Last Updated on April 1, 2025 by Kevin Chen
ORP (Oxidation-Reduction Potential) is a vital monitoring parameter for wastewater treatment in anoxic tanks. Measured in millivolts (mV), it reveals a real-time balance between oxidizing and reducing agents in the system. By maintaining optimal ORP values, operators can effectively enhance biological nitrogen removal efficiency, stabilize treatment performance, and meet environmental compliance standards—all critical for optimizing plant operations.
Why Monitor ORP in Anoxic Tanks?
1. Tracking Denitrification Efficiency
The primary function of anoxic basins is denitrification—converting nitrate nitrogen (NO₃⁻) into nitrogen gas (N₂). However, this biochemical process is invisible to the naked eye. ORP serves as a real-time “reaction monitor”:
- Mechanism: Active denitrification strengthens reductive conditions, lowering ORP values (e.g., stable at -100mV under regular operation).
- Alarm Signal: A sudden ORP rise (e.g., to -50mV) indicates process disruption—likely due to insufficient carbon sources or microbial inhibition.
2. Assessing Microbial Vitality
ORP fluctuations reflect the metabolic activity of denitrifying bacteria:
- High activity & vigorous denitrification: ORP remains stable.
- Erratic ORP fluctuations: Likely environmental stressors (e.g., temperature swings, pH deviations) impairing microbial metabolism.
Continuous ORP monitoring enables prompt adjustments to optimize microbial “working conditions,” ensuring peak performance.
3. Early-Warning System for Process Failures
ORP deviations predict system-wide risks:
- Persistent ORP decline: Indicates worsening anaerobic tendencies, risking byproducts (e.g., H₂S, phosphorus release) and downstream impacts.
- Continuous high ORP: Confirms failed anoxia, preventing denitrification and causing effluent non-compliance.
- Proactive ORP analysis allows preemptive corrections, averting operational crises and ensuring stable plant performance.
What is the Optimal ORP Range for Anoxic Tanks?
ORP control ranges lack a “universal standard” and must be determined based on operational conditions. However, in ideal denitrification scenarios, anoxic tanks typically maintain an ORP between -50 mV and -250 mV.
- Lower limit (-250 mV): If ORP drops below -250 mV, the tank exhibits excessive reducing conditions and likely transitions to an anaerobic state. While microbial activity persists, this environment risks generating malodorous gases like hydrogen sulfide (H₂S), triggering phosphorus release, compromising downstream treatment, and accelerating equipment corrosion.
- Upper limit (-50 mV): ORP above -50 mV indicates elevated dissolved oxygen levels, signaling inadequate anoxic conditions. This suppresses denitrifying bacteria activity, leaving nitrate nitrogen underprocessed and causing effluent total nitrogen (TN) exceedances.
| Wastewater Type | Recommended ORP Range | Success Rate* |
| Municipal Wastewater | -100mV to -50mV | 92% |
| High-Nitrogen Industrial | -250mV to -150mV | 86% |
| Mixed Industrial | -180mV to -100mV | 89% |
How to Adjust ORP Levels in Anoxic Tanks?
1. Adjust dissolved oxygen
If the ORP is too high, it indicates excessive oxygen in the tank, meaning insufficient anoxic conditions. The most direct solution is to reduce aeration. For example, using blowers for aeration decreases the air output by adjusting the blower settings. If aerators are in use, reduce the number of aerators in operation. Conversely, if the ORP is too low, it might signal overly anaerobic conditions, requiring a controlled increase in aeration to reintroduce oxygen. Always make gradual adjustments—avoid drastic changes—and retest ORP after 30-60 minutes to assess effectiveness.
2. Manage influent quality and flow rate
Variations in influent quality and flow rate significantly impact ORP. Insufficient carbon sources in the influent weaken denitrification (microbes “starve”), causing ORP to rise. In such cases, supplement carbon sources like methanol or sodium acetate to support microbial activity. Sudden increases in flow rate shorten hydraulic retention time, leading to incomplete reactions and ORP fluctuations. Stabilize inflow by coordinating flow controls or adjusting water levels in equalization tanks to ensure consistent influent conditions.
3. Monitor sludge status
Sludge acts as the “workforce” in anoxic tanks. Low sludge concentration reduces microbial populations, weakening denitrification and potentially elevating ORP. Address this by discharging aged sludge and replenishing it with active sludge. Conversely, excessively high sludge concentrations accelerate oxygen depletion, lowering ORP. Increase sludge withdrawal to maintain optimal levels. Regularly inspect sludge characteristics—dark, foul-smelling sludge indicates operational issues requiring immediate intervention to stabilize ORP.
4. Regulate pH and temperature
pH and temperature directly affect microbial activity and ORP. Maintain pH between 6.5-8.0 in anoxic tanks. Add sodium hydroxide for low pH (acidity); for high pH, introduce hydrochloric acid. Keep temperatures at 20-30°C for optimal microbial performance. Low temperatures slow microbial activity, potentially raising ORP—implement insulation or heating. High temperatures stress microbes—use cooling water circulation to lower temperatures. Adjustments should be incremental, followed by ORP retesting after stabilization periods.
The above is the information about the ORP in anoxic tanks. If you still have related questions about the membrane bioreactor, please feel free to contact SPERTA.
Shanghai SPERTA Environmental Technology Co., Ltd. has specialized in producing water treatment products for many years. The company has its own MBR membrane technology, a complete technical team, and pre-sales and after-sales service. If you have any needs, please feel free to contact us.
