3.3 Flocculation & Jar Testing

Flocculation & Jar Testing

Flocculation Kinetics & Particle Agglomeration Principles

Following rapid destabilization in the flash mix basin, raw water enters the flocculation basin. Flocculation is the physical process of providing gentle, continuous mixing to promote interparticle collisions, causing destabilized sub-micron pin-flocs to aggregate into large, visible, rapid-settling macroflocs (0.1 mm to 3.0 mm in diameter).

Particle collision kinetics occur via two distinct physical regimes:

1. Perikinetic Flocculation: Particle collisions driven solely by thermal Brownian motion. This mechanism operates on particles smaller than $1\,\mu\text{m}$ and is complete within seconds after flash mixing.
2. Orthokinetic Flocculation: Particle collisions induced by fluid velocity gradients (fluid shear) and bulk turbulence created by mechanical impellers or hydraulic baffles. Orthokinetic flocculation dominates the growth of macrofloc ($>1\,\mu\text{m}$).

The Hydrodynamic Shear Threshold

Flocculation design represents a delicate balance between collision frequency and floc shear breakup:

$$\text{Rate of Agglomeration} \propto G \cdot N_1 \cdot N_2$$ $$\text{Rate of Floc Breakup} \propto G^{1.5} \cdot d_p$$

Where $G$ is the velocity gradient, $N$ is particle concentration, and $d_p$ is floc diameter. If mixing energy ($G$) is too low, collision frequency drops, leaving un-agglomerated pin-floc that carries over onto filters. If $G$ is too high, hydrodynamic shear forces exceed the internal tensile strength of the floc, tearing macroflocs into non-settleable fragments that will not reform.

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Camp-Stein Velocity Gradient ($G$) & Dimensionless Camp Number ($Gt$)

In water treatment plant design, mixing intensity and energy input in flocculation basins are governed by the Camp-Stein equations:

$$G = \sqrt{\frac{P}{\mu V}}$$ $$Gt = G \times t$$

Where:

• $G = \text{Velocity gradient } (s^{-1})$
• $P = \text{Power dissipated into fluid } (\text{ft-lb/s or Watts})$
• $\mu = \text{Absolute dynamic viscosity of water } (lb\cdot s/ft^2 \text{ or } Pa\cdot s)$
• $V = \text{Basin volume } (ft^3 \text{ or } m^3)$
• $t = \text{Hydraulic Retention Time (HRT) in seconds } (\text{minutes} \times 60)$
• $Gt = \text{Dimensionless Camp Number (total mixing energy parameter)}$

                      [Flocculation Design Window]
     Gt < 20,000                   20,000 - 100,000                  Gt > 100,000
  ◄───────────────► ◄───────────────────────────────────────────────► ◄──────────────►
  Under-Mixing      OPTIMAL MACROFLOC AGGLOMERATION & SETTLING       Floc Shearing
  (Pin-Floc Escape)  Stage 1: G=50-70 | Stage 2: G=30-50 | Stage 3: G=10-25 (Destruction)

Standard Flocculation Design Criteria

• Velocity Gradient ($G$): $10\text{ to }70\,s^{-1}$
• Total Hydraulic Retention Time ($t$): $20\text{ to }45\,\text{minutes}$
• Dimensionless Camp Number ($Gt$): $20,000\text{ to }100,000$

Impact of Water Temperature on Viscosity: As water temperature approaches freezing ($0^\circ\text{C}$ to $4^\circ\text{C}$), the dynamic viscosity of water ($\mu$) increases by nearly $80\%$ compared to summer temperatures ($20^\circ\text{C}$). Because $G = \sqrt{P / (\mu V)}$, an increase in viscosity dampens the velocity gradient for a given motor power output, requiring operators to increase variable-speed mixer RPM in winter to maintain target $G$-values.

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Tapered Flocculation Architecture

Modern conventional treatment plants utilize Tapered Flocculation, dividing the flocculation basin into three or four compartmentalized stages connected in series:

  Raw Water + Coagulant
        │
        ▼
  ┌─────────────────┬─────────────────┬─────────────────┐
  │ STAGE 1         │ STAGE 2         │ STAGE 3         │
  │ G = 50–70 s⁻¹   │ G = 30–50 s⁻¹   │ G = 10–25 s⁻¹   │
  │ Rapid Collision │ Macrofloc Growth│ Gentle Maturation│ ──► To Clarifier / Settling
  │ Pin-Floc Form   │ Interparticle   │ Zero Floc Shear │
  │                 │ Bridging        │                 │
  └─────────────────┴─────────────────┴─────────────────┘
         HRT: 10 min       HRT: 10 min       HRT: 10 min
         ◄──────────────── Total HRT: 30–45 min ─────────────►

Stage-by-Stage Operating Goals

• Stage 1 ($G = 50\text{ to }70\,s^{-1}$): High mixing energy drives rapid initial collisions between destabilized micro-particles, rapidly forming visible pin-floc.
• Stage 2 ($G = 30\text{ to }50\,s^{-1}$): Intermediate mixing energy allows pin-flocs to aggregate into larger macroflocs while preventing shear.
• Stage 3 ($G = 10\text{ to }25\,s^{-1}$): Gentle stirring provides final floc maturation and densification without exceeding the fragile shear threshold of large macroflocs.

