3.2 Coagulation Chemistry, Flocculation & Sedimentation Basins
Key Takeaways
- Colloidal particles in natural waters remain stably suspended due to negative surface zeta potentials (-15 to -35 mV); chemical coagulation destabilizes colloids via double layer compression, charge neutralization, interparticle bridging, or sweep enmeshment.
- Aluminum sulfate (alum) operates optimally between pH 5.8 and 7.5 and consumes ~0.5 mg/L alkalinity as CaCO3 per mg/L alum fed, whereas ferric salts tolerate a wider pH range (4.0 to 11.0) and form heavier, faster-settling flocs.
- Polyaluminum chloride (PACl) provides pre-hydrolyzed aluminum polymers that consume 50% to 70% less alkalinity than alum, maintain process efficiency in cold winter waters (< 10°C), and produce lower sludge volumes.
- Tapered flocculation basins gradually reduce mixing intensity across sequential compartments (e.g., from 50–70 s⁻¹ down to 15–25 s⁻¹ over 20–45 minutes) to maximize particle collision frequency while preventing hydraulic shear of delicate macro-flocs.
- Sedimentation basin design relies on Surface Overflow Rate (SOR = Q / As, typically 500–1,000 gpd/ft² for conventional basins) and Weir Overflow Rate (WOR ≤ 20,000 gpd/ft), while inclined tube and plate settlers multiply effective settling area to reduce basin footprint.
3.2 Coagulation Chemistry, Flocculation & Sedimentation Basins
Coagulation, flocculation, and sedimentation constitute the chemical clarification core of conventional surface water treatment plants. While raw water intake and screening physically exclude bulk debris, clarification removes microscopic suspended solids, turbidity, natural organic matter, color, and pathogenic microorganisms prior to granular filtration.
1. Colloid Science & Destabilization Mechanisms
Turbidity in surface water consists primarily of inorganic silt and clay minerals (such as kaolinite), organic macromolecules (humic and fulvic acids), and microorganisms (bacteria, protozoan cysts, viruses). These particles fall predominantly within the colloidal size range (0.001 to 1.0 micrometer).
The Electrical Double Layer and Zeta Potential
Colloidal particles carry a permanent net negative surface electrical charge. In clay minerals, this charge originates from isomorphic substitution within the crystal lattice (e.g., Al³⁺ substituting for Si⁴⁺) and the ionization of structural hydroxyl groups.
Because like charges repel, electrostatic forces prevent colloidal particles from colliding and agglomerating. Surrounding every suspended colloid is an Electrical Double Layer (EDL) consisting of two regions:
- Stern Layer: A tightly bound, immobilized inner layer of positive counter-ions attracted directly to the electronegative particle surface.
- Gouy-Chapman Diffuse Layer: An outer, looser zone of counter-ions extending into the bulk solution where electrostatic attraction competes with thermal diffusion.
ELECTRICAL DOUBLE LAYER & ZETA POTENTIAL
[ - ] [ + ] [ + ]
[ - ] [ + ] [ + ] [ + ]
Colloid [ - ] [ + ] [ + ] [ + ] Bulk Water Solution
Surface [ - ] [ + ] [ + ] [ + ]
[ - ] [ + ] [ + ]
| | |
| Stern Layer |
| Slipping Plane (Plane of Shear) --> ZETA POTENTIAL
|<---------- Diffuse Layer ----------->|
When a particle moves through water, the tightly held fluid within the Stern layer moves with it, while the outer liquid shears away. The boundary between this moving fluid envelope and the bulk water is the slipping plane (plane of shear). The electrical potential measured at this slipping plane is the Zeta Potential.
In raw surface waters, the zeta potential of suspended colloids typically measures between -15 mV and -35 mV. To achieve agglomeration, this repulsive force must be overcome so that attractive short-range Van der Waals forces can bind particles together.
