2.2 Primary Sedimentation Principles & Design Parameters

Primary Sedimentation Principles & Design Parameters

Primary sedimentation—carried out in primary clarifiers or settling tanks—is the first major solid-liquid separation stage in conventional wastewater treatment. Positioned directly downstream of preliminary screening and grit removal, primary clarifiers rely on quiescent gravity settling to separate heavy settleable organic solids from the liquid stream and surface skimmers to remove floatable greases and oils before the wastewater undergoes secondary biological treatment.

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1. Fundamentals & Objectives of Primary Sedimentation

Primary treatment is primarily a physical unit process. Its core objectives are:

1. Organic Load Reduction: Removing settleable particulate carbonaceous material, which directly reduces the biological oxygen demand (BOD₅) that must be stabilized downstream in energy-intensive aeration basins.
2. Solids Separation: Concentrating settled solids into a dense raw primary sludge (typically 4% to 8% total solids) suitable for anaerobic or aerobic digestion.
3. Scum and Grease Capture: Collecting floatable fats, oils, greases, and buoyant plastics to prevent fouling downstream trickling filter media, clogging aeration diffusers, or proliferating biological foam.

Typical Removal Efficiencies

Under normal design loading and proper operational control, a conventional primary clarifier achieves the following performance thresholds:

• Total Suspended Solids (TSS): 50% to 70% removal.
• Biochemical Oxygen Demand (BOD₅): 30% to 40% removal (note: primary clarification only removes particulate, settleable BOD; dissolved organic BOD passes through unaffected to secondary units).
• Settleable Solids: 90% to 95% removal (measured using a 1-liter Imhoff cone after 60 minutes of settling).
• Oil, Grease, and Floatable Scum: 50% to 80% removal.

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2. Settling Regimes: Sedimentation Physics

Sedimentation behavior in water and wastewater engineering is classified into four distinct physical regimes based on particle concentration and inter-particle interaction:

+-------------------------------------------------------------------------+
|                         SEDIMENTATION REGIMES                           |
+-------------------+-----------------------------------------------------+
| Type I Settling   | Discrete, unhindered particle settling. Constant    |
| (Grit Chambers)   | velocity governed by Stokes' Law. No coalescence.   |
+-------------------+-----------------------------------------------------+
| Type II Settling  | Flocculent settling. Particles collide, aggregate,  |
| (Primary Basins)  | increase mass, and accelerate downward.             |
+-------------------+-----------------------------------------------------+
| Type III Settling | Zone / Hindered settling. Particles form a blanket  |
| (Secondary Units) | that settles as a unit with a distinct interface.   |
+-------------------+-----------------------------------------------------+
| Type IV Settling  | Compression settling. Solids matrix compacts under  |
| (Thickeners/Bottom| weight of overlying solids; water squeezed upward.  |
+-------------------+-----------------------------------------------------+

Type I: Discrete Particle Settling

Type I sedimentation describes the settling of individual, non-cohesive particles in dilute suspensions. Each particle maintains its discrete identity, shape, and size without aggregating with neighboring particles during descent. The terminal settling velocity ($v_s$) of a spherical particle under laminar flow conditions ($Re < 1$) is governed 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 $32.2 \text{ ft/s}^2$)
• $\rho_p$ = particle density; $\rho$ = fluid density
• $d$ = particle diameter; $\mu$ = fluid dynamic viscosity

Type I settling governs sand removal in velocity-controlled grit channels.

Type II: Flocculent Settling (Primary Clarification)

Type II sedimentation governs the settling of dilute organic suspensions in raw wastewater within primary clarifiers. As organic particles settle through the water column, vertical velocity gradients and turbulent eddies cause particles to collide. Coalescence occurs, forming larger, heavier aggregates (flocs):

• Because the settling velocity of a floc increases with its effective diameter ($v_s \propto d^2$), the downward settling velocity accelerates as the particle travels deeper toward the basin floor.
• Consequently, Type II removal efficiency is a function of both surface overflow rate and tank depth / detention time.

