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Ask the Expert Question-and-Answer Archive (Wastewater Treatment)

by Mike McGinness, EcoShield Environmental Systems, Inc.
April, 2010

Hex Residual

Q. After my last hex conversion treatment (Cr+6 = .00008mg/L), we have discovered that the hex residual begins to climb before our precipitation phase (Cr+6 = .0045m/L). The precipitation part of the plant is a 1/2 a mile from the conversion part of the process. The parameters at a sample spout just before the lime addtion is 2-2.1 pH and mid 300s - 400s ORP. The parameters leaving the conversion plant are 2-2.1 ph and 175ish ORP. 1st question would be why is the ORP higher a 1/2 mile down the road and the second question is do you think that the triavelent is converting back to hex in some strange way?

Additional info is that the precipitate tank has a ph of 9.0 & an ORP of 170ish. The floc tank has a pH of 9.0 & an ORP of 160ish and finally, the clarifier has a pH of 9.1 with an ORP around 120. The effluent Cr+6 = an average of .012mg/L. With tougher limits, we are close to violating them. Also, we have searched for any type of bypass of the conversion phase and have found nothing.

Any help is greatly appriciated.

A. While I could use more detailed information about your process, the simple answer is yes, as the ORP rises there is a reaction equilibrium shift than allows some tri conversion back to hex. So the question is why is the ORP increasing. My first guess would be reactions that are ongoing as it travels 1/2 mile down the pipe, such as O2 in the air dissolving in the water, or other contaminates in the waste water, continuing to consume the reducing agent over time and thus allowing the ORP to rise.

Let me know if you have more questions.


Mike McGinness

Q2. I wanted to thank you for your quick response and the information you provided. It was nice to see that someone with expertise in the field agreed with my theory of what was happening. Because Im on the union side of things, it was hard for management to accept this concept. And even harder for them to accept that the chem vendor was just trying to sell them more product (which would not have worked).

Anyways, after reducing the ORP to an acceptable range (250ish) before the pH adjust tank, the residual was non-detectable in our clarifier. The aquatic life in our area cannot thank you enough.

A2. Thanks, always nice to get a pat on the back (they are sometimes rare in this business) THANKS. Glad I could help.



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