8.3 Biological Nutrient Removal: Nitrogen & Phosphorus

8.3 Biological Nutrient Removal: Nitrogen & Phosphorus

Regulatory Context: The discharge of untreated nitrogen and phosphorus species into New Jersey surface waters accelerates cultural eutrophication, stimulates toxic harmful algal blooms (HABs), and depletes critical dissolved oxygen resources in sensitive receiving streams, coastal estuaries, and the Delaware River Basin. NJPDES permits issued under N.J.A.C. 7:14A establish stringent limits on ammonia-nitrogen ($NH_3\text{-N}$), Total Nitrogen (TN), and Total Phosphorus (TP), requiring licensed operators to master Biological Nutrient Removal (BNR) microbiology and stoichiometry.

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1. Biological Nitrogen Removal: Nitrification

Nitrification is a two-step biological oxidation process through which reduced nitrogen (ammonia and ammonium) is converted sequentially into nitrite and then nitrate. Nitrification is performed exclusively by obligate aerobic, autotrophic bacteria. Unlike heterotrophs, which obtain carbon from organic compounds ($BOD$), nitrifiers use inorganic carbon (carbon dioxide $CO_2$ or dissolved bicarbonate $HCO_3^-$) for cellular synthesis.

The Two-Step Biochemical Reactions

+-------------------------------------------------------------------------+
|                     TWO-STEP NITRIFICATION PATHWAY                      |
|                                                                         |
| STEP 1: Ammonia Oxidation to Nitrite (AOB: Nitrosomonas)                |
|         2 NH4+  +  3 O2  ------->  2 NO2-  +  4 H+  +  2 H2O            |
|                                                                         |
| STEP 2: Nitrite Oxidation to Nitrate (NOB: Nitrobacter / Nitrospira)    |
|         2 NO2-  +  O2    ------->  2 NO3-                               |
|                                                                         |
| OVERALL STOICHIOMETRIC REACTION:                                        |
|         NH4+  +  2 O2    ------->  NO3-  +  2 H+  +  H2O                |
+-------------------------------------------------------------------------+

1. Step 1: Ammonia Oxidation: Carried out by Ammonia-Oxidizing Bacteria (AOB), primarily Nitrosomonas europaea:

$2\text{NH}_4^+ + 3\text{O}_2 \xrightarrow{\text{Nitrosomonas}} 2\text{NO}_2^- + 4\text{H}^+ + 2\text{H}_2\text{O} + \text{Energy}$

1. Step 2: Nitrite Oxidation: Carried out by Nitrite-Oxidizing Bacteria (NOB), primarily Nitrobacter winogradskyi and Nitrospira species:

$2\text{NO}_2^- + \text{O}_2 \xrightarrow{\text{Nitrobacter}} 2\text{NO}_3^- + \text{Energy}$

Critical Stoichiometric & Kinetic Demands

• Dissolved Oxygen Demand: Oxidation requires 4.57 lbs of $O_2$ per lb of $NH_3\text{-N}$ oxidized (3.43 lbs $O_2$/lb for nitrite conversion, plus 1.14 lbs $O_2$/lb for nitrate conversion). At facilities performing year-round nitrification, biological nitrogen oxidation can account for 40% to 50% of the plant's total blower power demand.
• Alkalinity Consumption: Nitrification consumes 7.14 lbs of $\text{CaCO}_3$ alkalinity per lb of $NH_3\text{-N}$ oxidized. As ammonia is oxidized, two moles of hydrogen ions ($H^+$) are produced for each mole of ammonia oxidized. These hydrogen ions react directly with dissolved bicarbonate buffer:

$\text{H}^+ + \text{HCO}_3^- \longrightarrow \text{CO}_2 \uparrow + \text{H}_2\text{O}$

