9.2 Phosphorus Removal & Chesapeake Bay Compliance

9.2 Phosphorus Removal & Chesapeake Bay Compliance

[!NOTE] The Chesapeake Bay Watershed Mandate: The Chesapeake Bay is the largest and most ecologically productive estuary in North America, yet it suffers from pervasive nutrient over-enrichment. Approximately half of the freshwater entering the Bay flows from Pennsylvania via the Susquehanna River basin, with additional contributions from the Potomac River basin. Under the federal Clean Water Act and the Chesapeake Bay Total Maximum Daily Load (TMDL), excess discharges of phosphorus and nitrogen trigger massive blooms of phytoplankton and cyanobacteria. As these algal mats die and settle, microbial decomposition consumes dissolved oxygen, creating widespread benthic hypoxic "dead zones" that decimate aquatic habitats. To halt this degradation, the Pennsylvania Department of Environmental Protection (DEP) enforces stringent nutrient load limits on municipal wastewater dischargers.

Unlike carbon and nitrogen, phosphorus cannot be converted into an inert atmospheric gas. Complete phosphorus removal requires transforming soluble phosphorus into a solid, particulate state—either through chemical precipitation or intracellular biological accumulation—and physically separating and removing that solid fraction from the wastewater stream through sedimentation and sludge wasting.

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Phosphorus Speciation in Wastewater

Total Phosphorus (TP) in raw municipal wastewater typically ranges between $4.0\text{ and }8.0\text{ mg/L as P}$, originating from human metabolic waste, synthetic detergents, food residues, and industrial cleaning formulations. Wastewater phosphorus exists in three primary chemical forms:

1. Soluble Orthophosphate ($\text{PO}_4^{3-}, \text{HPO}_4^{2-}, \text{H}_2\text{PO}_4^-$): The simplest, fully oxidized inorganic form, accounting for $50%\text{ to }70%$ of raw wastewater phosphorus. Orthophosphate is directly bioavailable to microorganisms and algae without requiring biological hydrolysis, and is the specific species that reacts with chemical metal precipitants.
2. Polyphosphates (Condensed Phosphates): Inorganic molecular polymers featuring repeating phosphorus-oxygen chains (including pyrophosphate, tripolyphosphate, and trimetaphosphate). In collection systems and biological reactors, bacterial enzymes slowly hydrolyze polyphosphates into soluble orthophosphates.
3. Organic Phosphorus: Phosphorus bound chemically into nucleic acids (DNA, RNA), phospholipids, and adenosine triphosphate (ATP). Biological secondary treatment oxidizes and mineralizes organic phosphorus into orthophosphate.

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|                             Wastewater Phosphorus Speciation                                      |
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|  Total Phosphorus (TP) = Orthophosphates + Polyphosphates + Organically Bound Phosphorus          |
|                                                                                                   |
|  - Orthophosphate: Bioavailable, directly reactive with coagulants (Al3+, Fe3+)                   |
|  - Polyphosphate:  Hydrolyzes biologically into orthophosphate in activated sludge basins         |
|  - Organic-P:      Mineralized by heterotrophic bacteria into orthophosphate                      |
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Chemical Phosphorus Removal: Coagulant Chemistry & Dosing Stoichiometry

Chemical phosphorus removal involves adding trivalent metallic coagulant salts—predominantly aluminum or iron compounds—to react with soluble orthophosphate, forming insoluble, settleable metallic phosphate flocs.

1. Aluminum Sulfate (Alum)

Aluminum sulfate (commercial alum, $\text{Al}_2(\text{SO}_4)_3 \cdot 14\text{H}_2\text{O}$) dissociates in water to yield trivalent aluminum cations ($Al^{3+}$) that precipitate orthophosphate:

