How Oxidation Reduction Potential Probes Can Optimize Your Wastewater Facility

Effluent requirements are becoming more stringent than ever with total maximum daily loads (TMDLs) and water quality-based effluent limitations (WQBELs) replacing longstanding, technology-based limits such as biochemical oxygen demand (BOD), total suspended solids (TSS), ammonia and phosphorus.

Here, we reveal how an oxidation reduction potential (ORP) probe can help you meet compliance regulations and optimize operations.

  • First, effluent compliance requirements are driven by discharge location, receiving water classification and level of impairment. These variables can result in vastly different effluent quality requirements that must be met to comply with regulations.
  • Second, increasingly stringent effluent limitations are causing operators and engineers alike to reconsider longstanding biological operating parameters in order to optimize existing treatment processes. It is also causing them to adapt to new treatment schemes and automate process control all while reducing energy consumption.
Brendan Wolohan, SEH wastewater engineer, demonstrates how to use an ORP probe to optimize a wastewater treatment plant facility.

According to Dan Schaefer, SEH wastewater engineer, longtime operators are often familiar with parameters like flow, pH, mixed liquor suspended solids (MLSS), solids retention time (SRT) and dissolved oxygen (DO) to monitor and control secondary treatment for BOD removal and nitrification of influent ammonia. However, regulatory agencies are incorporating new limits for phosphorus and total nitrogen into reissued permits. This is leading operators to reconsider existing operations or implement upgrades to their facilities in order to be both compliant and optimize operations.

Related Content: 6 Innovative Approaches to Phosphorus Regulation Compliance

To accomplish both, some operators are turning to instruments like ORP probes to monitor and optimize their treatment process. When doing so, they not only satisfy limits, but also reduce annual chemical and power costs.

wastewater treatment tank
Wastewater treatment facilities can benefit from using an ORP probe to help measure the concentration of oxidizers and reducing agents.

What is ORP?

According to the Molecular Hydrology Institute, Oxidation Reduction Potential (ORP) is a way to measure water’s ability to either release or accept electrons during a chemical reaction. To measure it, the process uses a probe with one inert electrode and one reference electrode attached to a pole. The device uses millivolts (mV) to measure electrical potential.

The more positive an ORP reading is, the more concentrated the oxidizers such as dissolved oxygen are and more likely the system is to be aerobic. Conversely, the more negative the reading, the more concentrated reducing agents like hydrogen sulfide are present, making the system more anaerobic.

Positive ORP readings show dissolved oxygen is present in the water, which is ideal in aeration basins. But, in anaerobic or anoxic basins looking to achieve biological nutrient removal, these are not ideal conditions. Wastewater operators can adjust return activated sludge flow rates and internal nitrate recycle flow rates to the anaerobic selector and/or anoxic basins to maintain targeted conditions in each type of basin. In cases where there are multiple cells in the anaerobic selector basins, wastewater operators can modify the cells where raw wastewater and return activated sludge flows are introduced into the anaerobic selector basins to develop anaerobic conditions for phosphorus accumulating organisms to survive.

ORP probes are most commonly used to extend the measurement range of DO probes. DO probes cannot accurately report results less than about 0.5 mg/L. Operations staff that manage facilities designed for BNR (either nitrogen, phosphorus or both) may already be familiar with the use of ORP probes in that application, but there are many more functional uses for ORP probes. By measuring these processes, operators get a better understanding of how their system is performing to help them optimize their operation.

What’s more, according to Schaefer, ORP measurements can be a huge benefit when designing new plants or treatment processes.

A day in the field using an ORP probe can gain valuable data that can help the design process.
Dan Schaefer

Following are some of the ways in which wastewater treatment systems can be optimized by using ORP probes.

ORP probe
An ORP probe with a cover. The device is lowered into wastewater treatment basins to measure the water’s ability to release or accept electrons. This data can help operators optimize their systems.

