11.1 Biological Nutrient Removal (N & P)

11.1 Biological Nutrient Removal (N & P)

Quick Answer: Biological nutrient removal (BNR) combines nitrification (NH₃/NH₄⁺ → NO₂⁻ → NO₃⁻ under aerobic conditions with adequate DO, SRT, and alkalinity), denitrification (anoxic conversion of nitrate to N₂ gas with a carbon source), and often enhanced biological phosphorus removal by PAOs that need a true anaerobic zone. Florida AWT plants live or die on stable BNR control plus chemical polishing when needed. Track NH₃-N, NOₓ, TN, TP, DO, ORP, and alkalinity—not just CBOD₅ and TSS.

Secondary treatment alone removes most carbonaceous BOD and solids but leaves ammonia and phosphorus largely intact. Those nutrients fuel algal blooms, deplete dissolved oxygen after algae die, and threaten Florida springs, lakes, and estuaries. Biological nutrient removal is the operator’s main process path to the 5-5-3-1 AWT world (Chapter 11.2) and to high-quality reclaimed water (Chapter 11.4). Exam questions favor zone purpose, monitoring parameters, and what fails when DO/SRT/carbon/alkalinity are wrong.

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1. Why Nitrogen and Phosphorus Matter

TKN (total Kjeldahl nitrogen) is organic-N plus ammonia-N. After full nitrification, most remaining nitrogen is nitrate; after full denitrification, TN drops because N₂ gas leaves the water. Operators who only watch ammonia can pass an “ammonia-looking” day while still failing TN if denitrification is weak.

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2. Nitrification — NH₃ → NO₂ → NO₃

Nitrification is a two-step aerobic biological oxidation performed mainly by autotrophic bacteria:

1. Ammonia-oxidizing bacteria (AOB): NH₄⁺ / NH₃ → NO₂⁻ (nitrite)
2. Nitrite-oxidizing bacteria (NOB): NO₂⁻ → NO₃⁻ (nitrate)

Overall teaching line for exams: ammonia → nitrite → nitrate under aerobic conditions. Complete nitrification means low residual ammonia and low nitrite; incomplete nitrification (nitrite “blips”) can signal shock loading, low DO, low SRT, temperature drops, or inhibition.

Process conditions operators control

Alkalinity demand (high-yield number)

A classic operator estimate is that complete nitrification consumes about 7.14 mg alkalinity as CaCO₃ per mg NH₃-N oxidized. Soft, low-alkalinity Florida groundwaters and plants that already alkalinity-strip for other reasons can run out of buffering. If the aerobic zone pH crashes and ammonia climbs, alkalinity supplementation (often lime, caustic, or soda ash per plant design) is a process-control tool—not a decorative chemical.

Monitoring nitrification success

• NH₃-N should fall across the aerobic system (influent ammonia high → effluent ammonia low).
• NOₓ-N (nitrite + nitrate) rises as ammonia falls if denitrification is not yet removing nitrate.
• Nitrite alone is a red flag when elevated—nitrification is incomplete or inhibited.

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3. Denitrification — NO₃ → N₂ (Anoxic + Carbon)

Denitrification converts nitrate (and nitrite) to nitrogen gas that strips to the atmosphere. It is performed primarily by heterotrophic bacteria that use nitrate as an electron acceptor when free oxygen is scarce but a biodegradable carbon source is available.

Anoxic vs anaerobic (do not mix them up)

Exam trap: “no free oxygen” is not enough to call a zone anaerobic. If nitrate is still present, the zone is anoxic, and PAOs do not get the true anaerobic advantage they need for strong biological P removal.

Carbon source

Denitrifiers need readily biodegradable carbon. Sources include:

• Internal carbon — influent wastewater BOD directed to anoxic zones (common in pre-anoxic designs).
• Recycle of nitrate-rich mixed liquor to an upstream anoxic zone (internal mixed-liquor recycle in A²O / Bardenpho-type plants).
• External carbon — methanol, acetate, glycerol, or other approved carbon when influent BOD is insufficient for the nitrate load (common when polishing to very low TN).

If nitrate remains high in final effluent while carbon is limiting, TN will miss AWT targets even if ammonia is near zero.

