testing

Chlorine and other relatable compounds are highly reactive, oxidizing other species when used in water treatment systems, and play an important role in microbial killing. Chlorine is typically used when treating effluent water sources like those found at a Wastewater Treatment Facility.

While Chlorine and other oxidizers are very successful at reducing or eliminating microbial species, they can also interfere when testing for other compounds in the water.

At many wastewater treatment facilities, operators will use handheld meters to test for nitrate levels. The testing procedure consists of adding packets containing cadmium and a color reagent, to the sample. The cadmium promotes the reduction of nitrate to nitrite. The higher the nitrate level, the higher the reduction that occurs, and the color darkens as the level rises. This simple is then read on a handheld meter and then the operator uses that result to determine where the plant is currently operating at.

When chlorine is present in the sample, it will oxidize the cadmium reducing the effectiveness. This will result in a lower value for nitrate, which is not a true representation of the current conditions of the wastewater treatment plant.

There are several options to help improve the accuracy of the nitrate data.

  1. If able, take the sample upstream from the chlorine addition. While this will eliminate chlorine interferences, it will not remove other oxidizing agents that may be present in the system.
  2. Sodium thiosulfate will neutralize chlorine and some other oxidizing agents. The facility could keep the sodium thiosulfate on the property and use it when testing.
  3. Hach has an effective method for testing nitrate in the field that eliminates interferences from chlorine and other oxidizing agents. The Chromotropic acid testing procedure from Hach will solve the problem but can be cost prohibitive.
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