• Catastrophic pH Collapse: If raw wastewater lacks sufficient natural alkalinity, the unchecked generation of $H^+$ will strip the bicarbonate buffer, driving the aeration basin pH down below 6.5. Nitrifiers are severely inhibited below pH 6.8 and cease metabolizing entirely at pH $\le 6.0$. Operators must maintain a minimum residual alkalinity of 50 to 100 mg/L as $\text{CaCO}_3$ in the final aeration effluent. If natural alkalinity is depleted, operators must dose supplemental base chemicals: hydrated lime ($Ca(OH)_2$), sodium hydroxide (caustic soda, $NaOH$), or sodium bicarbonate ($NaHCO_3$).
• Dissolved Oxygen Setpoint: Maintain DO at $\ge 2.0\text{ mg/L}$ throughout the aeration zone. Nitrifiers possess a high oxygen half-saturation constant ($K_O \approx 0.5\text{ to } 1.0\text{ mg/L}$) compared to heterotrophic carbonaceous bacteria ($K_O \approx 0.15\text{ to } 0.25\text{ mg/L}$). If DO drops below 1.5 mg/L, nitrification kinetics drop off sharply while $BOD_5$ removal continues unimpeded.
• Optimal pH Range: Strictly 7.5 to 8.5.
• Temperature Sensitivity: Nitrifiers exhibit extreme temperature sensitivity governed by the Arrhenius kinetic relationship. Maximum specific growth rate ($\mu_{max}$) declines exponentially as water cools. Below 12°C (54°F), nitrification rates decrease by more than 50%; below 5°C (41°F), biological nitrification virtually halts. To prevent nitrifier washout during New Jersey winters, operators must elevate the Mean Cell Residence Time (MCRT / sludge age) from 8 to 10 days in summer up to 15 to 25 days in winter.

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2. Biological Nitrogen Removal: Denitrification

Denitrification is the biological reduction of oxidized nitrogen species (nitrate and nitrite) into inert, harmless elemental nitrogen gas ($N_2$), which vents safely into the atmosphere. Denitrification is performed by facultative heterotrophic bacteria (including species of Pseudomonas, Paracoccus, and Alcaligenes).

The Stepwise Reduction Pathway

$\text{NO}_3^- \;(\text{Nitrate}) \longrightarrow \text{NO}_2^- \;(\text{Nitrite}) \longrightarrow \text{NO} \;(\text{Nitric Oxide}) \longrightarrow \text{N}_2\text{O} \;(\text{Nitrous Oxide}) \longrightarrow \text{N}_2 \uparrow \;(\text{Nitrogen Gas})$

Environmental Prerequisites for Denitrification

1. Strictly Anoxic Conditions: Denitrification requires an anoxic environment—defined as the total absence of dissolved oxygen (DO strictly $< 0.2\text{ mg/L}$), while chemically bound oxygen (nitrate $\text{NO}_3^-$ and nitrite $\text{NO}_2^-$) remains present. Because facultative bacteria harvest more thermodynamic energy ($ATP$) by respiring dissolved oxygen than by reducing nitrate, the presence of even low DO ($> 0.5\text{ mg/L}$) suppresses nitrate reductase enzyme synthesis, shutting down denitrification immediately.
2. Readily Biodegradable Organic Carbon (Electron Donor): Heterotrophic denitrifying bacteria require organic carbon molecules to supply electrons for nitrate reduction. Carbon can be supplied from:
   • Internal / Influent BOD: Readily biodegradable soluble COD (rbCOD) present in incoming raw wastewater.
   • Supplemental External Carbon: When raw wastewater $BOD:TKN$ ratios are low ($< 4:1$), supplemental carbon must be metered directly into anoxic zones. Common commercial carbon sources include methanol ($\text{CH}_3\text{OH}$), MicroC (proprietary agricultural carbohydrate/glycerol co-products), pure glycerol, and acetic acid.

Stoichiometric Recovery Credits

Denitrification provides critical operational cost-recovery offsets for wastewater utilities:

• Alkalinity Recovery: Denitrification produces hydroxyl ions ($OH^-$), recovering 3.57 lbs of $\text{CaCO}_3$ alkalinity per lb of $\text{NO}_3^-\text{-N}$ reduced. This restores exactly 50% of the alkalinity destroyed during upstream nitrification, dramatically lowering supplemental chemical costs.
• Oxygen Credit: By utilizing bound nitrate oxygen for organic carbon respiration, denitrification yields an oxygen credit of 2.86 lbs of equivalent $O_2$ per lb of $\text{NO}_3^-\text{-N}$ reduced, reducing blower power demand in downstream aerobic zones.

