6.3 Biological Nutrient Removal: Nitrification, Denitrification & Phosphorus Removal

Biological Nutrient Removal (BNR) represents the pinnacle of modern municipal wastewater process engineering. Wastewater discharges containing elevated concentrations of nitrogen (ammonia, nitrate) and phosphorus stimulate rapid environmental degradation of surface receiving waters, requiring operators to master advanced biochemical transformations to meet stringent National Pollutant Discharge Elimination System (NPDES) permit limitations.

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1. Environmental & Human Health Impacts of Nitrogen and Phosphorus

1. Accelerated Eutrophication: In freshwater lakes, impoundments, and slow-moving rivers, phosphorus is typically the growth-limiting nutrient, whereas nitrogen is limiting in estuarine and marine coastal waters. Discharges containing excessive nutrients cause massive algal blooms. When these blooms die, heterotrophic bacteria consume dissolved oxygen during biomass decomposition, inducing widespread aquatic hypoxia ($DO < 2.0\;\text{mg/L}$), fish kills, and loss of biodiversity.

2. Un-Ionized Ammonia Toxicity to Aquatic Life: Total Ammonia Nitrogen ($TAN$) exists in aqueous solution as an equilibrium between ionized ammonium ($NH_4^+$) and un-ionized ammonia ($NH_3$):

$NH_4^+ \rightleftharpoons NH_3 + H^+ \quad (pK_a \approx 9.25 \text{ at } 25^\circ\text{C})$

While ionized ammonium ($NH_4^+$) is relatively non-toxic to aquatic fauna, un-ionized ammonia ($NH_3$) is a neutral, lipid-soluble gas molecule that readily diffuses across the gill membranes of fish. Once inside, it disrupts cellular osmoregulation, damages gill lamellae, and causes acute mortality at concentrations as low as 0.05 to 0.5 mg/L. High water pH (> 8.0) and elevated temperature shift the chemical equilibrium heavily toward toxic $NH_3$.

1. Methemoglobinemia ("Blue Baby Syndrome"): Nitrate ($NO_3^-$) discharged into surface waters that recharge drinking water aquifers poses a life-threatening risk to human infants under six months of age. In the infant digestive tract, neutral pH allows bacteria to reduce nitrate to nitrite ($NO_2^-$). Nitrite absorbs into the bloodstream and oxidizes the ferrous iron ($Fe^{2+}$) in hemoglobin to ferric iron ($Fe^{3+}$), forming methemoglobin. Methemoglobin cannot bind or transport oxygen, causing systemic asphyxiation (cyanosis or "blue baby syndrome"). The EPA Primary Maximum Contaminant Level (MCL) for drinking water is strictly 10 mg/L as $\text{NO}_3\text{-N}$ (or 45 mg/L as total nitrate).

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2. Biological Nitrification: Two-Step Autotrophic Oxidation

Nitrification is the biological conversion of reduced nitrogen (ammonia/ammonium) to oxidized nitrogen (nitrate). It is carried out by specialized, slow-growing, obligate aerobic autotrophic bacteria that utilize inorganic carbon ($CO_2$ or bicarbonate $HCO_3^-$) for cell synthesis.

The Two Biochemical Steps

• Step 1: Ammonia Oxidation to Nitrite: Carried out by Ammonia-Oxidizing Bacteria (AOB), predominantly belonging to the genus Nitrosomonas:

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

• Step 2: Nitrite Oxidation to Nitrate: Carried out by Nitrite-Oxidizing Bacteria (NOB), predominantly belonging to the genus Nitrobacter (and Nitrospira):

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

• Overall Combined Stoichiometric Reaction:

