3.4 Biological & Chemical Nutrient Removal (BNR/ENR)

3.4 Biological & Chemical Nutrient Removal (BNR/ENR)

Eutrophication & The Chesapeake Bay Regulatory Framework

Excessive discharges of nitrogen (N) and phosphorus (P) into aquatic environments accelerate cultural eutrophication. In estuaries and coastal waters such as the Chesapeake Bay, nitrogen is typically the primary limiting nutrient, while phosphorus is the limiting nutrient in freshwater tributaries and rivers. Nutrient over-enrichment triggers massive cyanobacterial and dinoflagellate algal blooms. When these algal populations die, heterotrophic decomposition consumes dissolved oxygen, creating vast hypoxic (<2.0 mg/L DO) and anoxic dead zones that decimate benthic habitats, oysters, blue crabs, and finfish.

Virginia DEQ Watershed Regulations

To restore water quality under the federal Clean Water Act and the multi-state Chesapeake Bay Watershed Implementation Plan (WIP), the Virginia Department of Environmental Quality (DEQ) enforces stringent Enhanced Nutrient Removal (ENR) standards through the Virginia Pollutant Discharge Elimination System (VPDES) Watershed General Permit for Nutrient Discharges (9 VAC 25-820):

• Standard Biological Nutrient Removal (BNR) Target: $\text{TN} \le 8.0\text{ mg/L}$, $\text{TP} \le 1.0\text{ mg/L}$
• Enhanced Nutrient Removal (ENR) Target: $\text{Total Nitrogen (TN)} \le 3.0 - 4.0\text{ mg/L}$, $\text{Total Phosphorus (TP)} \le 0.30\text{ mg/L}$

Meeting these limits requires continuous optimization of multi-stage biological reactors and chemical precipitation systems.

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Biological Nitrogen Removal (BNR)

Biological nitrogen removal is a two-step biochemical sequence: aerobic autotrophic nitrification followed by anoxic heterotrophic denitrification.

                                  NITRIFICATION (Aerobic Zone)
                       ┌──────────────────────────────────────────────────┐
Raw Ammonia (NH3/NH4+) │  Nitrosomonas:  2 NH4+ + 3 O2 ➔ 2 NO2- + 4 H+     │ Nitrate
─────────────────────► │  Nitrobacter:   2 NO2- + O2 ➔ 2 NO3-             │ (NO3-)
                       │  Consumes: 7.14 mg Alk & 4.57 mg O2 per mg N     │ ───┐
                       └──────────────────────────────────────────────────┘    │
                                                                               │
                                  DENITRIFICATION (Anoxic Zone)                │
                       ┌──────────────────────────────────────────────────┐    │
 Nitrogen Gas (N2 ^)   │  Heterotrophs + Carbon (BOD):                    │ ◄──┘
 (Discharged to Atmos) │  NO3- ➔ NO2- ➔ NO ➔ N2O ➔ N2 (gas)               │ (Internal Recycle
 ◄──────────────────── │  Recovers: 3.57 mg Alk per mg NO3-N reduced       │  NRCY: 200–400%)
                       └──────────────────────────────────────────────────┘

Step 1: Two-Stage Aerobic Nitrification

Nitrification is performed by specialized, slow-growing obligate autotrophic bacteria that derive carbon from inorganic alkalinity ($\text{HCO}_3^-$) rather than organic carbon:

1. Ammonia-Oxidizing Bacteria (Nitrosomonas): $2\text{NH}_4^+ + 3\text{O}_2 \xrightarrow{\text{Nitrosomonas}} 2\text{NO}_2^- + 4\text{H}^+ + 2\text{H}_2\text{O} + \text{Energy}$
2. Nitrite-Oxidizing Bacteria (Nitrobacter): $2\text{NO}_2^- + \text{O}_2 \xrightarrow{\text{Nitrobacter}} 2\text{NO}_3^- + \text{Energy}$
3. Overall Combined Nitrification Reaction: $\text{NH}_4^+ + 2\text{O}_2 + 2\text{HCO}_3^- \to \text{NO}_3^- + 2\text{CO}_2 + 3\text{H}_2\text{O} + \text{Biomass}$

