8.7 Biological Nutrient Removal & Tertiary Treatment

Biological Nitrogen Removal (BNR): Nitrification

Nitrification is the biological oxidation of ammonia-nitrogen ($\text{NH}_4^+-\text{N}$) to nitrite ($\text{NO}_2^--\text{N}$) and subsequently to nitrate ($\text{NO}_3^--\text{N}$). The process is carried out by obligate autotrophic nitrifying bacteria that derive energy from nitrogen oxidation and use inorganic carbon ($\text{CO}_2$ or bicarbonate) for cell synthesis.

Two-Step Biochemical Reactions

1. Step 1: Ammonia Oxidation to Nitrite by Ammonia-Oxidizing Bacteria (Nitrosomonas): $2\text{NH}_4^+ + 3\text{O}_2 \xrightarrow{\text{Nitrosomonas}} 2\text{NO}_2^- + 4\text{H}^+ + 2\text{H}_2\text{O}$
2. Step 2: Nitrite Oxidation to Nitrate by Nitrite-Oxidizing Bacteria (Nitrobacter / Nitrospira): $2\text{NO}_2^- + \text{O}_2 \xrightarrow{\text{Nitrobacter}} 2\text{NO}_3^-$
3. Combined Nitrification Reaction: $\text{NH}_4^+ + 2\text{O}_2 \rightarrow \text{NO}_3^- + 2\text{H}^+ + \text{H}_2\text{O}$

Stoichiometric Operating Requirements

• Oxygen Demand: Nitrification requires $4.57 \text{ lbs of O}_2$ per lb of $\text{NH}_4^+-\text{N}$ oxidized ($3.43 \text{ lbs}$ for Nitrosomonas $+ 1.14 \text{ lbs}$ for Nitrobacter).
• Alkalinity Consumption: Nitrification releases hydrogen ions ($H^+$), consuming $7.14 \text{ lbs of CaCO}_3 \text{ alkalinity}$ per lb of $\text{NH}_4^+-\text{N}$ oxidized. If wastewater buffer capacity is insufficient, basin pH drops below $6.5$, severely inhibiting nitrifying activity.
• Dissolved Oxygen Target: Maintain DO $> 2.0 \text{ mg/L}$ in aerobic basins. Nitrifiers have a low affinity for oxygen and are easily outcompeted by heterotrophs at low DO.
• Sludge Age (SRT / MCRT): Autotrophic nitrifiers grow slowly. Requires a minimum SRT $> 8 - 10 \text{ days}$ at $20^\circ\text{C}$, expanding to $> 15 - 20 \text{ days}$ at cold temperatures ($< 10^\circ\text{C}$).
• Optimal pH: Target $7.5 - 8.5$. Nitrification rates drop by $50%$ at $\text{pH } 7.0$ and cease below $\text{pH } 6.0$.

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Biological Nitrogen Removal (BNR): Denitrification

Denitrification is the biological reduction of nitrate ($\text{NO}_3^--\text{N}$) to inert nitrogen gas ($\text{N}_2$) by facultative heterotrophic bacteria (Pseudomonas, Paracoccus, Alcaligenes) under anoxic conditions (absence of dissolved oxygen, DO $< 0.2 \text{ mg/L}$, but presence of nitrate oxygen).

$\text{Denitrification Reaction: } 6\text{NO}_3^- + 5\text{CH}_3\text{OH} \text{ (Methanol)} \rightarrow 3\text{N}_2 \uparrow + 5\text{CO}_2 + 7\text{H}_2\text{O} + 6\text{OH}^-$

Key Stoichiometric & Process Benefits

• Alkalinity Recovery: Denitrification releases hydroxyl ions ($\text{OH}^-$), recovering $3.57 \text{ lbs of CaCO}_3 \text{ alkalinity}$ per lb of $\text{NO}_3^--\text{N}$ reduced (recovering exactly $50%$ of alkalinity consumed during nitrification!).
• Oxygen Credit: Heterotrophs utilize nitrate oxygen, saving equivalent to $2.86 \text{ lbs of O}_2$ per lb of $\text{NO}_3^--\text{N}$ reduced, lowering aeration blower energy requirements.
• Process Configuration (MLE Process): In the Modified Ludzack-Ettinger (MLE) configuration, primary effluent enters an un-aerated Anoxic Basin equipped with submerged mixers. An Internal Nitrate Recirculation (IMLR / NIT) pump recycles nitrified mixed liquor from the end of the Aerobic Basin back to the Anoxic Basin at rates of $200%$ to $400%$ of influent flow $Q$.

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Enhanced Biological Phosphorus Removal (EBPR)

EBPR relies on specialized bacteria known as Polyphosphate Accumulating Organisms (PAOs), such as Candidatus Accumulibacter phosphatis, to concentrate orthophosphate ($\text{PO}_4^{3-}$) inside cell biomass, which is subsequently wasted.

