10.1 Biological & Chemical Phosphorus Removal

10.1 Biological & Chemical Phosphorus Removal

Phosphorus is a critical limiting nutrient in freshwater aquatic ecosystems. When excessive quantities of phosphorus enter rivers, lakes, and reservoirs through municipal wastewater discharges, it triggers rapid eutrophication—leading to massive blooms of toxic cyanobacteria, severe dissolved oxygen depletion during nocturnal respiration and algal decay, degradation of drinking water supplies, and catastrophic fish kills. Under the Clean Water Act National Pollutant Discharge Elimination System (NPDES) and Missouri Department of Natural Resources (MoDNR) regulations, wastewater treatment facilities increasingly face stringent effluent total phosphorus limits, typically ranging from $0.5\text{ to }1.0\text{ mg/L}$, and as low as $0.05 - 0.10\text{ mg/L}$ in sensitive watersheds such as Table Rock Lake and the Ozark Plateau receiving basins.

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Forms of Phosphorus in Wastewater

Total Phosphorus (TP) in raw municipal wastewater typically ranges from $4\text{ to }12\text{ mg/L as P}$. Phosphorus occurs in three primary chemical forms:

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|                                    PHOSPHORUS FRACTIONS IN WASTEWATER                                   |
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| 1. ORTHOPHOSPHATES (PO4^3-, HPO4^2-, H2PO4-, H3PO4):                                                    |
|    - Represents 50% to 70% of influent phosphorus; the only form directly bioavailable to organisms.    |
|    - Soluble and immediately reactive with chemical coagulants (alum, ferric, lime).                    |
|    - Measured directly in the laboratory via the standard Ascorbic Acid / Molybdenum Blue colorimetric   |
|      test without preliminary acid digestion.                                                           |
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| 2. POLYPHOSPHATES (CONDENSED PHOSPHATES):                                                               |
|    - Includes pyrophosphate (P2O7^4-), tripolyphosphate (P3O10^5-), and hexametaphosphate.              |
|    - Originates from commercial detergents, cleaning formulations, boiler water additives, and food.    |
|    - Naturally and gradually hydrolyze in collection systems and biological reactors into soluble       |
|      orthophosphate through bacterial enzymatic action (acid hydrolysis).                               |
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| 3. ORGANICALLY BOUND PHOSPHORUS:                                                                        |
|    - Bound within cellular materials: nucleic acids (DNA, RNA), phospholipids, and nucleotides (ATP).   |
|    - Exists in both particulate (cell fragments, feces) and soluble organic forms.                      |
|    - Mineralized into orthophosphates through biological heterotrophic decomposition in secondary units.|
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To measure Total Phosphorus (TP), the wastewater sample must first undergo acid-persulfate digestion at high temperature and pressure (autoclave at $121^\circ\text{C}$ for 30 minutes) to hydrolyze all polyphosphates and oxidize organic phosphorus compounds completely into dissolved orthophosphate.

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Chemical Phosphorus Removal (Precipitation & Coagulation)

Chemical phosphorus removal involves dosing trivalent metallic salts (aluminum or ferric iron), divalent iron salts (ferrous iron), or calcium compounds (hydrated lime) to convert soluble orthophosphate into insoluble metal phosphate precipitates, which are subsequently removed via sedimentation or granular media filtration.

1. Aluminum Sulfate (Alum)

Commercial alum ($\text{Al}_2(\text{SO}_4)_3 \cdot 14\text{H}_2\text{O}$) reacts directly with orthophosphate to form solid aluminum phosphate ($\text{AlPO}_4$):

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

• Stoichiometry: On a pure molar basis, $1.0\text{ mole of Al}^{3+}$ precipitates $1.0\text{ mole of PO}_4^{3-}$ (a weight ratio of $0.87\text{ lb Al / lb P}$). However, side reactions with natural bicarbonate alkalinity consume alum by forming insoluble aluminum hydroxide ($\text{Al(OH)}_3$):

$\text{Al}^{3+} + 3\text{HCO}_3^- \longrightarrow \text{Al(OH)}_3\downarrow + 3\text{CO}_2$

• Practical Dosing: Because of competing hydroxide precipitation, the practical operating dosage requires a molar ratio of $1.2 : 1\text{ to }2.5 : 1\text{ Al : P}$ ($1.0 - 2.2\text{ lb Al / lb P}$, corresponding to $10 - 25\text{ lb commercial dry alum / lb P}$). Optimal precipitation occurs between $\text{pH } 5.5 - 6.5$.
• Alkalinity Consumption: Alum is an acidic coagulant. Every $1.0\text{ mg of Al}^{3+}$ dosed destroys $5.56\text{ mg of total alkalinity as CaCO}_3$ (or $0.50\text{ mg alkalinity as CaCO}_3$ per mg of dry commercial alum).

