11.3 Biological Nutrient Removal: Nitrification, Denitrification & Phosphorus Removal

4.4 Biological Nutrient Removal: Nitrification, Denitrification & Phosphorus Removal

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Environmental & Regulatory Drivers in Oregon

1. Ammonia Toxicity to Salmonids: Total ammonia exists in aqueous equilibrium between un-ionized ammonia ($\text{NH}_3$) and ammonium ions ($\text{NH}_4^+$):

$\text{NH}_4^+ + \text{OH}^- \rightleftharpoons \text{NH}_3 + \text{H}_2\text{O}$

Un-ionized ammonia ($\text{NH}_3$) is an uncharged lipid-soluble gas that easily penetrates gill membranes of migrating Pacific salmon and steelhead, causing severe neurotoxicity, gill tissue damage, and mortality. As water temperature and pH rise, the equilibrium shifts dramatically toward toxic $\text{NH}_3$. Consequently, Oregon DEQ NPDES permits enforce stringent seasonal ammonia limits (often $< 1.0-2.0 \text{ mg/L}$ as N during low-flow summer months).

1. Eutrophication & Hypoxia: Bioavailable nitrogen and phosphorus stimulate explosive blooms of algae and macrophytes. When these algal mats die, bacterial decay exerts an immense oxygen demand, driving dissolved oxygen below Oregon's criteria (for example 8.0 mg/L for cold-water aquatic life and 11.0 mg/L during salmonid spawning).

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Biological Nitrification: Biochemistry & Stoichiometry

Nitrification is a two-step biological oxidation performed by obligate aerobic, chemolithoautotrophic bacteria that utilize inorganic carbon (carbon dioxide and bicarbonate) for cellular synthesis:

+-----------------------------------------------------------------------------+
|                     TWO-STEP BIOLOGICAL NITRIFICATION                       |
+-----------------------------------------------------------------------------+
|  AMMONIUM (NH4+)                                                            |
|         |                                                                   |
|         |  Nitrosomonas bacteria (Ammonia-Oxidizing Bacteria - AOB)        |
|         |  Consumes 3.43 lbs O2 / lb NH3-N                                  |
|         v  Releases H+ ions (consumes alkalinity)                           |
|  NITRITE (NO2-)                                                             |
|         |                                                                   |
|         |  Nitrobacter bacteria (Nitrite-Oxidizing Bacteria - NOB)          |
|         |  Consumes 1.14 lbs O2 / lb NH3-N                                  |
|         v                                                                   |
|  NITRATE (NO3-)                                                             |
+-----------------------------------------------------------------------------+

1. Biochemical Reactions

• Step 1: Ammonia Oxidation to Nitrite (by Nitrosomonas):

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

• Step 2: Nitrite Oxidation to Nitrate (by Nitrobacter):

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

• Overall Combined Nitrification Reaction:

$\text{NH}_4^+ + 2\text{O}_2 \rightarrow \text{NO}_3^- + 2\text{H}^+ + \text{H}_2\text{O}$

2. Foundational Stoichiometric Constants

Every wastewater operator must know these theoretical constants for exam calculations and plant control:

• Oxygen Demand: $4.57 \text{ lbs } \text{O}_2$ are consumed per pound of ammonia nitrogen ($\text{NH}_3\text{-N}$) oxidized to nitrate ($3.43 \text{ lbs } \text{O}_2$ by Nitrosomonas and $1.14 \text{ lbs } \text{O}_2$ by Nitrobacter). Including cellular biomass synthesis, actual field demand ranges from $4.2 \text{ to } 4.6 \text{ lbs } \text{O}_2 / \text{lb } \text{NH}_3\text{-N}$.
• Alkalinity Destruction: $7.14 \text{ lbs of alkalinity as } \text{CaCO}_3$ are destroyed per pound of $\text{NH}_3\text{-N}$ oxidized. This occurs because the reaction produces two moles of hydrogen ions ($2\text{H}^+$) for every mole of ammonium oxidized, neutralizing bicarbonate alkalinity ($\text{HCO}_3^-$):

