5.1 Coagulation, Flocculation & Jar Testing Dynamics

3.2 Coagulation, Flocculation & Jar Testing Dynamics

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Colloidal Chemistry in Drinking Water

Suspended matter in raw surface water consists predominantly of colloids—particles ranging in size from $0.001 \ \mu\text{m}$ to $1.0 \ \mu\text{m}$ (including clays, silt, natural organic macromolecules, viruses, and bacteria). Colloidal suspensions are electrostatically stable and will not settle naturally due to two primary physical factors:

1. Brownian Motion: Continuous thermal kinetic bombardment by surrounding water molecules keeps microscopic colloids permanently agitated and suspended against gravitational forces.
2. Negative Surface Charge: Colloidal surfaces possess net negative electrical charges resulting from:
   • Isomorphic Substitution: Within clay mineral crystalline lattices, trivalent aluminum ions ($\text{Al}^{3+}$) frequently substitute for tetravalent silicon ($\text{Si}^{4+}$), leaving an unneutralized negative crystal lattice charge.
   • Surface Ionization: Functional carboxylic ($-\text{COOH} \rightarrow -\text{COO}^- + \text{H}^+$) and phenolic ($-\text{OH} \rightarrow -\text{O}^- + \text{H}^+$) groups on natural humic and fulvic matter ionize at environmental pH levels, generating negative surface sites.

+-----------------------------------------------------------------------------+
|                   ELECTRICAL DOUBLE LAYER & ZETA POTENTIAL                  |
+-----------------------------------------------------------------------------+
|                                                                             |
|   [ COLLOID SURFACE ]   |     STERN LAYER     |     DIFFUSE LAYER           |
|                         |                     |                             |
|         (-)             |         (+)         |        (+)      (-)         |
|         (-)             |         (+)         |             (+)             |
|         (-)             |         (+)         |        (-)      (+)         |
|         (-)             |         (+)         |                             |
|                         |                     |                             |
|                         |<--- Stern Plane     |                             |
|                                               |<--- Shear Plane             |
|                                                     (Zeta Potential, ζ)     |
|                                                                             |
|   Natural Water:  ζ = -15 to -30 mV  (Electrostatic Repulsion / Stable)     |
|   Coagulated:     ζ = -5 to +5 mV    (Destabilized / Rapid Aggregation)     |
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The Electrical Double Layer & Zeta Potential

When immersed in water, a negatively charged colloid attracts dissolved positive counter-ions (cations) from the bulk solution, creating an electrical double layer:

• Fixed Stern Layer: A rigid, tightly bound monolayer of positive cations held firmly against the particle surface by electrostatic attraction.
• Gouy-Chapman Diffuse Layer: A broader, dynamic outer zone containing an excess of cations that gradually decreases in concentration until ionic equilibrium with the bulk solution is established.
• Shear Plane (Slipping Plane): The hydrodynamic boundary separating the fluid layer that travels with the moving colloid from the stationary bulk fluid.
• Zeta Potential ($\zeta$): The measurable electrostatic potential at the shear plane. In untreated surface water, colloids exhibit zeta potentials ranging from $-15 \text{ mV to } -30 \text{ mV}$. This negative potential produces strong electrostatic repulsive forces that exceed short-range van der Waals attractive forces, keeping particles separated. Coagulation requires lowering the zeta potential to between $-5 \text{ mV and } +5 \text{ mV}$ to allow van der Waals attraction to dominate upon collision.

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Primary Mechanisms of Coagulation

Coagulation destabilizes colloidal suspensions through four distinct physical-chemical pathways:

1. Charge Neutralization

Positively charged trivalent metal hydrolysis products (e.g., $\text{Al}^{3+}$, $\text{Al(OH)}^{2+}$, $\text{Al}_7(\text{OH})_{17}^{4+}$, $\text{Fe}^{3+}$, $\text{Fe(OH)}^{2+}$) adsorb directly onto the negatively charged colloidal surfaces within $0.01 \text{ to } 0.1 \text{ second}$ of rapid mixing. This neutralizes surface charge, collapses the diffuse double layer, and reduces zeta potential toward zero, allowing particles to bond upon contact.

