8.2 Biological Phosphorus Removal (EBPR) & Chemical Precipitation

Phosphorus Forms & Environmental Regulations

Phosphorus is a primary limiting nutrient in freshwater aquatic ecosystems. Uncontrolled discharge of municipal wastewater containing phosphorus triggers accelerated cultural eutrophication, severe cyanobacteria blooms, toxin production (microcystin), and widespread hypolimnetic dissolved oxygen depletion.

┌────────────────────────────────────────────────────────────────────────┐
│                    Phosphorus Fractions in Wastewater                  │
├────────────────────────────┬───────────────────────────────────────────┤
│ Orthophosphate (PO43-)     │ Soluble, chemically reactive inorganic P  │
├────────────────────────────┼───────────────────────────────────────────┤
│ Polyphosphates             │ Condensed inorganic polymers (detergents) │
├────────────────────────────┼───────────────────────────────────────────┤
│ Organic Phosphorus         │ Bound in cellular DNA, RNA, ATP, proteins │
├────────────────────────────┼───────────────────────────────────────────┤
│ Total Phosphorus (TP)      │ Sum of all soluble and particulate forms  │
└────────────────────────────┴───────────────────────────────────────────┘

• Typical Concentrations: Raw municipal wastewater contains $4\text{ to }12\text{ mg/L of Total Phosphorus (TP)}$, consisting of approximately 50% to 70% soluble orthophosphate, 20% to 30% polyphosphates, and 10% to 20% organic phosphorus.
• Discharge Standards: While conventional secondary treatment removes only 10% to 25% of influent phosphorus through normal cell synthesis (heterotrophic biomass typically contains $1.5%\text{ to }2.0%$ phosphorus on a dry weight basis), modern water quality permits in sensitive California watersheds mandate Total Phosphorus limits ranging from $0.1\text{ to }1.0\text{ mg/L}$ (or $<0.05\text{ mg/L}$ for direct reuse and lake outfalls).

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The Biochemistry of Enhanced Biological Phosphorus Removal (EBPR)

Enhanced Biological Phosphorus Removal (EBPR) relies on the selective cultivation of specialized bacterial populations known as Phosphorus Accumulating Organisms (PAOs), with the principal identified genus being Candidatus Accumulibacter phosphatis (along with Tetrasphaera). EBPR relies on cycling mixed liquor through sequential anaerobic and aerobic zones.

                  EBPR BIOCHEMICAL CYCLE (ACCUMULIBACTER)

       ANAEROBIC ZONE (No DO, No NO3-)              AEROBIC ZONE (DO >= 2.0 mg/L)
┌───────────────────────────────────────────┐ ┌───────────────────────────────────────────┐
│  1. Volatile Fatty Acids (VFAs, Acetate)  │ │  1. Stored PHAs / PHB Oxidized with O2    │
│     Assimilated & Stored as Intracellular │ │     Generating ATP Energy for Growth    │
│     Polyhydroxyalkanoates (PHAs / PHB)    │ │                                         │
│                                           │ │  2. Massive Orthophosphate (PO43-)      │
│  2. High-Energy Polyphosphate Bonds       │ │     Taken Up from Liquid (Luxury Uptake)│
│     Hydrolyzed to Provide Energy          │ │     Rebuilding Polyphosphate Granules   │
│                                           │ │                                         │
│  3. Orthophosphate (PO43-) Excreted       │ │  3. Mixed Liquor P Content Reaches      │
│     into Bulk Liquid (P-Release Phase)    │ │     4% - 8% Dry Weight (Wasted via WAS) │
└───────────────────────────────────────────┘ └───────────────────────────────────────────┘

1. Anaerobic Selector Zone Dynamics (P-Release Phase)

For EBPR to occur, the mixed liquor must first pass through a strictly anaerobic zone characterized by the complete absence of dissolved oxygen ($\text{DO} = 0.0\text{ mg/L}$) and the complete absence of oxidized nitrogen ($\text{NO}_3^- = 0.0\text{ mg/L}$, $\text{NO}_2^- = 0.0\text{ mg/L}$).