Type III & Type IV Settling

• Type III (Zone / Hindered Settling): High solids concentrations (such as activated sludge mixed liquor in secondary clarifiers) cause inter-particle electrostatic forces to hinder individual settling. Particles remain in fixed relative positions and settle as a unified mass or "blanket," creating a distinct liquid-solid clarification interface.
• Type IV (Compression Settling): Occurs at the bottom of the sludge blanket in secondary clarifiers and gravity thickeners. Particles form a physical structural matrix where further settlement occurs only as water is mechanically squeezed out of interstitial pores by the compressive weight of overlying solids.

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3. Primary Clarifier Design & Operating Parameters

To ensure effective solid-liquid separation, primary clarifiers are engineered around three critical hydraulic parameters: Hydraulic Detention Time (HDT), Surface Overflow Rate (SOR), and Weir Overflow Rate (WOR).

1. Hydraulic Detention Time (HDT)

Detention time represents the theoretical average time a parcel of wastewater remains inside the settling basin:

$\text{Detention Time (hours)} = \frac{\text{Basin Volume (gallons)}}{\text{Influent Flow Rate (gallons/day)}} \times 24 \text{ hours/day}$

• Standard Operational Range: 1.5 to 2.5 hours at design average daily flow.
• Baseline Design Value: 2.0 hours.
• Operational Pitfalls:
  • Under-detention (<1.5 hours): High flow rates or storm inflows flush settleable organics out of the tank before they can settle to the floor, resulting in high TSS carryover to secondary aeration basins.
  • Over-detention (>3.0 hours): In oversized clarifiers during low nighttime flows, excessive detention time causes the wastewater and settled sludge to become anaerobic and septic, generating foul hydrogen sulfide ($H_2S$) odors and gasification that floats sludge to the surface.

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

The Surface Overflow Rate represents the volume of wastewater applied daily per square foot of clarifier surface area:

$\text{SOR (gpd/sq ft)} = \frac{\text{Daily Flow Rate (gallons/day)}}{\text{Clarifier Surface Area (sq ft)}}$

• Design Range at Average Daily Flow: 600 to 1,200 gallons/day/sq ft (24.4 to 48.9 m³/m²/day).
• Allowable Peak Hourly Flow Range: 1,500 to 2,000 gallons/day/sq ft.
• Theoretical Significance: The SOR equals the theoretical critical settling velocity ($v_c$) of the slowest-settling particle that will achieve 100% removal in an ideal settling basin. Any particle with a downward settling velocity ($v_s$) equal to or greater than the upward fluid velocity represented by the SOR will settle to the floor before reaching the effluent weirs.

3. Weir Overflow Rate (WOR)

The Weir Overflow Rate governs the velocity of the liquid passing over the effluent discharge weirs:

$\text{WOR (gpd/linear ft)} = \frac{\text{Daily Flow Rate (gallons/day)}}{\text{Total Active Weir Length (feet)}}$

• Typical Design Range: 10,000 to 20,000 gallons/day/linear foot.
• Operational Significance: If weir length is insufficient, the localized velocity of the approach currents converging on the weirs becomes excessively high. These high-velocity "draw-off currents" generate suction that scours settled solids off the sludge blanket and carries them over the weirs into downstream units.

4. Sidewater Depth (SWD)

Typical primary clarifier sidewall water depths range from 10 to 15 feet (3.0 to 4.5 m). Adequate water depth is required to provide three discrete physical zones:

1. A top clarification zone yielding clean supernatant liquid.
2. A middle settling zone allowing flocculent coalescence.
3. A bottom thickening and storage zone where rake arms consolidate sludge into the hopper.

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4. Flow Equalization Basins

Municipal wastewater treatment plants experience severe diurnal variations in influent flow and organic loading. Flow peaks typically occur in mid-morning and early evening, while deep troughs occur in early morning hours (2:00 to 5:00 AM). In addition, wet-weather inflow and infiltration (I/I) can swamp plant capacity.

Functions of Flow Equalization

Flow equalization basins dampen these hydraulic and organic surges before wastewater reaches primary clarifiers and secondary biological reactors:

• Stabilizes Clarifier Hydraulics: Prevents high-velocity hydraulic surges from exceeding primary clarifier SOR and washing out settleable solids.
• Dampens Shock Loads: Equalizes spikes in BOD₅, ammonia, suspended solids, and industrial pH shocks, delivering a steady organic feed rate to downstream activated sludge basins.
• Configuration: Equalization basins may be in-line (all plant flow routes through the basin) or side-stream (flow exceeding a predetermined threshold is diverted into the equalization basin and pumped back into the plant during low-flow nighttime periods).