2. Sweep Flocculation (Enmeshment in Precipitate)

When inorganic coagulant is dosed in excess of the solubility limit of the metal hydroxide at neutral pH, it rapidly precipitates voluminous, amorphous clouds of aluminum hydroxide [$\text{Al(OH)}_3\text{(s)}$] or ferric hydroxide [$\text{Fe(OH)}_3\text{(s)}$]. As this dense chemical precipitate settles through the water column, it physically entraps and enmeshes colloidal particles. Sweep flocculation is the primary operational mechanism in low-turbidity Pacific Northwest waters ($< 5 \text{ NTU}$), where particle concentrations are too sparse for frequent inter-particle collisions via charge neutralization alone.

3. Inter-Particle Bridging

High-molecular-weight synthetic polymers or long-chain organic molecules adsorb simultaneously onto multiple colloidal particles. The polymer chain extends across the electrostatic boundary, forming a physical "bridge" that binds particles into robust macroflocs.

4. Patch Coagulation (Charge Mosaic)

Highly charged cationic polymers adsorb in localized, concentrated positive "patches" on a negatively charged colloid. These positively charged zones attract bare, negatively charged sites on adjacent particles, binding them in a checkerboard electrostatic mosaic.

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Common Inorganic Coagulants: Chemistry & Hydraulics

Aluminum Sulfate Dynamics & Alkalinity Depletion

Commercial dry alum has an approximate molecular weight of $594 \text{ g/mol}$. When introduced into water containing natural bicarbonate alkalinity, it undergoes the following reaction:

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

• Alkalinity Consumption: Each $1.0 \text{ mg/L}$ of commercial dry alum consumes approximately $0.50 \text{ mg/L}$ of natural alkalinity (as $\text{CaCO}_3$). In poorly buffered Cascade mountain rivers (where baseline alkalinity may be only $12-16 \text{ mg/L}$), feeding an alum dose of $25 \text{ mg/L}$ during a winter storm will consume $12.5 \text{ mg/L}$ of alkalinity. This completely exhausts the water's buffering capacity, releasing excess carbonic acid ($\text{CO}_2$) and driving the pH down to $4.8-5.2$.
• Failure Modes: At $\text{pH} < 5.8$, aluminum hydroxide cannot precipitate; instead, soluble toxic aluminum ions ($\text{Al}^{3+}$) persist, causing total coagulation failure and finished water aluminum carryover. Operators treating low-alkalinity waters must feed supplemental alkali—such as hydrated lime [$\text{Ca(OH)}_2$], caustic soda [$\text{NaOH}$], or soda ash [$\text{Na}_2\text{CO}_3$]—to maintain coagulation pH within the $6.2-7.2$ window.

Polyaluminum Chloride (PACl) & Aluminum Chlorohydrate (ACH)

PACl and ACH are pre-polymerized, pre-hydrolyzed aluminum salts featuring high basicity ($50-83\%$). Because the aluminum is already partially reacted with hydroxyl ions ($\text{OH}^-$), they offer tremendous advantages in Oregon:

1. Superior Cold-Water Performance: In icy mountain waters ($< 4^\circ\text{C}$), traditional alum hydrolysis kinetics stall. Pre-hydrolyzed PACl reacts almost independently of temperature.
2. Alkalinity Preservation: Consumes $30-70\%$ less natural alkalinity than alum, often eliminating the capital expense and hazard of supplemental lime or caustic systems.
3. Reduced Sludge Footprint: Produces a tighter, denser floc that settles faster and reduces clarifier sludge disposal volumes by up to $40\%$.

Iron Coagulants

Ferric chloride ($\text{FeCl}_3$) and ferric sulfate [$\text{Fe}_2(\text{SO}_4)_3$] operate across a vast pH range ($4.0-11.0$). Their heavier molecular weight creates dense, shear-resistant flocs that settle up to twice as fast as alum flocs. They are exceptionally effective at removing natural organic color at acidic pH levels ($4.5-5.5$). However, ferric salts are highly corrosive (requiring specialized PVDF, Teflon, or rubber-lined pipe systems) and can cause severe rust staining if carried over into the clearwell.