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3. BNR System Configurations: MLE & 4-Stage Bardenpho

+-------------------------------------------------------------------------+
|              MODIFIED LUDZACK-ETTINGER (MLE) CONFIGURATION              |
|                                                                         |
| Raw Influent ---> [ ANOXIC ZONE ] ---------> [ AEROBIC ZONE ] --------> |
| (Carbon Source)   * DO < 0.2 mg/L            * DO > 2.0 mg/L            |
|                   * Denitrification          * Carbon BOD Oxidation     |
|                   * Restores Alkalinity      * Full Nitrification       |
|                          ^                            |                 |
|                          |      Internal Mixed        |                 |
|                          +----- Liquor Recycle (IMR) -+                 |
|                          |      (200% to 400% Q)      |                 |
|                          |                            v                 |
|                          |                    [ CLARIFIER ] ----------> |
|                          |                            |       Effluent  |
|                          +==== RAS Return (50-100% Q) =                 |
+-------------------------------------------------------------------------+

The Modified Ludzack-Ettinger (MLE) Process

The MLE configuration is the most widely utilized biological nitrogen removal architecture in modern municipal wastewater treatment. It consists of a pre-anoxic tank placed directly upstream of an aerobic aeration tank, followed by a secondary clarifier.

• Internal Mixed Liquor Recycle (IMR): High-capacity, low-head axial flow pumps continuously recycle nitrate-rich mixed liquor from the discharge of the aerobic basin back to the influent of the pre-anoxic tank at 200% to 400% of forward plant influent flow ($Q$).
• Process Synergy: Placing the anoxic tank first exploits the high concentration of readily biodegradable $BOD$ in raw influent as the organic carbon source for denitrification. This eliminates the expense of purchasing supplemental methanol or MicroC, while reducing the carbonaceous oxygen demand entering the aerobic basin by 30%.
• Nitrogen Removal Efficiency: The theoretical total nitrogen (TN) removal efficiency of an MLE process is limited by the recycle ratios:

$\text{Fraction TN Removed} = \frac{R_{\text{IMR}} + R_{\text{RAS}}}{1 + R_{\text{IMR}} + R_{\text{RAS}}}$

(where $R_{\text{IMR}}$ is the internal recycle ratio and $R_{\text{RAS}}$ is the return sludge ratio). At typical recycles ($R_{\text{IMR}} = 3.0$ and $R_{\text{RAS}} = 0.75$), theoretical TN removal is approximately 79%, yielding effluent TN concentrations of 6 to 10 mg/L.

The 4-Stage Bardenpho Process

For facilities required to meet ultra-low effluent Total Nitrogen limits ($< 3.0\text{ mg/L}$), the 4-stage Bardenpho process adds a second anoxic and aerobic stage:

+-------------------------------------------------------------------------+
|                        4-STAGE BARDENPHO TRAIN                          |
|                                                                         |
| Influent ---> [ 1. Primary ] ---> [ 2. Primary ] ---> [ 3. Secondary ]  |
|               [    Anoxic  ]      [    Aerobic ]      [    Anoxic    ]  |
|                     ^                 |  |                    |         |
|                     +--- IMR (400% Q)-+  +---> [ 4. Re-aeration ]       |
|                     |                             (Stripping/DO)        |
|                     |                                     |             |
|                     +========== RAS Return ===============+==== Clarifier
+-------------------------------------------------------------------------+

1. Stage 1 (Primary Anoxic): Denitrifies the large nitrate load recycled via IMR (300% to 400% Q) using raw influent BOD as carbon.
2. Stage 2 (Primary Aerobic): Accomplishes complete carbonaceous BOD oxidation and complete biological nitrification.
3. Stage 3 (Secondary Anoxic): Polishes out residual nitrate that bypassed the primary anoxic loop. Because influent carbon was consumed upstream, denitrification here relies on slow endogenous respiration or precise metering of supplemental carbon (MicroC or methanol).
4. Stage 4 (Re-aeration / Polishing): A small aeration zone (hydraulic retention time 30 to 60 minutes) that provides two critical functions: (1) strips entrained nitrogen gas ($N_2$) microbubbles from biological flocs to prevent rising sludge in secondary clarifiers; and (2) raises dissolved oxygen to 4.0 to 6.0 mg/L to prevent denitrification inside secondary clarifier sludge blankets.

