Nitrification and Denitrification in SBR and Extended Aeration Process Comparative Analysis
Nitrification and denitrification are two fundamental biological processes for removing nitrogen from wastewater. While both processes rely on microbial activity, their implementation and effectiveness differ significantly between continuous systems like Extended Aeration and batch systems like Sequencing Batch Reactors (SBRs).
Nitrification and Denitrification in the Extended Aeration Process
Extended Aeration is a type of activated sludge process characterized by a long hydraulic retention time (HRT) and very long sludge retention time (SRT), typically 20-30 days. This long SRT is crucial for achieving nitrification.
1. Nitrification in Extended Aeration:
Mechanism: Nitrification (conversion of ammonia to nitrate) occurs in the aeration tank, which is designed to be fully aerobic. The long HRT and SRT of extended aeration systems inherently provide the necessary conditions for nitrifying bacteria (Nitrosomonas and Nitrobacter) to grow and thrive.
Oxygen: Continuous and sufficient aeration maintains high dissolved oxygen (DO) levels (typically 2.0 - 4.0 mg/L) throughout the aeration basin, which is essential for these obligate aerobic bacteria.
Sludge Age: The very long SRT ensures that the slow-growing nitrifying bacteria are retained in the system long enough to perform their function and establish a stable population. This is a primary advantage of extended aeration for nitrification.
Carbon Removal: Carbonaceous BOD/COD removal also happens in the same aeration tank. Often, the organic load is relatively low in extended aeration, which minimizes competition for oxygen between heterotrophs (BOD removers) and nitrifiers.
Drawbacks for Nitrification: While generally effective, if the influent load is too high or if there are sudden drops in temperature or pH, nitrification efficiency can be impacted. Energy consumption for continuous aeration can also be significant.
2. Denitrification in Extended Aeration:
Achieving effective denitrification in a standard, single-tank extended aeration process is challenging and often incomplete without specific modifications.
The Challenge: Denitrification requires anoxic conditions (no free oxygen) and a source of readily biodegradable organic carbon.
In a typical extended aeration tank, the entire volume is kept aerobic for nitrification and BOD removal. Therefore, anoxic conditions are generally not present.
Even if the oxygen supply is occasionally reduced, the readily available organic carbon from the influent would have mostly been consumed by the aerobic bacteria in the highly aerated environment.
Limited Denitrification (Incidental):
Clarifier Denitrification: Some incidental denitrification can occur in the secondary clarifier if the sludge blanket remains deep for prolonged periods, leading to anoxic zones. However, this is usually uncontrolled, can lead to "rising sludge" (nitrogen gas bubbles causing sludge to float), and is generally not considered reliable for nitrogen removal.
Clarifier Denitrification: Some incidental denitrification can occur in the secondary clarifier if the sludge blanket remains deep for prolonged periods, leading to anoxic zones. However, this is usually uncontrolled, can lead to "rising sludge" (nitrogen gas bubbles causing sludge to float), and is generally not considered reliable for nitrogen removal.
Achieving Denitrification in Extended Aeration (with Modifications): To achieve significant denitrification in extended aeration, the system typically needs to be modified by adding:
An Anoxic Zone: A separate, un-aerated tank placed upstream of the main aeration tank. Mixed liquor containing nitrates from the aerobic zone is recycled back to this anoxic zone, where it mixes with raw influent (containing readily biodegradable carbon) to facilitate denitrification.
Intermittent Aeration (for single tank): Some extended aeration systems can be operated with on/off aeration cycles to create alternating aerobic and anoxic conditions within the same tank, similar to an SBR. This requires sophisticated control.
How Nitrification and Denitrification Happen in SBR
The Sequencing Batch Reactor (SBR) is inherently well-suited for biological nutrient removal, including both nitrification and denitrification, due to its time-sequenced operation within a single tank.
1. Nitrification in SBR:
Mechanism: Nitrification occurs primarily during the "React" (Aeration) phase of the SBR cycle.
Aerobic Conditions: During this phase, air blowers are active, providing ample dissolved oxygen for AOB and NOB to oxidize ammonia to nitrate.
High MLSS and Sludge Age: SBRs can operate at high MLSS concentrations and long sludge ages, providing stable populations of nitrifying bacteria. The hydraulic selection for dense, well-settling sludge (which often correlates with good biological health) further aids nitrification.
Optimal Environment: By controlling the duration of the aeration phase and the DO level, operators can ensure complete nitrification of the incoming ammonia.
This MLSS is actively managed by controlling the sludge wasting rate (Waste Activated Sludge - WAS) and the Return Activated Sludge (RAS) rate.
2. Denitrification in SBR:
Denitrification is efficiently achieved in SBRs by incorporating specific anoxic or anaerobic mixing phases into the operating cycle:
Pre-Anoxic/Mix-Fill Denitrification:
Typical Strategy: After the "Fill" phase (where raw wastewater enters the tank), the SBR might have an initial "Mix" or "Anoxic Fill" phase where the tank is mixed but not aerated.
Carbon Source: During this phase, the incoming raw wastewater provides a rich source of readily biodegradable organic carbon.
Nitrate Source: The mixed liquor already present in the tank from the previous cycle (which settled during the previous "Settle" and "Decant" phases) contains nitrates produced during the previous aeration cycle.
Result: Denitrifying bacteria utilize the influent carbon and the stored nitrates to reduce nitrates to nitrogen gas. This is a very efficient way to achieve denitrification.
Intermittent Aeration Denitrification (Post-Anoxic):
Some SBR cycles might incorporate an "Anoxic React" phase after an initial aerobic period.
In this approach, aeration is turned off, creating anoxic conditions. Nitrates that were formed during the preceding aerobic phase are then denitrified using any remaining residual BOD or internally stored carbon compounds by the bacteria. This is less efficient than pre-anoxic denitrification if external carbon isn't added or if the influent C:N ratio is low.
Simultaneous Nitrification-Denitrification (SND) in SBR:
In certain SBR operations, particularly with large floc sizes or granular sludge, SND can occur even during aerated periods. The outer layers of the floc/granule are aerobic (for nitrification), while the inner core becomes anoxic (for denitrification) due to oxygen diffusion limitations. This contributes to overall nitrogen removal.
Comparison: Extended Aeration vs. SBR for Nitrification & Denitrification
Conclusion:
While standard Extended Aeration is very good at nitrification due to its long sludge age, it is inherently poor at achieving denitrification without significant structural or operational modifications (like adding a pre-anoxic tank with internal recycle).
Sequencing Batch Reactors (SBRs), by their very nature of batch operation and time-sequenced phases, are exceptionally well-suited for both nitrification and denitrification. Their ability to create distinct aerobic and anoxic periods within the same tank, and to effectively utilize influent carbon for denitrification, makes them a highly efficient and compact choice for biological nutrient removal, which would be a significant advantage for any STP aiming for comprehensive wastewater treatment.