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Coagulant Aids & Polymer Conditioning

Polymers are long-chain organic macromolecules classified by their ionic charge:

• Cationic Polymers: Carry positive charges (e.g., polyDADMAC, polyamines). Used as primary coagulants or coagulant aids. They reduce inorganic coagulant dosages by up to $50\%$, add no dissolved salts, and do not consume natural alkalinity.
• Anionic Polymers: Carry negative charges (e.g., hydrolyzed polyacrylamides). Used strictly as flocculant aids ($0.05-0.20 \text{ mg/L}$) added at the discharge of the rapid mix or first flocculation stage to bind microfloc into massive, tough macrofloc aggregates.
• Nonionic Polymers: Neutral polyacrylamides that function via mechanical bridging and hydrogen bonding.
• Regulatory Limits (EPA treatment technique and NSF/ANSI/CAN 60): Treatment chemicals must be certified to NSF/ANSI/CAN 60. Separately, the federal acrylamide and epichlorohydrin treatment technique (40 CFR 141.111) requires certification that residual monomer is limited: acrylamide monomer is restricted to $\le 0.05\%$ at a maximum application dose of $1.0 \text{ mg/L}$, and epichlorohydrin is limited to $\le 0.01\%$ at a maximum dose of $20 \text{ mg/L}$.

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Rapid Mixing & Flocculation Basin Hydraulics

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|                        RAPID MIX vs. FLOCCULATION                           |
+-----------------------------------+-----------------------------------------+
|          RAPID MIXING             |         TAPERED FLOCCULATION            |
|  - Detention Time: 1 to 5 seconds |  - Detention Time: 20 to 45 minutes     |
|  - Velocity Gradient: 700-1000/s  |  - Velocity Gradient: 50 down to 20/s   |
|  - Flash mix, Static mixer        |  - Stage 1: G = 50-70/s (Microfloc)     |
|  - Goal: Instantaneous chemical   |  - Stage 2: G = 30-40/s (Growth)        |
|    dispersion before hydrolysis   |  - Stage 3: G = 15-20/s (Shear Prevent) |
|    species polymerize             |  - Goal: Agglomerate large macroflocs   |
+-----------------------------------+-----------------------------------------+

Rapid Mixing Hydraulics

The objective of the rapid mix chamber is to disperse chemical coagulants throughout the raw water stream instantaneously (within fractions of a second), before metal ions can precipitate as unattached inert hydroxides:

• Detention Time ($t$): $1 \text{ to } 5 \text{ seconds}$ for inline static mixers; $10 \text{ to } 30 \text{ seconds}$ for mechanical chambers. Prolonged rapid mixing ($> 60 \text{ seconds}$) is detrimental, as it shears early microscopic nuclei.
• Velocity Gradient ($G$-value): Maintained between $700 \text{ and } 1000 \text{ s}^{-1}$ (defined by Camp and Stein's equation: $G = \sqrt{P / (\mu V)}$, where $P$ is power dissipated, $\mu$ is dynamic viscosity, and $V$ is basin volume).
• Mixer Configurations: Inline static mixers with internal helical elements, pressurized injection nozzles, Parshall flumes (hydraulic jumps), and vertical high-speed mechanical turbine impellers.

Flocculation Hydraulics & Tapered Mixing

Flocculation provides gentle, prolonged agitation to bring destabilized microflocs into repeated physical contact without tearing them apart:

• Detention Time: $20 \text{ to } 45 \text{ minutes}$.
• Camp Number ($Gt$): A dimensionless parameter calculated as velocity gradient multiplied by detention time. Optimal drinking water flocculation targets a total $Gt$ value of $10,000 \text{ to } 100,000$. If $Gt < 10,000$, particle collisions are inadequate; if $Gt > 100,000$, excessive hydraulic turbulence breaks apart floc.
• Tapered Energy Gradient: As chemical flocs grow larger, their physical shear strength declines. Flocculation basins utilize three or more sequential chambers with tapered $G$-values:
  • Stage 1: $G = 50 \text{ to } 70 \text{ s}^{-1}$ (rapid microfloc formation)
  • Stage 2: $G = 30 \text{ to } 40 \text{ s}^{-1}$ (intermediate floc enlargement)
  • Stage 3: $G = 15 \text{ to } 20 \text{ s}^{-1}$ (gentle macrofloc conditioning to prevent shear)
• Equipment: Horizontal-shaft paddle wheels with variable-frequency drives (VFDs), vertical axial-flow hydrofoil impellers, and baffled serpentine channels.