Operational Zones & Mechanisms

1. Anaerobic Zone (DO $< 0.1 \text{ mg/L}$, $\text{NO}_3^- < 0.2 \text{ mg/L}$):
   • PAOs break intracellular polyphosphate bonds to generate energy, releasing orthophosphate into the bulk liquid (P concentration in liquid increases).
   • PAOs use this energy to absorb Volatile Fatty Acids (VFAs) (acetate, propionate) from wastewater, storing them internally as Polyhydroxyalkanoates (PHAs).
2. Aerobic Zone:
   • PAOs oxidize stored internal PHAs using dissolved oxygen.
   • Generated energy powers cell growth and "luxury uptake" of orthophosphate from solution, storing excess P as intracellular polyphosphate granules. Cell phosphorus content rises from normal $1-2%$ up to $5-7%$ of dry cell weight.
   • Phosphorus is permanently removed from the system by wasting excess sludge (WAS) from the secondary clarifier.

Chemical Phosphorus Removal Backup

When biological P removal is insufficient, metal coagulants are added (alum, $\text{Al}_2(\text{SO}_4)_3$, or ferric chloride, $\text{FeCl}_3$) to precipitate soluble orthophosphate as insoluble metal phosphate solids:

$\text{Al}^{3+} + \text{PO}_4^{3-} \rightarrow \text{AlPO}_4 \downarrow$

$\text{Fe}^{3+} + \text{PO}_4^{3-} \rightarrow \text{FePO}_4 \downarrow$

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Tertiary Filtration & Effluent UV Disinfection

Tertiary Filtration Systems

Tertiary filters polish secondary effluent to meet ultra-low limits ($\text{TSS} < 5 \text{ mg/L}$, $\text{TP} < 0.5 \text{ mg/L}$).

• Cloth Media Disc Filters: Submerged pile-cloth discs with a nominal pore size of $5 - 10 \mu\text{m}$. Water flows outside-in through cloth media. Backwashing occurs via surface vacuum heads while discs rotate without interrupting forward treatment flow. Low operational headloss ($< 12 \text{ inches}$).
• Deep Bed Sand Filters: Mono-medium or dual-media (anthracite/sand) gravity filters. Backwashing is triggered by high headloss ($> 6 - 8 \text{ ft}$), high effluent turbidity ($> 2.0 \text{ NTU}$), or timer (24 hrs). Combined air scour and water backwash fluidize the bed to purge entrapped solids.

Effluent Ultraviolet (UV) Disinfection

UV disinfection is a physical process utilizing germicidal UV-C light at a peak wavelength of $254 \text{ nm}$.

• Germicidal Mechanism: UV light penetrates cell walls of pathogenic bacteria, viruses, and protozoa (Giardia, Cryptosporidium), dimerizing thymine bases in DNA/RNA. This prevents cellular replication and renders pathogens non-infectious without generating toxic disinfection byproducts (DBPs like trihalomethanes).
• UV Dose Equation: $\text{UV Dose } (\text{mJ/cm}^2) = \text{UV Intensity } (\text{mW/cm}^2) \times \text{Exposure Time } (t \text{ seconds})$
• Critical Operating Factors:
  • UV Transmittance (UVT): Percentage of $254 \text{ nm}$ light passing through $1 \text{ cm}$ of water. Target UVT $> 65%$. Low UVT (caused by iron, color, or TSS) attenuates UV light rapidly.
  • Quartz Sleeve Maintenance: Mineral scale (calcium, iron) coats quartz protective sleeves, reducing UV intensity. Sleeves require automated mechanical wipers and routine chemical cleaning with citric acid.
  • Solids Shielding: Pathogens enclosed within large TSS flocs are shielded from UV rays, making effective upstream tertiary filtration vital.

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Worked Calculation Example (BNR Alkalinity Balance)

Problem Statement: A wastewater facility processes $Q = 2.0 \text{ MGD}$ with an influent $\text{NH}_4^+-\text{N} = 30 \text{ mg/L}$ and effluent target $\text{NH}_4^+-\text{N} = 1.0 \text{ mg/L}$. Influent alkalinity is $220 \text{ mg/L as CaCO}_3$. The MLE denitrification system achieves $75%$ reduction of generated nitrate. Calculate the net effluent alkalinity.

Step 1: Calculate ammonia-nitrogen oxidized. $\Delta \text{NH}_4^+-\text{N} = 30 \text{ mg/L} - 1.0 \text{ mg/L} = 29 \text{ mg/L oxidized}$

Step 2: Calculate alkalinity consumed by Nitrification. $\text{Alkalinity Consumed} = 29 \text{ mg/L} \times 7.14 \text{ mg CaCO}_3/\text{mg NH}_4^+-\text{N} = 207.06 \text{ mg/L as CaCO}_3$

Step 3: Calculate alkalinity recovered by Denitrification ($75%$ of $29 \text{ mg/L} = 21.75 \text{ mg/L } \text{NO}_3^--\text{N}$ reduced). $\text{Alkalinity Recovered} = 21.75 \text{ mg/L} \times 3.57 \text{ mg CaCO}_3/\text{mg NO}_3^--\text{N} = 77.65 \text{ mg/L as CaCO}_3$

Step 4: Calculate net effluent alkalinity. $\text{Net Effluent Alkalinity} = 220 - 207.06 + 77.65 = 90.59 \text{ mg/L as CaCO}_3 \quad (\text{Sufficient residual buffering capacity})$