8.2 Primary Sedimentation & Clarifier Operations

4.2 Primary Sedimentation & Clarifier Operations

Primary sedimentation represents the first major liquid-phase unit process in a municipal wastewater treatment plant designed to remove settleable organic solids and floatable materials. Positioned immediately downstream of preliminary screening and grit removal, primary clarifiers rely on gravity settling to separate suspended solids from the liquid stream. By removing a substantial portion of the organic loading before secondary biological treatment (such as activated sludge or trickling filters), primary clarification drastically reduces oxygen demand, lowers blower power consumption, and produces concentrated primary sludge suitable for anaerobic digestion or mechanical dewatering.

---

1. Primary Clarification Hydraulics & Mechanical Configurations

Primary clarifiers operate on the principle of discrete and flocculent gravity settling. Wastewater velocity is reduced as it enters the basin, allowing solids with a specific gravity greater than water ($>1.0$) to settle to the bottom floor, while materials with a specific gravity less than water ($<1.0$) float to the surface.

Rectangular vs. Circular Clarifier Configurations:

---

2. Key Operational Parameters & Process Calculations

Proper operation of primary clarifiers requires continuous monitoring of hydraulic loading rates. Exceeding design thresholds compromises settling efficiency, resulting in solids carryover to secondary biological systems.

1. Hydraulic Detention Time (HDT):

Hydraulic Detention Time represents the average theoretical time a volume of water remains inside the clarifier basin.

• Standard Operational Range: 1.5 to 2.5 hours (Target design standard: 2.0 hours at average design flow).
• Formula:

$\text{HDT (hours)} = \frac{\text{Clarifier Volume (gal)} \times 24\text{ hr/day}}{\text{Influent Flow Rate (gpd)}}$

2. Surface Overflow Rate (SOR):

Surface Overflow Rate (also known as surface loading rate) measures the volume of wastewater applied per square foot of clarifier surface area per day. SOR dictates the maximum settling velocity of particles that can be completely captured.

• Standard Operational Range: 800 to 1,200 gallons per square foot per day (gpd/ft²) for primary clarifiers treating raw wastewater alone. (Reduced to 600 to 800 gpd/ft² if co-settling returned waste activated sludge).
• Formula:

$\text{SOR (gpd/ft}^2\text{)} = \frac{\text{Influent Flow Rate (gpd)}}{\text{Surface Area (sq ft)}}$

3. Weir Loading Rate (WLR):

Weir Loading Rate measures the volume of effluent passing over each linear foot of overflow weir per day. Excessive weir loading causes high localized take-off velocities, pulling settled solids upward into the effluent launder (weir scouring).

• Standard Operational Range: 10,000 to 20,000 gallons per day per linear foot (gpd/ft).
• Formula:

$\text{WLR (gpd/ft)} = \frac{\text{Influent Flow Rate (gpd)}}{\text{Total Weir Length (ft)}}$

---

3. Worked Operational Calculations

Worked Example 1: Clarifier Hydraulics Evaluation

A municipality operates a circular primary clarifier with a diameter of 60 feet and a side water depth of 12 feet. The plant receives a constant influent raw wastewater flow of 2.5 MGD (2,500,000 gpd). Evaluate the SOR, HDT, and WLR.

1. Calculate Surface Area ($A$):

$A = \pi \times r^2 = 3.14159 \times (30\text{ ft})^2 = 2,827.43\text{ sq ft}$

1. Calculate Surface Overflow Rate (SOR):

$\text{SOR} = \frac{2,500,000\text{ gpd}}{2,827.43\text{ sq ft}} = 884.2\text{ gpd/sq ft} \quad (\text{Complies with 800–1,200 standard})$

1. Calculate Basin Volume ($V$):

$\text{Volume (cu ft)} = A \times \text{Depth} = 2,827.43\text{ sq ft} \times 12\text{ ft} = 33,929.16\text{ cu ft}$

$\text{Volume (gal)} = 33,929.16\text{ cu ft} \times 7.48\text{ gal/cu ft} = 253,790.1\text{ gal}$

1. Calculate Hydraulic Detention Time (HDT):

$\text{HDT (hours)} = \frac{253,790.1\text{ gal} \times 24\text{ hr/day}}{2,500,000\text{ gpd}} = \frac{6,090,962.4}{2,500,000} = 2.44\text{ hours} \quad (\text{Complies with 1.5–2.5 hr standard})$

1. Calculate Weir Length ($L$) & Weir Loading Rate (WLR): For a peripheral outer weir, $L = \pi \times D = 3.14159 \times 60\text{ ft} = 188.5\text{ ft}$.

