2.2 Primary Coagulants, Polymers & Alkalinity Demand

2.2 Primary Coagulants, Polymers & Alkalinity Demand

Water treatment operators must understand the chemical characteristics, feed mechanics, and stoichiometric reactions of primary coagulants and polymer aids. Selecting the appropriate chemical and calculating its impact on alkalinity and pH is vital to prevent process failure, distribution system corrosion, and regulatory violations.

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Aluminum-Based Primary Coagulants

Aluminum coagulants are the most widely used primary coagulants in municipal water treatment for removing particulate turbidity and color.

Dry Aluminum Sulfate (Alum)

• Formula: $\text{Al}_2(\text{SO}_4)_3 \cdot 14\text{H}_2\text{O}$ (hydrate number typically ranges from 14 to 18; standard commercial grade assumes 14 waters of hydration).
• Molecular Weight: Approximately 594.4 g/mol.
• Active Content: Standard commercial dry alum contains 17.0% to 17.5% water-soluble alumina ($\text{Al}_2\text{O}_3$), equivalent to ~9.0% to 9.2% elemental aluminum ($Al$).
• Feed Operation: Supplied as powder, granules, or lumps. Dry alum is fed using gravimetric or volumetric dry feeders discharging into a dissolving tank equipped with mechanical mixers to maintain a 1% to 10% solution.

Liquid Aluminum Sulfate (Liquid Alum)

• Concentration: Shipped as a clear, amber-colored aqueous solution containing 48.0% to 48.5% dry alum equivalent by weight (approximately 8.25% to 8.5% active $\text{Al}_2\text{O}_3$).
• Specific Gravity: Ranges from 1.31 to 1.34 at 60°F (15.5°C).
• Solution Density: Approximately 11.1 lb/gal at typical ambient temperatures.
• Active Dry Alum per Gallon:

$\text{Dry Alum Equivalent} = 11.1\text{ lb/gal} \times 0.485 = \mathbf{5.4\text{ lb dry alum per gallon}}$

• Crystallization Temperature: Liquid alum begins crystallizing (salting out) at approximately 30°F (-1°C). Bulk chemical storage tanks located outdoors must be insulated and heat-traced to prevent crystallization and line plugging.
• Corrosivity: Solution pH ranges from 2.0 to 2.5. Requires corrosion-resistant storage materials such as 316 stainless steel, fiberglass-reinforced plastic (FRP), cross-linked high-density polyethylene (XLPE), or rubber-lined steel.

Pre-Hydrolyzed Aluminum Coagulants: PACl and ACH

Pre-hydrolyzed coagulants are manufactured by partially reacting aluminum chloride with an alkaline base under controlled conditions, forming pre-formed polynuclear aluminum complexes with the general formula $\text{Al}n\text{Cl}{3n-m}(\text{OH})_m$.

• Basicity Rating: Basicity is defined as the percentage of total aluminum valence satisfied by bound hydroxyl groups ($[\text{OH}] / 3[\text{Al}] \times 100%$):
  • Polyaluminum Chloride (PACl): Basicity typically ranges from 50% to 70%.
  • Aluminum Chlorohydrate (ACH): Represents the highest basicity PACl formulation at 83% to 84% basicity [approximate formula $\text{Al}_2(\text{OH})_5\text{Cl}$].
• Operational Advantages over Standard Alum:
  • Reduced Alkalinity Demand: Because hydroxyl groups are already incorporated into the molecular complex, PACl and ACH consume 50% to 80% less raw water alkalinity than alum.
  • Minimal pH Depression: Treated water maintains higher pH stability, reducing or eliminating the need for post-treatment caustic or lime feed.
  • Cold-Water Superiority: Pre-hydrolyzed complexes do not rely on in-situ temperature-sensitive hydrolysis reactions, delivering rapid flocculation in freezing waters (< 4°C).
  • Reduced Sludge Production: Produces 20% to 40% less chemical sludge volume due to lower overall chemical dose and denser floc structure.

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Iron-Based Primary Coagulants

Iron coagulants (ferric and ferrous salts) are utilized extensively when raw waters have high organic color, wide pH fluctuations, or when simultaneous sulfide and arsenic removal is required.

