8.3 Primary Clarification & Sedimentation Operations

8.3 Primary Clarification & Sedimentation Operations

Primary clarification (or primary sedimentation) is the first major unit process in a conventional wastewater treatment plant designed to remove settleable particulate solids and floatable materials. By exploiting gravity settling in quiescent, low-velocity basins, primary clarifiers physically separate the heavier suspended organic solids from the liquid wastewater stream. Efficient primary clarification is vital to the overall economics of the treatment facility: every pound of organic solids removed as raw primary sludge in the clarifier represents one less pound of organic load that must be oxidized biologically in downstream aeration basins, saving substantial electrical blower energy.

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Mechanics & Process Removal Objectives

Sedimentation Settling Mechanics

Primary sedimentation operates under Type II (Flocculent) Settling. As suspended organic particles settle through the quiescent water column, they collide, coalesce, and aggregate into larger, denser agglomerations (flocs). Because larger flocs possess a higher settling velocity than individual discrete particles, the settling rate of flocculent solids accelerates as they sink toward the basin floor.

Typical Primary Clarifier Removal Efficiencies

Under optimal hydraulic and operational conditions, a well-operated primary clarifier achieves:

• Total Suspended Solids (TSS) Removal: 50% - 70%
• Biochemical Oxygen Demand (BOD5) Removal: 25% - 40%
• Settleable Solids Removal: 90% - 95%
• Fats, Oils, and Grease (FOG) Removal: 50% - 60%

Primary clarification does not significantly remove dissolved solids, colloidal non-settleable particles, or soluble nutrients (ammonia, nitrate, orthophosphate).

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Basin Geometry & Mechanical Configurations

Primary clarifiers are constructed in two predominant geometries: rectangular longitudinal-flow tanks and circular center-feed or peripheral-feed tanks.

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|                        PRIMARY CLARIFIER CONFIGURATIONS                                 |
+-----------------------------------------------------------------------------------------+
|  1. RECTANGULAR LONGITUDINAL-FLOW TANKS:                                                |
|     - Flow enters through influent distribution baffles and moves horizontally along     |
|       the tank length to effluent weirs. Length-to-width ratio: 3:1 to 5:1.             |
|     - Chain-and-flight collector mechanism: continuous polymer/wooden flights move       |
|       sludge along the tank floor toward an influent hopper at 2 to 3 ft/min.            |
|     - Return run of flights rides along the liquid surface, pushing scum to an effluent |
|       scum trough.                                                                      |
+-----------------------------------------------------------------------------------------+
|  2. CIRCULAR CENTER-FEED CLARIFIERS:                                                    |
|     - Influent enters through a central vertical pipe into a submerged center stilling  |
|       well (energy dissipating inlet) and flows radially outward to perimeter weirs.   |
|     - Rotating bridge mechanism with angled bottom scraper plows sweeps settled sludge  |
|       to a central bottom collection sump/hopper.                                       |
|     - Surface skimmer arm sweeps floating scum into a hinged scum box / scum beach.     |
+-----------------------------------------------------------------------------------------+

  Circular Center-Feed Primary Clarifier Cross-Section:

                     Rotating Bridge Drive & Surface Skimmer
                               ┌───────┴───────┐
                               │               │
   Center Stilling Well ───────┼───────┐       │
   (Energy Dissipator)         │       │       │
   Influent Pipe ──► ──┐       │       │       │         Scum Baffle & Perimeter Weirs
                       │       │       │       │               ┌───┐   ┌───┐
  ═════════════════════╪═══════╪═══════╪═══════╪═══════════════╡   ╞═══╡ V ╞════ Effluent
                       │       │       │       │               └───┘   └───┘ Flow ──►
                       │       │       │       │               Scum     V-Notch
                       │       │       │       │               Baffle   Weir
                       ▼       ▼       ▼       ▼
                              Radial Flow Outward
                       ────────────────────────────────►
                           
                           Settling Sludge Flocs (Type II)
                            •   •   •   •   •   •   •   •
                             ▼   ▼   ▼   ▼   ▼   ▼   ▼
  ────────────────────┐                                 ┌─────────────────────────
   Tank Floor (1:12)   \  Bottom Rotating Scraper Plows /  Tank Floor Slope
                        \◄─────────────────────────────/ 
                         \                            /
                          \                          /
                           └───┐                ┌──┘
                               │ SLUDGE HOPPER  │
                               └───┬────────────┘
                                   ▼
                            To Raw Sludge Pump (4% - 8% TS)

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Hydraulic Loading & Design Operational Parameters

Operators must monitor three fundamental hydraulic parameters to ensure clarifier efficiency and prevent solids wash-out:

1. Hydraulic Detention Time (HDT)

Hydraulic Detention Time is the average theoretical length of time wastewater remains inside the clarifier basin:

HDT (hours) = [Basin Volume (gallons) × 24 hr/day] / Influent Flow Rate (gallons/day)

• Standard Operational Range: 1.5 to 2.5 hours at design average flow.
• If HDT < 1.0 hr: High velocity prevents light flocs from settling, causing solids carryover over effluent weirs.
• If HDT > 3.5 hr: Stagnant, quiescent conditions allow sludge blankets to turn septic and anaerobic, producing gases that lift sludge.

