6.2 Primary Clarification & Sedimentation

6.2 Primary Clarification & Sedimentation

Core Function: Primary sedimentation basins (clarifiers) follow preliminary screening and grit removal. They provide quiescent gravity settling to separate settleable particulate organic solids and floatable grease from the wastewater stream. By removing the readily settleable organic fraction ahead of secondary treatment, primary clarifiers substantially diminish the electrical aeration demand, solids loading, and microbial stress imposed on downstream activated sludge systems.

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1. Sedimentation Theory & Settling Classes

Sedimentation is a physical unit operation that separates suspended solids from water under the influence of gravitational acceleration. In environmental engineering, settling behavior is classified into four discrete regimes (the Camp/Fitch settling classes):

Mechanics of Type II Flocculent Settling

Primary clarification of municipal wastewater is governed by Type II flocculent settling. Raw sewage contains millions of fine, non-discrete organic particles. As these particles slowly sink through the water column, natural Brownian motion, fluid velocity gradients, and differential settling velocities cause particles to collide. When organic particles collide, adhesive surface polymers cause them to coalesce into larger, denser composite flocs.

Because settling velocity ($v_s$) is proportional to particle diameter squared ($v_s \propto d^2$), flocculation significantly accelerates sedimentation as particles travel downward. Consequently, depth plays an active role in Type II settling: a deeper clarifier provides greater vertical opportunity for particle collisions and agglomeration.

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2. Performance Expectations & Removal Efficiencies

A properly designed and operated conventional primary clarifier achieves predictable solid-liquid separation:

• Total Suspended Solids (TSS) Removal: 50% to 70% (industry operational benchmark: ~60%).
• Biochemical Oxygen Demand ($BOD_5$) Removal: 25% to 40% (industry benchmark: ~30% to 35%). Clarification removes exclusively the particulate (insoluble) organic fraction; dissolved organics and colloidal compounds pass through the basin unaffected.
• Settleable Solids Removal: 90% to 95%, as measured in an Imhoff cone after 60 minutes.
• Nutrient Removal (Nitrogen and Phosphorus): Incidental only; typically 10% to 20% Total Kjeldahl Nitrogen (TKN) and 5% to 15% Total Phosphorus (TP) are removed, corresponding directly to the particulate organic fraction settled in the sludge.

Chemically Enhanced Primary Treatment (CEPT)

When municipal plants face hydraulic overloads, stringent seasonal phosphorus discharge limits, or seek to divert maximum carbon to anaerobic digesters for biomethane energy production, utilities implement Chemically Enhanced Primary Treatment (CEPT):

• Coagulants—typically metal salts such as ferric chloride ($FeCl_3$) or aluminum sulfate (alum, $Al_2(SO_4)_3$)—are injected into the aerated grit chamber effluent or primary influent flash mixer at doses of 15 to 40 mg/L.
• An anionic polymer (0.1 to 0.5 mg/L) is added downstream to build heavy, shear-resistant flocs.
• CEPT Performance: Boosts TSS removal to 80% to 90%, $BOD_5$ removal to 50% to 65%, and precipitates soluble orthophosphate, achieving > 85% Total Phosphorus removal in the primary stage.

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3. Critical Design & Operating Parameters

Primary clarifier performance is governed by three primary hydraulic criteria established under Ten States Standards and NJDEP engineering guidelines:

+-------------------------------------------------------------------------+
|                    PRIMARY CLARIFIER HYDRAULIC METRICS                   |
+--------------------+--------------------------------+-------------------+
| Parameter          | Formula                        | Typical Range     |
+--------------------+--------------------------------+-------------------+
| Hydraulic          | HRT (hr) = (V * 24) / Q        | 1.5 - 2.5 hours   |
| Retention Time     |                                | (min 1.0 hr peak) |
+--------------------+--------------------------------+-------------------+
| Surface Overflow   | SOR (gpd/ft2) = Q / A          | 800 - 1,200 avg   |
| Rate (SOR)         |                                | 1,500 - 2,500 peak|
+--------------------+--------------------------------+-------------------+
| Weir Overflow      | WOR (gpd/ft) = Q / L           | 10,000 - 20,000   |
| Rate (WOR)         |                                | gpd/linear foot   |
+--------------------+--------------------------------+-------------------+

