3.2 Coagulation Chemistry & Flash Mixing

Coagulation Chemistry & Flash Mixing

Fundamentals of Colloid Chemistry & Particle Stability

Raw surface waters contain vast quantities of suspended and colloidal matter that impart turbidity, color, and microbiological risk. Colloids are microscopic particles ranging in diameter from 0.001 micrometers ($1,\text{nm}$) to 1.0 micrometer ($1,\mu\text{m}$). Because of their minute mass and immense surface-area-to-volume ratio, gravity settling is negligible; a $0.1,\mu\text{m}$ clay colloid would take decades to settle 1 foot under quiescent conditions.

Colloids in natural waters are classified into two broad categories:

1. Hydrophobic Colloids (Water-Fearing): Clays, silts, and metal oxides that possess no affinity for water molecules. Their stability in suspension is maintained strictly by surface electrostatic charges.
2. Hydrophilic Colloids (Water-Loving): Natural organic matter (NOM), humic and fulvic acids, proteins, and cellular polymers that possess chemically bound water sheaths, requiring both charge neutralization and chemical destabilization.

                      [Colloid Suspension Forces]
                 ◄── [Electrostatic Repulsion] ──►
                      (-) Colloid     Colloid (-)
                            ▲             ▲
                            └───► [Van der Waals] ◄───┘
                                (Attractive Force)

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The Electrical Double Layer & Zeta Potential

Virtually all naturally occurring colloids carry a net negative surface charge due to isomorphous substitution in clay crystal lattices, ionization of surface hydroxyl/carboxyl groups, and adsorption of natural organic anions.

The Double Layer Structure

To balance this negative surface charge, dissolved cations in the surrounding water arrange themselves around the colloid, forming an Electrical Double Layer consisting of:

• Stern Layer (Fixed Layer): Strongly bound, rigid layer of positive counter-ions directly attached to the colloid surface.
• Gouy-Chapman Diffuse Layer: A surrounding dynamic cloud where positive ion concentration gradually decreases outward into the bulk solution.
• Plane of Shear (Slipping Plane): The boundary where the liquid moves with the particle as it moves through the water.

  Colloid Surface | Stern Layer | Diffuse Layer (Gouy-Chapman) | Bulk Water
  ════════════════╪═════════════╪══════════════════════════════╪═══════════
    Negative (-)  │ Positive(+) │ Cloud of (+) and (-) ions    │ Neutral
    Charge        │ Fixed Ions  │ Moving dynamic layer         │ Solution
                  │             │                              │
                  ▲             ▲                              ▲
             Stern Plane   Slipping Plane (Zeta Potential)   Bulk Boundary

Zeta Potential Dynamics

The electrical potential measured at the slipping plane is the Zeta Potential ($\zeta$), expressed in millivolts (mV): $\text{Typical Raw Water Colloid Zeta Potential} = -15\,\text{mV to } -30\,\text{mV}$ $\text{Coagulation Target Range} = -3\,\text{mV to } 0\,\text{mV (Isoelectric Point)}$

According to DLVO Theory (Derjaguin, Landau, Verwey, and Overbeek), particle stability is governed by the net balance between repulsive electrostatic forces and attractive Van der Waals forces. When the negative Zeta Potential exceeds $-15,\text{mV}$, repulsive forces prevent particles from colliding. The primary purpose of chemical coagulation is to compress the diffuse layer, neutralize the negative Zeta Potential toward zero, and allow attractive Van der Waals forces to bond colliding particles.

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The Four Primary Coagulation Mechanisms

Coagulation occurs through four distinct physical-chemical mechanisms depending on coagulant dosage, pH, and raw water quality:

┌────────────────────────────────────────────────────────────────────────┐
│                     FOUR COAGULATION MECHANISMS                         │
├──────────────────────────┬─────────────────────────────────────────────┤
│ 1. Charge Neutralization │ Trivalent metal ions (Al³⁺, Fe³⁺) compress  │
│    (Double Layer Comp.)  │ diffuse layer; drops Zeta Potential to ~0 mV│
├──────────────────────────┼─────────────────────────────────────────────┤
│ 2. Sweep Floc            │ High coagulant dose forms Al(OH)₃/Fe(OH)₃   │
│    (Precipitate Enmesh)  │ precipitates that physically trap colloids  │
├──────────────────────────┼─────────────────────────────────────────────┤
│ 3. Interparticle         │ Long-chain synthetic polymers bind multiple │
│    Bridging              │ colloids together across fluid gaps         │
├──────────────────────────┼─────────────────────────────────────────────┤
│ 4. Adsorption & Charge   │ Cationic polymer patches create localized   │
│    Patch Neutralization  │ (+) zones attracting adjacent (-) surfaces  │
└──────────────────────────┴─────────────────────────────────────────────┘