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4. Enhanced Biological Phosphorus Removal (EBPR)

Enhanced Biological Phosphorus Removal (EBPR) relies on specialized heterotrophic microorganisms called Polyphosphate Accumulating Organisms (PAOs), predominantly Candidatus Accumulibacter phosphatis. In conventional activated sludge, phosphorus comprises only 1.5% to 2.0% of dry bacterial cell weight; under EBPR cycling, PAOs store luxury quantities of intracellular polyphosphate, accumulating 5.0% to 7.0% phosphorus by dry weight.

+-------------------------------------------------------------------------+
|                 EBPR BIOCHEMICAL MECHANISM ACROSS BASINS                |
|                                                                         |
| ANAEROBIC ZONE (No DO, No Nitrate, ORP < -150 mV)                       |
| * Stress Environment: Microbes starved of terminal electron acceptors   |
| * Energy Generation: PAOs break intracellular Polyphosphate bonds       |
| * PHOSPHORUS RELEASE: Soluble Orthophosphate (PO4) is released into     |
|   solution, elevating bulk liquid PO4 to 20 to 40 mg/L                  |
| * Carbon Storage: PAOs absorb Volatile Fatty Acids (VFAs: acetate) and  |
|   polymerize them into intracellular Poly-Hydroxybutyrate (PHB) granules|
|                                                                         |
|                               v                                         |
|                                                                         |
| AEROBIC ZONE (DO > 2.0 mg/L)                                            |
| * Abundant Oxygen: Terminal electron acceptor restored                  |
| * Energy Generation: PAOs oxidize stored intracellular PHB              |
| * LUXURY PHOSPHORUS UPTAKE: PAOs harvest immense energy from PHB        |
|   oxidation and absorb soluble orthophosphate from solution in luxury   |
|   quantities, rebuilding internal polyphosphate chains                  |
| * EFFLUENT POLISHING: Soluble PO4 in liquid drops to < 0.5 mg/L         |
|                                                                         |
|                               v                                         |
|                                                                         |
| SLUDGE WASTING (WAS)                                                    |
| * Wasting biomass from the aerobic zone PERMANENTLY REMOVES phosphorus  |
|   from the facility before it can be re-released                        |
+-------------------------------------------------------------------------+

The Anaerobic Zone: Stress, VFA Uptake & Phosphorus Release

• Environmental Conditions: Strictly anaerobic. There must be zero dissolved oxygen and zero nitrate/nitrite ($ ext{NO}_x < 0.1\text{ mg/L}$, Oxidation-Reduction Potential $\text{ORP} < -150\text{ to } -250\text{ mV}$). If nitrate is present, denitrifiers will consume incoming VFAs, starving the PAOs.
• Biochemical Mechanism: Under the metabolic stress of having no electron acceptors, PAOs break down stored high-energy intracellular polyphosphate bonds. The energy released ($ATP$) is used to transport Volatile Fatty Acids (VFAs)—primarily acetate and propionate produced by upstream fermentation—across their cell membranes. PAOs polymerize these VFAs into internal carbon storage polymers known as poly-hydroxybutyrate (PHB). As the polyphosphate chains are broken, inorganic orthophosphate ($PO_4^{3-}$) is expelled into solution, causing soluble phosphorus in the anaerobic zone to spike dramatically to 20 to 40+ mg/L.

The Aerobic Zone: Luxury Uptake

• Biochemical Mechanism: When the mixed liquor enters the aerobic basin ($ ext{DO} \ge 2.0\text{ mg/L}$), PAOs utilize oxygen to catabolize their stored intracellular PHB granules. The immense cellular energy produced allows PAOs to absorb soluble orthophosphate from the bulk liquid in quantities far exceeding their baseline metabolic requirements (luxury uptake). Soluble phosphorus concentrations in the liquid drop below 0.1 to 0.5 mg/L.
• Permanent Removal via Waste Sludge (WAS): Phosphorus is permanently eliminated from the facility only when waste activated sludge (WAS) containing phosphorus-rich PAO cells is physically removed from the system. Sludge must be wasted from the aerobic zone. If WAS or secondary sludge is allowed to sit un-aerated in gravity thickeners or holding tanks, the sludge blanket turns anaerobic, causing PAOs to immediately re-release stored phosphorus back into the liquid supernatant, recycling massive nutrient loads to the plant headworks.