2. Ferric Chloride ($\text{FeCl}_3$) & Ferric Sulfate ($\text{Fe}_2(\text{SO}_4)_3$)

Trivalent iron reacts directly with orthophosphate to precipitate ferric phosphate ($\text{FePO}_4$):

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

• Stoichiometry: The theoretical molar ratio is $1.0\text{ mole Fe : } 1.0\text{ mole P}$ ($1.8\text{ lb Fe / lb P}$). Competing reactions with alkalinity form ferric hydroxide ($\text{Fe(OH)}_3$):

$\text{Fe}^{3+} + 3\text{HCO}_3^- \longrightarrow \text{Fe(OH)}_3\downarrow + 3\text{CO}_2$

• Practical Dosing: Practical molar ratios range from $1.5 : 1\text{ to }2.5 : 1\text{ Fe : P}$ ($2.7 - 4.5\text{ lb Fe / lb P}$). Optimal precipitation occurs between $\text{pH } 6.5 - 7.5$.
• Alkalinity Consumption: Every $1.0\text{ mg of Fe}^{3+}$ dosed destroys $2.69\text{ mg of total alkalinity as CaCO}_3$.

3. Ferrous Salts ($\text{FeSO}_4$ & $\text{FeCl}_2$)

Divalent ferrous iron is frequently obtained as pickle liquor from steel manufacturing. In aeration basins, dissolved oxygen oxidizes $\text{Fe}^{2+}$ to $\text{Fe}^{3+}$, which then precipitates orthophosphate. Each pound of $\text{Fe}^{2+}$ oxidized consumes $0.14\text{ lb of dissolved oxygen (O}_2\text{)}$.

4. Hydrated Lime ($\text{Ca(OH)}_2$)

Lime functions differently by reacting with natural bicarbonate hardness to precipitate calcium carbonate ($\text{CaCO}_3$), raising the wastewater pH above $10.0$. At $\text{pH } > 10.5$, excess calcium ions react with orthophosphate to precipitate hydroxyapatite ($[\text{Ca}_5(\text{OH})(\text{PO}_4)_3]$):

$5\text{Ca}^{2+} + 3\text{PO}_4^{3-} + \text{OH}^- \longrightarrow \text{Ca}_5(\text{OH})(\text{PO}_4)_3\downarrow$

Lime precipitation generates voluminous, dense chemical sludge and requires downstream recarbonation (injecting carbon dioxide, $\text{CO}_2$) to neutralize high pH prior to biological treatment or discharge.

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Chemical Dosing Locations & Sludge Production

Chemical coagulants can be injected at various stages within the wastewater treatment flowsheet, each offering distinct operational trade-offs:

Chemical Sludge Production Impact

Chemical phosphorus precipitation significantly increases total plant sludge generation. In addition to normal biological and primary solids, chemical addition produces:

• $Alum$: Produces approximately $4.0\text{ to }5.5\text{ lbs of dry sludge solids}$ per lb of phosphorus removed.
• $Ferric Chloride$: Produces approximately $4.5\text{ to }6.0\text{ lbs of dry sludge solids}$ per lb of phosphorus removed.
• $Lime$: Produces $15\text{ to }25\text{ lbs of dry sludge solids}$ per lb of phosphorus removed due to massive $\text{CaCO}_3$ co-precipitation.

Overall, chemical addition increases total facility dry solids production by $20%\text{ to }45%$, requiring increased capacity in digesters, dewatering presses, and hauling budgets.

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Enhanced Biological Phosphorus Removal (EBPR)

Enhanced Biological Phosphorus Removal (EBPR) is a sustainable, low-chemical process that relies on specialized bacteria called Polyphosphate Accumulating Organisms (PAOs)—predominantly Candidatus Accumulibacter phosphatis and Tetrasphaera. Under engineered alternating anaerobic and aerobic environments, PAOs accumulate phosphorus within their cellular structure far in excess of normal bacterial nutritional requirements.

