Biological Nutrient Removal

Biological nutrient removal, or BNR, uses controlled biological conditions to remove nitrogen and phosphorus from wastewater. Instead of relying only on aeration for carbonaceous BOD removal, BNR systems create different process zones so specific groups of microorganisms can perform nitrification, denitrification, and biological phosphorus removal.

Operators need to understand how aerobic, anoxic, and anaerobic conditions differ, how internal recycle streams move nitrogen through the process, how dissolved oxygen affects each zone, and how carbon, alkalinity, sludge age, and wasting influence nutrient removal.

Why Nutrient Removal Matters

Wastewater nutrients can contribute to excessive biological growth in receiving waters.

Important nutrients include:

• nitrogen;
• phosphorus.

BNR systems use biological reactions to reduce these nutrients before final discharge.

Main Nitrogen Forms

Important nitrogen forms in wastewater treatment include:

• organic nitrogen;
• ammonia nitrogen;
• nitrite nitrogen;
• nitrate nitrogen;
• nitrogen gas.

Nitrogen Transformations

Biological nitrogen removal commonly involves two major steps:

1. nitrification converts ammonia toward nitrate;
2. denitrification converts nitrate toward nitrogen gas.

Nitrification

Nitrification is the aerobic biological oxidation of ammonia.

A simplified sequence is:

Ammonia → Nitrite → Nitrate

Nitrifying Organisms

Nitrifying microorganisms grow more slowly than many heterotrophic organisms that remove carbonaceous BOD.

They therefore require careful control of:

• solids retention time;
• dissolved oxygen;
• temperature;
• pH;
• alkalinity.

Nitrification Requires Aerobic Conditions

Nitrification occurs in zones where dissolved oxygen is available.

If DO becomes too low, ammonia oxidation can slow or stop.

Solids Retention Time and Nitrification

Nitrifiers grow relatively slowly.

If SRT becomes too low, nitrifying organisms can be washed out of the process faster than they reproduce.

Temperature and Nitrification

Cold wastewater slows biological growth.

A nitrifying activated-sludge system may therefore require a higher SRT during cold weather than during warm conditions.

pH and Nitrification

Nitrification can be inhibited when pH becomes unfavorable.

Operators should review pH together with alkalinity because nitrification consumes alkalinity.

Alkalinity Consumption

Nitrification consumes approximately:

7.14 mg/L alkalinity as CaCO3 per mg/L ammonia-nitrogen oxidized

Alkalinity Example

If 20 mg/L ammonia-nitrogen is nitrified:

Alkalinity consumed = 20 × 7.14

Alkalinity consumed = 142.8 mg/L as CaCO3

This relationship helps explain why pH can fall as nitrification increases.

Nitrification Oxygen Demand

Nitrification creates additional oxygen demand beyond carbonaceous BOD removal.

Operators should expect air requirements to change with ammonia loading.

Ammonia Load

Ammonia mass loading can be calculated using:

Load, lb/day = Flow, MGD × Concentration, mg/L × 8.34

Ammonia-Load Example

Flow is 2.5 MGD and influent ammonia is 30 mg/L.

Load = 2.5 × 30 × 8.34

Load = 625.5 lb/day

The plant receives approximately 626 lb/day of ammonia-nitrogen.

Denitrification

Denitrification is the biological reduction of nitrate to nitrogen gas under anoxic conditions.

A simplified sequence is:

Nitrate → Nitrite → Nitrogen Gas

Anoxic Conditions

An anoxic zone has little or no free dissolved oxygen but contains nitrate or nitrite that microorganisms can use in place of dissolved oxygen.

Denitrifying Organisms

Many denitrifying organisms are heterotrophic bacteria that require biodegradable carbon as an energy source.

Carbon Source for Denitrification

Carbon may come from:

• influent wastewater;
• internal process carbon;
• supplemental external carbon where used.

Insufficient Carbon

If nitrate is present but biodegradable carbon is insufficient:

• denitrification may be incomplete;
• effluent nitrate may remain high.

Too Much DO in the Anoxic Zone

Free dissolved oxygen competes with nitrate as an electron acceptor.

If too much DO enters an anoxic zone, microorganisms may use oxygen instead of nitrate, reducing denitrification efficiency.

Mixing Without Aeration

Anoxic zones usually require mixing to keep solids suspended and distribute nitrate and carbon.

They are generally mixed without intentionally adding significant oxygen.