Critical Stoichiometry & Operating Conditions

• Oxygen Demand: Nitrification requires 4.57 mg $\text{O}_2$ for every 1.0 mg of $\text{NH}_3\text{-N}$ oxidized to nitrate (3.43 mg for Nitrosomonas + 1.14 mg for Nitrobacter).
• Alkalinity Consumption: Nitrification produces hydrogen ions ($4\text{H}^+$), destroying 7.14 mg of alkalinity (as $\text{CaCO}_3$) per mg of $\text{NH}_3\text{-N}$ oxidized. If influent alkalinity is insufficient, mixed liquor pH plummets.
• pH Range: Optimal nitrification occurs at pH 7.5 to 8.5. Nitrification rates decline sharply below pH 7.0 and cease below pH 6.0. Facilities with soft water must feed supplemental alkalinity (sodium hydroxide $\text{NaOH}$, hydrated lime $\text{Ca(OH)}_2$, or sodium bicarbonate $\text{NaHCO}_3$).
• Temperature Sensitivity: Nitrifiers are highly temperature-sensitive. Microbial growth rates drop by 50% for every 10°C drop in wastewater temperature. In winter (<12°C), operators must increase MCRT to 12 to 20+ days to prevent washout.
• Dissolved Oxygen: Basin DO must be maintained at $\ge 2.0\text{ mg/L}$ throughout the nitrifying zone.

Step 2: Anoxic Heterotrophic Denitrification

Denitrification is performed by common facultative heterotrophic bacteria (Pseudomonas, Bacillus, Paracoccus) in an anoxic environment (dissolved oxygen absent $<0.2\text{ mg/L}$, but chemically bound nitrate oxygen present):

$\text{NO}_3^- \to \text{NO}_2^- \to \text{NO} \uparrow \to \text{N}_2\text{O} \uparrow \to \text{N}_2 \uparrow \text{ (Inert Nitrogen Gas)}$

$\text{NO}_3^- + 0.35\text{ BOD (Methanol)} + \text{H}^+ \to 0.5\text{ N}_2 \uparrow + \text{CO}_2 + 1.2\text{ H}_2\text{O} + \text{Alkalinity}$

Operational Significance of Denitrification

• Alkalinity Production: Denitrification produces hydroxyl ions, restoring 3.57 mg of alkalinity (as $\text{CaCO}_3$) for every 1.0 mg of $\text{NO}_3\text{-N}$ reduced to $\text{N}_2$ gas—recovering exactly 50% of the alkalinity lost during nitrification.
• Carbon (BOD) Requirement: Heterotrophic denitrifiers require a source of readily biodegradable organic carbon (RBCOD). If raw influent BOD is insufficient (BOD:TKN ratio < 4:1), plants must dose external supplemental carbon such as methanol ($\text{CH}_3\text{OH}$), glycerol (MicroC), or acetic acid ($\text{CH}_3\text{COOH}$).

BNR Reactor Configurations

Modified Ludzack-Ettinger (MLE) Process Flow Diagram:

                 ┌────────────────────────────────────────────────────────┐
                 │   Internal Nitrate Recycle (NRCY: 200%–400% of Q)      │
                 ▼                                                        │
 Influent (Q) ┌──────────────┐     ┌──────────────┐     ┌─────────────┐   │ Clarified Effluent
════════════► │ Anoxic Basin │ ──► │ Aerobic Tank │ ──► │  Secondary  │ ──┴────────────────►
              │ (Denitrify)  │     │ (Nitrify)    │     │  Clarifier  │ (TN: 6–8 mg/L)
              └──────────────┘     └──────────────┘     └──────┬──────┘
                     ▲                                         │
                     │                                         │
                     └──────── Return Activated Sludge (RAS) ──┘

1. Modified Ludzack-Ettinger (MLE) Process

The MLE configuration places an anoxic basin upstream of the aerobic aeration tank:

• Mechanism: Raw influent enters the anoxic zone, supplying readily available carbon (BOD) to fuel denitrification. High-nitrate mixed liquor from the aerobic zone discharge is pumped back to the anoxic zone via a dedicated high-capacity pump line known as the Internal Nitrate Recycle (NRCY / IMLR) at a rate of 200% to 400% of forward influent flow ($Q$).
• Effluent Capability: Achieves effluent Total Nitrogen of 6.0 to 8.0 mg/L (limited by the physical fraction of nitrate recycled versus that escaping to the secondary clarifier).