• VFA Uptake & PHA Storage: Under anaerobic conditions, strict aerobic heterotrophs are inactive. However, PAOs possess specialized transport enzymes that allow them to rapidly take up soluble Volatile Fatty Acids (VFAs)—primarily acetic acid ($CH_3COOH$) and propionic acid ($CH_3CH_2COOH$)—from the raw wastewater. Inside the PAO cells, these VFAs are synthesized and stored as insoluble intracellular carbon reserve polymers known as Polyhydroxyalkanoates (PHAs), predominantly Poly-$\beta$-hydroxybutyrate (PHB).
• Polyphosphate Hydrolysis & P-Release: Synthesizing and polymerizing PHAs under anaerobic conditions requires biochemical energy (ATP). PAOs generate this energy by hydrolyzing their internal storage chains of high-energy polyphosphate ($poly\text{-}P$). As the polyphosphate chains are cleaved, soluble orthophosphate ($PO_4^{3-}$) and associated stabilizing cations (magnesium, $Mg^{2+}$, and potassium, $K^+$) are excreted across the cell membrane into the liquid.
• Observed Indicator: In a healthy anaerobic selector, bulk liquid soluble orthophosphate concentrations surge dramatically (P-release), often rising from an influent 6 mg/L to $20\text{ to }40+\text{ mg/L}$ in the anaerobic basin.

2. Aerobic Zone Dynamics (Luxury P-Uptake Phase)

When the mixed liquor exits the anaerobic zone and enters the aerobic aeration basin (where dissolved oxygen is maintained at $\ge 2.0\text{ mg/L}$):

• PHA Oxidation & Energy Generation: PAOs metabolize their stored intracellular PHAs/PHB via aerobic respiration, using oxygen as the terminal electron acceptor to generate vast quantities of ATP and synthesize glycogen.
• Luxury Orthophosphate Uptake: Powered by excess ATP, PAOs actively transport dissolved orthophosphate from the bulk liquid across their cell membranes, linking the molecules into high-energy polyphosphate storage chains within their cytoplasm. This hyper-accumulation is termed luxury uptake.
• Biomass Phosphorus Enrichment: PAOs draw bulk liquid orthophosphate down to ultra-low levels ($<0.1\text{ to }0.5\text{ mg/L}$). The resulting biological sludge contains $4.0%\text{ to }8.0%$ phosphorus by dry weight, compared to only $1.5%\text{ to }2.0%$ in conventional activated sludge.

[!IMPORTANT] The Waste Activated Sludge (WAS) Mandate: EBPR does not convert phosphorus into a gas; phosphorus is simply concentrated inside the PAO cells. Phosphorus is permanently removed from the wastewater facility solely by wasting biomass (WAS) from the aerobic zone or secondary clarifier underflow. If biological sludge is allowed to sit stagnant under anoxic/anaerobic conditions (e.g., deep clarifier sludge blankets or gravity thickeners), PAOs will release orthophosphate back into the liquid (secondary P-release), contaminating the plant effluent.

Volatile Fatty Acids (VFAs), Sludge Fermentation & GAO Competition

1. The VFA Substrate Requirement

PAO metabolism is strictly dependent on the availability of short-chain volatile fatty acids (VFAs). Readily Biodegradable COD (rbCOD) present in raw sewage is fermented into acetate and propionate by facultative bacteria in the collection system or anaerobic selector.

• Operational Ratios for Successful EBPR:
  • $\text{rbCOD} / \text{Total Phosphorus (TP)} \ge 20\text{ to }25\text{ mg rbCOD / mg TP removed}$
  • $\text{VFA (as acetic acid)} / \text{TP} \ge 7\text{ to }10\text{ mg VFA / mg TP removed}$
  • $\text{BOD}_5 / \text{TP} \ge 25\text{ to }30\text{ mg BOD}_5 / \text{ mg TP}$

2. Primary Sludge Fermenters

When raw wastewater is dilute or deficient in organic carbon (common in California during wet weather infiltration/inflow), facilities operate on-site primary sludge fermenters (complete-mix fermenters with thickener elutriation or static gravity fermenters):

• Primary sludge is retained anaerobically for an MCRT of 1 to 3 days at mesophilic temperatures.
• Acidogenic bacteria convert complex settled primary solids into a concentrated, VFA-rich supernatant (acetic and propionic acid concentrations of $1,000\text{ to }3,000\text{ mg/L}$), which is metered directly into the anaerobic selector basin.

