3.1 Coagulation & Flocculation Process Control

Primary Coagulants & Chemical Auxiliary Aids

Coagulation is the chemical process of destabilizing small, non-settleable suspended particles and colloidal matter by neutralizing their surface electrical charges. Without chemical coagulation, microscopic clay particles ($0.001$ to $1.0 \ \mu\text{m}$) would take years to settle by gravity alone.

1. Aluminum Sulfate (Alum)

Aluminum sulfate ($\text{Al}_2(\text{SO}_4)_3 \cdot 14\text{H}_2\text{O}$) is the most widely used primary coagulant in drinking water treatment. Commercial liquid alum is delivered as a 48.5% solution containing approximately 5.4 lbs of dry alum per gallon with a specific gravity of 1.33.

When added to water containing natural bicarbonate alkalinity, alum reacts to form insoluble aluminum hydroxide ($\text{Al(OH)}_3$) precipitation flocs:

$\text{Al}_2(\text{SO}_4)_3 + 3\text{Ca(HCO}_3)_2 \longrightarrow 2\text{Al(OH)}_3\downarrow + 3\text{CaSO}_4 + 6\text{CO}_2$

Key chemical constraints for alum coagulation:

• Optimal pH Range: Alum functions best within a pH window of 5.5 to 7.5, with an absolute sweet spot of 6.0 to 6.8.
• Alkalinity Consumption: Every 1.0 mg/L of dry alum added consumes 0.45 to 0.50 mg/L of natural alkalinity as $\text{CaCO}_3$ and generates approximately 0.44 mg/L of carbon dioxide gas ($\text{CO}_2$).
• Alkalinity Deficiency: If raw water alkalinity is under 20 mg/L as $\text{CaCO}_3$, alum addition will deplete remaining buffering capacity, crashing water pH below 5.0. At low pH, aluminum remains in soluble ionic form ($\text{Al}^{3+}$), leading to incomplete coagulation, elevated residual aluminum in finished water, and "pin-floc" pass-through on filters. Hydrated lime ($\text{Ca(OH)}_2$) or caustic soda ($\text{NaOH}$) must be fed to maintain alkalinity.

2. Ferric Chloride & Ferric Sulfate

Iron salts, including ferric chloride ($\text{FeCl}_3$) and ferric sulfate ($\text{Fe}_2(\text{SO}_4)_3$), operate across a significantly broader pH spectrum (4.0 to 11.0):

• Heavy Hydroxide Floc: Ferric ions form dense ferric hydroxide ($\text{Fe(OH)}_3\downarrow$) flocs that settle up to 50% faster than aluminum flocs.
• Alkalinity Consumption: Ferric chloride consumes 0.92 mg/L of alkalinity as $\text{CaCO}_3$ per 1.0 mg/L of $\text{FeCl}_3$ fed.
• Corrosivity: Liquid ferric chloride is extremely acidic and corrosive; storage tanks and chemical lines must be constructed of fiberglass-reinforced plastic (FRP) or PVC.

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

PAC and ACH are pre-hydrolyzed aluminum polymer salts containing high-charge polymeric species such as $\text{Al}_{13}\text{O}4(\text{OH}){24}^{7+}$.

• Cold Water Performance: PAC maintains high coagulation efficacy in cold water ($< 5^\circ\text{C}$) where alum kinetics stall.
• Lower Sludge & Alkalinity Impact: PAC produces denser sludge volume and consumes 50% to 80% less alkalinity than alum, minimizing the need for supplemental lime dosing.

4. Polyelectrolytes & Polymer Aids

Synthetic organic polymers are long-chain carbon molecules utilized as chemical aids:

• Cationic Polymers (Positively charged): Used as primary coagulants or coagulant aids to increase particle charge neutralization.
• Anionic Polymers (Negatively charged): Used as flocculant aids to form bridges between microflocs, creating large, heavy macroflocs.
• Non-ionic Polymers (Neutral): Used primarily as filter aids at doses of 0.01 to 0.10 mg/L to increase bed clarity.

[!CAUTION] Overdosing Hazard: Over-dosing cationic polymers causes restabilization of colloidal particles by charge reversal (turning particles positive), completely arresting flocculation. Excess polymer also blinds filter media beds, resulting in rapid head loss accumulation.

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Coagulation Physics & Zeta Potential

Raw water colloidal particles carry negative surface electrical charges created by chemical adsorption of ions and broken mineral crystal bonds. Because like charges repel, colloids stay suspended indefinitely in a state of colloidal stability.

  [-] Negative Colloid  ◄── Repulsion ──►  [-] Negative Colloid
                       (Zeta Potential: -15 to -30 mV)
                                  │
                 Coagulant Addition (Al3+ / Fe3+)
                                  ▼
  [0] Neutralized Floc  ◄── Agglomeration ──► [0] Neutralized Floc
                       (Zeta Potential: -3 to +3 mV)

Zeta Potential

Zeta potential is the electrical potential (measured in millivolts, mV) at the slipping plane of a colloid's double layer. Raw surface water typically exhibits a negative zeta potential between -15 mV and -30 mV.

• Adding cationic coagulants reduces negative surface potential toward zero (the isoelectric point).
• Optimal charge neutralization occurs when zeta potential is driven into the target range of -3 mV to +3 mV.