$Al^{3+} + PO_4^{3-} \rightarrow AlPO_4\downarrow$

• Stoichiometric Molar Ratio: The chemical equation requires exactly $1.0\text{ mole of } Al^{3+}\text{ per } 1.0\text{ mole of } P$ (a mass ratio of $0.87\text{ lbs of } Al\text{ per lb of } P$).
• Competing Hydroxide Reactions: In real wastewater, aluminum ions react competitively with natural bicarbonate alkalinity to form insoluble aluminum hydroxide precipitates: $Al^{3+} + 3HCO_3^- \rightarrow Al(OH)_3\downarrow + 3CO_2$
• Actual Field Dosing Ratio: Because of competitive hydroxide precipitation, operating facilities must feed alum at actual molar dosing ratios between $1.5:1\text{ and }2.5:1\text{ Al:P}$ (equivalent to $15\text{ to }25\text{ lbs of commercial liquid alum per lb of P removed}$).

2. Ferric Chloride & Iron Salts

Ferric chloride ($\text{FeCl}_3$) and ferrous sulfate ($\text{FeSO}_4$) are widely utilized iron coagulants. Trivalent ferric iron reacts with orthophosphate to form insoluble ferric phosphate:

$Fe^{3+} + PO_4^{3-} \rightarrow FePO_4\downarrow$

• Stoichiometric Molar Ratio: $1.0\text{ mole of } Fe^{3+}\text{ per } 1.0\text{ mole of } P$ (a mass ratio of $1.8\text{ lbs of } Fe\text{ per lb of } P$).
• Competing Hydroxide Reactions: Iron cations similarly react with alkalinity to form ferric hydroxide: $Fe^{3+} + 3HCO_3^- \rightarrow Fe(OH)_3\downarrow + 3CO_2$
• Actual Field Dosing Ratio: Practical operational dosing requires molar ratios of $1.5:1\text{ to }2.5:1\text{ Fe:P}$.

3. Polyaluminum Chloride (PACl) & Sodium Aluminate

• Polyaluminum Chloride (PACl): Pre-polymerized, highly charged aluminum hydroxychloride complexes. PACl consumes significantly less alkalinity than alum, performs exceptionally well in cold winter water, and generates a denser, faster-settling floc.
• Sodium Aluminate ($\text{NaAlO}_2$): An alkaline aluminum compound that supplies aluminum cations while adding caustic alkalinity rather than consuming it, making it ideal for facilities treating low-alkalinity wastewater.

Chemical Application Points in the Treatment Train

Chemical precipitants can be applied at three strategic locations within the liquid process train:

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|                       Chemical Phosphorus Coagulant Dosing Locations                              |
+---------------------------------------------------------------------------------------------------+
| Point 1: Pre-Precipitation  | Added ahead of primary clarifiers; removes 50-70% TP; increases     |
|                             | primary sludge volume; reduces organic load to aeration basins.     |
| Point 2: Co-Precipitation   | Added directly into aeration basin effluent or secondary clarifier  |
|                             | feed; biological floc acts as a sweep coagulant; most common layout.|
| Point 3: Post-Precipitation | Added after secondary clarifiers ahead of tertiary sand or disc     |
|                             | filters; achieves ultra-low effluent phosphorus (< 0.10 mg/L).      |
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Impact on Solids Handling and Alkalinity

While chemical precipitation is reliable and straightforward to automate, it introduces significant operational side effects:

1. Chemical Sludge Generation: Precipitating phosphorus generates substantial inorganic chemical sludge mass. On average, chemical phosphorus removal increases total dry solids production by $4.0\text{ to }6.0\text{ lbs of dry sludge per lb of P removed}$.
2. Alkalinity Depletion: Dosing acidic metal salts consumes natural alkalinity:
   • $1.0\text{ lb of alum}$ consumes approximately $0.45\text{ lbs of alkalinity as } \text{CaCO}_3$.
   • $1.0\text{ lb of ferric chloride}$ consumes approximately $0.92\text{ lbs of alkalinity as } \text{CaCO}_3$.
3. Dewatering Polymer Demand: Chemical sludges contain gelatinous metal hydroxides that bind water tightly, typically increasing dewatering polymer conditioning dosages by $15%\text{ to }30%$.

