2.2 Coagulation Chemistry & Rapid Mixing

2.2 Coagulation Chemistry & Rapid Mixing

Coagulation is the chemical process of destabilizing suspended colloids and fine particles so they can aggregate during subsequent flocculation. Because natural contaminants carry electrical charges that resist gravitational settling, chemical coagulants must be added to neutralize repulsive forces. Certified operators must master colloidal surface chemistry, metal hydrolysis reactions, alkalinity consumption stoichiometry, flash mix hydraulics, and jar testing protocols.

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1. Colloidal Stability & Electrical Double Layer

Natural surface waters contain suspended colloids ranging from 0.001 to 1.0 micrometer (µm) in size (clays, silts, humic/fulvic acids, bacteria, viruses). Due to minute particle mass, gravitational settling velocities are negligible, while thermal molecular motion (Brownian motion) keeps them suspended indefinitely.

Origin of Surface Charge & Zeta Potential

Almost all natural colloids carry a net negative electrical surface charge originating from:

1. Isomorphic Substitution: Imperfections in clay mineral lattices (e.g., $\text{Al}^{3+}$ substituting for $\text{Si}^{4+}$ in silicate sheets).
2. Ionization of Functional Groups: Dissociation of carboxyl ($-\text{COOH} \rightleftharpoons -\text{COO}^- + \text{H}^+$) and hydroxyl ($-\text{OH}$) groups on organic matter.
3. Preferential Adsorption: Adsorption of natural organic anions onto mineral surfaces.

Dissolved ions organize around each particle, forming an electrical double layer:

• Stern Layer: Inner, rigid layer of positive counter-ions bound tightly to the colloidal surface by electrostatic attraction.
• Gouy-Chapman Diffuse Layer: Outer dynamic zone of cations and anions whose electrostatic attraction diminishes into the bulk solution.
• Slipping Plane: Hydrodynamic boundary separating fluid bound to the moving colloid from bulk water.
• Zeta Potential ($\zeta$): The electrical potential measured at the slipping plane. In natural raw water, colloidal zeta potential ranges between -15 mV and -30 mV.

Under DLVO theory, stability reflects the balance between attractive van der Waals forces and repulsive electrostatic forces. The negative zeta potential creates an energy barrier preventing particle contact. Coagulation compresses or neutralizes this charge, shifting zeta potential toward zero (-5 to +5 mV) to allow van der Waals attraction to dominate.

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2. Four Coagulation Destabilization Mechanisms

Coagulants destabilize colloidal suspensions through four pathways:

1. Double Layer Compression: Increasing bulk water ionic strength collapses the Gouy-Chapman layer. Under the Schulze-Hardy Rule, counter-ion coagulating power increases with the sixth power of valence ($z^6$). Trivalent cations ($\text{Al}^{3+}$, $\text{Fe}^{3+}$) are 700 to 1,000 times more effective than monovalent ions ($\text{Na}^+$).
2. Charge Neutralization: Positively charged metal hydrolysis species ($\text{Al(OH)}^{2+}$, $\text{Fe(OH)}^{2+}$) adsorb directly onto negative colloidal surfaces. Caution: Overdosing coagulant reverses particle charge to positive ($>+10\text{ mV}$), re-stabilizing colloids and elevating finished turbidity.
3. Inter-Particle Bridging: High-molecular-weight polymers attach simultaneously to multiple colloids, forming physical bridges that bind them into a settleable matrix.
4. Sweep Flocculation: In low-turbidity waters ($<5\text{ NTU}$), coagulant is dosed in excess of solubility to precipitate amorphous metal hydroxides ($\text{Al(OH)}_3$ or $\text{Fe(OH)}_3$). As these sticky precipitates settle, they physically enmesh and sweep colloidal impurities from the water column.

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3. Primary Coagulants & Alkalinity Stoichiometry

Metallic salts of aluminum and iron serve as primary chemical coagulants, reacting directly with natural alkalinity.

Aluminum Sulfate (Commercial Alum)

Commercial alum has the nominal formula $\text{Al}_2(\text{SO}_4)_3\cdot14\text{H}_2\text{O}$. When added to water, it hydrolyzes and consumes natural bicarbonate alkalinity: $\text{Al}_2(\text{SO}_4)_3\cdot14\text{H}_2\text{O} + 3\text{Ca(HCO}_3)_2 \longrightarrow 2\text{Al(OH)}_3\downarrow + 3\text{CaSO}_4 + 6\text{CO}_2 + 14\text{H}_2\text{O}$

• Alkalinity Consumption: Every $1.0\text{ mg/L}$ of commercial alum consumes approximately $0.50\text{ mg/L}$ of natural alkalinity as $\text{CaCO}_3$.
• pH Depression: The reaction generates carbon dioxide ($\text{CO}_2$), forming carbonic acid and depressing process pH.
• Optimum Coagulation pH: Alum precipitates insolubly between pH 5.8 and 7.5. If raw alkalinity is deficient ($<30\text{--}45\text{ mg/L as }\text{CaCO}_3$), pH plunges below 5.8, causing aluminum hydroxide to dissolve into soluble ionic aluminum ($\text{Al}^{3+}$) that passes into finished water. Supplemental alkali (lime $\text{Ca(OH)}_2$, soda ash $\text{Na}_2\text{CO}_3$, or caustic $\text{NaOH}$) must be fed to maintain buffering capacity.

