6.1 Biological Nutrient Removal: Nitrification, Denitrification & Anoxic Zones

Biological Nutrient Removal: Nitrogen Transformations & Process Control

Discharge of nitrogen compounds into Colorado's receiving waters creates severe environmental and public health hazards. Ammonia nitrogen ($NH_3 / NH_4^+$) exerts an immediate dissolved oxygen demand on surface streams and exhibits acute toxicity to cold-water salmonid fish species at ultra-low concentrations. Nitrate nitrogen ($NO_3^-$) stimulates eutrophication, accelerates nuisance aquatic macrophyte growth, and violates the primary drinking water standard Maximum Contaminant Level (MCL) of 10.0 mg/L $NO_3\text{-}N$ under Colorado Regulation 11. Under Colorado's Nutrients Management Control Regulation (Regulation 85 / 5 CCR 1002-85) and stream classification standards (Regulation 31 / 5 CCR 1002-31), domestic wastewater treatment facilities face stringent effluent nitrogen limits requiring advanced Biological Nutrient Removal (BNR).

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1. Nitrogen Forms & The Wastewater Nitrogen Cycle

Nitrogen exists in domestic wastewater in four primary chemical oxidation states, continually transforming through biological assimilation, mineralization, oxidation, and reduction pathways:

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|                              WASTEWATER NITROGEN CYCLE                                  |
+-----------------------------------------------------------------------------------------+
| Raw Influent: Organic Nitrogen (Urea, Proteins, Amino Acids)                            |
|       │                                                                                 |
|       ▼  [ Ammonification / Hydrolysis by Heterotrophs ]                                |
| Ammonia / Ammonium (NH3 / NH4+)  ─── Total Kjeldahl Nitrogen (TKN = Org-N + NH4-N)      |
|       │                                                                                 |
|       ▼  [ Nitrification Step 1: Nitrosomonas (AOB) + O2 ]                              |
| Nitrite (NO2-)                                                                          |
|       │                                                                                 |
|       ▼  [ Nitrification Step 2: Nitrobacter / Nitrospira (NOB) + O2 ]                  |
| Nitrate (NO3-)                                                                          |
|       │                                                                                 |
|       ▼  [ Denitrification: Facultative Heterotrophs + Carbon Source (DO < 0.2 mg/L) ]  |
| Nitrogen Gas (N2 ↑) ── Released harmlessly into atmosphere (78% of ambient air)         |
+-----------------------------------------------------------------------------------------+

Nitrogen Analytical Fractions

1. Organic Nitrogen: Bound organically in fecal matter, food waste, proteins, and cellular biomass.
2. Ammonia Nitrogen ($NH_3 / NH_4^+$): The reduced inorganic form resulting from urea hydrolysis and biological deamination. In aqueous solution, ammonia exists in a chemical equilibrium governed by temperature and pH:

$NH_3 + H_2O \rightleftharpoons NH_4^+ + OH^- \quad \text{or} \quad NH_4^+ \rightleftharpoons NH_3 + H^+ \quad (pK_a = 9.25 \text{ at } 25^\circ\text{C})$

• Ammonium Ion ($NH_4^+$): Ionized, non-volatile, and non-toxic to fish at typical neutral pH levels.
• Un-ionized Ammonia ($NH_3$): Neutral gas molecule that readily diffuses across gill membranes of fish, causing respiratory failure and acute aquatic mortality. As pH or temperature rises, the equilibrium shifts sharply toward toxic un-ionized $NH_3$.

1. Total Kjeldahl Nitrogen (TKN): The analytical sum of organic nitrogen and ammonia nitrogen ($\text{TKN} = \text{Org-N} + NH_4\text{-}N$). Typical municipal raw wastewater exhibits a TKN of $30–50\text{ mg/L}$.
2. Nitrite Nitrogen ($NO_2^-$): A transient, unstable intermediate oxidized form (typically $<0.1–0.5\text{ mg/L}$ in stable systems). Nitrite exerts significant chlorine demand ($5.0\text{ lbs } Cl_2$ per lb $NO_2\text{-}N$) during final disinfection.
3. Nitrate Nitrogen ($NO_3^-$): The fully oxidized end product of aerobic nitrification.
4. Total Nitrogen (TN): The sum of all inorganic and organic forms: $\text{TN} = \text{TKN} + NO_2\text{-}N + NO_3\text{-}N$.