3. Nitrate Intrusion & GAO Competition

Two major biological mechanisms cause EBPR failure:

1. Nitrate Intrusion into the Anaerobic Selector: If nitrate ($NO_3^-$) enters the anaerobic zone via the Return Activated Sludge (RAS) stream or internal recycles, facultative denitrifying heterotrophs consume the available VFAs for denitrification. Denitrifiers metabolize VFAs significantly faster than PAOs can assimilate them anaerobically, starving the PAOs and halting luxury uptake downstream.
2. Glycogen Accumulating Organisms (GAOs): GAOs (principally Candidatus Competibacter and Defluviicoccus) compete directly with PAOs for VFAs in the anaerobic zone. GAOs assimilate VFAs using stored glycogen energy without accumulating or releasing polyphosphate. GAOs thrive over PAOs at higher temperatures ($>25^\circ\text{C}$) and lower pH ($<7.0$). Operators maintain EBPR dominance by operating at slightly higher pH (7.2–7.6) and shorter anaerobic retention times.

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EBPR Process Configurations

                      1. A/O PROCESS (PHOREDOX - PHOSPHORUS ONLY)

Influent ────► [ ANAEROBIC ZONE ] ────► [ AEROBIC ZONE ] ────► [ SECONDARY CLARIFIER ] ──► Effluent
                     ▲                         │                        │
                     │                         │                        │
                     └────────────── RAS ──────┴────────────────────────┘


                      2. A2/O PROCESS (COMBINED NITROGEN & PHOSPHORUS)

          ┌────────────────── Internal Nitrate Recycle (INR 200 - 400% Q) ─────────────────┐
          │                                                                                │
          ▼                                                                                │
Influent ──► [ ANAEROBIC ] ──► [ ANOXIC ] ──► [ AEROBIC ] ──────────────► [ CLARIFIER ] ──► Effluent
               (P-Release)      (Denitrification)  (Nitrification/Uptake)       │
                   ▲                   ▲                                        │
                   │                   └────────────── RAS (NO3- present) ──────┘
                   └────────────────────────────────── (Nitrate can slip to Anaerobic)


                      3. UNIVERSITY OF CAPE TOWN (UCT) PROCESS

          ┌────────────────── Internal Nitrate Recycle (INR 200 - 400% Q) ─────────────────┐
          │                                                                                │
          ▼                                                                                │
Influent ──► [ ANAEROBIC ] ──► [ ANOXIC ] ──► [ AEROBIC ] ──────────────► [ CLARIFIER ] ──► Effluent
                   ▲               │               (Nitrification/Uptake)       │
                   │               │                                            │
                   │ (100-200% Q)  │ (Fully Denitrified Recycle)                │
                   └── Anox-to-Ana ┘                                            │
                                   ▲                                            │
                                   └────────────────── RAS ─────────────────────┘
                                                   (Recycled to ANOXIC Zone)

1. A/O Process (Anaerobic / Oxic)

• Layout: An anaerobic selector basin (HRT 0.5 to 1.5 hours) followed directly by an aerobic aeration basin (HRT 3 to 6 hours). Return Activated Sludge (RAS) is recycled to the anaerobic tank.
• Application: Used strictly for phosphorus removal where biological nitrification is not required or permitted. If nitrification occurs, nitrate in the RAS enters the anaerobic zone, deteriorating P-removal.