Ferric Chloride ($\text{FeCl}_3$)

• Properties: Shipped as a dark brown, aqueous solution containing 35% to 45% $\text{FeCl}_3$ with a specific gravity of 1.35 to 1.45 (weighing 11.2 to 12.1 lb/gal).
• Operating pH Window: Highly effective across a broad spectrum of pH 4.0 to 9.0.
• Handling Hazards: Solution pH is less than 1.0 (strongly acidic and oxidizing). It causes severe chemical burns to human tissue and stains concrete, tanks, and equipment indelible reddish-brown. It releases corrosive hydrochloric acid fumes. Piping and storage must be constructed of PVDF, PTFE, PVC, FRP, or rubber-lined steel; metals such as carbon steel, brass, and copper are rapidly dissolved.

Ferric Sulfate [$\text{Fe}_2(\text{SO}_4)_3$]

• Properties: Supplied as dry granular material (containing ~20% soluble iron) or as a 50% liquid solution with specific gravity ~1.50.
• Performance: Forms dense, rapid-settling ferric hydroxide flocs. Less corrosive and less prone to fuming than ferric chloride, but produces acidic solutions that demand corrosion-resistant handling.

Ferrous Sulfate (Copperas, $\text{FeSO}_4 \cdot 7\text{H}_2\text{O}$)

• Properties: Granular green crystals containing iron in the divalent ($Fe^{2+}$) state.
• Oxidation Requirement: In municipal surface water treatment at neutral pH, $Fe^{2+}$ hydrolyzes too slowly to serve as an effective primary coagulant. It requires oxidation to trivalent ferric iron ($Fe^{3+}$) using chlorine (chlorinated copperas) or dissolved oxygen at high pH:

$6\text{FeSO}_4 \cdot 7\text{H}_2\text{O} + 3\text{Cl}_2 \rightarrow 2\text{Fe}_2(\text{SO}_4)_3 + 2\text{FeCl}_3 + 42\text{H}_2\text{O}$

Chlorinated copperas provides both ferric sulfate and ferric chloride, forming heavy flocs useful in softening applications where pH exceeds 9.0.

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Alkalinity Consumption Stoichiometry & pH Depletion

Primary metal coagulants do not coagulate as bare ions; they undergo hydrolysis reactions that extract hydroxide ions ($\text{OH}^-$) from water, releasing hydronium ions ($\text{H}^+$) that neutralize natural bicarbonate alkalinity ($\text{HCO}_3^-$).

Alum Reaction Stoichiometry

$\text{Al}_2(\text{SO}_4)_3 \cdot 14\text{H}_2\text{O} + 3\text{Ca}(\text{HCO}_3)_2 \rightarrow 2\text{Al}(\text{OH})_3\downarrow + 3\text{CaSO}_4 + 6\text{CO}_2 + 14\text{H}_2\text{O}$

• Molecular weight of Alum = ~594 g/mol.
• 3 moles of Calcium Bicarbonate expressed as $\text{CaCO}_3$ = $3 \times 100.08 = 300.24$ g/mol.
• Ratio: $300.24 / 594.4 = \mathbf{0.505}$
• Rule of Thumb: Every 1.0 mg/L of alum consumes 0.45 to 0.50 mg/L of natural alkalinity (as $\text{CaCO}_3$).

Ferric Chloride Reaction Stoichiometry

$2\text{FeCl}_3 + 3\text{Ca}(\text{HCO}_3)_2 \rightarrow 2\text{Fe}(\text{OH})_3\downarrow + 3\text{CaCl}_2 + 6\text{CO}_2$

• Molecular weight of 2 moles $\text{FeCl}_3$ = $2 \times 162.2 = 324.4$ g/mol.
• 3 moles of $\text{CaCO}_3$ equivalent = 300.24 g/mol.
• Ratio: $300.24 / 324.4 = \mathbf{0.925}$
• Rule of Thumb: Every 1.0 mg/L of ferric chloride consumes ~0.92 mg/L of natural alkalinity (as $\text{CaCO}_3$).