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2. Biochemistry & Stoichiometry of Nitrification

Biological nitrification is a two-step aerobic conversion of ammonia to nitrate mediated by specialized chemoautotrophic bacteria (organisms that derive energy from the oxidation of inorganic nitrogen and obtain cellular carbon from inorganic carbon dioxide / bicarbonate):

Step 1: Ammonia Oxidation to Nitrite

Mediated primarily by Ammonia-Oxidizing Bacteria (AOB), predominantly the genus Nitrosomonas:

$2 NH_4^+ + 3 O_2 \xrightarrow{\text{Nitrosomonas}} 2 NO_2^- + 4 H^+ + 2 H_2O + \text{Energy}$

Step 2: Nitrite Oxidation to Nitrate

Mediated by Nitrite-Oxidizing Bacteria (NOB), including Nitrobacter and Nitrospira:

$2 NO_2^- + O_2 \xrightarrow{\text{Nitrobacter}} 2 NO_3^- + \text{Energy}$

Combined Overall Nitrification Reaction

$NH_4^+ + 2 O_2 \longrightarrow NO_3^- + 2 H^+ + H_2O$

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|                        CRITICAL NITRIFICATION STOICHIOMETRIC CONSTANTS                  |
+-----------------------------------------------------------------------------------------+
| • Oxygen Demand:      4.57 lbs O2 required per 1.0 lb NH4-N oxidized                     |
|                       (3.43 lbs O2 for Nitrosomonas + 1.14 lbs O2 for Nitrobacter)      |
| • Alkalinity Consumed: 7.14 lbs CaCO3 destroyed per 1.0 lb NH4-N oxidized               |
|                       (Due to release of 2 moles of H+ per mole of NH4+ oxidized)        |
| • Biomass Yield:      0.15 - 0.20 lbs dry volatile solids produced per lb NH4-N         |
+-----------------------------------------------------------------------------------------+

Environmental & Operational Controls for Nitrification

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3. Biochemistry & Stoichiometry of Denitrification

Denitrification is the biological reduction of nitrate to harmless dinitrogen gas ($N_2\uparrow$) executed by widespread facultative heterotrophic bacteria (such as Pseudomonas, Paracoccus, Alcaligenes, and Bacillus). When dissolved oxygen is depleted ($DO < 0.2\text{ mg/L}$), these organisms switch their metabolic respiratory pathways, utilizing nitrate ($NO_3^-$) or nitrite ($NO_2^-$) instead of dissolved oxygen as their terminal electron acceptor to metabolize organic carbon.

The Denitrification Reduction Cascade

$NO_3^- \;(\text{Nitrate}) \longrightarrow NO_2^- \;(\text{Nitrite}) \longrightarrow NO \;(\text{Nitric Oxide}) \longrightarrow N_2O \;(\text{Nitrous Oxide}) \longrightarrow N_2\uparrow \;(\text{Dinitrogen Gas})$

Stoichiometric Principles & Carbon Demands

Denitrification requires an electron donor (organic carbon source). This can be provided by the readily biodegradable chemical oxygen demand (rbCOD / BOD) naturally present in raw wastewater, or via supplemental external carbon sources such as methanol ($CH_3OH$), sodium acetate ($CH_3COONa$), glycerin, or micro-brewing byproducts.

Using methanol as the carbon source, the overall balanced redox reaction is:

$6 NO_3^- + 5 CH_3OH + \text{Carbonic Acid} \longrightarrow 3 N_2\uparrow + 5 CO_2 + 7 H_2O + 6 OH^- + \text{Biomass}$

+-----------------------------------------------------------------------------------------+
|                        CRITICAL DENITRIFICATION RECOVERY CONSTANTS                      |
+-----------------------------------------------------------------------------------------+
| • Alkalinity Recovery: 3.57 lbs CaCO3 produced per 1.0 lb NO3-N reduced                 |
|                       (Recovers exactly 50% of the alkalinity destroyed during         |
|                        nitrification: 3.57 / 7.14 = 50.0%)                              |
| • Oxygen Credit:       2.86 lbs equivalent O2 credit per 1.0 lb NO3-N reduced           |
|                       (Reduces aeration blower power demand in downstream basins)       |
| • Carbon Requirement:  3.0 - 4.5 lbs BOD (or 2.47 lbs pure methanol) per lb NO3-N       |
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4. Biological Nutrient Removal Configurations

To achieve simultaneous carbon oxidation, nitrification, and denitrification within an activated sludge process, wastewater treatment plants employ multi-stage zoned reactor configurations featuring designated anoxic zones (un-aerated, mechanically mixed, $DO < 0.2\text{ mg/L}$, nitrate present) and aerobic zones (aerated, $DO \ge 2.0\text{ mg/L}$).