Aeration and Mixing Requirements

Equalization basins must be equipped with mechanical mixers or diffused aeration systems:

• Air Supply Rate: Typically 1.25 to 2.0 cfm per 1,000 gallons of basin volume.
• Dual Purpose:
  1. Suspension of Solids: Mixing prevents heavy organic solids from settling and caking on the basin floor, eliminating the need for periodic manual cleaning.
  2. Prevention of Septicity: Introducing air maintains dissolved oxygen levels, preventing the wastewater from turning anaerobic, which would otherwise generate severe hydrogen sulfide ($H_2S$) odors and toxic atmospheric hazards.

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5. Step-by-Step Operator Calculations

Class I wastewater certification exams regularly test hydraulic loading calculations. Master the following standard problem types:

Calculation Example 1: Hydraulic Detention Time (Circular Clarifier)

Problem: A circular primary clarifier has a diameter of 70 feet and an average sidewall water depth of 12 feet. If the facility treats a daily influent wastewater flow of 3.2 MGD (million gallons per day), what is the hydraulic detention time in hours?

Step 1: Calculate the surface area of the circular clarifier. $\text{Radius } (r) = \frac{\text{Diameter}}{2} = \frac{70 \text{ ft}}{2} = 35 \text{ ft}$ $\text{Area } (A) = \pi \times r^2 = 3.1416 \times (35 \text{ ft})^2 = 3.1416 \times 1,225 \text{ sq ft} = 3,848.5 \text{ sq ft}$

Step 2: Calculate the tank volume in cubic feet. $\text{Volume } (V_{\text{cu ft}}) = \text{Area} \times \text{Depth} = 3,848.5 \text{ sq ft} \times 12 \text{ ft} = 46,182 \text{ cu ft}$

Step 3: Convert the volume from cubic feet to gallons. $\text{Volume } (V_{\text{gal}}) = 46,182 \text{ cu ft} \times 7.48 \text{ gal/cu ft} = 345,441 \text{ gallons}$

Step 4: Convert daily flow rate to hourly flow rate. $\text{Flow Rate } (Q_{\text{gph}}) = \frac{3,200,000 \text{ gal/day}}{24 \text{ hr/day}} = 133,333 \text{ gal/hr}$

Step 5: Calculate detention time in hours. $\text{Detention Time} = \frac{\text{Volume (gal)}}{\text{Flow Rate (gal/hr)}} = \frac{345,441 \text{ gal}}{133,333 \text{ gal/hr}} = 2.59 \text{ hours}$

Operational Evaluation: A detention time of 2.59 hours is slightly above the 2.0-hour standard baseline but well within acceptable operational limits (1.5 to 2.5–3.0 hours).

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Calculation Example 2: Surface Overflow Rate and Weir Overflow Rate

Problem: Using the same 70-foot diameter circular primary clarifier treating 3.2 MGD, with an effluent weir installed around the entire outer circumference of the tank, calculate:

1. The Surface Overflow Rate (SOR) in gpd/sq ft.
2. The Weir Overflow Rate (WOR) in gpd/linear ft.

Part 1: Surface Overflow Rate (SOR) $\text{SOR} = \frac{\text{Flow Rate (gpd)}}{\text{Surface Area (sq ft)}} = \frac{3,200,000 \text{ gpd}}{3,848.5 \text{ sq ft}} = 831.5 \text{ gpd/sq ft}$

Evaluation: 831.5 gpd/sq ft falls squarely in the middle of the recommended 600 to 1,200 gpd/sq ft design window.

Part 2: Weir Overflow Rate (WOR) $\text{Weir Length } (L) = \text{Circumference} = \pi \times \text{Diameter} = 3.1416 \times 70 \text{ ft} = 219.9 \text{ linear feet}$ $\text{WOR} = \frac{\text{Flow Rate (gpd)}}{\text{Weir Length (ft)}} = \frac{3,200,000 \text{ gpd}}{219.9 \text{ linear ft}} = 14,552 \text{ gpd/linear ft}$

Evaluation: 14,552 gpd/linear ft falls well within the standard 10,000 to 20,000 gpd/ft guideline, ensuring low approach velocities that will not pull solids over the weir crest.