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

3. Critical Environmental & Kinetic Parameters

• Dissolved Oxygen (DO): Nitrifiers have a much lower oxygen affinity than heterotrophs. Aeration basin DO must be maintained at $\ge 2.0 \text{ mg/L}$ ($2.0 \text{ to } 3.0 \text{ mg/L}$). If DO drops below $1.0 \text{ mg/L}$, nitrification slows significantly; below $0.5 \text{ mg/L}$, it ceases.
• pH Range: Optimum nitrification occurs between pH $7.5 \text{ and } 8.5$. Nitrification kinetics decline steeply below pH $7.0$; at pH $< 6.5$, rates drop by $75\%$; below pH $6.0$, nitrification halts completely. In Pacific Northwest surface-water-fed collection systems with naturally low raw alkalinity ($20-50 \text{ mg/L}$), utilities must feed supplemental alkalinity (caustic soda $\text{NaOH}$, soda ash $\text{Na}_2\text{CO}_3$, or lime $\text{Ca(OH)}_2$) to maintain a residual aeration alkalinity $> 50-80 \text{ mg/L as } \text{CaCO}_3$.
• Temperature Sensitivity ($Q_{10}$ Effect): Nitrifiers reproduce very slowly. Their growth rate is highly sensitive to water temperature. When temperature drops from $20^\circ\text{C}$ to $10^\circ\text{C}$, the minimum MCRT required to prevent nitrifier washout doubles or triples (e.g., from $5 \text{ days}$ in summer to $12-18 \text{ days}$ in Pacific Northwest winters).

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Biological Denitrification: Anoxic Reduction

Biological denitrification is the biological conversion of oxidized nitrogen (nitrate $\text{NO}_3^-$ and nitrite $\text{NO}_2^-$) into inert dinitrogen gas ($\text{N}_2$), which safely vents to the atmosphere.

1. Microbial Ecology & Anoxic Conditions

Denitrification is carried out by facultative heterotrophic bacteria (Pseudomonas, Paracoccus, Alcaligenes). These organisms are ordinary heterotrophs that utilize organic carbon for energy and growth. In an anoxic environment—defined as a zone where dissolved oxygen is absent ($DO < 0.2 \text{ mg/L}$) but chemically bound oxygen in the form of nitrate ($\text{NO}_3^-$) is present—these bacteria substitute nitrate for molecular oxygen as their terminal respiratory electron acceptor:

$\text{NO}_3^- \rightarrow \text{NO}_2^- \rightarrow \text{NO}\uparrow \rightarrow \text{N}_2\text{O}\uparrow \rightarrow \text{N}_2\uparrow$

2. Carbon Source Requirements

Denitrifiers require an electron donor (biodegradable organic carbon) to reduce nitrate:

• Raw Wastewater BOD (Pre-Anoxic): Readily biodegradable soluble COD (rbCOD) present in raw influent wastewater. Highly economical as it requires no purchased chemical carbon.
• External Carbon Feed (Post-Anoxic): When an anoxic tank is situated downstream of the aerobic zone (where influent BOD has already been consumed), an external carbon source must be injected. Common electron donors include methanol ($\text{CH}_3\text{OH}$), sodium acetate, or micro-blended glycerin:

$6\text{NO}_3^- + 5\text{CH}_3\text{OH} \rightarrow 3\text{N}_2\uparrow + 5\text{CO}_2 + 7\text{H}_2\text{O} + 6\text{OH}^-$

3. Stoichiometric Credits of Denitrification

Denitrification partially offsets the intense resource demands of nitrification:

• Alkalinity Recovery: Recovers $3.57 \text{ lbs of alkalinity as } \text{CaCO}_3$ per pound of nitrate nitrogen ($\text{NO}_3\text{-N}$) reduced. This represents exactly $50\%$ of the alkalinity destroyed during nitrification.
• Oxygen Credit (Energy Savings): Heterotrophs utilize the chemically bound oxygen in nitrate to oxidize organic carbon, crediting/saving $2.86 \text{ lbs of } \text{O}_2$ equivalent per pound of $\text{NO}_3\text{-N}$ reduced. This reduces mechanical blower electrical power requirements by $15-25\%$.

