10.2 Primary Clarification & Sedimentation Hydraulics

4.2 Primary Clarification & Sedimentation Hydraulics

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Primary Sedimentation Performance Benchmarks

Unlike secondary clarifiers, which must flocculate and settle living biological cultures (biomass), primary sedimentation basins process raw wastewater containing discrete and flocculent solids:

• Total Suspended Solids (TSS) Removal: $50\% \text{ to } 60\%$. The settled fraction consists primarily of dense fecal solids, paper fibers, and fine food particles.
• Biochemical Oxygen Demand ($\text{BOD}_5$) Removal: $25\% \text{ to } 35\%$. Settling removes only the particulate organic matter. Soluble and colloidal $\text{BOD}_5$ (dissolved starches, sugars, volatile organic acids) passes directly through the primary clarifier unaffected.
• Nutrient Removal Efficiency: Minimal. Primary clarifiers achieve only $10\% \text{ to } 15\%$ Total Nitrogen (TN) removal and $10\% \text{ to } 20\%$ Total Phosphorus (TP) removal. This nutrient capture corresponds entirely to the particulate organic nitrogen and phosphorus chemically bound within the settled solids matrix.

+-----------------------------------------------------------------------------+
|                      PRIMARY CLARIFIER PERFORMANCE                          |
+-----------------------------------------------------------------------------+
|  RAW SEWAGE INFLUENT                  PRIMARY CLARIFIER EFFLUENT            |
|  - TSS: 200 - 300 mg/L   -------->   - TSS: 80 - 120 mg/L   (50-60% Removal)|
|  - BOD5: 200 - 280 mg/L  -------->   - BOD5: 140 - 180 mg/L (25-35% Removal)|
|  - TN: 35 - 50 mg/L      -------->   - TN: 30 - 45 mg/L     (10-15% Removal)|
|  - TP: 5 - 8 mg/L        -------->   - TP: 4 - 6.5 mg/L     (10-20% Removal)|
|                                                 |                           |
|                                                 v                           |
|                                        PRIMARY SLUDGE TO DIGESTION          |
|                                        - 2% to 6% Dry Solids                |
|                                        - High Volatile Content (70-80%)     |
+-----------------------------------------------------------------------------+

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Clarifier Basin Configurations & Mechanics

Primary sedimentation basins are engineered into two primary geometrical configurations: rectangular longitudinal tanks and circular center-feed or peripheral-feed tanks.

1. Rectangular Longitudinal Basins

Rectangular basins are laid out side-by-side with common concrete dividing walls, maximizing land-use efficiency and piping economy:

• Geometry: Length-to-width ratios typically range from $3:1 \text{ to } 5:1$, with water depths of $3.0 \text{ to } 4.5 \text{ m}$ ($10 \text{ to } 15 \text{ ft}$).
• Chain-and-Flight Scrapers: Continuous loops of heavy plastic or non-metallic chains carry wooden or fiberglass scraper flights spaced every $3 \text{ m}$ ($10 \text{ ft}$). Along the basin floor, the flights move slowly forward at $0.6 \text{ to } 1.2 \text{ m/min}$ ($2 \text{ to } 4 \text{ ft/min}$) in the direction of or counter to wastewater flow, gently pushing settled sludge into a deep sludge hopper located at the tank inlet.
• Surface Scum Travel: As the continuous chain loops return along the surface tracks, the flights ride partially submerged, pushing floating grease, oils, and scum toward an effluent scum trough or slotted dip pipe.

2. Circular Center-Feed Clarifiers

Circular clarifiers deliver wastewater through a central submerged vertical riser pipe into an energy-dissipating center feed well:

• Hydraulic Flow: Water exits the feed well radially, moving outward toward peripheral effluent launders at continually decelerating horizontal velocities.
• Mechanical Collectors: A motorized rotating bridge carries submerged rake arms equipped with angled plow blades. The arms rotate at tip speeds of $2.4 \text{ to } 3.0 \text{ m/min}$ ($8 \text{ to } 10 \text{ ft/min}$), plowing settled sludge into a central circular sludge hopper.
• Surface Skimming: A surface skimmer blade attached to the rotating bridge sweeps floating scum up a stationary beach plate into a collection scum box connected to a dedicated scum pump.