Internal Mixed-Liquor Recycle

Many nitrogen-removal systems use an internal recycle to move nitrate-rich mixed liquor from an aerobic zone back to an anoxic zone.

Purpose of Internal Recycle

The recycle brings nitrate into contact with:

• denitrifying organisms;
• available carbon.

Internal Recycle Too Low

An insufficient recycle rate can limit the amount of nitrate returned for denitrification.

Internal Recycle Too High

Excessive recycle can:

• increase pumping energy;
• carry excessive dissolved oxygen into the anoxic zone;
• reduce effective anoxic conditions.

Pre-Anoxic Denitrification

In a pre-anoxic configuration, nitrate-rich mixed liquor is recycled to an anoxic zone located before the main aerobic zone.

Incoming wastewater provides biodegradable carbon for denitrification.

Post-Anoxic Denitrification

Some systems use an anoxic zone after the main aerobic zone.

Because much of the readily biodegradable carbon may already be consumed, supplemental carbon may sometimes be required.

Nitrogen Gas

Successful denitrification converts nitrate to nitrogen gas, which leaves the wastewater.

Denitrification in the Wrong Location

Denitrification is useful in a designed anoxic zone but can create problems if it occurs in a secondary clarifier sludge blanket.

Rising Sludge

If nitrate-rich sludge remains in the clarifier under low-oxygen conditions, nitrogen gas can form within the sludge blanket.

Gas bubbles can attach to solids and cause:

• sludge flotation;
• solids carryover;
• rising sludge.

Reducing Clarifier Denitrification

Operators may review:

• sludge blanket depth;
• RAS rate;
• sludge residence time;
• nitrate entering the clarifier.

Biological Phosphorus Removal

Enhanced biological phosphorus removal, or EBPR, uses specialized microorganisms to remove more phosphorus through biological solids wasting.

Phosphorus-Accumulating Organisms

Phosphorus-accumulating organisms, or PAOs, are microorganisms that can store phosphorus within their cells under appropriate process conditions.

Anaerobic Zone

An anaerobic zone contains:

• no significant free dissolved oxygen;
• no significant nitrate available for biological respiration.

Anaerobic Is Not the Same as Anoxic

Anoxic conditions may contain nitrate.

Anaerobic conditions should lack both free DO and significant nitrate.

Why the Anaerobic Zone Matters for EBPR

PAOs undergo characteristic metabolic changes in anaerobic conditions that help give them a competitive advantage later in the process.

The overall biological cycle allows phosphorus to become concentrated in the biomass.

Phosphorus Leaves Through Wasting

Biological phosphorus removal is completed when phosphorus-rich biomass is removed through sludge wasting.

If the phosphorus-rich solids remain in the system indefinitely, phosphorus is not permanently removed from the treatment process.

Wasting and Phosphorus Removal

WAS operation therefore influences biological phosphorus removal.

Operators should control wasting based on:

• solids inventory;
• SRT;
• phosphorus-removal goals;
• overall process stability.

Nitrate in the Anaerobic Zone

Nitrate entering an anaerobic zone can interfere with EBPR because denitrifying organisms may use available carbon before PAOs can use it effectively.

Sources of Nitrate Carryover

Nitrate may enter the anaerobic zone through:

• RAS;
• internal recycle;
• improper zone configuration.

DO Carryover into the Anaerobic Zone

Dissolved oxygen entering the anaerobic zone can also reduce the desired anaerobic conditions.

Carbon Availability

Readily biodegradable carbon is important for both:

• denitrification;
• biological phosphorus removal.

Competition for carbon can therefore affect BNR performance.

Aerobic Zone

The aerobic zone supports:

• carbonaceous BOD removal;
• nitrification;
• phosphorus uptake by PAOs in EBPR systems.

Anoxic Zone

The anoxic zone supports:

• denitrification;
• nitrate removal.

Anaerobic Zone

The anaerobic zone supports the biological conditions required for enhanced phosphorus removal.

Zone Separation Matters

BNR depends on maintaining different biological conditions in different parts of the process.

Poor hydraulic separation can allow:

• DO carryover;
• nitrate carryover;
• short-circuiting.

DO Control in BNR

Aerobic zones need adequate oxygen, while anoxic and anaerobic zones should avoid excessive oxygen.

This makes DO distribution more important than simply maintaining a single plantwide DO value.

DO Profile

Operators may monitor DO at several points to identify:

• under-aeration;
• over-aeration;
• oxygen carryover;
• uneven zone performance.