2. A2/O Process (Anaerobic / Anoxic / Oxic)

• Layout: Adds an intermediate anoxic basin between the anaerobic and aerobic zones, combined with an Internal Nitrate Recycle (INR) from the aerobic to the anoxic tank.
• Advantage: Achieves simultaneous nitrogen and phosphorus removal. Nitrate produced in the aerobic zone is denitrified in the anoxic basin, reducing the nitrate concentration in the RAS stream before it returns to the anaerobic selector.
• Limitation: In low COD:TKN wastewaters, incomplete anoxic denitrification leaves residual nitrate in the RAS, which enters the anaerobic zone and impairs EBPR.

3. University of Cape Town (UCT) Process

• Layout: Designed specifically to protect the anaerobic selector from nitrate contamination. The RAS stream is returned directly to the anoxic basin (not the anaerobic basin). A separate Anoxic-to-Anaerobic recycle stream (100% to 200% Q) pumps fully de-nitrified mixed liquor from the anoxic zone back into the anaerobic selector.
• Modified UCT: Splits the anoxic zone into two sequential stages. RAS is returned to Anoxic Basin 1 (which receives only RAS and achieves 100% complete denitrification), and mixed liquor from Anoxic Basin 1 is recycled to the anaerobic zone, guaranteeing zero nitrate entry.

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Chemical Phosphorus Precipitation Chemistry

Chemical precipitation involves the addition of trivalent multivalent metal salts to react with soluble orthophosphate, forming insoluble metal phosphate precipitates that are separated via sedimentation or filtration.

┌────────────────────────────────────────────────────────────────────────┐
│                     Coagulant Precipitation Reactions                  │
├────────────────────────────────────────────────────────────────────────┤
│ Aluminum Sulfate (Alum):                                               │
│   Al3+ + PO43- ──► AlPO4 (solid precipitate)                           │
│   (Stoichiometric Molar Ratio: 1.0 Al : 1.0 P  |  0.87 lb Al / lb P)   │
├────────────────────────────────────────────────────────────────────────┤
│ Ferric Chloride:                                                       │
│   Fe3+ + PO43- ──► FePO4 (solid precipitate)                           │
│   (Stoichiometric Molar Ratio: 1.0 Fe : 1.0 P  |  1.80 lb Fe / lb P)   │
└────────────────────────────────────────────────────────────────────────┘

1. Stoichiometric vs. Practical Chemical Dosages

While the theoretical precipitation reaction requires a 1:1 molar ratio of metal to phosphorus, practical chemical dosing in municipal wastewater requires $1.5:1\text{ to }2.5:1\text{ molar ratio}$ (or higher for low effluent limits $<0.1\text{ mg/L}$).

• Competing Hydroxide Reactions: Practical excess dosage is necessary because metal cations react competitively with wastewater water molecules and hydroxide ($OH^-$), forming metal hydroxides:

$\text{Fe}^{3+} + 3\text{H}_2\text{O} \rightarrow \text{Fe(OH)}_3 \downarrow + 3\text{H}^+$ $\text{Al}^{3+} + 3\text{H}_2\text{O} \rightarrow \text{Al(OH)}_3 \downarrow + 3\text{H}^+$

• These amorphous metal hydroxide flocs provide valuable sweep-floc enmeshment of fine suspended particles and colloidal phosphorus.

2. Chemical Feed Locations & Operational Trade-offs

3. Impact on Alkalinity and Sludge Production

• Alkalinity Destruction:
  • Dosing Alum consumes $5.54\text{ lbs of alkalinity as }CaCO_3$ per $1.0\text{ lb of }Al^{3+}$ dosed.
  • Dosing Ferric Chloride consumes $2.69\text{ lbs of alkalinity as }CaCO_3$ per $1.0\text{ lb of }Fe^{3+}$ dosed.
• Sludge Mass Generation: Chemical phosphorus precipitation produces 20% to 40% additional dry solids sludge mass consisting of dense metal phosphates ($FePO_4, AlPO_4$) and unreacted metal hydroxides ($Fe(OH)_3, Al(OH)_3$). Chemical sludges increase overall sludge specific gravity, improve thickening density, but increase polymer demand during mechanical dewatering.