It’s Not Black Magic

Oxidation Reduction Potential (ORP) can be a valuable tool for seeing what is happening in unit processes like anoxic and anaerobic tanks
It’s Not Black Magic
A small ORP meter shows a negative mV reading in an anoxic basin.

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During my 28 years in wastewater treatment, I’ve worked at treatment plants in positions from utility worker to operations manager. One of my best jobs was with a Rural Water Association as a wastewater circuit rider. During that time, I met a fellow named Charlie who was having problems with a new sequencing batch reactor (SBR) treatment plant.

A circuit rider helps out-of-compliance plants get back into compliance. Charlie was having a tough time meeting the state and federal standards for nutrients discharged to a local creek. The facility was a two-tank SBR designed to meet advanced standards of 5-5-3-1 mg/l annual averages (CBOD-TSS-total nitrogen-total phosphorus). The plant couldn’t consistently meet the less than 3 mg/l total nitrogen and/or the less than 1 mg/l total phosphorus values.

A sequencing batch reactor treats wastewater as a batch process, and each tank operates independently. While a batch of wastewater is treated, a programmable logic controller (PLC) regulates the various valves, motors, mixers, blowers and pumps. The timing of these devices is critical, especially when trying to meet strict effluent standards.


Flows running low

Charlie’s plant was designed to treat 1.1 mgd, but due to water conservation and reduction of I&I, and because of over-design, the plant was well below capacity in its early years. The influent flow and BOD loadings remained well below design parameters, until a state prison opened years later and began sending its wastewater to the plant. This low flow and loading caused compliance problems.

Charlie had been trained by the manufacturer representatives to use dissolved oxygen as the main process control tool and to always maintain a 0.2 mg/l or less DO reading when the tanks were in the anaerobic or anoxic phases. He was not familiar with using oxidation reduction potential (ORP) as a process control tool.

I loaned him a portable ORP meter and described how he could use it to determine if the cycles of the SBR batch were anaerobic, anoxic or aerobic. These respiratory environments are necessary for bacteria to perform certain nutrient removal activities.

Charlie’s facility was experiencing two issues. First, the plant was able to nitrify the influent ammonium, but could not completely denitrify, causing the excursions in total N. Second, total effluent phosphorus was high, and the facility did not appear to be removing much phosphorus at all.


Two-part reaction

The biological removal of nitrogen is a two-part reaction called nitrification and denitrification. Nitrification takes place under aerated conditions, where nitrifying bacteria known as nitrosomonas convert the influent ammonium to nitrite, and then another group of nitrifying bacteria known as nitrobacter convert the nitrite to nitrate. These bacteria require free oxygen, as well as alkalinity, correct pH, temperature and time.

To finally remove the nitrogen, we allow facultative bacteria to consume organic matter (raw wastewater) under anoxic (no free dissolved oxygen) conditions. Facultative bacteria can use free dissolved oxygen, nitrate, sulfate and carbon dioxide as a sort of oxygen source. They prefer free DO, since it’s not combined with anything like nitrate or sulfate.

If free DO is not available, they break apart the chemical bond holding the nitrogen and oxygen together in a nitrate molecule (NO3) and utilize the oxygen, freeing the nitrogen to become nitrogen gas. This process of reducing the nitrate to nitrogen gas is called denitrification.

Charlie’s treatment plant was having problems denitrifying. The original SBR cycle times set by the process engineers were too short for the anoxic phase to become truly anoxic — the blowers would come on before denitrification could take place.

Using the ORP meter, Charlie monitored the anoxic time cycle. He kept the aeration blower off until the ORP meter read -50 mV, then allowed the aeration blower to come on. In the original setting, the blower would have come on after just 15 minutes of off time, and the DO meter showed 0.2 or less mg/l. Charlie found that the blower needed to remain off for an additional 45 minutes to get into the anoxic environment. He made adjustments to the PLC’s time settings based on these findings, and then set his sights on the total phosphorus issues.


Attacking phosphorus

For phosphate-accumulating organisms (PAOs) to release the maximum polyphosphate during the anaerobic (fermentation) phase, there must be zero dissolved oxygen and no nitrate available to these obligate aerobic bacteria. The bacteria utilize the incoming BOD containing volatile fatty acids as a food source.

