6.2 Biological & Chemical Phosphorus Removal (EBPR & Coagulant Precipitation)

Phosphorus Removal Processes: Biological Mechanisms & Chemical Coagulation

Phosphorus is the primary limiting nutrient controlling algal blooms and freshwater eutrophication across Colorado's pristine rivers, reservoirs, and mountain watersheds. Excess phosphorus stimulates massive blooms of cyanobacteria, which deplete nocturnal dissolved oxygen, generate cyanotoxins (microcystins), produce taste and odor compounds (geosmin and MIB), and disrupt aquatic life.

Under Colorado Regulation 85 (Nutrients Management Control Regulation / 5 CCR 1002-85), domestic wastewater treatment facilities with design capacities $\ge 1.0\text{ MGD}$ (or equivalent industrial dischargers) must comply with technology-based effluent limitations, typically establishing a running annual median of $1.0\text{ mg/L Total Phosphorus (TP)}$ and $15.0\text{ mg/L Total Nitrogen (TN)}$. Furthermore, water quality-based effluent limits promulgated under Regulation 31 (The Basic Standards and Methodologies for Surface Water / 5 CCR 1002-31) establish stream-specific phosphorus standards ranging from $0.02\text{ to } 0.10\text{ mg/L Total Phosphorus}$ for sensitive lakes and reservoir catchments.

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1. Principles of Enhanced Biological Phosphorus Removal (EBPR)

Enhanced Biological Phosphorus Removal (EBPR) harnesses specialized bacteria termed Phosphorus Accumulating Organisms (PAOs) (most notably Candidatus Accumulibacter phosphatis and Tetrasphaera). In a standard conventional activated sludge process, biomass contains approximately $1.5%–2.0%$ phosphorus by dry weight (used for cell membranes and nucleic acids). Under the cyclic environmental stress of alternating anaerobic and aerobic zones, PAOs outcompete ordinary heterotrophs and accumulate $4.0%–8.0%$ phosphorus by dry weight inside their cells as inorganic polyphosphate granules.

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|                         EBPR DUAL-PHASE BIOCHEMICAL CYCLE                               |
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| 1. ANAEROBIC ZONE (DO = 0.0 mg/L, NO3-N < 0.1 mg/L, ORP < -150 mV)                      |
|    • Substrate Uptake: PAOs absorb Volatile Fatty Acids (VFAs, e.g., acetate/propionate)|
|    • Internal Storage: VFAs converted into Polyhydroxyalkanoates (PHAs / PHB)           |
|    • Energy Source:    High-energy intracellular Poly-P bonds are broken (hydrolyzed)   |
|    • Solution Change:  Orthophosphate (PO4(3-)) is RELEASED into liquid (P-Release)     |
|                                                                                         |
| 2. AEROBIC ZONE (DO >= 2.0 mg/L, ORP > +100 mV)                                         |
|    • Metabolism:       PAOs oxidize stored intracellular PHAs for growth and energy     |
|    • Luxury Uptake:    Excess orthophosphate absorbed from liquid into cells (P-Uptake) |
|    • Internal Storage: Rebuilds massive intracellular Poly-P volutin granules           |
|    • Permanent Removal:Wasting phosphorus-rich sludge (WAS) containing 4-8% dry weight P|
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The Anaerobic Phase Mechanics

To establish true anaerobic conditions, the selector zone must be strictly depleted of both dissolved oxygen ($DO = 0.0\text{ mg/L}$) and oxidized nitrogen ($NO_3\text{-}N < 0.1\text{ mg/L}$), driving the Oxidation-Reduction Potential (ORP) down to $-150\text{ to } -250\text{ mV}$:

1. VFA Uptake & Fermentation: Fermentative bacteria convert complex soluble organic matter in raw sewage into short-chain Volatile Fatty Acids (VFAs), predominantly acetic acid ($CH_3COOH$) and propionic acid ($CH_3CH_2COOH$).
2. Energy Coupling & Poly-P Hydrolysis: PAOs rapidly absorb VFAs and synthesize Polyhydroxyalkanoates (PHAs), such as poly-$\beta$-hydroxybutyrate (PHB). To generate the cellular energy (ATP) required for active VFA transport across the cell membrane without external electron acceptors, PAOs hydrolyze intracellular inorganic polyphosphate ($[PO_4]_n$) chains.
3. Phosphorus Release: Orthophosphate ($PO_4^{3-}$) and associated cations ($Mg^{2+}, K^+, Ca^{2+}$) are expelled into the bulk liquid. Consequently, soluble orthophosphate concentrations in the anaerobic zone surge dramatically, often reaching $20–50\text{ mg/L } PO_4\text{-}P$ (a phenomenon called P-Release).