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Nitrogen Removal Process Configurations

+-----------------------------------------------------------------------------+
|                    MODIFIED LUDZACK-ETTINGER (MLE) PROCESS                  |
+-----------------------------------------------------------------------------+
|                          Internal Nitrate Recycle (MLR)                     |
|                    +---------------------------------------+                |
|                    |             (200% - 400% Q)           |                |
|                    v                                       |                |
|  Raw Influent  -->[ Anoxic ]----->[     Aerobic     ]------+--> Secondary   |
|  Wastewater       [  Zone  ]      [  Nitrification  ]           Clarifier   |
|                   [ DO<0.2 ]      [     DO > 2.0    ]               |       |
|                        ^                                            |       |
|                        |                 RAS                        |       |
|                        +--------------------------------------------+       |
+-----------------------------------------------------------------------------+

1. Modified Ludzack-Ettinger (MLE) Process

The MLE process is the industry standard for biological nitrogen removal:

• Basin Layout: A pre-anoxic tank is placed directly upstream of the aerobic nitrification tank.
• Internal Nitrate Recycle (MLR / NRCY): High-capacity axial flow pumps recycle nitrate-rich mixed liquor from the discharge end of the aerobic basin back to the pre-anoxic tank at $200\% \text{ to } 400\%$ of influent flow ($2Q \text{ to } 4Q$).
• Operational Advantage: The incoming raw wastewater BOD provides the free organic carbon needed for denitrification in the pre-anoxic zone, eliminating external carbon chemical costs while recovering $50\%$ of lost alkalinity before the water reaches the aerobic zone.

2. 4-Stage Bardenpho Process

Designed to achieve ultra-low Total Nitrogen ($< 3.0 \text{ mg/L}$):

1. Primary Anoxic Zone: Receives raw influent and internal recycle ($200-400\% Q$); achieves $70-80\%$ nitrate removal using influent carbon.
2. Primary Aerobic Zone: Completes BOD oxidation and nitrifies ammonia to nitrate.
3. Secondary (Post) Anoxic Zone: Denitrifies residual nitrate escaping the first stage via endogenous microbial respiration or supplemental external carbon feed (methanol/acetate).
4. Re-Aeration Zone: A short ($15-30 \text{ minute}$) aerobic stage that strips entrained nitrogen gas, adds dissolved oxygen (commonly to about 2 mg/L) and prevents septic or rising sludge in the secondary clarifier.

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Phosphorus Removal Mechanisms

Phosphorus exists in wastewater as orthophosphate ($\text{PO}_4^{3-}$), polyphosphates, and organically bound phosphorus. It cannot be converted into an inert gas; it can only be removed from wastewater by physically incorporating it into a solid phase and wasting it out in the sludge.

1. Enhanced Biological Phosphorus Removal (EBPR)

EBPR employs specialized bacteria known as Phosphorus Accumulating Organisms (PAOs), such as Candidatus Accumulibacter phosphatis, cycled through alternating anaerobic and aerobic zones:

+-----------------------------------------------------------------------------+
|                        EBPR BIOCHEMICAL CYCLE                               |
+-------------------------------------+---------------------------------------+
|        ANAEROBIC SELECTOR           |            AEROBIC ZONE               |
|  - ZERO Dissolved Oxygen            |  - Dissolved Oxygen > 2.0 mg/L        |
|  - ZERO Nitrate (NO3-)              |  - PAOs oxidize stored PHAs           |
|  - PAOs take up VFAs (Acetate)      |  - "LUXURY PHOSPHATE UPTAKE"          |
|  - Store VFAs as intracellular PHA  |  - Intracellular Poly-P storage       |
|  - Polyphosphate bonds cleaved      |  - Soluble ortho-P drops < 0.1 mg/L   |
|  - "ORTHOPHOSPHATE RELEASE"         |  - Permanent P removal via WAS wasting|
+-------------------------------------+---------------------------------------+