1. Charge Neutralization (Double Layer Compression): Highly charged trivalent metal cations ($Al^{3+}$, $Fe^{3+}$) adsorb onto colloid surfaces within microseconds, compressing the electrical double layer. This mechanism dominates at low-to-moderate coagulant dosages under acidic/neutral pH.
2. Sweep Floc / Precipitate Enmeshment: At higher coagulant dosages and optimal pH, metal coagulants rapidly exceed the solubility limit of metal hydroxides, forming amorphous precipitates of Aluminum Hydroxide ($Al(OH)_3(s)$) or Ferric Hydroxide ($Fe(OH)_3(s)$). These heavy, sticky precipitates physically enmesh and sweep colloidal particles and pathogens from suspension. This is the primary mechanism in low-turbidity raw waters.
3. Interparticle Bridging: Long-chain, high-molecular-weight synthetic polymers adsorb simultaneously onto two or more colloidal particles, forming physical molecular bridges.
4. Adsorption and Charge Patch Mechanism: Highly charged cationic polymers adsorb to localized patches on negatively charged colloids, creating alternating positive and negative surface regions that attract neighboring particles.

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Primary Coagulants: Chemistry, Reactions & Alkalinity Demand

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

Commercial alum is the most widely used primary coagulant in municipal drinking water. When added to water, alum hydrolyzes rapidly to form soluble monomeric and polymeric species, ultimately precipitating as insoluble aluminum hydroxide: $Al_2(SO_4)_3 \cdot 14H_2O + 3Ca(HCO_3)_2 \rightarrow 2Al(OH)_3(s) \downarrow + 3CaSO_4 + 6CO_2 \uparrow + 14H_2O$

• Optimal pH Range: $5.8\text{ to }7.5$ (Optimal sweep floc occurs at pH 6.5–7.2; optimal TOC removal occurs at pH 5.8–6.3).
• Alkalinity Consumption Stoichiometry: $1.0\,\text{mg/L of commercial alum consumes } 0.50\,\text{mg/L of natural alkalinity as } CaCO_3$
• Operating Rule: If raw water alkalinity is insufficient (<30–45 mg/L), the reaction exhausts natural bicarbonate buffering, causing plant pH to plummet (<5.2). Incomplete hydrolysis occurs, leaving soluble $Al^{3+}$ in finished water (causing post-precipitation turbidity in distribution systems). Supplemental alkalinity (hydrated lime $Ca(OH)_2$, soda ash $Na_2CO_3$, or caustic soda $NaOH$) must be dosed.

2. Ferric Chloride ($FeCl_3$)

Ferric chloride is an aggressive, heavy metal coagulant widely utilized across California surface waters, especially for high-TOC and variable-pH supplies: $2FeCl_3 + 3Ca(HCO_3)_2 \rightarrow 2Fe(OH)_3(s) \downarrow + 3CaCl_2 + 6CO_2 \uparrow$

• Optimal pH Range: Broad operating window of $4.0\text{ to }11.0$.
• Alkalinity Consumption Stoichiometry: $1.0\,\text{mg/L of } FeCl_3 \text{ consumes } 0.92\,\text{mg/L of natural alkalinity as } CaCO_3$
• Key Advantages: Forms denser, heavier flocs with specific gravities ($\approx 1.34$) significantly higher than alum flocs ($\approx 1.05$), yielding faster clarifier settling rates and superior cold-water performance.

3. Polyaluminum Chloride (PAC) & Aluminum Chlorohydrate (ACH)

PAC and ACH are pre-hydrolyzed polymeric aluminum coagulants manufactured by reacting aluminum chloride with base, yielding pre-formed high-charge polymers such as the $Al_{13}$ Keggin ion ($[Al_{13}O_4(OH){24}(H_2O){12}]^{7+}$).

• Alkalinity Consumption: Consumes only $0.15\text{ to }0.25,\text{mg/L}$ alkalinity as $CaCO_3$ per mg/L chemical.
• Key Advantages: Minimal pH depression, superior performance in cold water (<5°C), lower sludge production, and significantly reduced residual dissolved aluminum in finished water.