The Aerobic Phase Mechanics

When the mixed liquor flows into the aerobic zone ($DO \ge 2.0\text{ mg/L}$):

1. PHA Oxidation: PAOs utilize dissolved oxygen to metabolize their stored intracellular PHAs, producing ATP, carbon dioxide, water, and new cellular biomass.
2. Luxury Phosphorus Uptake: Energized by PHA catabolism, PAOs absorb all the orthophosphate released in the anaerobic zone plus all the background influent phosphorus, polymerizing it back into dense polyphosphate granules (known as luxury uptake).
3. Sludge Wasting: Final secondary effluent achieves ultra-low dissolved orthophosphate ($<0.1–0.5\text{ mg/L}$). The accumulated phosphorus is permanently removed from the treatment system by wasting a controlled mass of Waste Activated Sludge (WAS).

Competing Organisms: Glycogen Accumulating Organisms (GAOs)

Under upset conditions (such as warm wastewater temperatures $>25^\circ\text{C}$ or low $\text{pH} < 6.5$), Glycogen Accumulating Organisms (GAOs) (e.g., Candidatus Competibacter) can proliferate. GAOs consume available VFAs in the anaerobic zone and store PHAs without storing or releasing polyphosphates. When GAOs outcompete PAOs for VFAs, biological phosphorus removal collapses.

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2. EBPR System Configurations

Designing effective EBPR facilities requires strict hydraulic and nitrate isolation to prevent oxidized nitrogen ($NO_3^-$) from entering the anaerobic selector. If nitrate enters the anaerobic zone, denitrifying heterotrophs will consume the available VFAs before PAOs can utilize them, suppressing P-release.

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3. Chemical Phosphorus Precipitation

When biological phosphorus removal alone cannot achieve stringent discharge limits (e.g., $<0.5\text{ mg/L}$ or $<0.05\text{ mg/L}$), wastewater plants apply chemical precipitation utilizing multivalent metal salts or lime.

Primary Coagulants & Precipitation Chemistry

1. Aluminum Sulfate (Alum - $Al_2(SO_4)_3 \cdot 14H_2O$):

$Al^{3+} + PO_4^{3-} \longrightarrow AlPO_4\downarrow \quad (\text{Aluminum Phosphate Precipitate})$

1. Ferric Chloride ($FeCl_3$):

$Fe^{3+} + PO_4^{3-} \longrightarrow FePO_4\downarrow \quad (\text{Ferric Phosphate Precipitate})$

1. Ferrous Sulfate ($FeSO_4$): Requires dissolved oxygen or alkaline pH to oxidize ferrous iron ($Fe^{2+}$) to ferric iron ($Fe^{3+}$) to precipitate $FePO_4$ or vivianite $[Fe_3(PO_4)_2]$.
2. Hydrated Lime ($Ca(OH)_2$): Reacts with natural bicarbonate alkalinity to precipitate calcium carbonate, elevating pH above $10.5$ to precipitate calcium hydroxyapatite:

$10 Ca^{2+} + 6 PO_4^{3-} + 2 OH^- \longrightarrow Ca_{10}(PO_4)_6(OH)_2\downarrow \quad (\text{Hydroxyapatite})$

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|                    THEORETICAL VS. ACTUAL CHEMICAL DOSING REALITIES                     |
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| • Stoichiometric Molar Ratio:  1.0 mole Al3+ or Fe3+ per 1.0 mole Phosphorus (1.0 : 1.0)|
| • Stoichiometric Mass Ratio:   0.87 lbs Al3+ per lb P  |  1.80 lbs Fe3+ per lb P        |
|                                (9.6 lbs Alum / lb P)   |  (5.2 lbs dry FeCl3 / lb P)    |
|                                                                                         |
| • ACTUAL Operating Molar Ratio:1.5 : 1.0 to 2.5 : 1.0 (Fe/Al to P)                      |
|                                (Side reactions with alkalinity consume metal ions to    |
|                                 form Al(OH)3 and Fe(OH)3 hydroxide flocs)               |
| • Ultra-Low Limits (<0.1 mg/L):Actual Molar Ratio surges to 3.0 : 1.0 to 5.0 : 1.0      |
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Points of Chemical Addition