Four Primary Destabilization Mechanisms
Chemical coagulants destabilize colloidal suspensions through four distinct physical and chemical mechanisms:
- Double Layer Compression: When high concentrations of simple electrolytes (salts) are added to water, the increased ionic strength compresses the diffuse layer toward the particle surface. This reduces the spatial reach of electrostatic repulsion, allowing Van der Waals forces to pull particles together. However, double layer compression alone requires impractical chemical quantities for drinking water treatment.
- Charge Neutralization (Adsorption-Neutralization): Positively charged metal hydrolysis species (e.g., Al(OH)²⁺, Fe(OH)²⁺, cationic polymers) adsorb directly onto the negatively charged colloid surface. This neutralizes the negative surface charge, driving the zeta potential from -30 mV toward zero (the isoelectric point, typically optimized between -3 mV and +3 mV). Once neutralized, repulsive forces vanish, and normal particle agitation induces rapid agglomeration.
- Interparticle Bridging: Extremely long-chain synthetic organic polymers (polyelectrolytes) contain active chemical groups along their molecular backbone. When dosed into water, a single polymer molecule adsorbs onto multiple colloidal particles simultaneously, forming a physical "bridge" that binds particles into large, durable networks.
- Sweep Coagulation (Enmeshment): When metal coagulants (alum or iron) are added in concentrations exceeding the solubility limit of the metal hydroxide, amorphous, gelatinous precipitates form (Al(OH)3 or Fe(OH)3). As these heavy precipitates form and settle, they physically collide with, envelop, and "sweep" colloidal particles and natural organic matter out of the water column like a falling net. Sweep coagulation is the dominant mechanism used in low-turbidity waters where particle collision frequency is otherwise too low for simple charge neutralization.
2. Primary Chemical Coagulants & Solution Chemistry
Water treatment facilities rely primarily on metallic salts of aluminum or iron as primary coagulants. Selecting the correct coagulant depends on raw water pH, alkalinity, temperature, organic carbon concentration, and sludge disposal costs.
PRIMARY COAGULANTS
ALUMINUM SALTS IRON SALTS
--------------------------------- ---------------------------------
* Aluminum Sulfate (Alum) * Ferric Chloride (FeCl3)
- Al2(SO4)3 · 14H2O - Liquid solution (38% - 42%)
- Optimum pH: 5.8 - 7.5 - Optimum pH: 4.0 - 11.0
- Consumes 0.5 mg/L Alk / mg Alum - Consumes 0.92 mg/L Alk / mg FeCl3
- Sluggish in cold water - Dense, rapid settling flocs
* Polyaluminum Chloride (PACl) * Ferric Sulfate (Fe2(SO4)3)
- Pre-hydrolyzed polymeric salt - Granular or liquid
- Consumes 50%-70% less alkalinity - Widely used for heavy organics
- Superior cold water performance - Corrosive, staining
Aluminum Sulfate (Alum)
Commercial dry alum has the approximate chemical formula Al2(SO4)3 · 14H2O (molecular weight ~594 g/mol), while liquid alum is supplied as an approximately 48% to 50% aqueous solution.
When alum dissolves in water containing natural calcium bicarbonate alkalinity, it reacts according to the following stoichiometric equation: Al2(SO4)3 · 14H2O + 3 Ca(HCO3)2 → 2 Al(OH)3 ↓ + 3 CaSO4 + 14 H2O + 6 CO2 ↑
Operational Chemistry of Alum:
- Alkalinity Consumption: Every 1.0 mg/L of commercial dry alum consumes approximately 0.45 to 0.50 mg/L of natural alkalinity as CaCO3.
- pH Depression and Carbon Dioxide: The reaction releases dissolved carbon dioxide (CO2), creating carbonic acid and driving raw water pH downward.
- Optimum pH Range: Alum operates effectively only within an optimum pH window of 5.8 to 7.5. The point of minimum solubility for aluminum hydroxide occurs between pH 6.0 and 6.8.
- Risks of Operating Outside the Window:
- At pH < 5.5: Aluminum hydroxide precipitates dissolve back into soluble trivalent aluminum ions (Al³⁺). Soluble aluminum passes through sand filters into the finished water, where it causes post-precipitation floc in the distribution network and presents neurotoxicity concerns.