ORP

Some facilities use oxidation-reduction potential, or ORP, as an additional process indicator.

ORP can help operators evaluate changing oxidation conditions across process zones, but it should be interpreted with other process data.

Ammonia Monitoring

Ammonia provides direct information about nitrification performance.

Operators may compare ammonia at:

• influent;
• intermediate process locations;
• final effluent.

Nitrate Monitoring

Nitrate measurements can help evaluate:

• nitrification;
• denitrification;
• internal recycle performance;
• nitrate carryover into anaerobic zones.

Phosphorus Monitoring

Phosphorus trends can help evaluate:

• EBPR performance;
• chemical phosphorus removal where also used;
• solids wasting effects.

pH Monitoring

Because nitrification consumes alkalinity, pH should be watched closely in nitrifying systems.

Alkalinity Monitoring

Low alkalinity can lead to:

• falling pH;
• reduced nitrification;
• process instability.

Denitrification Can Recover Some Alkalinity

Denitrification can restore part of the alkalinity consumed during nitrification.

This is one operational benefit of complete nitrogen removal.

SRT and BNR

BNR processes must maintain an SRT appropriate for the slow-growing organisms needed by the treatment objectives.

Nitrification is especially sensitive to insufficient SRT.

Temperature and BNR

Cold weather can affect:

• nitrification rate;
• required SRT;
• oxygen demand;
• overall nutrient-removal performance.

Influent Carbon Variability

Changes in influent biodegradable carbon can affect:

• denitrification;
• EBPR;
• F/M;
• oxygen demand.

Wet Weather

Wet weather can change BNR performance through:

• higher hydraulic flow;
• dilution of carbon;
• shorter detention time;
• higher clarifier loading.

Internal Recycle Monitoring

Operators should verify:

• pump status;
• actual flow;
• valve position;
• DO in the recycle stream where relevant.

RAS and BNR

RAS returns biomass from the secondary clarifier but can also return:

• nitrate;
• dissolved oxygen.

These recycled constituents can affect anaerobic and anoxic zones.

Example: Effluent Ammonia Rising

Review:

• aerobic-zone DO;
• SRT;
• temperature;
• pH;
• alkalinity;
• influent ammonia load;
• toxicity.

Example: Ammonia Low but Nitrate High

This pattern indicates nitrification may be occurring while denitrification is incomplete.

Review:

• anoxic-zone conditions;
• carbon availability;
• internal recycle;
• DO carryover.

Example: Nitrate High and Anoxic DO High

Excess oxygen may be suppressing denitrification.

Review:

• aeration control;
• internal recycle flow;
• mixing;
• DO measurement accuracy.

Example: Nitrate High and Anoxic DO Low

If the anoxic zone is truly anoxic, investigate:

• carbon availability;
• detention time;
• internal recycle rate;
• temperature;
• biological activity.

Example: pH Falling as Nitrification Improves

This can occur because nitrification consumes alkalinity.

Review alkalinity before pH becomes limiting.

Example: Rising Sludge in Secondary Clarifier

If nitrate is present, investigate denitrification within the sludge blanket.

Review:

• blanket depth;
• RAS withdrawal;
• sludge residence time;
• nitrate concentration.

Example: Effluent Phosphorus Rising

Review:

• anaerobic-zone conditions;
• nitrate carryover;
• DO carryover;
• carbon availability;
• WAS operation;
• solids separation.

Example: Anaerobic Zone Contains Nitrate

Possible causes include:

• high nitrate in RAS;
• incorrect internal recycle routing;
• hydraulic short-circuiting.

Example: Anaerobic Zone Contains DO

Review:

• upstream aeration;
• recycle streams;
• mixer operation;
• probe accuracy.

Example: BNR Performance Changes After Wet Weather

Review:

• flow;
• influent carbon concentration;
• ammonia load;
• zone detention time;
• clarifier loading.

Example: Internal Recycle Pump Failure

A failed internal recycle can reduce nitrate transfer to the anoxic zone and increase effluent nitrate.

Example: Internal Recycle Too High

Possible effects include:

• higher pumping energy;
• DO carryover into anoxic zones;
• reduced denitrification efficiency.

Biological and Chemical Phosphorus Removal

Some facilities combine biological phosphorus removal with chemical precipitation.

Operators should understand which part of phosphorus removal is provided by:

• biology;
• chemical feed;
• solids separation.