Ferric Salts (Ferric Chloride & Ferric Sulfate)

• Ferric Chloride ($\text{FeCl}_3$): Hydrolyzes to insoluble ferric hydroxide, consuming $0.92\text{ mg/L}$ of alkalinity as $\text{CaCO}_3$ per $1.0\text{ mg/L}$ of $\text{FeCl}_3$.
• Ferric Sulfate ($\text{Fe}_2(\text{SO}_4)_3$): Consumes $0.75\text{ mg/L}$ of alkalinity per $1.0\text{ mg/L}$.
• Operational Advantages: Ferric salts operate across a wide pH range (4.0 to 11.0). Ferric flocs are denser and heavier than alum flocs, producing faster settling. Crucially, ferric coagulants perform reliably in cold water ($<4^\circ\text{C} / 39^\circ\text{F}$), where alum hydrolysis kinetics slow down significantly.

Polyaluminum Chloride (PACl) & Aluminum Chlorohydrate (ACH)

PACl and ACH are pre-hydrolyzed aluminum polymers with basicities ranging from 50% to 85%. Because hydroxyl bonds are pre-incorporated into the chemical complex, PACl consumes 50% to 80% less natural alkalinity than alum, minimizes pH depression, generates up to 40% less chemical sludge, and functions effectively in cold water.

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4. Coagulant Aids & Adjuvants

Coagulant aids improve floc density, settleability, and shear resistance:

• Synthetic Polymers: Cationic polymers possess high positive charge density, serving as primary coagulants or aids to reduce metal doses. Anionic polymers feature high molecular weights and are applied after rapid mix as bridging aids. Nonionic polymers provide inter-particle bridging.
• Activated Silica: Polysilicic acid prepared by partially neutralizing sodium silicate with acid or chlorine. It strengthens alum flocs against hydraulic shear during high-rate filtration.
• Weighting Agents (Bentonite Clay): Fed to cold, low-turbidity waters ($<2\text{ NTU}$) to supply particulate nuclei, accelerating collisions for sweep flocculation.

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5. Flash Mixing Hydraulics & Velocity Gradients

Flash mixing (rapid mix) achieves instantaneous, homogeneous dispersion of coagulant chemicals across raw water. Primary metal hydrolysis occurs within fractions of a second ($<0.1\text{ to }1.0\text{ second}$). Inadequate mixing leads to local overdosing and untreated bypassed colloids.

• Mixing Systems: Mechanical high-speed turbine impellers in baffled tanks; static in-line mixers utilizing pipeline head loss across helical elements; and hydraulic jumps in Parshall flumes or overflow weirs.
• Velocity Gradient ($G$-value): Measures turbulence intensity ($G = \sqrt{P / (\mu \cdot V)}$). Maintained between $700\text{ and }1,000\text{ sec}^{-1}$ (static mixers can exceed $1,500\text{ sec}^{-1}$).
• Detention Time: Strictly limited to $10\text{ to }60\text{ seconds}$ (typically 15 to 30 seconds).

Operational Constraint: Detention times exceeding 60 seconds or excessive mechanical shear will prematurely shatter emerging polymeric microflocs, permanently impairing downstream flocculation.

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6. Jar Testing Procedure & Optimization Curves

The laboratory jar test simulates full-scale treatment to evaluate coagulant selection, optimize dosages, and set operating parameters.

Standard Apparatus & Protocol

A 6-station gang stirrer with illuminated base operates inside 1-liter to 2-liter square B-Ker jars:

1. Sample Collection: Fill jars with fresh raw water at ambient stream temperature.
2. Chemical Addition: Add staggered dosages of coagulant (e.g., 10, 15, 20, 25, 30, 35 mg/L) simultaneously across all six jars.
3. Rapid Mix Stage: Agitate paddles at $100\text{ to }150\text{ rpm}$ for 60 seconds (simulating $G = 700\text{--}1,000\text{ sec}^{-1}$).
4. Flocculation Stage: Lower agitation to simulate tapered flocculation: $40\text{ rpm}$ for 10 minutes, then $20\text{ rpm}$ for 10 minutes.
5. Settling Stage: Allow quiescent settling for $15\text{ to }20\text{ minutes}$.

Analytical Evaluation Criteria

• Floc Characteristics & Settling: Record time to initial pinhead formation, observe macrofloc structure, and measure interface settling speed (inches/minute).
• Supernatant Turbidity & pH: Collect clarified water 10 cm below the surface and analyze with a calibrated ratio nephelometer (target: $<1.0\text{ to }2.0\text{ NTU}$). Measure settled water pH and residual alkalinity.
• UV-254 Absorbance: Quantifies natural organic matter (NOM) removal for Disinfection Byproduct Rule TOC precursor compliance.
• Optimization Curve: Plotting settled turbidity against coagulant dose produces an optimization curve. The optimal dose is selected at the "knee" of the curve—the lowest chemical concentration achieving compliance turbidity. Dosing beyond this threshold wastes chemicals, accelerates sludge accumulation, and risks colloidal charge reversal.