Consequences of Depleted Alkalinity

If raw water alkalinity is insufficient to satisfy the coagulant demand, the following operational crises occur:

1. pH Plunge: The water loses buffering capacity, causing pH to crash below 5.5 (or below 4.5 with ferric).
2. Soluble Metal Pass-Through: In acidic conditions, metal hydroxides cannot precipitate into solid flocs and remain in soluble ionic form ($Al^{3+}$, $Fe^{2+/3+}$). These unprecipitated ions pass straight through filter media.
3. Post-Precipitation Haze: When finished water pH is adjusted upward prior to distribution, dissolved aluminum precipitates inside transmission mains as a milky-white colloidal haze, while unreacted iron precipitates as insoluble rust particles, causing widespread red-water complaints and distribution line tuberculation.
4. Residual Alkalinity Standard: To ensure chemical stability and prevent pH collapse, water plants must maintain a settled water residual alkalinity of at least 20 to 30 mg/L as $\text{CaCO}_3$.

Supplemental Alkalinity Chemicals

When raw water alkalinity is inadequate, supplemental base must be added ahead of or at the point of coagulant feed:

• Hydrated Lime [$\text{Ca}(\text{OH})_2$]: 90% purity powder fed as a slurry. Adds calcium hardness; consumes carbon dioxide.
• Quicklime [$\text{CaO}$]: 90% to 95% purity. Requires a slaker to generate hydrated lime in an exothermic reaction ($CaO + H_2O \rightarrow Ca(OH)_2 + \text{Heat}$). Economical for large facilities (> 10 MGD).
• Soda Ash (Sodium Carbonate, $\text{Na}_2\text{CO}_3$): Readily soluble dry chemical. Adds carbonate alkalinity without adding calcium hardness; easy to handle and dissolve.
• Caustic Soda (Sodium Hydroxide, $\text{NaOH}$): Liquid chemical supplied as 25% or 50% solution. Critical Operational Fact: 50% caustic soda crystallizes and freezes at 54°F (12°C). Bulk tanks and delivery pipes must be housed in heated rooms or equipped with heating pads and insulated heat-tracing.
• Sodium Bicarbonate ($\text{NaHCO}_3$): Safest chemical to handle; raises alkalinity without sharply overshooting pH.

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Coagulant Aids and Synthetic Polyelectrolytes

Polymers are high-molecular-weight synthetic organic compounds consisting of long repeating chains of monomers. They are categorized based on electrical charge:

• Cationic Polymers: Positively charged, lower-to-medium molecular weight ($10^4$ to $10^6$ Da). Used as primary coagulants or coagulant aids to neutralize negative colloidal charges. Can reduce required alum doses by 30% to 50% and dramatically reduce sludge volume.
• Anionic Polymers: Negatively charged, very high molecular weight ($10^6$ to $10^7$ Da). Function strictly as flocculant aids and bridging agents to agglomerate pin floc into large macro-flocs for rapid sedimentation.
• Nonionic Polymers: Neutral charge, high molecular weight. Function via hydrogen bonding and physical interparticle bridging.

Polymer Feed, Aging, and Handling Rules

1. Wetting and Activation: Dry polymers must be introduced through a specialized wetting cone or eductor to hydrate individual particles instantly. Incomplete wetting produces "fish-eyes"—gel-like clumps with dry, unhydrated cores that plug feed systems and waste expensive chemical.
2. Aging Time: Once wetted, polymer solutions must mix gently in a dedicated aging tank for 30 to 60 minutes before feeding. This allows tightly coiled polymer macromolecules to fully uncoil and expose their reactive binding sites.
3. Solution Viscosity: Working polymer solutions are prepared at low concentrations—typically 0.1% to 0.5% by weight. Concentrations above 0.5% to 1.0% become excessively viscous and resist accurate metering.
4. Shear Degradation: Long polymer chains are fragile. High-speed centrifugal pumps or violent mechanical impellers will tear (shear) the molecular chains in half, destroying their bridging capability. Plants must use low-shear progressive cavity pumps or diaphragm metering pumps to transfer polymer solutions.

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Chemical Reference Tables

Table 1: Primary Coagulant Characteristics and Operational Specifications

Table 2: Supplemental Alkalinity Chemicals Comparison