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

Surface Overflow Rate is the volume of wastewater passing through each square foot of clarifier surface area per day. It represents the upward liquid velocity that settling particles must exceed to reach the floor:

SOR (gpd/sq ft) = Flow Rate (gallons/day) / Clarifier Surface Area (sq ft)

• Average Design Flow Range: 800 - 1,200 gpd/sq ft
• Peak Wet Weather Hourly Flow Range: 2,000 - 3,000 gpd/sq ft

3. Weir Overflow Rate (WOR / Weir Loading Rate)

Weir Overflow Rate measures the volume of effluent passing over each linear foot of effluent weir crest per day. It ensures that discharge velocity near the weir is low enough to prevent suction currents from pulling settled sludge off the floor:

WOR (gpd/linear ft) = Flow Rate (gallons/day) / Total Active Weir Length (feet)

• Standard Design Range: 10,000 - 20,000 gpd/linear ft
• Peripheral clarifiers use 90° V-notch weirs because the triangular notches maintain a consistent, evenly distributed discharge crest even during low-flow periods, eliminating dead zones and short-circuiting.

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Scum Management & Floatables Collection

Scum consists of low-density materials with a specific gravity less than 1.0 that float to the clarifier surface, including animal fats, vegetable oils, grease (FOG), mineral wax, floatable plastics, cigarette tips, and rubber materials.

• Scum Baffles: Continuous metal or fiberglass baffles submerged 12 - 18 inches below the water surface, positioned 6 - 12 inches in front of the effluent weirs. The baffle blocks floating scum from escaping over the weir crest while allowing clarified liquid to pass underneath.
• Skimming Mechanisms:
  • Rectangular Tanks: The return run of chain-and-flight scrapers sweeps surface scum to a slotted, rotatable scum pipe or transverse scum trough located at the effluent end.
  • Circular Tanks: A radial skimmer blade mounted to the rotating bridge arm sweeps scum up a sloping metal ramp (scum beach) into a collection hopper or scum well.
• Scum Disposal: Collected scum is pumped via positive displacement scum pumps to anaerobic digesters (where lipids provide high methane yields) or concentrated and hauled for landfill disposal/rendering.

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Raw Primary Sludge Pumping & Process Optimization

+-----------------------------------------------------------------------------------------+
|                        RAW PRIMARY SLUDGE PUMPING CHARACTERISTICS                       |
+-----------------------------------------------------------------------------------------+
|  Target Sludge Density:                                                                 |
|  - Ideal Raw Sludge Concentration: 4.0% to 8.0% Total Solids (TS).                      |
|  - Minimum Acceptable Density: 3.0% TS.                                                 |
|  - Thin Sludge (< 3.0% TS): Indicates over-pumping; excess water chills digesters.      |
+-----------------------------------------------------------------------------------------+
|  Pumping Cycles & Operational Strategy:                                                 |
|  - Pumping Mode: Frequent, short-duration cycles (e.g., 1 to 3 minutes every 15 to 30   |
|    minutes) rather than long, infrequent pumping runs.                                  |
|  - Equipment: Progressive cavity pumps or positive displacement piston/diaphragm pumps. |
|  - Sludge Blanket Target Depth: 1.0 to 2.0 feet over the sludge hopper floor.          |
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The Dangers of Over-Pumping vs. Under-Pumping

1. Over-Pumping (Pumping Too Long or Too Frequently):
   • Pulls water through the center of the sludge hopper ("ratholing" or "coning"), withdrawing thin, dilute sludge (< 2% - 3% TS).
   • Pumping excess water hydraulic overloads anaerobic digesters, cooling the digester heating system, shortening digestion retention times, and dramatically increasing sludge heating and dewatering chemical costs.
2. Under-Pumping (Leaving Sludge in Basin Too Long):
   • In the absence of dissolved oxygen, raw primary sludge undergoes anaerobic acid fermentation within hours, especially in warm weather.
   • Anaerobic bacteria produce methane (CH4), carbon dioxide (CO2), and hydrogen sulfide (H2S) gases.
   • Rising Sludge (Gasification): Entrained gas bubbles attach to settled sludge particles, reducing their effective density and lifting large, black, foul-smelling mats of septic sludge to the clarifier surface. These mats break up and carry over the effluent weirs into secondary treatment.

Primary Clarifier Troubleshooting Matrix