1. Hydraulic Retention Time (HRT)

Hydraulic Retention Time represents the average theoretical duration that a parcel of wastewater spends inside the sedimentation basin:

$\text{HRT (hours)} = \frac{\text{Clarifier Volume } V \text{ (gallons)}}{\text{Daily Flow Rate } Q \text{ (gallons/day)}} \times 24 \text{ hr/day}$

• Standard Operating Range: 1.5 to 2.5 hours at average daily design flow.
• Operational Constraints:
  • If HRT drops below 1.0 hour (during peak wet weather surges), fluid velocity prevents flocculent particles from settling, causing massive solids carryover over effluent weirs.
  • If HRT exceeds 3.0 to 3.5 hours (during hot summer low-flow conditions), dissolved oxygen is completely exhausted. The sludge blanket turns septic, generating foul odors and causing gas-buoyed solids to float.

2. Surface Settling Rate / Surface Overflow Rate (SOR)

The Surface Overflow Rate represents the volumetric loading applied per unit of clarifier water surface area per day:

$\text{SOR (gpd/ft}^2\text{)} = \frac{\text{Daily Flow Rate } Q \text{ (gallons/day)}}{\text{Clarifier Surface Area } A \text{ (square feet)}}$

• Design Standards:
  • Average Daily Flow: 800 to 1,200 gpd/ft²
  • Peak Hourly Flow: 1,500 to 2,500 gpd/ft²
  • With CEPT Chemical Addition: Up to 1,500 gpd/ft² average and 3,000 gpd/ft² peak
• Physical Significance: Under ideal sedimentation theory, the Surface Overflow Rate equals the critical settling velocity ($v_c$). Any individual particle whose terminal settling velocity ($v_s$) equals or exceeds the SOR will theoretically be 100% removed. Particles with settling velocities less than the SOR are removed in proportion to the ratio $v_s / \text{SOR}$.

3. Weir Overflow Rate (WOR)

The Weir Overflow Rate measures the volume of clarified effluent passing over each linear foot of effluent weir crest per day:

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

• Design Standards:
  • Average Daily Flow: 10,000 to 15,000 gpd/ft
  • Peak Hourly Flow: $\le$ 20,000 gpd/ft (small plants $\le$ 1 MGD typically limit peak WOR to 10,000–15,000 gpd/ft)
• Operational Importance: If weir length is insufficient, the exit velocity near the weir troughs increases drastically. High localized approach velocities generate upward suction currents that pull settled sludge up off the tank floor, scouring solids directly over the V-notch weirs.

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4. Basin Configurations: Rectangular vs. Circular Basins

Primary clarifiers are constructed in either rectangular or circular geometries, each with distinct mechanical and hydraulic characteristics.

Rectangular Clarifiers

• Geometry: Long, narrow concrete basins with length-to-width ratios of 3:1 to 5:1 (lengths 75 to 250 ft, widths 15 to 40 ft, water depths 10 to 15 ft). Flow moves in a horizontal plug-flow path from the inlet distribution baffle to the effluent launders.
• Sludge Collection Mechanics: Longitudinal chain-and-flight collectors (polymeric non-metallic chains with structural fiberglass flights spaced 10 ft apart) travel along the tank floor toward the influent end. The flights travel at a slow, non-turbulent speed of 2 to 4 ft/min (0.01 to 0.02 m/s) to scrape settled sludge into a transverse sludge hopper located at the inlet floor.
• Scum Skimming: On their return journey along the liquid surface, the flights push floating scum, oils, and grease toward the effluent end, sweeping it into a rotating slotted scum pipe or beach trough.
• Sludge Hoppers: A transverse cross-collector or screw auger inside the hopper conveys sludge to a single central sump connected to the sludge pump suction.