+-----------------------------------------------------------------------------+
|                   CIRCULAR CENTER-FEED CLARIFIER ZONING                     |
+-----------------------------------------------------------------------------+
|                             Rotating Bridge & Skimmer                       |
|                         +------------------------------+                    |
|    Influent             |      Center Feed Well        |    Effluent Launder|
|    Riser Pipe           |    (Inlet Baffle Zone)       |       & V-Notch    |
|   +----------+          |          |     |             |         Weirs      |
|   |          |--------->|          v     v             |---+     +-----+    |
|   |          |          |                              |   |     |     |    |
|   +----------+          +------------------------------+   v     +-----+    |
|                                Settling Zone                ~~~~~~~|        |
|              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~|        |
|                                                                    | Scum   |
|                      Rotating Scraper Arm                          | Baffle |
|              \_______________________________________________/     +--------+
|                                      |                                      |
|                                      v                                      |
|                             Central Sludge Hopper                           |
+-----------------------------------------------------------------------------+

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Fundamental Hydraulic Loading Formulas & Design Criteria

Primary clarifier design and daily process control depend on three interrelated hydraulic formulas: Surface Overflow Rate, Weir Overflow Rate, and Hydraulic Detention Time.

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

The Surface Overflow Rate represents the upward hydraulic velocity of water exiting the basin, expressed as volume per unit surface area per day:

$\text{SOR } (\text{gpd/ft}^2) = \frac{\text{Flow } Q (\text{gpd})}{\text{Surface Area } A_s (\text{ft}^2)}$

$\text{For Rectangular Basins: } A_s = \text{Length } L \times \text{Width } W$ $\text{For Circular Basins: } A_s = \pi \cdot r^2 = \frac{\pi \cdot D^2}{4}$

• Theoretical Settling Significance: In ideal sedimentation theory, any particle whose terminal settling velocity ($v_s$) equals or exceeds the design SOR ($v_s \ge \text{SOR}$) will achieve $100\%$ capture. Particles with settling velocities less than the SOR are captured at a fractional ratio of $v_s / \text{SOR}$.
• Design Standards:
  • Average Dry Weather Flow: $800 \text{ to } 1200 \text{ gpd/ft}^2$ ($33 \text{ to } 49 \text{ m}^3/\text{m}^2\cdot\text{d}$)
  • Peak Hourly Wet Weather Flow: $2000 \text{ to } 3000 \text{ gpd/ft}^2$ ($81 \text{ to } 122 \text{ m}^3/\text{m}^2\cdot\text{d}$)

2. Weir Overflow Rate (WOR)

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

$\text{WOR } (\text{gpd/ft}) = \frac{\text{Flow } Q (\text{gpd})}{\text{Total Active Weir Length } L_w (\text{ft})}$

• Operational Importance: If weirs are excessively short or unevenly leveled, the localized approach velocity immediately upstream of the weir increases dramatically. This localized suction sweeps settled sludge upward off the blanket into the effluent launder—a phenomenon known as weir sweeping or channel scouring.
• Design Standards:
  • Standard Design Average: $10,000 \text{ to } 20,000 \text{ gpd/ft}$ ($124 \text{ to } 248 \text{ m}^3/\text{m}\cdot\text{d}$)
  • Peak Wet Weather Flow: Up to $30,000 \text{ gpd/ft}$ ($372 \text{ m}^3/\text{m}\cdot\text{d}$)

3. Hydraulic Detention Time (HDT)

Detention time represents the theoretical duration that a slug of water resides within the basin volume:

$\text{HDT } (\text{hours}) = \frac{\text{Basin Volume } V (\text{gallons}) \times 24 \text{ hr/day}}{\text{Flow } Q (\text{gpd})}$

$\text{Where: } V (\text{gallons}) = \text{Volume } (\text{ft}^3) \times 7.48 \text{ gal/ft}^3$

• Design Standard: $1.5 \text{ to } 2.5 \text{ hours}$ (target $2.0 \text{ hours}$ under average dry weather flow).
• Hydraulic Thresholds: An HDT below $1.0 \text{ hour}$ leads to severe solids wash-through. Conversely, an HDT exceeding $3.0 \text{ hours}$ in warm weather induces septic, anaerobic conditions within the settling sludge.

