3.3 Sedimentation Basin Design, Clarifiers & Solids Separation

Principles of Gravitational Sedimentation

Sedimentation (clarification) is the solid-liquid separation process that removes settleable floc particles from water under quiescent gravitational conditions before filtration. Effective sedimentation removes $80%\text{ to }95%$ of total suspended solids, protecting downstream granular media filters from rapid particulate clogging, surface blinding, and premature turbidity breakthrough.

Stokes' Law and Settling Velocity

Terminal settling velocity ($v_s$) of an individual spherical particle in laminar flow is defined by Stokes' Law: $v_s = \frac{g (\rho_p - \rho) d^2}{18 \mu}$ Where:

• $g$ = Acceleration due to gravity ($9.81\text{ m/s}^2$ or $32.2\text{ ft/s}^2$)
• $\rho_p$ = Density of the floc particle (typically $1.005\text{ to }1.05\text{ g/cm}^3$ for alum flocs; $1.10\text{ to }1.30\text{ g/cm}^3$ for ferric flocs)
• $\rho$ = Density of water ($1.000\text{ g/cm}^3$ at $20^\circ\text{C}$)
• $d$ = Particle diameter ($m$ or $ft$)
• $\mu$ = Dynamic viscosity of water ($Pa\cdot s$ or $\text{lb}\cdot\text{s/ft}^2$)

Key Engineering Insights from Stokes' Law

1. Diameter Squared ($d^2$): Doubling particle diameter quadruples settling velocity, demonstrating why effective coagulation and flocculation are vital.
2. Density Differential ($(\rho_p - \rho)$): Because alum floc is mostly bound water, its density is only slightly greater than water. Ferric and lime-softening precipitates are substantially denser, settling at much higher velocities.
3. Viscosity Sensitivity ($\mu$): In cold winter waters ($2^\circ\text{C}$), viscosity increases by nearly $60%$, reducing settling rates proportionally and causing clarifier performance to drop unless chemical aids are applied.

The Four Particle Settling Regimes

Functional Zones and Clarifier Configurations

Every sedimentation basin is engineered around four discrete functional zones:

1. Inlet Zone: Dissipates incoming pipeline kinetic energy, dampens turbulence, and distributes flow uniformly across the basin's entire cross-sectional area using perforated diffuser baffle walls ($10%\text{ to }20%\text{ open area}$) or slotted distribution flumes.
2. Settling Zone: The main quiescent volume where gravity separation occurs. Must maintain stable, laminar horizontal flow without convective or wind-induced turbulence.
3. Sludge (Solids Storage) Zone: The bottom floor and collection hoppers where settled flocs accumulate. Sludge collection equipment scrapes solids to sumps for regular evacuation.
4. Outlet Zone: Collects clarified supernatant uniformly across the surface, using effluent launders (troughs) equipped with V-notch weirs or submerged orifice ports to prevent high-velocity exit currents that scour bottom sludge.

Basin Geometry and Sludge Collection Systems

• Rectangular Basins:
  • Length-to-width ratio of at least $4:1$ (typically $5:1\text{ to }7:1$); water depth $10\text{ to }16\text{ feet}$.
  • Sludge collection: Chain-and-flight collectors (polymer flights moving along the floor at $1\text{ to }3\text{ ft/min}$, pushing sludge into an influent-end hopper, then returning along the surface to skim floating scum) or traveling bridge scrapers.
  • Advantages: Excellent plug flow hydraulics, minimal short-circuiting, shared common-wall construction.
• Circular Clarifiers (Center-Feed or Peripheral-Feed):
  • Center influent well dissipates kinetic energy; water radiates outward at decreasing velocity toward peripheral overflow weirs.
  • Sludge collection: Rotating bottom rake arms with angled scraper plows or suction-header arms sweeping sludge to a center hopper.
  • Advantages: Simple mechanical drive systems, lower maintenance, compact footprint.

High-Rate Clarification Technologies

1. Inclined Tube and Lamella Plate Settlers

In 1904, Allen Hazen established that sedimentation basin efficiency is independent of basin depth and governed strictly by available surface settling area ($v_0 = Q / A_s$).

Modern high-rate settlers install honeycomb modules of hexagonal tubes or parallel plates inclined at a $60^\circ\text{ angle}$ in the upper settling zone:

• Vertical Settling Distance: Decreased from $10\text{--}15\text{ feet}$ down to $2\text{ inches}$ ($5\text{ cm}$).
• Laminar Flow Regime: Small hydraulic diameter reduces the Reynolds number to $Re < 800$ (typically $Re < 100$), eliminating all turbulent mixing.
• Self-Cleaning Action: The $60^\circ\text{ incline}$ allows accumulated sludge to slide continuously downward countercurrent to upward water flow, dropping into the sludge zone.
• Capacity Upgrade: Retrofitting existing rectangular basins with tube settlers increases hydraulic capacity by $200%\text{ to }300%$ without expanding civil basin footprints.