- At pH > 8.0: Aluminum redissolves as soluble aluminate ions (Al(OH)4⁻), destroying floc structure and resulting in high settled water turbidity.
Iron Salts: Ferric Chloride and Ferric Sulfate
Iron coagulants utilize trivalent iron (Fe³⁺) to destabilize particles:
- Ferric Chloride (FeCl3): Typically delivered as a dense, dark reddish-brown acidic liquid (38% to 42% concentration by weight).
- Ferric Sulfate (Fe2(SO4)3): Available as dry granules or liquid solution.
The hydrolysis reaction of ferric chloride consumes alkalinity as follows: FeCl3 + 3 H2O → Fe(OH)3 ↓ + 3 HCl 3 HCl + 3 Ca(HCO3)2 → 3 CaCl2 + 6 H2O + 6 CO2 ↑
Advantages and Disadvantages of Iron Salts:
- Broad pH Tolerance: Ferric salts operate over an exceptionally wide pH range of 4.0 to 11.0, making them ideal for treating acidic swamp waters or high-pH waters without extensive chemical trimming.
- Dense, Rapid-Settling Floc: Ferric hydroxide (Fe(OH)3) flocs have a higher specific gravity than alum flocs, settling substantially faster.
- Cold Water Performance: Iron hydrolysis reactions proceed vigorously in cold winter waters where alum reactions stall.
- Higher Alkalinity Consumption: Every 1.0 mg/L of anhydrous FeCl3 consumes approximately 0.92 mg/L of alkalinity as CaCO3—nearly double the consumption of alum.
- Corrosivity and Staining: Ferric chloride is an aggressive acid that attacks concrete and stainless steel (requiring FRP or rubber-lined storage). Chemical leaks cause permanent reddish-brown rust stains on equipment, basins, and building facades.
Polyaluminum Chloride (PACl) and Aluminum Chlorohydrate (ACH)
PACl coagulants are manufactured by partially reacting aluminum chloride with a base, creating pre-formed, highly charged polynuclear aluminum complexes (most notably the Keggin ion, [Al13O4(OH)24]⁷⁺).
PACl formulations are defined by their basicity—the degree of pre-neutralization (typically 50% to 83% basicity).
Advantages for North Carolina Facilities:
- Low Alkalinity Consumption: Because PACl is already partially neutralized during manufacturing, it consumes 50% to 70% less natural alkalinity than alum, preventing severe pH crashes in weakly buffered Piedmont waters.
- Exceptional Cold-Water Performance: Because the polynuclear complexes are pre-formed, PACl does not rely on sluggish in-situ hydrolysis kinetics, outperforming alum during near-freezing winter operations.
- Reduced Sludge Volumes: PACl produces a denser, more compact chemical sludge with lower bound water content, reducing dewatering and disposal costs.
- Lower Finished Water Dissolved Aluminum: Yields significantly lower residual soluble aluminum concentrations across a wider operating pH range.
| Coagulant Property | Aluminum Sulfate (Alum) | Ferric Chloride (FeCl3) | Polyaluminum Chloride (PACl) |
|---|---|---|---|
| Physical Form | Liquid (48%) or dry granules | Liquid solution (38–42%) | Liquid solution |
| Active Species | Al³⁺ → Al(OH)3 | Fe³⁺ → Fe(OH)3 | Pre-formed [Al13O4(OH)24]⁷⁺ |
| Optimum pH Window | 5.8 – 7.5 | 4.0 – 11.0 | 5.5 – 8.5 |
| Alkalinity Consumed | ~0.50 mg/L per mg/L alum | ~0.92 mg/L per mg/L FeCl3 | ~0.15–0.25 mg/L per mg/L PACl |
| Cold Water Kinetics | Sluggish below 8°C | Good | Excellent |
| Floc Density | Light, feathery | Dense, heavy | Dense, compact |
3. Coagulant Aids & Flocculant Polymers
Coagulant aids are auxiliary chemicals added during rapid mix or early flocculation to strengthen flocs, increase settling rates, bridge micro-flocs, or weight down light suspensions.