The Modified Ludzack-Ettinger (MLE) Process

The MLE process is the most widely utilized pre-anoxic BNR configuration in Colorado:

1. Pre-Anoxic Basin: Positioned at the head of the secondary treatment process. Raw wastewater (rich in organic carbon / BOD) enters this basin alongside two recycled streams: Return Activated Sludge (RAS) from the clarifiers and a high-rate Internal Mixed Liquor Recycle (IMLR / NRCY) pumped directly from the tail end of the aerobic basin.
2. Aerobic Basin: Mixed liquor from the pre-anoxic basin flows into the aerobic zone, where heterotrophs oxidize remaining carbonaceous BOD and autotrophic nitrifiers (Nitrosomonas and Nitrobacter) convert ammonia to nitrate.
3. Internal Mixed Liquor Recycle (IMLR): Pumps nitrate-rich mixed liquor back to the pre-anoxic zone at recycle rates of $100%–400%$ of forward influent flow ($1.0–4.0 \times Q$).
4. Performance: Achieves $70%–85%$ Total Nitrogen removal, yielding typical effluent TN concentrations of $5.0–8.0\text{ mg/L}$.

The 4-Stage Bardenpho Process

For facilities required to meet ultra-low effluent total nitrogen standards ($TN < 3.0\text{ mg/L}$), the 4-Stage Bardenpho configuration adds secondary polishing zones:

1. Primary Anoxic Zone: Uses raw influent rbCOD and high-rate IMLR ($300%–400% Q$) for primary denitrification (removes ~70% of nitrogen).
2. Primary Aerobic Zone: Provides complete carbon oxidation and full nitrification of ammonia to nitrate.
3. Secondary Post-Anoxic Zone: Without raw carbon available, heterotrophs carry out endogenous denitrification (utilizing carbon from decaying bacterial cell mass) or feed on metered supplemental carbon (methanol/acetate) to reduce residual nitrate down to $<1.0–2.0\text{ mg/L}$.
4. Secondary Re-Aeration Zone: A small aerobic zone ($5–15\text{ minutes}$ hydraulic detention time) that strips nitrogen gas bubbles from the mixed liquor, raises DO to $4.0–6.0\text{ mg/L}$, and oxidizes any remaining ammonia or residual carbon before the mixed liquor enters the secondary clarifiers, preventing clarifier septicity and sludge floating.

+-----------------------------------------------------------------------------------------+
|                           4-STAGE BARDENPHO SYSTEM LAYOUT                               |
+-----------------------------------------------------------------------------------------+
|            +----------------------- IMLR (300-400% Q) -----------------------+          |
|            |                                                                 |          |
| Influent   ▼                   ┌──────────────┐             ┌─────────────┐  │          |
| ────────► [1. Pre-Anoxic] ───► │2. 1st Aerobic│ ──────────► │3. 2nd Anoxic│ ─┴────────┐ |
|               (Mixed)          └──────────────┘  (Nitrate)  │ (Endogenous │           │ |
|                  ▲               (Nitrify)                  │  / Methanol)│           │ |
|                  │                                          └─────────────┘           │ |
|                  │                                                 │                  │ |
|                  │                                                 ▼                  ▼ |
|                  │            Effluent ◄─── [Secondary Clarifier] ◄─── [4. Re-Aeration] |
|                  │                                  │                   (DO Strip)      |
|                  +───────────── RAS (50-100% Q) ────+                                   |
+-----------------------------------------------------------------------------------------+

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5. Operational Troubleshooting & Nitrogen Process Control

Clumping / Rising Sludge in Secondary Clarifiers

When mixed liquor entering the secondary clarifiers contains elevated nitrate ($NO_3\text{-}N > 5–8\text{ mg/L}$) and settles into an anoxic sludge blanket ($DO = 0\text{ mg/L}$), denitrifying bacteria reduce nitrate to dinitrogen gas ($N_2$).