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Enhanced Biological Phosphorus Removal (EBPR)

Enhanced Biological Phosphorus Removal (EBPR) achieves phosphorus reduction without high chemical coagulant costs by cultivating specialized bacterial cultures known as Polyphosphate Accumulating Organisms (PAOs), predominantly belonging to the genus Candidatus Accumulibacter phosphatis.

Standard heterotrophic bacteria contain approximately $1.5%\text{ to }2.0%$ phosphorus by dry cell weight for basic cellular functions. In contrast, PAOs under alternating anaerobic and aerobic stress conditions accumulate between $20%\text{ to }30%$ of their dry cell weight as internal polyphosphate storage granules.

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|                       The Cyclic Biochemistry of EBPR Microorganisms                              |
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|                                                                                                   |
|   [ ANAEROBIC ZONE: Stress & Release ]            [ AEROBIC ZONE: Luxury Uptake & Storage ]       |
|   - Conditions: DO = 0, Nitrate = 0               - Conditions: DO >= 2.0 mg/L                    |
|   - Microbes: PAOs break polyphosphate bonds      - Microbes: PAOs oxidize stored PHAs            |
|     to release energy                             - Orthophosphate is pulled from water           |
|   - Result: Orthophosphate is RELEASED into         at high rates ("luxury uptake")              |
|     liquid (Soluble P spikes: 20-40 mg/L)         - Cellular Poly-P granules are rebuilt          |
|   - Energy used to absorb Volatile Fatty          - Soluble P in water drops to < 0.2 mg/L        |
|     Acids (VFAs: acetate, propionate)             - Excess P permanently removed by WASTING       |
|   - VFAs stored internally as PHAs (PHB)            high-phosphorus sludge (WAS)                  |
|                                                                                                   |
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1. The Anaerobic Phase (Phosphorus Release & VFA Storage)

Mixed liquor containing PAOs enters an initial anaerobic selector zone characterized by the strict absence of both dissolved oxygen ($DO = 0.0\text{ mg/L}$) and oxidized nitrogen ($NO_3^- / NO_2^- < 0.2\text{ mg/L}$).

• Under these anaerobic conditions, PAOs cannot utilize normal respiration pathways. However, they possess a unique survival mechanism: they hydrolyze their intracellular high-energy polyphosphate bonds using polyphosphate kinase enzymes.
• This bond cleavage releases energy (ATP), which the PAOs use to take up soluble Volatile Fatty Acids (VFAs), predominantly acetic acid and propionic acid produced by the fermentation of raw wastewater organic matter.
• The absorbed VFAs are polymerized and stored intracellularly as insoluble carbon storage reserves called Polyhydroxyalkanoates (PHAs) or Poly-$\beta$-hydroxybutyrate (PHB).
• As a byproduct of polyphosphate bond cleavage, soluble orthophosphate is released across the cell membrane into the bulk liquid. Consequently, orthophosphate concentrations in the anaerobic zone surge dramatically, often reaching $20\text{ to }40\text{ mg/L}$.

2. The Aerobic Phase (Luxury Phosphorus Uptake)

The mixed liquor then flows into an aerated aerobic zone maintained at a dissolved oxygen concentration of $\ge 2.0\text{ mg/L}$.

• In the presence of oxygen, PAOs metabolize their stored intracellular PHAs via aerobic respiration, generating abundant cellular energy.
• The PAOs utilize this energy to take up soluble orthophosphate from the surrounding bulk liquid at rates far exceeding normal metabolic synthesis—a phenomenon termed "luxury uptake."
• The absorbed orthophosphate is synthesized into dense internal chains of polyphosphate granules, replenishing the cell's energy reserves.
• Luxury uptake rapidly strips orthophosphate from the water, reducing soluble effluent phosphorus concentrations to $< 0.1\text{ to }0.3\text{ mg/L}$.

3. Phosphorus Removal via Sludge Wasting

Phosphorus absorbed during luxury uptake remains trapped inside the PAO cells. The only mechanism by which phosphorus is permanently removed from the wastewater system is through the intentional wasting of phosphorus-rich Waste Activated Sludge (WAS).