Four Primary Mechanisms of Coagulation

1. Charge Neutralization: Positively charged trivalent ions ($\text{Al}^{3+}$, $\text{Fe}^{3+}$) compress the electrical double layer, neutralizing negative particle charges so short-range van der Waals attractive forces draw particles together.
2. Sweep Flocculation (Enmeshment): High doses of alum or iron added at $\text{pH} > 6.4$ cause rapid precipitation of amorphous $\text{Al(OH)}_3$ or $\text{Fe(OH)}_3$ solids. The heavy precipitate settles downward, sweeping colloids out of suspension like net enmeshment.
3. Interparticle Bridging: Long-chain polymer molecules adsorb onto multiple colloidal particles simultaneously, forming structural bridges across particle gaps.
4. Charge Patch Mechanism: High-charge density cationic polymers bind to localized sites on negative colloid surfaces, creating positive patches that attract negative areas on adjacent colloids.

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Rapid Flash Mixing Hydraulics

Flash mixing introduces coagulant chemicals into raw water and provides high-energy agitation to achieve uniform chemical dispersion before hydrolysis reactions complete ($< 1$ second for charge neutralization).

Velocity Gradient (G-Value)

The intensity of mixing energy imparted to water is quantified by the Velocity Gradient ($G$-value), measured in reciprocal seconds ($s^{-1}$):

$G = \sqrt{\frac{P}{\mu V}}$

Where:

• $P$ = Power dissipated into water ($\text{ft}\cdot\text{lb/s}$ or Watts)
• $\mu$ = Dynamic viscosity of water ($\text{lb}\cdot\text{s/ft}^2$ or $\text{Pa}\cdot\text{s}$)
• $V$ = Mixing basin volume ($\text{ft}^3$ or $\text{m}^3$)

Flash mixing design criteria:

• $G$-Value Range: 750 to 1,500 $s^{-1}$
• Detention Time: 1 to 30 seconds (preferably $< 5$ seconds for charge neutralization mode).
• Mixer Configurations: Mechanical impellers, in-line static pipe mixers, hydraulic jumps (Parshall flumes), or chemical injection diffusers.

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Flocculation Basin Design & Tapered Mixing

Flocculation follows rapid mixing. Its purpose is to promote physical collisions between destabilized microflocs so they aggregate into large, dense macroflocs ($0.1$ to $2.0 \text{ mm}$) without shearing fragile floc structures.

Tapered Velocity Gradient

Flocculation basins are partitioned into 3 or 4 sequential stages separated by baffled walls. Mixing intensity is progressively decreased from stage to stage to match growing floc size:

Total flocculation detention time ranges from 20 to 30 minutes under warm water conditions, extending up to 30 to 45 minutes in cold water ($< 5^\circ\text{C}$). The overall dimensionless Camp number ($G \times t$) must remain between 30,000 and 150,000.

Mechanical Flocculators

Flocculation energy is supplied by horizontal or vertical variable-speed paddle wheels or hydrofoil impellers.

[!IMPORTANT] Tip Speed Restriction: Paddle peripheral tip speed must be maintained below 1.5 ft/s (0.45 m/s). Tip speeds exceeding 1.5 ft/s generate localized shear forces that tear fragile alum flocs apart into un-settleable pin-floc.

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Step-by-Step Jar Testing Procedure

Jar testing is the primary bench-top procedure used to determine optimal coagulant dosage, optimum pH, and polymer feed rates.

Raw Sample ──► Fill 6 Jars (2L) ──► Flash Mix (150 rpm, 1 min) ──► Tapered Slow Mix (20 min) ──► Settling (15 min) ──► Sample Supernatant (2cm)

Step-by-Step Laboratory Protocol

1. Sample Collection: Collect a fresh 10 to 12 L representative raw water sample. Immediately measure and record raw temperature, pH, turbidity, total alkalinity, and color.
2. Jar Preparation: Fill six 2,000 mL square Beker jars precisely to the 2,000 mL mark. Square jars eliminate vortexing without requiring internal baffles.
3. Chemical Preparation: Prepare a 1.0% stock alum solution ($10.0 \text{ mg/mL}$). In a 2,000 mL water jar, dosing $1.0 \text{ mL}$ of 1.0% stock yields an exact concentration of 5.0 mg/L alum.
4. Rapid Mix Phase: Lower stirrer paddles into jars. Set gang stirrer to 100 to 150 rpm ($G \approx 300 \text{ s}^{-1}$). Inject multi-stage chemical doses simultaneously across Jars 1 through 6 (e.g., 10, 15, 20, 25, 30, and 35 mg/L alum). Rapid mix for precisely 1 minute.
5. Tapered Slow Mix Phase: Reduce stirrer speeds to simulate basin stages: 40 rpm for 7 minutes, 30 rpm for 7 minutes, and 20 rpm for 6 minutes (total 20 minutes slow mix). Observe and record:
   • Time of initial pin-floc appearance (seconds after slow mix start).
   • Floc growth rate, size category (small, medium, large), and structural appearance.
6. Quiescent Settling Phase: Stop stirrers, lift paddles carefully out of jars, and start timer for 15 minutes of undisturbed settling. Record floc settling rate (inches per minute) and sludge volume accumulated.
7. Supernatant Analysis: Using a pipette, carefully draw 100 mL supernatant samples from the sample port located 2 cm below the water surface in each jar without disturbing settled sludge.
8. Data Evaluation: Measure supernatant settled turbidity (NTU), final pH, residual alkalinity, and TOC. Plot dosage versus settled turbidity. Select the optimum dosage—the lowest chemical feed rate located at the "knee of the curve" achieving settled turbidity below 1.0 to 1.5 NTU while preserving adequate alkalinity buffer.