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|                                 THE TWO-PHASE BIOCHEMICAL CYCLE OF PAOs                                |
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| 1. ANAEROBIC SELECTOR ZONE (DO = 0.0 mg/L, NO3-N = 0.0 mg/L):                                           |
|    • Environmental Stress: Strict absence of both dissolved oxygen and chemically bound oxygen (nitrate).|
|    • VFA Fermentation: Heterotrophic bacteria ferment readily biodegradable COD (rbCOD) into Volatile   |
|      Fatty Acids (VFAs), primarily acetic acid and propionic acid.                                      |
|    • Substrate Uptake: PAOs rapidly transport VFAs across cell membranes and store them internally as   |
|      Poly-β-hydroxybutyrate (PHB) and Polyhydroxyalkanoates (PHA).                                      |
|    • Orthophosphate Release: To generate the biochemical energy (ATP) needed for VFA transport and     |
|      polymerization, PAOs cleave high-energy bonds in internal polyphosphate chains, releasing soluble   |
|      orthophosphate (PO4^3-) and associated cations (Mg2+, K+, Ca2+) into the bulk liquid.              |
|    • Result: Anaerobic bulk liquid orthophosphate concentration surges dramatically (to 20 - 40 mg/L). |
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| 2. AEROBIC ZONE (DO >= 2.0 mg/L):                                                                       |
|    • Substrate Oxidation: PAOs metabolize their stored intracellular PHB reserves using dissolved oxygen|
|      as the terminal electron acceptor, generating energy and building new biomass.                     |
|    • Luxury Phosphorus Uptake: Using energy from PHB catabolism, PAOs absorb all available soluble      |
|      orthophosphate from the bulk liquid, converting it into long intracellular polyphosphate chains    |
|      (volutin granules) within their cytoplasm.                                                         |
|    • Result: Soluble orthophosphate is scrubbed from the bulk water, dropping effluent P to < 0.2 mg/L. |
|    • Biomass Phosphorus Content: EBPR biomass contains 5% to 8% phosphorus by dry weight (compared to   |
|      only 1.5% to 2.0% in conventional activated sludge).                                               |
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Critical Operational Factors in EBPR

Successful biological phosphorus removal demands precise process control across several biochemical and physical parameters:

1. The VFA to Total Phosphorus Ratio ($\text{VFA : TP}$)

PAOs require sufficient carbon substrate in the anaerobic selector to drive polyphosphate release and PHB accumulation. Operators must maintain a minimum ratio of $25 - 45\text{ mg of COD}$ (or $7 - 10\text{ mg of VFA as acetic acid}$) per $1.0\text{ mg of influent TP}$ to be removed. If raw influent is deficient in volatile fatty acids, facilities operate in-line or side-stream primary sludge fermenters to generate volatile acids (acetic and propionic acids) from raw primary solids.

2. Nitrate and Dissolved Oxygen Intrusion

Nitrate ($\text{NO}_3^-\text{-N}$) or dissolved oxygen introduced into the anaerobic selector via RAS or influent flow will poison the EBPR process. Denitrifying heterotrophic bacteria (Pseudomonas) will utilize the available nitrate or oxygen to consume VFAs, outcompeting PAOs for scarce carbon. Anaerobic zone oxidation-reduction potential (ORP) must remain below $-150\text{ to }-250\text{ mV}$.

3. WAS Wasting as the Sole Removal Mechanism

Phosphorus is an elemental chemical; it cannot be converted into an inert gas like nitrogen. The ONLY mechanism by which phosphorus is permanently removed from an EBPR treatment plant is the physical wasting of phosphorus-rich Waste Activated Sludge (WAS). If solids inventory is not wasted daily, phosphorus remains in the plant and will ultimately break through into the final effluent.

4. Preventing Secondary Phosphorus Release

If PAO-laden sludge settles in secondary clarifiers for excessive detention periods, or is sent to un-aerated gravity thickeners, dissolved oxygen and nitrate are quickly depleted. Entering an anaerobic state without fresh food, PAOs hydrolyze their intracellular polyphosphates for survival maintenance, causing secondary phosphorus release. Soluble orthophosphate enters the clarifier effluent or thickener supernatant and recycles directly back to the headworks, overwhelming the treatment process.

  OPERATIONAL PROTOCOLS TO PREVENT SECONDARY P RELEASE:
  1. Maintain secondary clarifier sludge blanket depth at < 2.0 feet with adequate RAS rates.
  2. Avoid static gravity thickening for EBPR WAS; utilize mechanical thickening equipment
     such as Gravity Belt Thickeners (GBT) or Decanter Centrifuges.
  3. If sludge holding basins are used, maintain continuous diffuse aeration (DO > 1.0 mg/L).
  4. In anaerobic digesters treating EBPR sludge, dose ferric chloride to bind released
     orthophosphate and prevent massive struvite (MgNH4PO4·6H2O) pipe scale formation.