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Flash Mixing Design, Hydraulics & Velocity Gradients

Flash mixing (rapid mixing) is the critical physical process of instantly and uniformly dispersing the coagulant chemical into the raw water stream before hydrolysis species polymerize.

Coagulant Hydrolysis Kinetics

When metal coagulants contact water, the initial hydrolysis reactions (forming trivalent and polymeric hydroxo-complexes) occur within 100 to 1,000 milliseconds (0.1 to 1.0 second). If mixing is sluggish, the chemical reacts with itself, precipitating into inactive localized clumps rather than destabilizing raw colloids.

The Velocity Gradient ($G$-Value)

Mixing intensity is quantified by the Camp-Stein Velocity Gradient ($G$), expressed in reciprocal seconds ($s^{-1}$): $G = \sqrt{\frac{P}{\mu V}}$ Where:

• $P = \text{Power dissipated in the fluid (ft-lb/s or Watts)}$
• $\mu = \text{Absolute dynamic viscosity of water } (lb\cdot s/ft^2 \text{ or } Pa\cdot s)$
• $V = \text{Mixing chamber volume } (ft^3 \text{ or } m^3)$

Flash Mixing Engineering Criteria

• Target $G$-Value: $G = 700\text{ to }1000+,s^{-1}$
• Hydraulic Retention Time (HRT): $0.5\text{ to }5.0,\text{seconds}$ for charge neutralization; up to $10\text{ to }30,\text{seconds}$ for sweep floc.

                        [Flash Mix Configurations]
  ┌───────────────────────┬───────────────────────┬───────────────────────┐
  │ In-Line Static Mixer  │ Mechanical Rapid Mix  │ Hydraulic Jump Mixer  │
  │                       │                       │                       │
  │ [>>>><<<>>>>] Pipe    │ [  |===| Impeller ]   │ [~~~~\] Drop Weir     │
  │ Inst. energy, 0 power │ High G-value chamber  │ Natural head loss     │
  └───────────────────────┴───────────────────────┴───────────────────────┘

1. In-Line Static Mixers: Internal geometric fixed mixing elements within a pipeline that induce severe fluid shearing; achieves instantaneous mixing in <1 second with zero moving mechanical parts.
2. Mechanical Rapid Mixers: High-speed vertical turbine or radial impellers operating inside a compact, baffled chamber.
3. Hydraulic Jump Mixers: Utilizes the kinetic energy of supercritical hydraulic jumps at Parshall flumes or weir drops; eliminates mechanical mixer maintenance.

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Practical Operational Scenarios & Troubleshooting

Scenario 1: Alkalinity Exhaustion and Secondary Coagulation Failure

Situation: A water treatment plant treating an Sierra snowmelt river experiences a winter storm. Turbidity rises from 5 NTU to 65 NTU. The operator increases the alum dose from 12 mg/L to 45 mg/L. Suddenly, finished water turbidity increases, floc in the sedimentation basin disappears into a milky haze, and plant pH drops from 7.4 to 5.1. Diagnostic: Raw water alkalinity was only $20,\text{mg/L as } CaCO_3$. The $45,\text{mg/L}$ alum dose required $45 \times 0.50 = 22.5,\text{mg/L}$ of alkalinity, completely consuming the buffering capacity and depressing pH below the alum precipitation threshold. Operator Action:

1. Immediately start chemical feeding of Caustic Soda ($NaOH$) or Soda Ash ($Na_2CO_3$) at the flash mixer.
2. Target a coagulation pH of $6.8\text{ to }7.2$ and maintain a residual finished alkalinity $\ge 20,\text{mg/L as } CaCO_3$.

Scenario 2: Chemical Overdosing and Charge Reversal

Situation: An operator notices settled water turbidity rising and assumes the plant is under-dosed. The operator increases cationic polymer dosage. Settled turbidity worsens, and filtered water turbidity exceeds 0.3 NTU. Diagnostic: Excessive cationic coagulant has covered all colloid surfaces with excess positive charges, driving the Zeta Potential from $-20,\text{mV}$ past zero to $+15,\text{mV}$. This charge reversal causes positive-positive electrostatic repulsion, restabilizing the colloidal suspension. Operator Action: Immediately run a jar test, measure Zeta Potential, and reduce coagulant feed to re-establish the isoelectric operating point.