• Pre-Precipitation (Primary Clarifier Influent): Metal coagulant added ahead of primary settling tanks. Removes up to $50%–70%$ of influent phosphorus along with heavy organic BOD solids. Increases primary sludge volume and reduces biological loading to aeration basins, but requires higher chemical doses and may leave insufficient phosphorus for downstream biological synthesis.
• Simultaneous Precipitation (Aeration Basin / Secondary Clarifier Feed): Coagulant is injected directly into the aeration basin discharge or secondary clarifier feed channel. Metal phosphate precipitates co-settle with biological mixed liquor in the secondary clarifier. Highly flexible, requires low capital infrastructure, and captures both chemical and biological flocs.
• Post-Precipitation (Tertiary Clarification / Filtration): Coagulant is injected into secondary effluent ahead of tertiary sand filters, cloth media disc filters, or high-rate ballasted flocculators (e.g., Actiflo / CoMag). Essential for achieving ultra-low effluent phosphorus ($<0.02–0.05\text{ mg/L}$) to meet Colorado Regulation 31 standards.

Operational Impacts of Metal Salt Addition

• Alkalinity Consumption: Metal coagulants act as Lewis acids. Alum consumes $0.50\text{ lbs of alkalinity as } \text{CaCO}_3\text{ per lb of commercial alum}$; ferric chloride consumes $0.92\text{ lbs of alkalinity as } \text{CaCO}_3\text{ per lb of dry } FeCl_3$.
• Chemical Sludge Production: Metal coagulant addition increases overall plant sludge generation by $1.5\text{ to } 2.5\text{ lbs of total dry solids}$ for every pound of phosphorus removed.

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4. Worked Chemical Coagulant Dosing Calculation

A wastewater treatment plant treats a flow of $2.0\text{ MGD}$ with a secondary effluent orthophosphate concentration of $4.0\text{ mg/L } PO_4\text{-}P$. The target discharge limit is $0.5\text{ mg/L } PO_4\text{-}P$. Jar testing indicates an operating molar ratio of $2.0\text{ moles of } Fe^{3+}\text{ per mole of } P$ using a liquid ferric chloride solution ($FeCl_3$) containing $38%\text{ dry } FeCl_3\text{ by weight}$ with a specific gravity ($SG$) of $1.38$ (solution density $= 1.38 \times 8.34 = 11.51\text{ lbs/gal}$).

(Atomic weights: $Fe = 55.85\text{ g/mol}$, $P = 30.97\text{ g/mol}$, $Cl = 35.45\text{ g/mol}$; Molecular weight of $FeCl_3 = 162.2\text{ g/mol}$)

Step 1: Calculate Mass of Phosphorus to be Removed

$\Delta P = 4.0\text{ mg/L} - 0.5\text{ mg/L} = 3.5\text{ mg/L } P$

$\text{Mass of } P \text{ removed (lbs/day)} = 2.0\text{ MGD} \times 3.5\text{ mg/L} \times 8.34 = 58.38\text{ lbs/day } P$

Step 2: Calculate Required Dry Mass of Ferric Chloride ($FeCl_3$)

At a $1:1$ stoichiometric molar ratio:

$\text{Mass Ratio of } FeCl_3 / P = \frac{162.2\text{ g/mol } FeCl_3}{30.97\text{ g/mol } P} = 5.237\text{ lbs dry } FeCl_3 / \text{lb } P$

At an operating molar ratio of $2.0 : 1.0$:

$\text{Dry } FeCl_3 \text{ required (lbs/day)} = 58.38\text{ lbs } P / \text{day} \times (5.237 \times 2.0) = 611.47\text{ lbs dry } FeCl_3 / \text{day}$

Step 3: Calculate Gallons of Commercial Liquid Ferric Chloride Solution per Day

$\text{Lbs of active } FeCl_3 \text{ per gallon} = 11.51\text{ lbs/gal} \times 0.38 = 4.374\text{ lbs active } FeCl_3 / \text{gal}$

$\text{Feed Rate (gal/day)} = \frac{611.47\text{ lbs/day } FeCl_3}{4.374\text{ lbs/gal}} = 139.79\text{ gallons/day}$

$\text{Chemical Metering Pump Feed Rate (gal/hr)} = \frac{139.79\text{ gal/day}}{24\text{ hr/day}} = 5.82\text{ GPH}$