Compartmental Baffling: Each stage is separated by slotted redwood or fiberglass baffle walls to eliminate hydraulic short-circuiting and dead zones, ensuring all water receives the full design retention time.

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Flocculator Mechanical & Hydraulic Configurations

1. Mechanical Paddle Wheel Flocculators

• Horizontal Reel Paddle Wheels: Large rotating shafts oriented perpendicular or parallel to flow with paddle blades. Paddle tip speed is strictly limited to $0.5\text{ to }3.0\,\text{ft/s}$ ($0.15\text{ to }0.9\,\text{m/s}$) to eliminate localized shear at the blade edges.
• Vertical Turbine / Hydrofoil Impellers: Modern axial-flow impellers mounted on vertical shafts driven by Variable Frequency Drives (VFDs); offer superior energy efficiency and uniform velocity distribution.

2. Hydraulic Baffled Basins (Non-Mechanical)

• Around-the-End Baffles: Horizontal serpentine channels where water turns 180 degrees around walls. Ideal for shallow basins.
• Over-and-Under Baffles: Vertical serpentine channels where water flows over weirs and under bottom baffles. Ideal for deep basins.
• Head Loss Calculation: Velocity gradient is generated by hydraulic head loss ($h_L$) across baffles: $G = \sqrt{\frac{\rho g h_L}{\mu t}}$.

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Coagulant Aids, Polymers & Ballasted Flocculation

┌────────────────────────────────────────────────────────────────────────┐
│                     POLYMERS & COAGULANT AIDS                          │
├──────────────────────────┬─────────────────────────────────────────────┤
│ Cationic Polymers        │ High positive charge density, low MW. Acts  │
│                          │ as primary coagulant aid; reduces alum dose │
├──────────────────────────┼─────────────────────────────────────────────┤
│ Anionic Polymers         │ Negatively charged, very high MW. Bridges   │
│                          │ metal hydroxide flocs with multivalent ions │
├──────────────────────────┼─────────────────────────────────────────────┤
│ Non-Ionic Polymers       │ Polyacrylamides (neutral). Forms physical   │
│                          │ interparticle bridges; tough, dense flocs   │
├──────────────────────────┼─────────────────────────────────────────────┤
│ Bentonite Clay           │ High-density clay added to crystal-clear    │
│                          │ raw water (<2 NTU) to provide floc nuclei   │
├──────────────────────────┼─────────────────────────────────────────────┤
│ Ballasted Micro-Sand     │ Actiflo system: 100 µm silica sand + polymer│
│ (High-Rate Flocculation) │ Settling velocity increases 10x-20x         │
└──────────────────────────┴─────────────────────────────────────────────┘

Polymer Classifications

1. Cationic Polymers: Low-to-medium molecular weight, high positive charge density. Used as primary coagulant aids to reduce primary metal coagulant dosages by 30–50%.
2. Anionic Polymers: Extremely high molecular weight with negative charges along the polymer chain. Used as flocculant aids to bridge metal hydroxide flocs in the presence of divalent cations ($Ca^{2+}, Mg^{2+}$).
3. Non-Ionic Polymers: Neutral polyacrylamides that form long physical bridges, producing extremely tough, shear-resistant flocs suitable for direct filtration or centrifuge dewatering.

Ballasted High-Rate Flocculation (Actiflo)

In ballasted flocculation, fine silica micro-sand ($100\text{ to }150\,\mu\text{m}$) and an anionic polymer are injected into the flocculator alongside the primary coagulant. The micro-sand serves as a heavy, dense seed onto which floc binds. Settling velocities increase from $0.5\text{--}1.0\,\text{gpm/ft}^2$ (conventional) to $15\text{--}35\,\text{gpm/ft}^2$, reducing plant footprint by over $85\%$. Hydrocyclones separate and recycle the micro-sand.

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Standard Jar Testing Operating Procedure (SOP)

The Jar Test is the essential bench-scale laboratory procedure used by water treatment operators to simulate plant flash mixing, flocculation, and sedimentation to optimize chemical dosages.

