3.2 Sedimentation Basins & Clarification

Sedimentation Theory & Particle Settling Types

Sedimentation is the physical process of gravity separation designed to remove settleable solids and chemical flocs produced during coagulation and flocculation prior to filtration. Proper clarification removes 80% to 95% of suspended solids, reducing turbidity loading on granular filter beds.

Four Classifications of Particle Settling

Type 1: Discrete (Stokes' Law) ──► Type 2: Flocculent (Coalescing) ──► Type 3: Zone (Sludge Blanket) ──► Type 4: Compression

1. Type 1: Discrete Particle Settling: Unhindered settling of rigid particles (sand, grit) where particle size, shape, and density remain constant. Governed directly by Stokes' Law:

$v_s = \frac{g (\rho_p - \rho_w) d^2}{18 \mu}$

Where $v_s$ is settling velocity, $g$ is gravitational acceleration, $\rho_p$ is particle density, $\rho_w$ is water density, $d$ is particle diameter, and $\mu$ is dynamic fluid viscosity.

1. Type 2: Flocculent Settling: Settling of coalescing chemical flocs (alum or ferric flocs). As particles settle, they collide, aggregate, increase in mass and size, and accelerate downward. Their settling trajectory forms a downward-curving vector.
2. Type 3: Zone (Hindered) Settling: High solids concentration environments (such as sludge blankets in solids-contact units). Interparticle forces lock particles into a fixed matrix that settles as a single consolidated mass with a distinct interface between clarified water and sludge.
3. Type 4: Compression Settling: Compaction of the bottom sludge layer under the physical weight of overlying accumulated solids.

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Basin Configurations & Hydraulic Zones

Conventional Rectangular Sedimentation Basins

Rectangular basins are designed with length-to-width ratios of at least 4:1 (ideally 5:1 to 7:1) and depths of 10 to 16 feet.

Influent ──► [ Inlet Zone ] ──► [ Settling Zone ] ──► [ Outlet Zone ] ──► Effluent
                                [ Sludge Zone ]

Rectangular basins are divided into four distinct hydraulic zones:

• Inlet Zone: Receives flocculator effluent. Utilizes target baffles or perforated diffusion walls (10% to 20% open wall area) to distribute flow uniformly across the basin cross-section, dissipate kinetic energy, and prevent high-velocity short-circuiting.
• Settling Zone: Quiescent volume providing detention time for flocs to settle. Horizontal velocity ($v_h$) must be kept below 0.5 to 1.0 ft/min to prevent floor scour.
• Outlet Zone: Collects clarified supernatant water. Consists of effluent launder troughs equipped with adjustable 90-degree V-notch weirs.
• Sludge Zone: Floor area at the basin bottom sloped 1% to 2% toward sludge hoppers located at the inlet end.

Circular Clarifiers

Circular clarifiers utilize center-feed (radial flow) or peripheral-feed geometry:

• Influent enters a central feedwell designed to dissipate inlet velocity.
• Water flows radially outward toward a continuous perimeter effluent launder.
• The floor is sloped at 1:12 (1 inch drop per foot of radius) toward a central sludge hopper.
• A motor-driven rotating rake arm operating at 1 to 3 revolutions per hour sweeps settled sludge continuously into the center hopper.

High-Rate Sedimentation: Tube Settlers & Lamella Plates

High-rate clarifiers dramatically increase treatment capacity within small footprints by applying Hazen's sedimentation principle: settling efficiency depends strictly on basin surface area, not basin depth.

              Inclined Tubes (55°-60°)
              ┌───┬───┬───┬───┐  ◄── Water Flow (Upward)
              │ / │ / │ / │ / │
              └───┴───┴───┴───┘  ──► Sludge Slide (Downward)

• Configuration: Honeycomb plastic tubes or parallel lamella plates inclined at an angle of $55^\circ$ to $60^\circ$.
• Mechanism: Water flows upward through tubes while floc settles downward onto tube walls. Because settling height is reduced to just 2 to 4 inches, particles strike the tube surface in seconds and slide down into the sludge hopper below.
• Hydraulic Capacity: Increases effective surface area by 400% to 1,000%, allowing Surface Overflow Rates of 1.5 to 3.0 gpm/ft² compared to 0.35–0.70 gpm/ft² for conventional basins.

