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ORP vs. Total Nitrogen Removal
Ann Mastroianni
Posted: Friday, December 3, 2010 11:11 AM
Joined: 11/22/2010
Posts: 2

We use the MLE activated sludge process.  Recently we installed an ORP monitoring probe (HACH) in the anoxic tank and connected it to our SCADA system.  It has raised a lot of process questions.


Our question is:  why does the ORP go from around -150 during the day to     -70 and closer to zero at night?


Our final effluent ammonia is running between 0.5 and 1.5 mg/l (Nov) and effluent nitrate has been 1.7 to 2.7 mg/l(Nov).  Our RAS return rate has varied from approx. 65% to 90% Q. 


Any enlightment on ORP would be of great help.


Thanks in advance.





Posted: Monday, December 13, 2010 4:58 PM

ORP is oxidation-reduction Potential

When ammonia is oxidized you will see a positive mV reading.

When nitrate is reduced you will see a negative mV reading.

There are several papers and web sites that expound on the wonders of ORP usage in wastewater.  Usually between -100 to -250 you get biological phosphorous release, between -50 to -250 hydrogen sulfide formation, between -100 to -225 acid formation(fermentation).  YSI has an overview of ORP in an application note on their web site.  The note is titled "ORP Management in Wastewater as an Indicator of Process Efficiency" and their site is

Posted: Tuesday, December 14, 2010 12:32 PM

This is not unusual in a continuous flow activated sludge process such as the MLE. The phenomenon is related to the metabolic activity of the RAS entering the anoxic zone. In other work this can be measured as OUR or an OUR equivalent such as the Potential Oxygen Utilization Rate (POUR).In an SBR operation the rate of reduction of the ORP during an air off sequence is directly related to its OUR (the stored intracellular organics). The same situation occurs in the anoxic zone of an MLE configuration.You probably also have a higher DO in the RAS during the lower ORP periods.

What is the HRT in the anoxic zone?

The situation you are referring to is harnessed and used commercially for simultaneous nitrification denitrification operation in modified SBRs.

Robert Smith
Posted: Monday, June 11, 2012 9:55 AM
Joined: 10/8/2009
Posts: 5

Less negative ORP's observed could be related to recycle flows (RAS and IMLR).  If recycle flow rates are not paced to wastewater flow, more highly oxidized mixed liquor (higher ORP value) will be a greater proportion of the flow through the anoxic zone.  Is this consistent with your operation?


Cheers, RS