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

Stoichiometric Demands & Process Control Parameters

1. Theoretical Oxygen Demand (4.57 lb $O_2$ / lb N):
   • Step 1 (Ammonia to Nitrite): Consumes 3.43 lb $O_2$ per lb $NH_4\text{-N}$ oxidized.
   • Step 2 (Nitrite to Nitrate): Consumes 1.14 lb $O_2$ per lb $NO_2\text{-N}$ oxidized.
   • Total Theoretical Oxygen Required: $3.43 + 1.14 = \mathbf{4.57\;lb\;O_2\;/\;lb\;NH_4\text{-N}}$ oxidized. In full-scale activated sludge plants, actual oxygen demand is approximately 4.2 to 4.6 lb $O_2$/lb N due to a small amount of nitrogen incorporated into cell mass.
2. Alkalinity Consumption & pH Depression (7.14 lb Alkalinity / lb N):
   • Nitrification releases hydrogen ions ($H^+$), which directly neutralize the bicarbonate buffering capacity of wastewater:
   • 7.14 lb of Alkalinity (as $CaCO_3$) is destroyed for every 1.0 lb of $NH_4\text{-N}$ oxidized to nitrate.
   • If raw wastewater lacks sufficient natural alkalinity, aeration basin pH will plummet below 6.5. Nitrifiers are severely inhibited below pH 6.8 and cease functioning entirely below pH 6.0. Operators must maintain an aeration basin effluent alkalinity of at least 50 to 100 mg/L as $CaCO_3$ by dosing alkaline chemicals (hydrated lime $\text{Ca(OH)}_2$, sodium hydroxide $\text{NaOH}$, or sodium bicarbonate $\text{NaHCO}_3$).
3. Dissolved Oxygen Operating Range: Autotrophic nitrifiers have a much lower oxygen affinity than heterotrophic bacteria. Aeration basin DO must be maintained at ≥ 2.0 mg/L throughout the nitrification zone. If DO falls below 1.0 mg/L, nitrification efficiency collapses.
4. Temperature Kinetics: Nitrification rates are exquisitely sensitive to temperature ($Q_{10} \approx 2.0$). Optimal performance occurs between 20°C and 30°C. Below 15°C, nitrification slows noticeably; below 10°C, biological conversion drops by more than 50%.
5. Mean Cell Residence Time (MCRT / Sludge Age): Autotrophic nitrifiers have a very slow maximum specific growth rate ($\mu_{\max} \approx 0.3-0.5\;\text{day}^{-1}$) and a low cellular yield ($Y \approx 0.15\;\text{lb VSS/lb } NH_4\text{-N}$). If the system sludge wasting rate exceeds the bacterial growth rate, nitrifiers will be washed out of the plant. At 20°C, a minimum MCRT of 8 to 12 days is required; at winter water temperatures (< 12°C), operators must increase MCRT to 15 to 25+ days to prevent washout.

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3. Biological Denitrification: Anoxic Heterotrophic Reduction

Denitrification is the biological reduction of oxidized nitrogen (nitrate $NO_3^-$ and nitrite $NO_2^-$) into harmless, inert molecular nitrogen gas ($N_2$), which vents safely to the atmosphere.

Microbiology and Biochemical Requirements

• Facultative Heterotrophs: Denitrification is performed by ubiquitous heterotrophic bacteria (Pseudomonas, Paracoccus, Alcaligenes). Unlike nitrifiers, they do not require specialized cultivation.
• Anoxic Environment (Absence of Free Oxygen): Free dissolved oxygen inhibits the enzyme nitrate reductase. The bulk dissolved oxygen must be strictly maintained at $DO < 0.2\;\text{mg/L}$. Under anoxic conditions, heterotrophs use nitrate as their terminal electron acceptor.
• Carbon Source (Electron Donor): Heterotrophic denitrifiers require an organic carbon source. This can be readily biodegradable carbon present in raw influent wastewater ($BOD_5$) or supplemental external carbon (e.g., methanol, acetate, micro-C):

$6NO_3^- + 5CH_3OH \;(\text{Methanol}) \rightarrow 3N_2 \uparrow + 5CO_2 + 7H_2O + 6OH^-$

Stoichiometric Benefits of Denitrification

Denitrification provides two massive operational and economic benefits that offset the costs of nitrification:

1. Alkalinity Recovery (3.57 lb Alkalinity / lb N): Denitrification produces hydroxide ions ($OH^-$), recovering 3.57 lb of alkalinity (as $CaCO_3$) for every pound of $NO_3\text{-N}$ reduced to $N_2$. This restores exactly 50% of the alkalinity destroyed during nitrification.
2. Oxygen Credit (2.86 lb $O_2$ Equivalent / lb N): By utilizing the bound oxygen atoms in the nitrate molecule ($NO_3^-$) to oxidize organic carbon, denitrification recovers 2.86 lb of oxygen equivalent per lb of $NO_3\text{-N}$ reduced, reducing the required electrical aeration blower output by 15% to 25%.