Solids-Contact Clarifiers (Sludge Blanket Units)

Solids-contact units combine chemical flash mixing, flocculation, and sedimentation within a single compact tank. Raw water passes upward through a suspended slurry layer of previously formed solids (the sludge blanket).

• Process Control: Sludge blanket concentration is monitored using the 30-minute Settled Sludge Volume test ($\text{SSV}_{30}$). Target settled volume is 15% to 30% by volume.
• Vulnerability: Highly sensitive to sudden flow changes or thermal shifts ($> 1^\circ\text{C}$ per hour), which cause thermal density currents and blanket overturn.

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Critical Process Control Math & Operating Parameters

Plant operators must evaluate four primary hydraulic loading formulas:

1. Hydraulic Detention Time (DT)

Detention time represents the average theoretical time water remains inside the basin:

$\text{DT (hours)} = \frac{\text{Basin Volume (gallons)}}{\text{Flow Rate (gallons/day)}} \times 24 \text{ hr/day}$ $\text{DT (minutes)} = \frac{\text{Basin Volume (gallons)}}{\text{Flow Rate (gallons/minute)}}$

• Conventional Standard: 2.0 to 4.0 hours.

2. Surface Overflow Rate (SOR) / Hydraulic Loading Rate

SOR represents the upward fluid velocity at the basin surface. Any particle with a settling velocity ($v_s$) greater than the SOR will be captured:

$\text{SOR (gpd/ft}^2\text{)} = \frac{\text{Flow Rate (gpd)}}{\text{Basin Surface Area (ft}^2\text{)}} = \frac{Q}{L \times W}$ $\text{SOR (gpm/ft}^2\text{)} = \frac{\text{Flow Rate (gpm)}}{\text{Basin Surface Area (ft}^2\text{)}}$

• Conventional Rectangular/Circular Limit: 500 to 1,000 gpd/ft² (0.35 to 0.70 gpm/ft²).
• High-Rate Tube Settler Limit: 1,500 to 3,000 gpd/ft² (1.0 to 2.1 gpm/ft²).

3. Weir Loading Rate (WLR)

Weir loading rate measures the flow rate per linear foot of effluent weir trough:

$\text{WLR (gpd/ft)} = \frac{\text{Flow Rate (gpd)}}{\text{Total Length of Weir (feet)}}$

• Maximum Regulatory Standard: 10,000 to 15,000 gpd/lineal foot.

[!WARNING] Exam Trap: Exceeding maximum weir loading rates ($> 15,000 \text{ gpd/ft}$) creates high localized draw-down approach velocities at the weir crest. This localized suction pulls settled floc out of the sludge layer up into effluent launders ("weir draw-down carryover").

4. Mean Horizontal Velocity ($v_h$)

$v_h (\text{ft/min}) = \frac{\text{Flow Rate (ft}^3\text{/min)}}{\text{Cross-Sectional Area (ft}^2\text{)}} = \frac{Q}{W \times D}$

• Design Standard: Must remain $\le \mathbf{0.5 \text{ to } 1.0 \text{ ft/min}}$ to avoid scouring settled sludge.

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Worked Calculation Examples

Calculation 1: Detention Time & Overflow Rate

A rectangular sedimentation basin measures $120 \text{ ft}$ long, $40 \text{ ft}$ wide, and $12 \text{ ft}$ deep. The plant treats a flow rate of $3.0 \text{ MGD}$ ($3,000,000 \text{ gpd}$). Calculate the Detention Time (hours) and Surface Overflow Rate ($ ext{gpd/ft}^2$).

1. Calculate Basin Volume: $\text{Volume} = 120 \text{ ft} \times 40 \text{ ft} \times 12 \text{ ft} = 57,600 \text{ ft}^3$ $\text{Volume in Gallons} = 57,600 \text{ ft}^3 \times 7.48 \text{ gal/ft}^3 = 430,848 \text{ gallons}$

2. Calculate Detention Time: $\text{DT} = \frac{430,848 \text{ gal}}{3,000,000 \text{ gpd}} \times 24 \text{ hr/day} = 3.45 \text{ hours}$

3. Calculate Surface Area: $\text{Surface Area} = 120 \text{ ft} \times 40 \text{ ft} = 4,800 \text{ ft}^2$

4. Calculate Surface Overflow Rate: $\text{SOR} = \frac{3,000,000 \text{ gpd}}{4,800 \text{ ft}^2} = 625 \text{ gpd/ft}^2$

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Operational Troubleshooting