[!CAUTION] Secondary Phosphorus Release Hazard: If settled, phosphorus-rich sludge is permitted to sit stagnant under un-aerated or septic conditions in secondary clarifiers, gravity thickeners, or aerated sludge holding tanks, the PAOs will experience anaerobic stress and release their stored phosphorus back into the liquid phase. This generates a concentrated phosphorus recycle stream (supernatant or centrate containing $50\text{ to }200+\text{ mg/L of P}$) that returns to the plant headworks, completely overwhelming biological treatment capacity.

Biological Phosphorus Removal Configurations

To balance biological phosphorus removal with carbonaceous oxidation and biological nitrogen removal, specialized multi-stage reactor layouts are utilized.

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|                         EBPR Process Layouts: A/O vs. A2O Configurations                          |
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| A/O Process (Phosphorus Removal Only):                                                            |
|                                                                                                   |
|   Influent      +---------------+       +---------------+       +---------------------+           |
|   ============> | Anaerobic (A) | ====> |   Oxic (O)    | ====> | Secondary Clarifier | ===> Eff  |
|                 +---------------+       +---------------+       +---------------------+           |
|                         ^                                                  |                      |
|                         +================ RAS =============================+                      |
|                                                                                                   |
| A2O Process (Combined Nitrogen & Phosphorus Removal):                                             |
|                                                                                                   |
|   Influent      +---------------+       +---------------+       +---------------+       +----+    |
|   ============> | Anaerobic (A) | ====> |   Anoxic (A)  | ====> |   Oxic (O)    | ====> |Clar|==> |
|                 +---------------+       +---------------+       +---------------+       +----+    |
|                         ^                       ^                       |                 |       |
|                         |                       |<= IMLR (200-400% Q) ==|                 |       |
|                         +============================= RAS ===============================+       |
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1. The A/O Process (Anaerobic / Oxic)

The A/O process is a two-stage mainstream configuration consisting of an initial anaerobic selector (detention time $0.5\text{ to }1.5\text{ hours}$) followed by an oxic/aerobic basin ($1.5\text{ to }3.0\text{ hours}$). Return Activated Sludge (RAS) is recycled directly to the anaerobic selector. Because the process operates at a short MCRT without nitrification, the RAS contains zero nitrate, ensuring a true anaerobic environment. The A/O process is optimal for facilities requiring phosphorus reduction without total nitrogen limits.

2. The $A^2O$ Process (Anaerobic / Anoxic / Oxic)

When a facility must achieve both phosphorus removal and biological nitrogen removal, the $A^2O$ process introduces an intermediate anoxic zone between the anaerobic selector and the aerobic basin:

• Anaerobic Zone: Receives raw wastewater influent and RAS. PAOs release phosphorus and accumulate VFAs.
• Anoxic Zone: Receives mixed liquor from the anaerobic zone and high-rate nitrate recycle (IMLR) from the aerobic zone. Facultative heterotrophs reduce nitrate to nitrogen gas.
• Oxic Zone: Nitrifies ammonia, oxidizes carbonaceous BOD, and achieves luxury phosphorus uptake.

The Destructive Impact of Nitrate on EBPR

The critical operational challenge in combined BNR-EBPR facilities is preventing oxidized nitrogen ($NO_3^-, NO_2^-$) from entering the anaerobic selector. If excessive nitrate is returned to the anaerobic zone via the RAS stream, facultative denitrifying heterotrophs will consume the available volatile fatty acids (VFAs) to reduce the nitrate. Denitrifiers outcompete PAOs for organic carbon, starving the PAOs of the substrate needed to synthesize internal PHAs. Without stored PHAs, PAOs cannot execute luxury phosphorus uptake in the subsequent aerobic zone, causing EBPR failure.

3. University of Cape Town (UCT) Process

To completely insulate the anaerobic selector from nitrate intrusion, the UCT process redirects the RAS stream into the anoxic basin rather than the anaerobic basin. A separate anoxic recycle line then pumps mixed liquor (which has been completely stripped of nitrate by denitrifiers) from the anoxic zone into the anaerobic selector. This guarantees a nitrate-free environment for maximum VFA uptake by PAOs.