Circular Clarifiers

• Geometry: Cylindrical concrete tanks ranging from 30 to 150+ feet in diameter, with sidewater depths (SWD) of 10 to 16 feet and a floor sloped inward toward a center hopper at a pitch of 1:12 (1 inch per foot).
• Flow Mechanics: Center-feed configuration dominates. Raw wastewater enters through a central vertical influent pipe and discharges into a concentric center feed well (flocculating baffle). The feed well dissipates influent kinetic turbulence and directs flow downward into the settling zone. Clarified effluent travels radially outward to perimeter V-notch weir launders.
• Sludge Collection Mechanics: A central bridge rotates at a tip speed of 8 to 12 ft/min, driving two or four bottom scraper arms equipped with angled plows or continuous spiral scrapers that sweep settled sludge inward along the conical floor into the central sludge hopper.
• Surface Scum Collection: A surface skimmer arm attached to the rotating bridge skims floating grease radially outward, driving it up a ramped beaching plate into a scum collection box connected to an external scum wet well.

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5. Primary Sludge Pumping Protocols & Solids Concentration

Raw Primary Sludge Characteristics

Raw primary sludge is a dense, dark gray to brown slurry containing fibrous fecal solids, paper fibers, food remnants, and grease. It is highly putrescible, digests readily, and rapidly turns anaerobic.

• Target Solids Concentration: A well-managed primary clarifier yields raw primary sludge at 3.0% to 6.0% Total Solids (TS) by weight (30,000 to 60,000 mg/L). Under optimized operation and gravity pre-thickening, concentrations can reach 5.0% to 8.0% TS.

Sludge Pump Selection

1. Positive Displacement Pumps:
   • Progressive Cavity Pumps: A single-helix metallic rotor rotates eccentrically within a double-helix elastomeric stator, forming sealed progressing cavities. They deliver precise, pulsation-free flow against variable discharge heads. Caution: Running a progressive cavity pump dry for even 30 seconds will burn and permanently destroy the elastomeric stator.
   • Plunger / Diaphragm Pumps: Heavy-duty reciprocating plungers or air-operated double-diaphragms (AODD) with large ball check valves. Excellent self-priming capabilities and high suction lift, handling thick solids and rags without binding.
2. Recessed-Impeller Centrifugal Pumps (Vortex / Torque-Flow Pumps):
   • The impeller is completely recessed out of the casing flow path into the back of the volute. Rotation creates a powerful liquid vortex that draws solids through the suction and discharges them out the casing without direct contact with impeller vanes, completely eliminating rag-wrapping and impeller binding.

Pumping Strategy & Frequency Control

The fundamental operational rule of primary sludge pumping is: PUMP SHORT, FREQUENT CYCLES.

Rather than pumping for 60 continuous minutes twice per day, operators configure automated timers or ultrasonic sludge blanket controllers to pump for 3 to 10 minutes every 1 to 2 hours.

+-------------------------------------------------------------------------+
|                   PRIMARY SLUDGE PUMPING FAILURE MODES                  |
+---------------------+---------------------------------------------------+
| Operational Error   | Manifestations & Downstream Consequences          |
+---------------------+---------------------------------------------------+
| UNDER-PUMPING       | - Sludge blanket exceeds 2 to 3 feet in depth     |
| (Infrequent / Short | - Anaerobic decomposition in hopper floor         |
| pumping cycles)     | - Methanogenesis produces CH4, CO2, and H2S gases |
|                     | - Gas bubbles attach to sludge flocs              |
|                     | - "Sludge Gasification" / "Rising Sludge"         |
|                     | - Large black mats float and wash over weirs      |
|                     | - Massive TSS/BOD shock load to aeration basins   |
+---------------------+---------------------------------------------------+
| OVER-PUMPING        | - "Hydraulic Coning" / "Rat-Holing" occurs        |
| (Continuous / Long  | - Clear water is pulled through center of blanket |
| pumping cycles)     | - Sludge concentration drops below 2.0% TS        |
|                     | - Excessive water pumped to anaerobic digester    |
|                     | - Chills digester temperature, wastes fuel        |
|                     | - Dilutes digester alkalinity, causes souring     |
|                     | - Overloads downstream dewatering and decanting   |
+---------------------+---------------------------------------------------+