ORP as a denitrification aid

Oxidation-reduction potential (ORP) helps operators see the “electron pressure” of a zone. Rough teaching bands (plant-specific, not universal law):

• Strongly positive ORP → oxidizing / aerobic environment.
• Moderately negative ORP ranges often associated with anoxic/denitrifying conditions.
• More strongly negative ORP may indicate anaerobic conditions favorable for bio-P release (and for unwanted sulfide generation if out of control).

Use ORP with DO, nitrate, and visual process sense—not as a single magic number.

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4. BNR Configurations Operators Must Recognize

A²O (Anaerobic–Anoxic–Oxic)

A²O is a classic three-zone activated-sludge layout:

1. Anaerobic — PAOs release phosphorus and store organic carbon internally.
2. Anoxic — denitrification using nitrate recycled from the aerobic zone plus available carbon.
3. Oxic (aerobic) — nitrification, residual BOD removal, and luxury phosphorus uptake.

Internal nitrate recycle from oxic to anoxic and RAS return paths are design features operators must keep flowing at intended rates. Wrong recycle ratios starve denitrification or dilute anaerobic conditions.

Five-stage Bardenpho (concept level)

The five-stage Bardenpho concept extends nitrogen removal with a second anoxic/aerobic pair after the main A²O-like sequence:

1. Anaerobic
2. Anoxic
3. Aerobic
4. Second anoxic (polishing denitrification)
5. Second aerobic (strip residual N₂ bubbles, re-aerate, nitrify any residual ammonia)

Exam idea: more staged anoxic/aerobic contact improves the chance of very low TN needed for Florida AWT—especially when paired with filtration and careful recycle control. Exact tank names vary by vendor; focus on purpose of each redox environment.

Other related terms (recognition only)

• MLE (Modified Ludzack–Ettinger): pre-anoxic + aerobic with nitrate recycle—strong on nitrogen, not a full bio-P anaerobic zone by itself.
• Oxidation ditches / SBR with timed cycles: can create anoxic/aerobic (and sometimes anaerobic) conditions in time rather than space.
• IFAS / MBBR hybrids: media increases nitrifier inventory when SRT on flocs alone is tight.

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5. Biological Phosphorus Removal (PAOs)

Phosphorus-accumulating organisms (PAOs) perform enhanced biological phosphorus removal (EBPR) when the plant provides a true cycle:

1. Anaerobic zone: PAOs release orthophosphate to the liquid while taking up volatile fatty acids and storing internal carbon (often discussed as PHB/PHA storage).
2. Aerobic zone: PAOs oxidize stored carbon and take up more phosphorus than they released, packing it into cells as polyphosphate.
3. Sludge wasting: net P leaves in WAS. If you do not waste, P returns through the plant.

What breaks bio-P

• Nitrate or oxygen leaking into the “anaerobic” zone (it becomes anoxic/aerobic).
• Insufficient readily biodegradable carbon / VFA for PAO metabolism.
• Secondary release in thickeners, digesters, or long holding of sludge—recycle streams can dump massive orthophosphate back to the headworks.
• Competing glycogen-accumulating organisms (GAOs) under some carbon/pH/temperature conditions (advanced concept; know that “carbon is present but bio-P still fails” is a real field problem).

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6. Chemical Phosphorus Precipitation (Backup and Polish)

Even excellent bio-P plants keep chemical precipitation ready. Metal salts form insoluble phosphate precipitates removed with sludge:

Chemical P removal consumes alkalinity, adds sludge mass, and costs money—but it is often the insurance policy that keeps TP ≤ 1 mg/L annual average when bio-P is seasonal or unstable. Overdosing can drop pH and harm nitrification; underdosing leaves TP high. Jar testing and orthophosphate residual tracking beat guessing.

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7. Integrated Monitoring for BNR Control

Operator sequence when ammonia rises: check DO and aeration, SRT/inventory, toxic shock clues, then alkalinity/pH. When nitrate/TN stays high with low ammonia: check anoxic conditions, carbon availability, and recycle rates. When TP rises: check anaerobic zone integrity, secondary P release in solids handling, and chemical feed.

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Exam Bottom Line

BNR is redox choreography: aerobic for nitrification and P uptake, anoxic for denitrification, anaerobic for PAO phosphorus release. Florida exams connect this choreography directly to AWT nutrient limits. Know the NH₃→NO₂→NO₃ pathway, alkalinity demand, carbon need for denit, A²O vs five-stage Bardenpho concepts, and the monitoring suite NH₃-N, NOₓ, TN, TP, DO, ORP.