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Pennsylvania DEP Chesapeake Bay Tributary Strategy

Pennsylvania's regulatory framework for nutrient compliance is anchored in the Chesapeake Bay Tributary Strategy, codified under the state Clean Streams Law and integrated into the federal Phase 3 Watershed Implementation Plan (WIP).

1. Annual Mass Cap Load Allocations

Unlike standard municipal NPDES permits that govern compliance solely through monthly average concentration limits (such as $30\text{ mg/L BOD}$ and $30\text{ mg/L TSS}$), Pennsylvania wastewater facilities within the Susquehanna and Potomac River basins are governed by legally binding annual mass cap load allocations expressed strictly in pounds per year (lbs/year) for Total Nitrogen and Total Phosphorus.

$\text{Annual Mass Load (lbs/year)} = \sum \left[ \text{Daily Flow (MGD)} \times \text{Daily Concentration (mg/L)} \times 8.34 \right]$

• The Permitting Dynamic: Facilities are assigned fixed annual pound caps based on their design hydraulic capacity and designated target concentrations—typically an annual net average of $6.0\text{ mg/L TN}$ and $0.8\text{ mg/L TP}$ (or $3.0\text{ mg/L TN}$ and $0.1\text{ to }0.5\text{ mg/L TP}$ in enhanced tributary segments).
• Growth vs. Concentration: Because the allocation is a fixed mass ceiling, as a municipality's hydraulic discharge flow expands due to population growth, the facility must discharge lower and lower nutrient concentrations to remain beneath its legal pound-per-year ceiling.
• Compliance Year: In Pennsylvania, the Chesapeake Bay nutrient compliance year runs from October 1 through September 30.

2. Pennsylvania Nutrient Credit Trading Program

Codified in 25 Pa. Code Chapter 96 (Water Quality Standards Implementation), the Pennsylvania Nutrient Credit Trading Program provides an economically flexible compliance mechanism allowing point source dischargers to meet their annual cap load allocations.

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|                     Pennsylvania DEP Nutrient Credit Trading Framework                            |
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|  Facility A (Over-Performing Plant):                                                              |
|  Permitted Cap: 50,000 lbs TN/yr  |  Actual Discharge: 35,000 lbs TN/yr                           |
|  --> Generates 15,000 Certified Nutrient Credits (1 Credit = 1 lb/yr reduction below baseline)    |
|                                                                                                   |
|                                            || Sells Credits via PENNVEST Auction / Private Trade  |
|                                            \/                                                     |
|  Facility B (Under-Performing / Growing Plant):                                                   |
|  Permitted Cap: 40,000 lbs TN/yr  |  Actual Discharge: 52,000 lbs TN/yr                           |
|  --> Must Purchase 12,000 Certified Nutrient Credits to achieve legal NPDES compliance            |
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Program Mechanics and Regulatory Rules

• Definition of a Credit: One nutrient credit represents one pound of Total Nitrogen or Total Phosphorus reduction achieved beyond regulatory baseline requirements within a specific compliance year.
• Certification and Verification: Before credits can be traded, the DEP must formally certify the pollution-reduction activity and verify that the physical reductions occurred through validated Discharge Monitoring Report (DMR) analytical data.
• Trading Baselines: A wastewater treatment facility cannot generate sellable credits until it first satisfies its own statutory baseline performance requirements. Upgrades financed primarily through public taxpayer grants face stringent credit calculation adjustments.
• Credit Exchanges & Auctions: Facilities register credits on the Commonwealth's nutrient credit registry. Trading occurs through direct bilateral utility-to-utility contracts, qualified private credit aggregators, or state-facilitated reverse auctions managed by the Pennsylvania Infrastructure Investment Authority (PENNVEST).
• True-Up Period: Following the close of the compliance year on September 30, facilities have a mandatory 60-day true-up period (ending November 28) to reconcile annual discharge numbers, acquire necessary credits, and submit their final annual compliance report to the DEP.