$\text{WLR} = \frac{2,500,000\text{ gpd}}{188.5\text{ ft}} = 13,262.6\text{ gpd/ft} \quad (\text{Complies with 10,000–20,000 standard})$

Worked Example 2: Solids Removal Mass Balance

The plant influent has a TSS concentration of 250 mg/L. The primary clarifier achieves 65% TSS removal. Calculate the daily mass of TSS removed as primary sludge at a flow rate of 2.5 MGD.

$\text{Influent TSS Mass (lbs/day)} = 2.5\text{ MGD} \times 250\text{ mg/L} \times 8.34\text{ lbs/gal} = 5,212.5\text{ lbs/day}$

$\text{TSS Mass Removed (lbs/day)} = 5,212.5\text{ lbs/day} \times 0.65 = 3,388.1\text{ lbs/day}$

---

4. Performance Efficiency & Operational Troubleshooting

Under normal operating conditions, a well-managed primary clarifier achieves specific treatment efficiency goals:

Operational Troubleshooting Guide:

• Rising Sludge (Gasification): Large clumps of dark sludge pop up to the surface accompanied by gas bubbles. Cause: Sludge blanket held too long in the basin. Facultative and anaerobic bacteria convert organic matter to gases ($ ext{H}_2 ext{S}, ext{CO}_2, ext{CH}_4$) and cause denitrification (releasing $ ext{N}_2$ gas). Gas bubbles attach to sludge particles, buoying them upward. Corrective Action: Increase primary sludge pumping rate and frequency; reduce blanket depth.
• Short-Circuiting: Influent fluid travels directly from inlet to effluent weir without utilizing total basin volume. Cause: Damaged inlet baffles, unlevel V-notch weirs, or thermal/density currents. Corrective Action: Level weir plates using a carpenter's level; repair inlet diffusion baffles.
• Septic Odors: Foul rotten-egg odors emerging from basin surface. Cause: Septic raw influent, stagnant scum accumulation, or septic sludge blanket. Corrective Action: Increase scum skimming frequency; flush scum lines; pre-aerate influent or feed odor control chemicals (calcium nitrate, hydrogen peroxide).

---

5. Scum Removal Mechanics & Primary Sludge Pumping

Separating floatable scum and pumping concentrated primary sludge are vital daily operations.

Scum Removal Systems:

Scum consists of floatable grease, oils, fats (FOG), plastics, and matchsticks. In circular clarifiers, a skimmer arm attached to the center drive rotates across the liquid surface, sweeping scum up a beach plate into a scum trough. Scum drains into a scum pit where it is pumped to anaerobic digesters or concentrated for off-site disposal. Skimmer blades and scum troughs must be washed down daily with high-pressure hose sprays to prevent hardened grease buildup.

Primary Sludge Pumping & Density Control:

Primary sludge collected in bottom hoppers is thick, heavy, and fibrous, typically possessing a Total Solids (TS) concentration of 4.0% to 8.0%.

• Pumping Equipment: Positive Displacement Pumps (such as ram/plunger pumps, progressive cavity pumps, or heavy-duty diaphragm pumps) are required to move viscous primary sludge. Standard centrifugal pumps tend to air-bind or clog with rags and grease.
• Pumping Cycles: Pumping must be conducted on an intermittent, cyclic schedule (e.g., 5 to 15 minutes every hour) controlled by automatic timers.
• Avoiding Coning ("Rat-Holing"): Pumping continuously or at excessively high withdrawal rates pulls thin water directly through the center of the sludge blanket, leaving heavy solids behind. Pumping thin sludge ($<2.0%\text{ TS}$) dilutes anaerobic digesters, wastes heating energy, and hydraulically overloads sludge dewatering facilities.
• Monitoring Tools: Operators monitor sludge density using inline ultrasonic density sensors, optical sight glasses, or manual sample cocks and telescoping valves situated in sludge pit boxes.