5.2 Primary Clarification & Sedimentation

5.2 Primary Clarification & Sedimentation

Primary clarification is the principal gravitational separation process in municipal wastewater treatment. Following screening and grit removal, raw wastewater enters quiescent sedimentation basins where settleable organic particulates settle under gravity, while oils, grease, and buoyant floatables rise to the surface. Separating settleable organic carbon at this stage saves substantial operational expenditure: every pound of biochemical oxygen demand ($BOD_5$) captured in primary clarifiers directly eliminates the electrical power and blower capacity needed to oxidize that organic matter in downstream activated sludge basins.

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1. Clarifier Basin Configurations & Mechanics

Municipal facilities utilize either rectangular horizontal-flow or circular center-feed sedimentation tanks.

Rectangular Horizontal-Flow Basins

Rectangular clarifiers have length-to-width ratios of 3:1 to 5:1 and depths of 10 to 15 feet (3.0 to 4.5 m):

• Chain-and-Flight Scrapers: Continuous drive chains pull transverse fiberglass or wooden flights along the floor toward an influent sludge hopper at a speed of 2 to 3 ft/min (0.01 to 0.015 m/s), gently sweeping settled sludge without resuspending solids. On their return path along the liquid surface, flights push floating scum toward an effluent scum skimmer.
• Traveling Bridges: Motorized wheel-mounted bridges travel on rails along the tank walls, towing scraper blades or suction headers along the floor and pushing a surface scum squeegee.
• Advantages: Rectangular basins share interior concrete walls, creating a compact footprint ideal for multi-unit facilities.

Circular Center-Feed Clarifiers

Circular basins range from 30 to 150+ feet in diameter with side water depths of 10 to 16 feet and a floor slope of 1:12 descending to a center hopper:

• Inlet Dynamics: Influent enters through a central riser pipe into a circular center feed well. This baffle ring dissipates turbulent entry energy and directs wastewater downward into the radial settling zone.
• Rake Mechanism: Center-driven steel rake arms with angled plow blades rotate slowly (tip speed 8 to 12 ft/min) to roll settled sludge inward into the central conical hopper.
• Surface Skimming: A surface skimmer arm attached to the rotating center cage pushes floating grease up a radial scum beach into a collection trough once per revolution.

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2. Design & Operating Parameters

Primary clarifier sizing and operational control are governed by three primary hydraulic parameters:

Hydraulic Detention Time ($DT$)

Detention time represents the average duration wastewater remains within the settling basin: $\text{Detention Time (hours)} = \frac{\text{Basin Volume (gallons)}}{\text{Wastewater Flow Rate (gal/day)}} \times 24\text{ hr/day}$

• Standard Operating Range: 1.5 to 2.5 hours at design average flow.
• Operational Impacts: Detention times below 1.5 hours (during storm surges) scour unsettled solids over effluent weirs. Over-detention exceeding 3.0 hours (during dry-weather nighttime low flows) promotes anoxic conditions in the sludge blanket, triggering septic gasification.

Surface Overflow Rate ($SOR$)

The surface overflow rate measures the upward hydraulic loading per square foot of surface settling area ($A_{\text{surface}}$): $\text{SOR (gpd/sq ft)} = \frac{\text{Daily Flow Rate (gallons/day)}}{\text{Surface Area (sq ft)}}$

• Design Average Flow: 800 to 1,200 gpd/sq ft.
• Peak Hourly Flow: 2,000 to 3,000 gpd/sq ft.
• Sedimentation Physics: Any particle whose discrete settling velocity ($v_s$) equals or exceeds the $SOR$ will be captured. If high inflow raises the $SOR$ above particle settling velocity, solids carry over the weirs.

Weir Overflow Rate ($WOR$)

Weir overflow rate evaluates hydraulic discharge velocity per linear foot of effluent perimeter weir: $\text{WOR (gpd/linear ft)} = \frac{\text{Daily Flow Rate (gallons/day)}}{\text{Total Active Weir Length (feet)}}$

• Design Range: 10,000 to 15,000 gpd/linear foot at design average flow.
• V-Notch Weirs: Effluent launders utilize 90° V-notch weir plates to maintain uniform perimeter flow, preventing localized high-velocity approach currents that draw solids upward.

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3. Performance Expectations & Mass Balance

Under normal municipal operating conditions, primary sedimentation achieves:

• Total Suspended Solids (TSS) Removal: 50% to 70%.
• Biochemical Oxygen Demand ($BOD_5$) Removal: 25% to 40%.
• Settleable Solids Removal: 90% to 99% (leaving $<0.1\text{ to }0.3\text{ mL/L/hr}$ in settled effluent).

Primary sludge is carbon-rich, consisting of raw fecal matter and food waste. With an average Volatile Suspended Solids (VSS) fraction of 70% to 80%, it serves as an excellent feedstock for anaerobic digesters, yielding high volumes of methane ($CH_4$) biogas.

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4. Scum & Floatables Collection

Floatable materials comprise fats, oils, grease (FOG), plastics, and food remnants. If discharged downstream, floatables coat aeration diffusers and promote biological foaming:

• Scum Baffles: Solid fiberglass baffles extending 12 to 18 inches into the liquid are positioned 6 to 12 inches upstream of effluent weirs, preventing surface grease from spilling into effluent launders.
• Skimmers & Beaches: Circular skimmer arms push grease up an inclined metal ramp (scum beach) into a scum pit. In rectangular basins, transverse slotted pipes are rotated periodically to ingest the surface grease layer.
• Scum Concentration: Scum drains to an adjacent heated pit, where excess water is decanted before progressive cavity scum pumps transfer grease to digesters or dewatering containers.

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5. Primary Sludge Pumping & Process Control

Sludge withdrawal requires precise operational control to optimize solids concentration and prevent biological septicity.

Target Solids Concentration

Primary sludge should be thick and dark gray, with a target concentration of 4.0% to 6.0% Total Solids (TS) (40,000 to 60,000 mg/L). Concentrations below 3.0% TS indicate over-pumping, while concentrations above 8.0% TS risk plugging lines.

Pumping Equipment & Cycle Control

Because primary sludge is viscous and abrasive, positive displacement pumps (progressive cavity or reciprocating plunger pumps) are mandatory. Centrifugal pumps slip and cavitate on thick sludge.

Operators must configure automated pumping for short, frequent pumping cycles (e.g., 5 to 10 minutes every 1 to 2 hours) rather than extended pumping once per shift:

• Over-Pumping (Pumping Too Long): Pulls clarified water straight down through the sludge blanket—a phenomenon termed "rat-holing" or "coning"—pumping thin watery liquid ($<2%\text{ TS}$) to digesters. This excess water overloads digester volume, dilutes anaerobic buffers, and cools digester temperatures.
• Under-Pumping (Leaving Sludge Too Long): Exhausts dissolved oxygen in the sludge blanket. Anaerobic fermentation generates methane ($CH_4$) and carbon dioxide ($CO_2$) gas bubbles that adhere to sludge flocs, lifting large clumps of black putrid sludge to the surface ("sludge clumping" or "gasification"). Septicity also generates hydrogen sulfide ($H_2S$) odors, depresses wastewater pH, and overloads downstream aeration basins.