Synthetic Organic Polyelectrolytes
Polymers are high-molecular-weight organic chains synthesized from acrylamide monomers, classified by their net ionic charge:
- Cationic Polymers (Positively Charged): Have low-to-medium molecular weights and high charge densities. Used as primary coagulants or coagulant aids immediately following flash mix to neutralize negative colloidal charges, reducing primary metal coagulant demand by 30% to 50%.
- Anionic Polymers (Negatively Charged): Have exceptionally high molecular weights and long chain lengths. Dosed during intermediate flocculation stages. They function purely via interparticle bridging, capturing pin-flocs and knitting them into massive macro-flocs.
- Non-Ionic Polymers (Uncharged): Composed of pure polyacrylamide. Function through hydrogen bonding and mechanical interparticle bridging.
OPERATOR CAUTION ON POLYMER OVERFEEDING: Polymer aids must be dosed with extreme precision (typical doses: 0.05 to 0.5 mg/L). Overfeeding polymers can saturate all available adsorption sites on colloidal surfaces, causing restabilization where particles develop positive charges and repel each other. Furthermore, polymer carryover blinds sand filters within hours, causing rapid terminal head loss.
Weighting Agents
In waters with extremely low raw turbidity (< 5 NTU) and cold temperatures, particle collision frequency is insufficient to produce settleable floc. Operators introduce weighting agents:
- Bentonite Clay: Added at 10 to 30 mg/L ahead of coagulants to supply artificial mineral nucleation sites and add mass to the floc.
- Powdered Activated Silica: Sodium silicate partially neutralized with acid to form colloidal silica sols that accelerate flocculation.
- Microsand (Ballasted Flocculation): Utilized in high-rate proprietary systems (such as Actiflo), where fine silica sand (100–150 µm) is injected with polymer, creating massive flocs that settle at rates 10 to 20 times faster than conventional clarifiers.
4. Flocculation Basin Hydraulics & Tapered Mixing
Following rapid mixing, water enters the flocculation basin. The objective of flocculation is to provide gentle, sustained hydraulic agitation that maximizes collisions between destabilized micro-flocs, transforming them into large, dense macro-flocs capable of rapid settling in sedimentation basins.
THREE-STAGE TAPERED FLOCCULATION
Flash Mix ===> [ STAGE 1 ] =====> [ STAGE 2 ] =====> [ STAGE 3 ] ===> Clarifier
High Energy Medium Energy Gentle Energy
G = 50 - 70 s⁻¹ G = 30 - 40 s⁻¹ G = 15 - 25 s⁻¹
Small, tough Agglomerating Large macro-floc
micro-flocs pin-flocs (Shear sensitive)
The Need for Tapered Mixing
The shear resistance of a floc particle is inversely proportional to its diameter: microscopic pin-flocs are tough and shear-resistant, whereas large macro-flocs are fragile and tear apart easily under fluid shear.
If mixing energy is uniform throughout the entire basin:
- A high mixing speed creates large flocs in the beginning but violently shreds them in the exit chamber.
- A low mixing speed prevents floc shear at the end but fails to generate enough particle collisions in the beginning.
To resolve this dilemma, modern plants employ tapered flocculation across a minimum of three distinct, baffled compartments in series:
- Stage 1 (Inlet Compartment): High mixing energy (G = 50 to 70 s⁻¹). Promotes maximum collision frequency while flocs are small.
- Stage 2 (Intermediate Compartment): Moderate mixing energy (G = 30 to 40 s⁻¹). Flocs grow into visible pinhead structures.
- Stage 3 (Outlet Compartment): Gentle mixing energy (G = 15 to 25 s⁻¹). Assembles dense macro-flocs ("tea leaf" appearance) while strictly preventing hydraulic floc shear.
Design and Operational Specifications
- Hydraulic Detention Time: Typically 20 to 45 minutes at peak plant flow (nominally 30 minutes).