2. Solids-Contact / Upflow Sludge Blanket Clarifiers

Solids-contact clarifiers combine rapid mix, mechanical flocculation, and upflow sedimentation within a single circular tank:

• Raw water and chemicals enter a central reaction cone where a large turbine impeller recirculates pre-formed slurry ($20%\text{ to }40%\text{ solids by volume}$) into incoming water.
• Clarified water flows upward through a suspended fluidized sludge blanket that physically traps and enmeshes incoming microflocs through zone settling.
• Sludge Blanket Control: Controlled via automatic blowdown valves and internal sludge concentrators. Slurry concentration must be tested daily via 5-minute settleable solids tests ($100\text{ mL}$ cylinder).

Design Loading Parameters, Math, and Diagnostics

Water treatment operators must calculate and monitor three primary hydraulic loading parameters:

1. Hydraulic Detention Time ($DT$)

Theoretical time water resides in the basin under plug-flow conditions: $DT\text{ (hours)} = \frac{\text{Basin Volume (gallons)}}{\text{Plant Flow Rate (gallons/day)}} \times 24\text{ hr/day}$

• Conventional Basins: $2.0\text{ to }4.0\text{ hours}$.
• Tube Settler Basins: $1.0\text{ to }2.0\text{ hours}$.
• Solids-Contact Clarifiers: $1.0\text{ to }1.5\text{ hours}$.

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

The volume of water applied per day per square foot of surface settling area: $SOR\text{ (gpd/ft}^2\text{)} = \frac{Q\text{ (gallons/day)}}{A_s\text{ (surface area in ft}^2\text{)}}$

• Conventional Alum Floc: $500\text{ to }1,000\text{ gpd/ft}^2$ ($0.35\text{ to }0.70\text{ gpm/ft}^2$).
• Ferric / Lime Softening: $800\text{ to }1,200\text{ gpd/ft}^2$.
• Tube / Plate Settlers: $1,500\text{ to }3,000\text{ gpd/ft}^2$ (based on plan area).

3. Weir Overflow Rate ($WOR$)

The volume of clarified water passing over each linear foot of effluent weir per day: $WOR\text{ (gpd/linear ft)} = \frac{Q\text{ (gallons/day)}}{\text{Total Active Weir Length (ft)}}$

• Standard Operational Limit: $10,000\text{ to }20,000\text{ gpd/ft}$ (Ten States Standards maximum $21,600\text{ gpd/ft}$ for deep launders).
• Excessive WOR creates localized upward velocity currents near launders (weir draw), pulling settled sludge up off the basin floor and carrying it onto filters.

Hydraulic Short-Circuiting and Sludge Management

Diagnosing Hydraulic Short-Circuiting

Short-circuiting occurs when water bypasses the active settling volume, traveling from inlet to outlet in a fraction of the design detention time. Common causes include:

• Thermal Density Currents: Sinking cold raw water or warm surface skimming during seasonal weather changes.
• Wind-Induced Surface Currents: High winds creating surface waves that push water across the basin while generating bottom counter-currents.
• Unlevel Effluent Weirs: Settling or misaligned launder weirs causing heavy localized flow concentration.
• Diagnostic Method: Fluorometric dye tracer testing (Rhodamine WT) or step-dose salt addition to measure the time to peak concentration ($t_{10}$ and $t_{50}$) and calculate the Morrill Dispersion Index.

Sludge Withdrawal and Rising Sludge

Accumulated sludge flocs must be systematically withdrawn from collection hoppers:

• Consequences of Under-Withdrawal: Sludge stored too long undergoes anaerobic decomposition by heterotrophic bacteria, producing methane ($CH_4$), carbon dioxide ($CO_2$), and hydrogen sulfide ($H_2S$) gases. Microscopic gas bubbles attach to sludge particles, causing large, dark clumps of septic sludge to float to the surface (rising sludge / bulking), destroying effluent clarity and releasing soluble iron, manganese, and severe taste-and-odor compounds into finished water.
• Consequences of Over-Withdrawal: Wastes treated water and pumps dilute sludge ($< 0.5%\text{ solids}$) to dewatering lagoons, overwhelming sludge drying beds and diluting solids-contact sludge blankets below target operating concentrations.