Collection system diagnosis

As noted, ORP provides a measure of the oxidative or reducing ability of a solution, with reduced conditions being more anaerobic in nature. Collection systems can have several characteristics leading to anaerobic conditions such as:

  • Long travel times to the treatment plant
  • Sedimentation
  • Changes in pipe slope
  • Lower initial flows

Under any of the above conditions, the potential exists for odors to form, which can lead to complaints. It can be difficult to predict when and where odors form when complaints occur. By using a portable ORP probe and monitoring manholes around the complaint site at different times of day or days of the week, the source can be pinpointed. Once the location of the odor is determined, operations staff can be provided with information on how best to address the odor formation. The solution could be chemical dosing of oxidizers or other odor control agents, or it could point to a problem with an industrial discharger, which leads to the next ORP monitoring benefit.

ORP probe
Tape marks along the ORP probe’s cord help the operator get readings at different depths of the tank. In a properly functioning tank, the readings at the different depths should all be similar.

Monitoring industrial dischargers

Both toxic and slug loads are often prohibited by municipal sewer use ordinances. This is intended to protect both the collection system from deterioration and the treatment facility from treatment upsets caused by toxics or slug loads. In the odor complaint example above, an extremely negative ORP reading in the collection system downstream of an industrial discharger provides wastewater operations staff with valuable information regarding the relative strength of the discharge without the need to install a composite sampler to monitor the industrial discharger.

Many small to medium municipalities may only monitor industrial discharges monthly, quarterly, or as infrequent as semi-annually. Use of a portable ORP probe in industrial park manholes or at the treatment facility headworks provides staff a quick check of industrial discharge strength between sampling events. Spot ORP readings can also inform operators of changed conditions that may result in treatment process upsets if not addressed via process adjustments.

Brendan using an ORP probe
Because of their higher accuracy, ORP probe readings can supplement DO probe data to help optimize wastewater treatment schemes.

ORPs improve BNR operations

Specific conditions must be met within a BNR system for treatment to be both efficient and consistent. Biological removal of nitrogen and phosphorus require aerobic, anoxic and anaerobic conditions to be present at various locations in the treatment scheme. As mentioned, DO probes lose accuracy at low DO concentrations, and more biological systems are being optimized to operate at low DO levels to promote energy savings and in some cases simultaneous nitrification-denitrification. Under these conditions ORP readings are a critical monitoring parameter for periodic checks to diagnose treatment deficiencies and make process adjustments.

Gary Hanson, SEH operations specialist, has used portable ORP probes at several oxidation ditch facilities with the goal of optimizing the current system for BNR, or determining upgrades required to create the appropriate conditions to optimize BNR performance.

Case studies: ORP probes at work

In one case study, a facility was designed for BNR, but ORP probes were only permanently installed in the aerobic zones of the oxidation ditch. Plant operations staff had little knowledge of the oxidation state in the anaerobic selector basins upstream of the oxidation ditches. Hanson conducted a field ORP profile to determine the state of each basin. This provided design engineers with additional information necessary to recommend cost effective improvements aimed at reducing the facility’s dependence on chemical polishing for phosphorus removal.

In a second case study, Schaefer used the ORP probe to determine an ORP profile around a carrousel style oxidation ditch equipped only with DO probes to determine if the current aerator set points promoted a low DO condition along the basin where simultaneous nitrification and denitrification could occur. The outcome of the field profile indicated the facility was likely over-aerating, and the set points could be lowered to reduce energy consumption and improve the overall treatment efficiency.

Wrapping it up

Wastewater treatment facilities are complex and require precise adjustments to operate properly. With an array of tools at operators’ disposal, ORP probes can help bring effluent requirements under control, help be a good neighbor by managing odors and, in some cases, save on operating costs.

About the Authors

Dan Schaefer

Dan Schaefer, PE* is a senior wastewater engineer who assists municipal and industrial clients with treatment facility optimization as a means to address upsets, improve treatment performance and reduce annual operating and maintenance costs. Contact Dan

Gary Hanson

Gary Hanson is a veteran wastewater operations specialist dedicated to helping his clients understand how to get the most from their wastewater systems. Contact Gary

Brendan Wolohan

Brendan Wolohan, PE** is an SEH wastewater engineer dedicated to using technology to help communities make smart decisions. Contact Brendan

*Registered Professional Engineer in CO, NC, and WI.
**Registered Professional Engineer in MN.

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