Effluent Solids and Phosphorus

Phosphorus stored in biomass can leave with suspended solids.

Poor secondary clarification can therefore increase effluent phosphorus even when biological uptake is occurring.

Effluent TSS and Nutrient Performance

High effluent TSS can interfere with:

• total phosphorus performance;
• overall effluent quality.

Do Not Troubleshoot Nutrients in Isolation

Nutrient performance should be interpreted with:

• DO;
• SRT;
• MLSS;
• RAS;
• WAS;
• settling;
• clarifier performance;
• influent loading.

Common BNR Mistakes

• Confusing anoxic and anaerobic conditions.
• Assuming nitrification automatically means total nitrogen removal.
• Ignoring carbon availability during denitrification.
• Allowing excessive DO into anoxic zones.
• Allowing nitrate into an anaerobic EBPR zone.
• Ignoring alkalinity consumption during nitrification.
• Reducing SRT below the needs of nitrifying organisms.
• Ignoring internal recycle performance.
• Assuming high effluent phosphorus is always a biological problem when solids carryover may be involved.
• Changing several BNR controls at once without identifying the limiting condition.

A Practical Nitrification Review

1. Verify ammonia results.
2. Review aerobic-zone DO.
3. Review SRT.
4. Review wastewater temperature.
5. Review pH and alkalinity.
6. Review influent ammonia loading.
7. Check for toxic or inhibitory influent.
8. Review aeration and solids-control trends.

A Practical Denitrification Review

1. Verify nitrate results.
2. Review anoxic-zone DO.
3. Review biodegradable carbon availability.
4. Review internal recycle flow.
5. Check for excessive oxygen carryover.
6. Review anoxic mixing.
7. Review detention time.
8. Trend effluent nitrate response.

A Practical Biological Phosphorus Review

1. Verify phosphorus results.
2. Review anaerobic-zone DO.
3. Review nitrate entering the anaerobic zone.
4. Review readily biodegradable carbon availability.
5. Review WAS and solids inventory.
6. Review secondary clarifier solids loss.
7. Review process-zone hydraulics.
8. Compare biological performance with any chemical phosphorus-removal contribution.

A Practical BNR Process Review

1. Review influent flow and nutrient load.
2. Review aerobic, anoxic, and anaerobic zone conditions.
3. Review ammonia, nitrate, and phosphorus profiles.
4. Review DO distribution.
5. Review pH and alkalinity.
6. Review SRT, MLSS, RAS, and WAS.
7. Review internal recycle operation.
8. Review secondary clarification.
9. Identify the limiting condition.
10. Make a controlled process adjustment and trend the response.

What to Remember for the Exam

• Biological nutrient removal uses different process zones to remove nitrogen and phosphorus.
• Nitrification converts ammonia toward nitrate under aerobic conditions.
• Denitrification converts nitrate toward nitrogen gas under anoxic conditions.
• Anaerobic conditions lack both significant free DO and significant nitrate available for respiration.
• Anoxic and anaerobic conditions are not the same.
• Nitrifying organisms grow slowly and require adequate SRT.
• Low DO, low SRT, cold temperature, low alkalinity, unfavorable pH, high load, and toxicity can impair nitrification.
• Nitrification consumes approximately 7.14 mg/L alkalinity as CaCO3 per mg/L ammonia-nitrogen oxidized.
• Denitrification requires nitrate, suitable microorganisms, anoxic conditions, and an available carbon source.
• Excess DO in an anoxic zone can reduce denitrification efficiency.
• Internal mixed-liquor recycle commonly moves nitrate from aerobic zones back to anoxic zones.
• Excessive internal recycle can carry unwanted DO into the anoxic zone.
• Denitrification in a secondary clarifier can produce nitrogen gas and rising sludge.
• Enhanced biological phosphorus removal relies on phosphorus-accumulating organisms and properly maintained anaerobic and aerobic conditions.
• Phosphorus stored in biomass is permanently removed when phosphorus-rich sludge is wasted from the process.
• Nitrate and DO carryover into an anaerobic zone can interfere with biological phosphorus removal.
• Carbon availability affects both denitrification and biological phosphorus removal.
• High effluent phosphorus can result from biological problems, poor solids separation, or both.
• BNR troubleshooting should combine nutrient profiles with DO, alkalinity, SRT, recycle, solids control, and clarification data.
• Successful BNR depends on maintaining the correct biological conditions in the correct process zones.