2. Four-Stage Bardenpho Process

To achieve Enhanced Nutrient Removal (ENR) limits ($ ext{TN} < 3.0 ext{ mg/L}$), the 4-Stage Bardenpho process utilizes sequential anoxic and aerobic stages:

1. Primary Anoxic Zone: Receives raw influent BOD and 400% NRCY recycle; removes 70–80% of nitrate.
2. Primary Aerobic Zone: Completes carbonaceous BOD removal and full autotrophic nitrification.
3. Secondary Anoxic Zone: Microorganisms denitrify remaining nitrate via endogenous decay (without influent BOD) or via supplemental methanol dosing.
4. Re-Aeration Zone (Post-Aerobic): Short 15–30 minute aeration period to raise DO to 4–6 mg/L, purge entrained $ ext{N}_2$ gas bubbles (preventing clarifier sludge rising), and oxidize any residual carbon.

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Phosphorus Removal Mechanisms

Phosphorus is removed through either Enhanced Biological Phosphorus Removal (EBPR) or chemical precipitation.

Enhanced Biological Phosphorus Removal (EBPR) Cycle:

ANAEROBIC ZONE (DO = 0, NO3 = 0)              AEROBIC ZONE (DO > 2.0 mg/L)
┌──────────────────────────────────────┐     ┌──────────────────────────────────────┐
│ PAOs consume Volatile Fatty Acids    │     │ PAOs oxidize stored PHAs for energy; │
│ (VFAs / Acetate) and store as PHAs;  │ ──► │ Take up massive amounts of Ortho-P   │
│ Intracellular Poly-P bonds break:    │     │ ("LUXURY UPTAKE") into new cells;    │
│ PHOSPHORUS RELEASED INTO SOLUTION    │     │ P REMOVED BY WASTING SLUDGE (WAS)    │
└──────────────────────────────────────┘     └──────────────────────────────────────┘

1. Enhanced Biological Phosphorus Removal (EBPR)

EBPR relies on specialized bacteria known as Polyphosphate Accumulating Organisms (PAOs) (e.g., Candidatus Accumulibacter phosphatis):

• Anaerobic Selector Phase (True Anaerobic: $\text{DO} = 0$, $\text{NO}_3 = 0$): In the complete absence of dissolved oxygen and nitrate, PAOs break internal polyphosphate bonds to generate ATP energy. They use this energy to absorb Volatile Fatty Acids (VFAs) (such as acetic and propionic acids) from raw wastewater, storing them internally as polyhydroxyalkanoates (PHAs). In doing so, PAOs release orthophosphate into the water, causing soluble phosphorus concentrations in the anaerobic zone to spike to 20–40 mg/L.
• Aerobic Zone (Luxury Uptake): When PAOs enter the aerobic aeration zone, they metabolize their stored internal PHAs, generating large amounts of energy. They take up orthophosphate from the water at levels far exceeding their normal metabolic needs (luxury phosphorus uptake), accumulating polyphosphate chains inside their cells up to 5–15% of dry cell weight (compared to 1.5–2.0% in standard bacteria).
• Permanent Removal: Phosphorus is permanently removed from the facility by wasting PAO-rich sludge (WAS) directly from the aerobic zone. If WAS sits in an un-aerated clarifier or storage tank, the PAOs will experience anaerobic stress and re-release phosphorus back into the liquid stream.
• Critical EBPR Pitfall: If dissolved oxygen or nitrate is carried back into the anaerobic zone via RAS, denitrifying bacteria will consume the available VFAs before PAOs can store them, collapsing biological phosphorus removal.

2. Chemical Phosphorus Removal

Chemical removal relies on trivalent metal salt coagulants to precipitate soluble orthophosphate ($\text{PO}_4^{3-}$) into insoluble metal phosphate flocs:

$\text{Al}^{3+} + \text{PO}_4^{3-} \to \text{AlPO}_4 \downarrow \quad \text{(Aluminum Phosphate)}$ $\text{Fe}^{3+} + \text{PO}_4^{3-} \to \text{FePO}_4 \downarrow \quad \text{(Ferric Phosphate)}$

• Feed Locations: Metal salts can be fed at the headworks (co-precipitation in primary clarifiers), at the aeration basin effluent (simultaneous precipitation in secondary clarifiers), or ahead of tertiary sand/cloth filters (tertiary post-precipitation) to polish effluent Total Phosphorus down to $<0.10\text{ mg/L}$.