Tiny micro-bubbles of $N_2$ gas become trapped within the settling biological floc, decreasing its bulk density until large, brown clumps of sludge float to the clarifier surface (known as rising sludge or clumping).

Diagnostic vs. Bulking Sludge: Rising sludge is distinguished from filamentous bulking because the mixed liquor exhibits a normal, rapid initial settling rate (good Sludge Volume Index / $\text{SVI} < 120\text{ mL/g}$), but floats 30 to 90 minutes later as gas accumulates.

Corrective Actions:

• Increase the Return Activated Sludge (RAS) pumping rate to reduce sludge blanket detention time in the clarifier.
• Increase the IMLR recycle rate to ensure more nitrate is denitrified in the upstream anoxic basin.
• Increase DO in the aerobic basin tail / re-aeration basin to purge dissolved nitrogen gas and maintain positive DO in the clarifier blanket.

Alkalinity Depletion & pH Crash

Because nitrification destroys $7.14\text{ lbs of alkalinity as } \text{CaCO}_3\text{ per lb } NH_4\text{-}N$, poorly buffered wastewater (common in Colorado mountain streams with natural alkalinity $<50–80\text{ mg/L}$) will experience rapid pH drops. If pH drops below $6.8$, nitrification slows; if it drops below $6.0$, nitrification halts completely.

Corrective Actions:

• Dose supplemental chemical alkalinity into the aeration basin (Sodium Bicarbonate $NaHCO_3$, Soda Ash $Na_2CO_3$, Hydrated Lime $Ca(OH)_2$, or Magnesium Hydroxide $Mg(OH)_2$).
• Optimize upstream anoxic denitrification to maximize natural alkalinity recovery ($3.57\text{ lbs } \text{CaCO}_3\text{ / lb } NO_3\text{-}N$).

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6. Worked Stoichiometric Calculation Example

A domestic wastewater treatment facility treating a flow of $3.0\text{ MGD}$ has an influent ammonia concentration of $32.0\text{ mg/L } NH_4\text{-}N$ and an effluent target of $1.0\text{ mg/L } NH_4\text{-}N$. Raw wastewater alkalinity entering the aeration basin is $180.0\text{ mg/L as } \text{CaCO}_3$.

Step 1: Calculate Total Ammonia Mass Oxidized per Day

$\Delta NH_4\text{-}N = 32.0\text{ mg/L} - 1.0\text{ mg/L} = 31.0\text{ mg/L}$

$\text{Mass of } NH_4\text{-}N \text{ oxidized (lbs/day)} = \text{Flow (MGD)} \times \Delta \text{Concentration (mg/L)} \times 8.34$

$\text{Mass} = 3.0 \times 31.0 \times 8.34 = 775.62\text{ lbs/day } NH_4\text{-}N$

Step 2: Calculate Daily Oxygen Demand for Nitrification

$\text{Oxygen Demand} = 775.62\text{ lbs/day } NH_4\text{-}N \times 4.57\text{ lbs } O_2 / \text{lb } NH_4\text{-}N = 3,544.58\text{ lbs } O_2 / \text{day}$

Step 3: Calculate Total Alkalinity Consumed

$\text{Alkalinity Consumed} = 775.62\text{ lbs/day } NH_4\text{-}N \times 7.14\text{ lbs } \text{CaCO}_3 / \text{lb } NH_4\text{-}N = 5,537.93\text{ lbs } \text{CaCO}_3 / \text{day}$

$\text{Alkalinity Concentration Destroyed (mg/L)} = 31.0\text{ mg/L } NH_4\text{-}N \times 7.14 = 221.34\text{ mg/L as } \text{CaCO}_3$

Step 4: Evaluate Residual Alkalinity & Chemical Need

$\text{Residual Raw Alkalinity} = 180.0\text{ mg/L} - 221.34\text{ mg/L} = -41.34\text{ mg/L}$

Without chemical addition or anoxic denitrification recovery, the facility will exhaust all buffering capacity, causing a total pH crash and process failure. To maintain a safe residual of $60.0\text{ mg/L as } \text{CaCO}_3$, the operator must supply supplemental alkalinity or implement an MLE anoxic zone to recover $3.57\text{ lbs } \text{CaCO}_3 / \text{lb } NO_3\text{-}N$ reduced.