1. Anaerobic Selector Zone (True Anaerobic: No DO, No $\text{NO}_3^-$):
   • Under fermentation conditions, non-PAO heterotrophs convert raw BOD into Volatile Fatty Acids (VFAs), predominantly acetic acid and propionic acid.
   • PAOs absorb these VFAs and store them internally as carbon reserves called polyhydroxyalkanoates (PHAs).
   • To power this rapid VFA transport and storage without oxygen, PAOs break the high-energy bonds of their internal polyphosphate chains, discharging free orthophosphate into the bulk liquid (orthophosphate release). Liquid phosphorus concentrations in this zone can spike to $20-40 \text{ mg/L}$.
2. Aerobic Zone:
   • Entering the aerated basin, PAOs oxidize their stored internal PHAs for cellular energy and growth.
   • Using this energy, PAOs absorb dissolved orthophosphate from the water column in massive excess of normal biological nutritional requirements—a phenomenon known as luxury uptake.
   • PAOs re-synthesize intracellular polyphosphate granules, driving soluble orthophosphate in the bulk liquid down to $< 0.1-0.5 \text{ mg/L}$.
3. Sludge Wasting (WAS):
   • Phosphorus is permanently purged from the system solely by wasting PAO-rich Waste Activated Sludge (WAS).
   • Operational Warning: If P-rich WAS is allowed to sit in an anaerobic gravity thickener or non-aerated holding tank, the PAOs will experience anaerobic stress and re-release all stored orthophosphate back into the liquid phase, returning concentrated phosphorus to the plant headworks in the decant stream.

2. Common Biological Phosphorus Removal Process Configurations

• A/O (Anaerobic / Oxic): Mainstream anaerobic selector followed by aerobic basin. Used strictly for phosphorus removal where nitrification is not required.
• A2O (Anaerobic / Anoxic / Oxic): Incorporates an anoxic zone between the anaerobic and aerobic basins with internal nitrate recycle. Protects the anaerobic zone from nitrate contamination while achieving simultaneous nitrogen and phosphorus removal.
• University of Cape Town (UCT) Process: In high-nitrate systems, RAS contains substantial nitrate that would destroy anaerobic conditions if returned directly to the anaerobic selector. The UCT process routes RAS to the anoxic zone first to reduce nitrate, then recycles anoxic mixed liquor to the anaerobic selector.

3. Chemical Phosphorus Precipitation

When wastewater lacks sufficient influent VFAs for EBPR, or when permit limits require effluent Total Phosphorus $< 0.05-0.10 \text{ mg/L}$, chemical precipitation using trivalent metal salts is required:

• Precipitation Reactions:
  • Aluminum Sulfate (Alum):

$\text{Al}_2(\text{SO}_4)_3 \cdot 14\text{H}_2\text{O} + 2\text{PO}_4^{3-} \rightarrow 2\text{AlPO}_4\downarrow + 3\text{SO}_4^{2-} + 14\text{H}_2\text{O}$

• Ferric Chloride:

$\text{FeCl}_3 + \text{PO}_4^{3-} \rightarrow \text{FePO}_4\downarrow + 3\text{Cl}^-$

• Molar Dosing Requirements: Stoichiometrically, one mole of aluminum or iron precipitates one mole of orthophosphate ($1:1 \text{ molar ratio}$). However, because hydroxide ions ($\text{OH}^-$) compete aggressively for metal cations to form metal hydroxides ($\text{Al(OH)}_3$ or $\text{Fe(OH)}_3$), actual plant dosage requires a molar ratio of $1.5:1 \text{ to } 2.5:1 \text{ metal-to-phosphorus}$.
• Chemical Sludge Production: Coagulant addition generates substantial chemical precipitate, adding $4.0 \text{ to } 5.0 \text{ lbs of dry chemical sludge}$ for every pound of phosphorus removed. It also consumes alkalinity and depresses basin pH.