To absorb this material into their cells, they break apart an internal polyphosphate bond, providing energy to consume the fatty acids. In this cycle, or zone of treatment, a dissolved oxygen meter should read zero, whereas an ORP meter would read in the -150 to -250 mV range. In Charlie’s SBR tank during the mixed fill cycle, it took another 60 minutes over the allotted one hour for the ORP to reach this negative mV reading.

Once the ORP reached the target reading, the reactor was allowed to enter the react fill stage and the aeration blower came on to provide dissolved oxygen. In this aerated environment, 1.0 to 2.0 mg/l of DO is maintained, and the oxygen allows the PAOs to burn up the stored food they collected in the anaerobic zone.

They use the energy gained to maintain their cells and to reproduce. During cell maintenance, the PAOs reabsorb the polyphosphate they released earlier and even absorb more phosphorus than they started with. This extra phosphorus comes in with the influent wastewater as orthophosphate, (PO4-3), and we call this biological process “luxury uptake.” It can allow a facility to reach very low effluent total phosphorus. The phosphorus is now contained within the bacterial cells and is removed by wasting the solids out of the treatment flow scheme.


The whole picture

This example shows that while DO is a valuable process control tool, it has limits when used in an anoxic or anaerobic basin or in SBR phases. Once the proper environmental conditions were provided to the bacteria, the nutrient removal processes worked well, and Charlie soon met all the effluent limits. In fact, he noticed that the whole plant seemed to work much better — the floc changed characteristics, settling improved, effluent turbidity decreased, and BOD and TSS were well below permitted limits. Charlie later said, “ORP was a lifesaver.”

Some operators and engineers continue to look upon ORP as black magic, but it gives operators another method to see the whole picture of what is happening in unit processes like anoxic and anaerobic tanks, and collection systems as well. But ORP is like any other process control tool — it must be used correctly.

When the liquid environment contains more oxidizing agents than reducing agents, the ORP shows a high (positive) value. When there are more reducers than oxidizers, ORP reads in the negative range. ORP does not read DO or measure the amount of oxygen. It just tells us if the conditions are favorable for certain biological activities.


Different ranges

For example, raw wastewater contains much ammonia nitrogen and little DO. The ORP normally reads in the negative range (-50 to -150 mV). The more septic in the wastewater, the lower the ORP reading. If you read the ORP in a chlorine contact tank, you see the opposite: ORP climbs very high due to the amount of an oxidizer like chlorine. The ORP reading might be as high as +400 to +700 mV.

In aeration tanks, we see ORP values around +50 to +200, and if nitrate is also present, the ORP might go up to +300 mV. When trying to denitrify in an anoxic tank or zone we like to see ORP from +50 to -100 mV (see accompanying graph). There are various types of ORP probes, some using platinum and others silver. Know what type of probe you have and maintain it accordingly.

If you have a biological nutrient removal (BNR) process, learn more about ORP and its capabilities. Dr. James Barnard, the leading authority on BNR, stated the importance of ORP in a recent WEF webinar on phosphorus removal: “Operators should consider using ORP for control of anoxic conditions.”

If you’d like to learn more about using ORP or need more information on nitrogen control in wastewater treatment plants, contact me at the email address below.


About the author

Ron Trygar is senior training specialist in water and waste-water at the University of Florida TREEO Center and a certified environmental trainer (CET). He can be reached at


Barnard, Dr. James; WEF webcast, June 2011; “Phosphorus Removal: Tips for Operators, Trainers and Design Engineers.”

Dabkowski, Bob, Hach Co.; August 2010; “Applying Oxidation Reduction Potential Sensors in Biological Nutrient Removal Systems.”

EPA “Nutrient Control Design Manual” EPA/600/R-10/100, August 2010.

Trygar, Ron; University of Florida, TREEO Center; “Nitrogen Control in Wastewater Treatment Plants,” Second Edition; 2009.

WEF Manual of Practice #29; “Biological Nutrient Removal Operation in Wastewater Treatment Plants”; McGraw Hill, 2005.


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