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6. Scum Collection, Concentration & Management

Composition of Scum

Scum comprises all floatable substances with specific gravities less than water ($SG < 1.0$): animal fats, vegetable oils, grease (FOG), mineral oils, waxes, soaps, plastics, condoms, and floating organic matter.

Collection Mechanisms & Concentrators

• Skimming Mechanisms: In rectangular clarifiers, return flight passes or traveling bridge wipers sweep scum into a slotted scum pipe or dipping scum trough. In circular clarifiers, rotating skimmer arms sweep scum up an inclined beach plate into a scum hopper box.
• Flushing & Decanting Pits: Gravity scum flows contain vast volumes of carriage water (often 80% to 90% water by volume). Scum discharges into a dedicated scum decanting pit or concentration tank equipped with heated immersion coils or hot-water flushes to keep grease liquefied. The carriage water is drained from the bottom and returned to the clarifier influent, leaving concentrated grease (10% to 20% solids).
• Digester Utilization: Concentrated scum is pumped using progressive cavity or heated plunger pumps directly to anaerobic digesters. FOG contains high energy density and volatile fatty acids, generating superior biomethane ($CH_4$) gas yields compared to regular sludge. Alternatively, scum is collected by licensed commercial rendering haulers for biodiesel feedstock.

Strict Operational Rule: NEVER return collected scum back to the primary clarifier influent channel. Recycled grease will not settle; it immediately re-floats, coats clarifier concrete walls, blinds effluent V-notch weirs, causes intense odors, and passes into secondary activated sludge where it fosters massive blooms of foam-causing filamentous bacteria (Nocardia and Microthrix parvicella).

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7. Practical Operational Scenario & Exam Traps

Practical Operational Scenario

A treatment plant operator notices that Primary Clarifier No. 2 has developed large islands of foul-smelling, dark gray sludge floating across the effluent third of the basin. Gas bubbles are constantly erupting to the surface, and effluent TSS has spiked from 65 mg/L to 140 mg/L.

• Investigation: The operator inserts a core-sampling sludge blanket finder ("Sludge Judge") and discovers a 4.5-foot sludge blanket in the hopper. Reviewing the SCADA pump log, the operator discovers that the primary sludge pump run-time timer was mistakenly reduced from 8 minutes/hour to 2 minutes/hour following maintenance.
• Corrective Action: The operator immediately places the sludge pump on continuous manual run while monitoring pump discharge sight glasses and density meter readings. Once sludge solids drop below 3.0% TS, the pump is returned to automated timed service set at 10 minutes every hour. The operator uses a high-pressure hose to break up surface mats, allowing scum skimmers to capture floating solids, and checks dissolved oxygen in downstream aeration basins to compensate for the organic surge.

Critical Exam Traps

• Trap 1: Surface Settling Rate Calculation. When calculating SOR on an exam, always verify the surface area used: for circular tanks, use $A = \pi r^2$ (or $A = 0.7854 \times D^2$). Always divide by the number of clarifiers in service if total plant flow is given!
• Trap 2: Sludge Bulking vs. Rising Sludge. Do NOT confuse secondary clarifier filamentous bulking with primary clarifier rising sludge. Primary clarifier sludge rises due to anaerobic gasification ($CH_4, CO_2, H_2S$ generation from septic conditions). Secondary clarifier sludge rises either from denitrification ($N_2$ gas) or filamentous overgrowth.
• Trap 3: Effects of Over-Pumping on Digesters. Pumping thin sludge (<2% TS) does NOT help an anaerobic digester; it severely harms the digester by diluting volatile acids, lowering volatile solids retention time, chilling the vessel, and demanding massive additional heating fuel.