Process Layouts: MLE & 4-Stage Bardenpho

• Modified Ludzack-Ettinger (MLE) Process: The industry-standard pre-anoxic configuration. Raw wastewater (providing carbon) and Return Activated Sludge (RAS) enter an anoxic basin (DO < 0.2 mg/L) equipped with mechanical submerged mixers. The flow then enters an aerobic basin where carbonaceous BOD is oxidized and ammonia is nitrified to nitrate. A high-rate Internal Mixed Liquor Recycle (IMLR) pump continuously pumps nitrified mixed liquor from the discharge end of the aeration tank back to the pre-anoxic tank at 200% to 400% of forward influent flow ($2Q$ to $4Q$).

$\text{Total Nitrogen (TN) Removal Efficiency} = \frac{R_{\text{IMLR}} + R_{\text{RAS}}}{1 + R_{\text{IMLR}} + R_{\text{RAS}}}$

• At an IMLR of 300% ($3Q$) and RAS of 100% ($1Q$), theoretical TN removal is $(3+1)/(1+3+1) = 80\%$.

• 4-Stage Bardenpho Process: Provides complete total nitrogen removal down to $< 3.0\;\text{mg/L}$. Features: (1) Primary pre-anoxic tank with IMLR recycle, (2) Primary aerobic nitrification tank, (3) Secondary post-anoxic tank where endogenous respiration strips residual nitrate, and (4) Final re-aeration tank (10 to 15 min detention) that raises effluent DO and strips entrained nitrogen gas bubbles, preventing floating sludge blankets in secondary clarifiers.

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4. Enhanced Biological Phosphorus Removal (EBPR) & Chemical Precipitation

The EBPR Microbial Engine: Polyphosphate Accumulating Organisms (PAOs)

EBPR relies on the selective enrichment of specialized heterotrophs called Polyphosphate Accumulating Organisms (PAOs), predominantly Candidatus Accumulibacter phosphatis.

• Permanent Phosphorus Removal: Phosphorus is never converted into a gas. It is removed from the treatment system strictly by wasting activated sludge (WAS) from the aerobic basin while the PAOs are gorged with luxury polyphosphate. Normal activated sludge contains 1.5% to 2.0% phosphorus by dry weight; EBPR sludge contains 5.0% to 8.0% phosphorus by dry weight.

Critical Operating Safeguards for EBPR

1. Nitrate Intrusion Hazard: If the Return Activated Sludge (RAS) carries elevated nitrate ($NO_3^-$) into the upfront anaerobic zone, denitrifiers will consume the available VFAs before PAOs can absorb them. This starves the PAOs and causes complete failure of biological phosphorus removal.
2. Anaerobic Digester Recycle Danger: When EBPR waste sludge is sent to an anaerobic digester, the strict anaerobic environment causes PAOs to release all their accumulated polyphosphate back into solution. Dewatering centrate/filtrate will contain astronomical phosphorus and ammonia concentrations, forming severe struvite ($MgNH_4PO_4 \cdot 6H_2O$) scale on piping and overwhelming the headworks if recycled without chemical precipitation.

Chemical Phosphorus Precipitation

When biological removal alone cannot achieve ultra-low effluent limits ($< 0.1\;\text{mg/L}$ total P), metal coagulants are added directly into primary clarifiers, aeration basins, or tertiary filters:

• Aluminum Sulfate (Alum): $Al^{3+} + PO_4^{3-} \rightarrow AlPO_4 \downarrow$
• Ferric Chloride: $Fe^{3+} + PO_4^{3-} \rightarrow FePO_4 \downarrow$
• Dosing Realities: While the stoichiometric molar ratio is 1:1, real-world chemical dosing requires 1.5 to 2.5 moles of metal ion per mole of phosphorus due to competing reactions with wastewater alkalinity. Chemical precipitation consumes alkalinity and increases total dry plant sludge production by 20% to 40%.