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Clarifier Functional Zones

A properly functioning sedimentation tank consists of four distinct hydrodynamic zones:

1. Inlet Zone: Incorporates energy-dissipating target baffles or submerged perforated diffuser walls. It transforms high-velocity pipe discharge ($2-6 \text{ ft/s}$) into quiescent, uniform laminar sheet flow ($< 0.05 \text{ ft/s}$) distributed evenly across the tank cross-section.
2. Settling Zone: The main quiescent volume where gravity sedimentation takes place unimpeded by eddy turbulence or high horizontal velocities.
3. Sludge Zone & Hopper: Located in the lowest elevation of the basin. Serves as a consolidation reservoir where settled solids compact into a dense blanket before evacuation by primary sludge pumps.
4. Outlet Zone: Comprises effluent launders, adjustable V-notch weirs, and scum baffles. The scum baffle extends $150 \text{ to } 300 \text{ mm}$ ($6 \text{ to } 12 \text{ in}$) below the liquid surface and $50 \text{ to } 100 \text{ mm}$ above it, physically preventing floating oils and grease from spilling over the weirs into secondary treatment.

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Troubleshooting Clarifier Operations

1. Hydraulic Short-Circuiting

Short-circuiting occurs when wastewater passes through a clarifier along high-velocity paths, resulting in actual detention times far lower than theoretical HDT:

• Density Currents: In winter, cold river storm infiltration is denser than tank liquid and plunges to the floor, creating high-velocity bottom currents that sweep settled sludge forward. In summer, warm industrial discharges skim across the top, carrying un-settled solids directly to the effluent weirs.
• Wind-Induced Currents: Wind speeds exceeding $15-20 \text{ mph}$ across open circular tanks generate surface currents that push floatables over weirs and create reverse counter-currents on the tank floor.
• Dye Tracer Profiling: Operators verify short-circuiting by injecting a slug of Rhodamine WT dye at the influent and monitoring fluorometer readings at the effluent launder. A narrow, high-concentration spike appearing within 15–30 minutes confirms severe short-circuiting.

2. Septic Sludge & Gasification ("Ashing" and Rising Clods)

When primary solids are left in the hopper too long, heterotrophic anaerobic bacteria consume the organic matter, depleting all residual dissolved oxygen and nitrate. The bacteria ferment volatile fatty acids, producing methane ($\text{CH}_4$) and carbon dioxide ($\text{CO}_2$) gases:

• Microscopic gas bubbles nucleate within the settled sludge blanket, adhering to organic fibers.
• As the gas bubbles grow, the buoyant force overcomes the gravitational mass of the sludge clod, tearing sheets of sludge from the bottom and floating them to the surface.
• These floating rafts appear dark brown to black, generate foul hydrogen sulfide odors, and disintegrate upon contact with surface skimmers, discharging heavy suspended solids into the secondary process.

3. Sludge Pump Optimization: Pumping Thin Sludge vs. Thick Sludge

Primary sludge withdrawal requires balancing solids concentration against blanket depth:

• Target Solids Concentration: Primary sludge should ideally be pumped at $2.0\% \text{ to } 6.0\%$ Total Dry Solids ($20,000 \text{ to } 60,000 \text{ mg/L}$), with optimal operation at $3.0\% \text{ to } 5.0\%$.
• Consequences of Over-Pumping (Rat-Holing / Coning): If a primary sludge pump operates too long or at too high a flow rate, it pulls a high-velocity suction cone directly through the sludge blanket. Clean, dilute wastewater is drawn straight down from the supernatant, while dense, viscous sludge remains stranded along the hopper walls. Pumping dilute sludge ($< 1.5\%$ solids) introduces excess water into anaerobic digesters, consuming thermal heating energy, dropping digester detention time, and washing out volatile acids.
• Consequences of Under-Pumping: Under-pumping leads to high sludge blanket levels ($> 0.6 \text{ m}$ or $2 \text{ ft}$), septic gasification, solids carryover over weirs, and extreme mechanical torque that can snap scraper drive chains or trip overload switches.

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Surface Scum Skimming Mechanics

Raw wastewater carries significant quantities of hydrophobic materials known collectively as FOG (Fats, Oils, and Grease), along with wax, plastics, and floating food residues:

1. Scum Trapping: The effluent scum baffle blocks surface floatables from reaching the effluent weirs.
2. Collection Mechanisms:
   • Slotted Dip Pipe (Rotary Scum Pipe): A horizontal steel pipe with a longitudinal slot cut into its top. When rotated downward by an actuator or manual handwheel, the weir lip submerges slightly below the water surface, decanting the accumulated scum layer into a gravity drain.
   • Beach Plates & Skimmer Arms: In circular tanks, a neoprene-tipped wiper blade pushes scum up an inclined ramp (beach plate) and drops it into a collection hopper above water level, minimizing the volume of water decanted with the grease.
3. Disposal: Scum is pumped via non-clog recessed impeller vortex pumps or positive displacement progressive cavity pumps to anaerobic digesters, thermal incinerators, or dedicated off-site rendering facilities.