- Camp-Stein G · t Value: The dimensionless product of velocity gradient and time should total G · t = 30,000 to 100,000.
- Flocculator Equipment:
- Horizontal Shaft Paddle Wheels: Slow-turning paddle reels (1 to 5 RPM) equipped with wooden or fiberglass blades. Stator baffles are installed between reels to prevent bulk fluid rotation and short-circuiting.
- Vertical Turbine Flocculators: Top-mounted electric gearmotors driving large axial-flow hydrofoil impellers. Preferred in modern design because motors and bearings remain above the water line for ease of maintenance, and variable frequency drives (VFDs) allow precise electronic tuning of G-values.
5. Sedimentation Basin Dynamics & Zone Architecture
Sedimentation (clarification) is the solid-liquid separation process wherein flocculated water is held in a quiescent state, allowing heavy flocs to settle to the basin floor under gravitational force, producing a clarified supernatant for filtration.
CONVENTIONAL RECTANGULAR BASIN
Flocculated SETTLING ZONE Effluent Launders
Water Influent (Quiescent Flow) & V-Notch Weirs
| |
v v
[Diffuser Wall] ==============================================> [~~~~~~~]
(Perforated) |
| · · · · · v
| · · · · (Settling Particles) Clarified Effluent
v · · · to Filters
===============================================================
[Sludge Hopper] <===== [ Flight & Chain Scrapers ]
| (Moves Sludge to Hopper)
v
Sludge Blowdown
The Four Functional Zones of a Clarifier
Every sedimentation basin—whether rectangular or circular—contains four hydraulically distinct functional zones:
- Inlet Zone:
- Function: Smoothly transitions high-velocity water exiting the flocculation basin into uniform, non-turbulent horizontal flow across the entire cross-section of the clarifier.
- Architecture: Utilizes a perforated diffuser baffle wall located 3 to 5 feet downstream of the inlet channel. The wall features round ports (typically 4 to 6 inches in diameter) providing a 10% to 20% open area. This creates a small head loss (0.05 to 0.1 ft) that dampens approach velocity, eliminates density currents, and prevents jetting.
- Settling Zone:
- Function: Provides a quiescent volume where particles settle toward the floor without turbulent disruption.
- Hydraulics: Flow must remain strictly laminar (Reynolds Number Re < 2,000, and ideally Re < 500). Horizontal flow velocity (vh) must not exceed 0.5 feet per minute (0.0025 m/s) to prevent scouring settled solids back into suspension.
- Sludge Zone:
- Function: Collects and consolidates settled chemical flocs at the basin floor until removal.
- Removal Systems: Rectangular basins utilize continuous flight-and-chain scrapers or travelling bridge vacuum mechanisms to pull sludge into hoppers at the inlet end. Circular basins use rotating rakes with plow blades to push sludge toward a center sump. Regular automated blowdown is mandatory: if sludge remains too long, anaerobic decomposition generates methane and nitrogen gases that attach to sludge flocs, floating them to the surface as unsightly mats.
- Outlet Zone:
- Function: Collects clarified supernatant water uniformly across the surface without generating vertical upward velocity currents that pull settled floc off the floor.
- Architecture: Employs extensive networks of effluent launders (troughs) equipped with adjustable 90° V-notch weirs or submerged orifice plates.
6. Clarifier Sizing & Operational Mathematics
Proper operation of sedimentation basins requires mastery of three key design equations: Hydraulic Detention Time (DT), Surface Overflow Rate (SOR), and Weir Overflow Rate (WOR).
1. Hydraulic Detention Time (DT)
The theoretical time required for a unit volume of water to transit the basin: DT = (Basin Volume in gallons / Plant Flow Rate in gpd) * 24 hours/day DT = Basin Volume in gallons / Plant Flow Rate in gpm (in minutes)
- Conventional Basins: Designed for 2.0 to 4.0 hours of detention time.