  Jar 1 (0 mg/L)    Jar 2 (10 mg/L)   Jar 3 (20 mg/L)   Jar 4 (30 mg/L)   Jar 5 (40 mg/L)   Jar 6 (50 mg/L)
  ┌────────────┐    ┌────────────┐    ┌────────────┐    ┌────────────┐    ┌────────────┐    ┌────────────┐
  │ [Paddle]   │    │ [Paddle]   │    │ [Paddle]   │    │ [Paddle]   │    │ [Paddle]   │    │ [Paddle]   │
  │ Raw Water  │    │ Alum Dose  │    │ Alum Dose  │    │ Alum Dose  │    │ Alum Dose  │    │ Alum Dose  │
  └────────────┘    └────────────┘    └────────────┘    └────────────┘    └────────────┘    └────────────┘
  ◄─── 1 min Rapid Mix (100 rpm) ──► ◄─── 20 min Tapered Mix (50/30/15 rpm) ──► ◄─── 15 min Settling ──►

Step-by-Step SOP

1. Sample Collection & Characterization: Collect a fresh, representative raw water sample. Record initial temperature, pH, turbidity, total alkalinity, and UV254 absorbance.
2. Setup Apparatus: Fill six 2-liter square B-Ker jars (or round 1-liter beakers) with exactly 2.0 L of raw water. Mount onto the gang-stirrer apparatus.
3. Chemical Preparation & Dosing: Prepare fresh stock solutions ($1\,\text{mL of } 10\,\text{g/L solution in } 1\,\text{L} = 10\,\text{mg/L}$). Add varying coagulant doses across jars (e.g., 0, 10, 20, 30, 40, 50 mg/L).
4. Rapid Mix Phase: Operate stirrers at $100\,\text{rpm}$ ($G \approx 300\text{--}400\,s^{-1}$) for exactly $1.0\,\text{minute}$ while injecting chemicals.
5. Tapered Flocculation Phase: Step down paddle speeds to simulate plant basins:
   • $50\,\text{rpm}$ for 5 minutes ($G \approx 50\,s^{-1}$)
   • $30\,\text{rpm}$ for 10 minutes ($G \approx 30\,s^{-1}$)
   • $15\,\text{rpm}$ for 5 minutes ($G \approx 15\,s^{-1}$)
6. Visual Observations: Record the time to initial pin-floc formation (seconds) and evaluate floc size using a standard index (Pinpoint, Small, Medium, Large, Heavy).
7. Settling Phase: Raise paddles; allow quiescent settling for $10\text{ to }15\,\text{minutes}$. Observe settling velocity.
8. Supernatant Analysis: Draw settled water from the sample ports ($10\,\text{cm}$ below liquid surface). Measure and plot Settled Turbidity (NTU), final pH, residual alkalinity, and filtered UV254 absorbance.

Enhanced Coagulation for D/DBP Rule Compliance

Under the EPA/SWRCB Stage 1 Disinfectants and Disinfection Byproducts (D/DBP) Rule, utilities treating surface water must achieve specified percentage removals of Total Organic Carbon (TOC) prior to chlorination to minimize Trihalomethane (TTHM) and Haloacetic Acid (HAA5) formation.

Optimizing Enhanced Coagulation: To achieve mandatory TOC removal in low-alkalinity waters, operators use jar testing to identify the "point of diminishing returns" (PODR)—adding coagulant and acid (sulfuric acid $H_2SO_4$) to depress coagulation pH to 5.5 to 6.3, where humic and fulvic acids precipitate most efficiently.

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Operational Scenarios & Troubleshooting

Scenario 1: Cold Water Flocculation Failure

Situation: In January, raw water temperature drops to 3°C. Settled water turbidity doubles from 0.8 NTU to 2.4 NTU, and un-agglomerated pinpoint flocs pass directly through the sedimentation basin onto the filters. Diagnostic: High water viscosity dampens particle motion, and cold water slows the chemical hydrolysis kinetics of alum. Operator Action:

1. Perform a jar test with refrigerated raw water to simulate 3°C basin temperatures.
2. Evaluate switching from standard alum to Polyaluminum Chloride (PAC), which is pre-hydrolyzed and insensitive to cold water temperatures.
3. Add a low dose ($0.05\text{ to }0.10\,\text{mg/L}$) of non-ionic polymer flocculant aid in Stage 2 flocculation to bridge pin-flocs into robust macroflocs.

Scenario 2: Severe Floc Shearing from Excessive Mixing

Situation: A new operator increases the Stage 3 flocculator paddle VFD from 20% to 80% to "improve mixing." Settled water turbidity immediately surges, and filters rapidly blind with head loss. Diagnostic: Excessive $G$-values in Stage 3 ($G > 80\,s^{-1}$) exceeded the floc shear threshold, shattering fragile macroflocs into microscopic sheared fragments that cannot settle. Operator Action: Immediately throttle Stage 3 mixer speeds back to $G = 10\text{--}20\,s^{-1}$.