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The Laboratory Jar Test Protocol

The jar test remains the fundamental benchmark for evaluating coagulant type, optimal chemical dosage, coagulant aid selection, and process pH control.

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|                          STANDARD JAR TEST PROTOCOL                         |
+-----------------------------------------------------------------------------+
|  [1] Collect fresh raw water; measure baseline pH, NTU, Temp, Alkalinity    |
|  [2] Fill 6 square baffled B-KER jars (2.0 L each)                          |
|  [3] Dose coagulant array (e.g., 10, 15, 20, 25, 30, 35 mg/L)              |
|  [4] Rapid Mix: highest practical paddle speed for about 30-60 seconds      |
|  [5] Flocculation: Tapered paddle speeds (45 RPM -> 30 RPM -> 15 RPM, 30 m) |
|  [6] Record floc size (pinpoint, small, medium, large feathered)            |
|  [7] Quiescent Settling: Stop paddles; observe settleability at 5, 10, 15 m |
|  [8] Sample Supernatant (10 cm below surface); measure settled NTU and pH   |
|  [9] Plot Dose vs. Settled Turbidity to locate optimal "knee of the curve"  |
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Operational Procedure

1. Apparatus: A 6-gang variable-speed laboratory stirrer equipped with flat stainless steel paddles and six $2\text{-liter}$ square baffled jars (B-KER jars). Square jars prevent liquid vortex rotation, ensuring that mixing energy is converted into true hydraulic shear.
2. Baseline Testing: Measure and record raw water temperature, initial turbidity, pH, and total alkalinity.
3. Chemical Dosing: Dose an array of coagulant concentrations across the six jars (e.g., 10, 15, 20, 25, 30, and $35 \text{ mg/L}$). If polymers or alkali aids are tested, inject them at designated intervals.
4. Rapid Mix Phase: Spin paddles at the stirrer's highest practical speed (often 100 to 300 rpm) for about 30 to 60 seconds to disperse the coagulant. Bench jars cannot reach full-scale rapid-mix G values, so match plant conditions as closely as practical and keep the procedure consistent from test to test.
5. Flocculation Cycle: Step down paddle speeds to simulate the treatment plant's tapered hydraulics: $45 \text{ rpm}$ for 10 minutes, $30 \text{ rpm}$ for 10 minutes, and $15 \text{ rpm}$ for 10 minutes.
6. Visual Floc Evaluation: Note the time (in seconds) required for visible "pin-floc" to appear, and rate floc development, for example with the Willcomb floc-size index (0 = colloidal with no floc, 2 = visible, 4 = disperse, 6 = clear-formed, 8 = good, 10 = excellent).
7. Settling Phase: Lift paddles and allow jars to remain undisturbed for $15 \text{ to } 30 \text{ minutes}$. Record the settling velocity (inches per minute) and supernatant clarity.
8. Analytical Measurement: Withdraw supernatant samples from the side sampling ports located exactly $10 \text{ cm}$ below the water surface. Measure settled turbidity, final pH, and filtered turbidity (passed through Whatman #40 filter paper to simulate media filtration).
9. Optimization Curve: Plot coagulant dose versus settled turbidity. The optimal chemical dose represents the "knee of the curve"—the minimum dosage achieving target settled turbidity ($< 1.0-2.0 \text{ NTU}$). Dosing beyond this threshold wastes expensive chemicals, accelerates head loss accumulation on filters, generates excessive sludge, and risks charge reversal (where excess positively charged polymer restabilizes the colloidal suspension).