2. Surface Overflow Rate (SOR) / Surface Settling Rate (SSR)
Surface Overflow Rate is the primary hydraulic sizing parameter for clarifiers. It represents the flow rate divided by the horizontal surface area: SOR = Flow Rate (gpd) / Surface Area (ft²) = Q / (W * L)
- Expressed in gallons per day per square foot (gpd/ft²) or gallons per minute per square foot (gpm/ft², where 1.0 gpm/ft² = 1,440 gpd/ft²).
- Conventional Basin Sizing: Typically 500 to 1,000 gpd/ft² (0.35 to 0.70 gpm/ft²) for alum flocs.
Ideal Settling Theory (Hazen Principle):
In an ideal horizontal sedimentation basin, the removal of discrete settling particles depends solely on the surface area, completely independent of basin depth. Every particle has an individual settling velocity (vs). The Surface Overflow Rate (SOR) represents the critical upward fluid velocity of the basin:
- Any floc particle with a settling velocity greater than or equal to the SOR (vs ≥ SOR) will be 100% captured.
- Any particle with a settling velocity less than the SOR (vs < SOR) will be partially removed in the direct proportion of (vs / SOR).
3. Weir Overflow Rate (WOR)
Weir Overflow Rate measures the volume of water passing over each linear foot of effluent weir per day: WOR = Flow Rate (gpd) / Total Active Weir Length (feet) = Q / L_weir
- Regulatory Limits: Under Ten States Standards and NC DWR rules, the WOR must not exceed 20,000 gpd per linear foot of weir (and should be kept under 10,000 to 15,000 gpd/ft when treating light alum flocs).
- If weirs are too short, the localized upward velocity adjacent to the weir becomes so strong that it vacuums settled floc up into the effluent launders.
7. High-Rate Clarification: Tube Settlers & Lamella Plates
Conventional sedimentation basins require enormous land footprints and massive concrete volumes to achieve 2 to 4 hours of detention time. Modern plants and retrofits overcome this limitation through high-rate inclined settlers.
INCLINED TUBE / LAMELLA SETTLER
Clarified Supernatant Upflow ( ^ )
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/ / / / / / / / / / / / / / Inclined Plates / Modules
/ / / / / / / / / / / / / / Angle: 55° - 60°
/ / / / / / / / / / / / / / Vertical Fall: 2 inches (5 cm)
/ / / / / / / / / / / / / / Sludge slides down counter-current
=============================== to flow into lower hopper
Sludge Discharge ( v )
Operating Principle
According to Hazen's Settling Theory, clarification efficiency is maximized by increasing settling area and minimizing vertical fall distance.
Tube settlers (honeycomb PVC modules) and Lamella plate settlers (parallel stainless steel plates) are installed in the upper 3 to 4 feet of the settling zone, inclined at an angle of 55° to 60°:
- Drastic Depth Reduction: The distance a floc particle must fall to reach a solid boundary is reduced from a 12-foot basin depth to a mere 2 inches (5 cm) between adjacent plates.
- Self-Cleaning Counter-Current Slide: Water flows upward through the modules while floc settles downward onto the inclined surface. At a 60° angle, gravity overcomes friction, causing accumulated sludge sheets to continuously slide downward, counter-current to the upward water flow, falling into the bottom sludge hopper.
Engineering Benefits
- Multiplies effective settling area by 4 to 8 times the floor footprint.
- Permits Surface Overflow Rates of 2,000 to 3,000 gpd/ft² (2 to 3 times higher than conventional basins).
- Reduces required basin detention times to 45 to 60 minutes.
- Allows older NC plants to double treatment capacity within existing concrete basin structures without acquiring additional land.
What is the primary operational consequence of dosing aluminum sulfate (alum) into raw water with low natural alkalinity (e.g., < 20 mg/L as CaCO3) without feeding a supplemental base?
Why do conventional surface water treatment plants utilize tapered flocculation basins with decreasing G-values rather than uniform mixing throughout all stages?
A rectangular sedimentation basin is 120 feet long, 40 feet wide, and 12 feet deep. The water treatment plant processes a flow rate of 4.0 million gallons per day (MGD). What is the Surface Overflow Rate (SOR) of this basin?