Competitors: Glycogen Accumulating Organisms (GAOs)

Under process upsets, Glycogen Accumulating Organisms (GAOs), such as Candidatus Competibacter, compete directly with PAOs. GAOs consume VFAs in the anaerobic zone but store them without accumulating or removing polyphosphate. GAOs thrive and outcompete PAOs at high liquid temperatures ($> 25^\circ\text{C}$) and lower operating pH ($< 7.0$).

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5. Chemical Phosphorus Precipitation

When biological phosphorus removal cannot consistently achieve stringent water quality limits (e.g., NJPDES limits of $< 0.1\text{ to } 0.5\text{ mg/L}$ TP), chemical precipitation is used as a standalone or polishing process.

Chemical Dosing Realities

• Molar Ratios: While the theoretical reaction forms insoluble metal phosphates ($AlPO_4$ or $FePO_4$) at a 1:1 molar ratio, side reactions with natural wastewater alkalinity and hydroxide formation ($Al(OH)_3$, $Fe(OH)_3$) demand practical dosing of 1.5 to 2.5 moles of metal per mole of phosphorus.
• Alkalinity Depletion: Metal coagulants are acidic. Dosing alum or ferric chloride consumes 5.5 mg/L of alkalinity as $\text{CaCO}_3$ per mg/L of $Al^{3+}$ or $Fe^{3+}$ added, which can further compound pH depression in nitrifying systems.
• Sludge Generation: Chemical precipitation generates significant additional sludge mass—typically 2.0 to 2.5 lbs of dry chemical sludge per lb of metal salt dosed.

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6. Practical Operational Scenario & Exam Traps

Practical Operational Scenario

A 10 MGD MLE wastewater treatment facility in northern New Jersey faces a sudden winter drop in aeration tank temperature to 8°C (46°F). Laboratory analyses reveal that effluent ammonia has spiked from 0.8 mg/L to 7.4 mg/L, while effluent nitrate has dropped from 8.0 mg/L to 1.5 mg/L. Aeration basin pH has declined from 7.4 to 6.3, and residual alkalinity in the basin effluent is measured at only 28 mg/L as $\text{CaCO}_3$.

• Root Cause Diagnosis: Cold temperature reduced nitrifier growth rates ($\mu_{max}$). As nitrification slowed, the operator initially left blower output unchanged. However, the background alkalinity was depleted by ongoing acid generation, driving pH down to 6.3, which placed the remaining nitrifiers into severe metabolic inhibition.
• Corrective Action Plan:
  1. Immediately initiate supplemental alkalinity dosing: Feed liquid sodium bicarbonate ($NaHCO_3$) into the aeration basin influent to raise effluent residual alkalinity above 80 mg/L as $\text{CaCO}_3$ and restore pH to 7.4 to 7.6.
  2. Reduce waste activated sludge (WAS) pumping rates to elevate Mean Cell Residence Time (MCRT) from 10 days to 20 days, building the nitrifier biomass inventory to compensate for lower cold-weather metabolic kinetics.
  3. Verify that aeration DO is maintained at $\ge 2.5\text{ mg/L}$ in the nitrification zone.
  4. Within 5 days of restoring alkalinity and increasing MCRT, effluent ammonia returns to compliance at 0.6 mg/L.

Critical Exam Traps

• Trap 1: Nitrification Stoichiometry. Nitrification requires 4.57 lbs $O_2$ and consumes 7.14 lbs $\text{CaCO}_3$ alkalinity per lb $NH_3\text{-N}$ oxidized. Memorize these exact numbers.
• Trap 2: Denitrification Credits. Denitrification recovers 3.57 lbs $\text{CaCO}_3$ alkalinity (exactly 50% of the alkalinity lost in nitrification) and saves 2.86 lbs equivalent $O_2$ per lb $NO_3^-\text{-N}$ reduced.
• Trap 3: EBPR Phosphorus Mechanics. In the anaerobic zone, phosphorus is RELEASED into solution (elevating soluble $PO_4$ to 20–40 mg/L); luxury uptake occurs strictly in the aerobic zone. Wasting must occur from the aerobic zone to eliminate phosphorus from the facility.
• Trap 4: Anoxic vs. Anaerobic Definitions. Anoxic means no dissolved oxygen ($ ext{DO} < 0.2\text{ mg/L}$) but nitrate is present. Anaerobic means no dissolved oxygen AND no nitrate ($ ext{NO}_3^- = 0$, $ ext{ORP} < -150\text{ mV}$).