9.3 Primary Sedimentation

9.3 Primary Sedimentation

Quick Answer: Primary clarifiers remove settleable solids and floatable scum by gravity before secondary treatment. Classic textbook removals are about 25–40% BOD and 50–70% TSS (ranges vary with wastewater and design). Operators control detention time, surface overflow rate (SOR), sludge withdrawal, and scum removal—and avoid septic sludge that releases gas, odors, and floating solids.

After preliminary treatment, wastewater still carries high concentrations of settleable organic and inorganic solids. Primary sedimentation (primary clarification) uses gravity in large quiescent tanks to drop those solids as primary sludge and float grease/scum to the surface. On FDEP wastewater exams, primary clarifiers appear under process control, loadings, and troubleshooting (floating sludge, washout, septic conditions).

Purpose of Primary Clarifiers

Primary sedimentation aims to:

1. Reduce TSS and particulate BOD so secondary processes receive a more stable load.
2. Concentrate solids into a pumpable primary sludge for thickening/digestion.
3. Remove floatable FOG/scum that would otherwise foul aeration and secondary clarifiers.
4. Provide equalization of spikes to a limited degree (not a substitute for true equalization basins).

Primary treatment is physical—not biological. Microbes are present, but design detention is measured in hours, not the multi-day solids residence of activated sludge.

Typical Removals (Classic Exam Ranges)

Memorize the classic performance bands used in operator training materials:

These are order-of-magnitude teaching ranges, not a permit. Actual performance depends on temperature, septicity, industrial contributions, SOR, detention time, and sludge inventory. If wastewater is highly soluble (some industrial streams) or already septic with gas-buoyed solids, BOD/TSS removals fall.

Florida exam items may give influent/effluent data and ask removal efficiency:

[ %\ \mathrm{Removal} = \frac{C_{\mathrm{in}} - C_{\mathrm{out}}}{C_{\mathrm{in}}} \times 100 ]

Example: influent TSS 240 mg/L, primary effluent TSS 80 mg/L → removal = (160/240)×100 = 67% (within the classic TSS band).

Tank Types and Mechanisms

Common configurations:

• Rectangular clarifiers with chain-and-flight or traveling-bridge collectors pushing sludge to hoppers and scum to a beach/trough.
• Circular clarifiers with rotating sludge scrapers (plows) and surface skimmers discharging to a scum box.
• Imhoff tanks (historical/combined settling-digestion)—still appear in older literature and some small systems.

Mechanisms must run. A broken flight chain or stalled circular drive leaves sludge blankets that go septic or wash out.

Scum and Primary Sludge

Scum

Scum is the floating layer of FOG, plastics that passed screens, and buoyant debris. Skimmers must operate continuously or on a reliable cycle. Neglected scum:

• Dries into thick mats
• Creates odor and vector problems
• Breaks loose and loads secondary treatment
• Fouls weirs and launders

Primary Sludge

Settled solids collect in hoppers as primary sludge—typically higher in volatile solids and readily biodegradable material than secondary waste sludge. Operators pump primary sludge on a schedule that:

• Prevents excessive blanket depth
• Avoids pumping too thin (wastes water, overloads thickeners)
• Avoids leaving sludge so long it turns septic

Primary sludge is often co-thickened or sent to anaerobic digesters where its high energy content is valued—but only if it arrives before it becomes a gas-lift problem in the clarifier.

Detention Time and Surface Overflow Rate (SOR)

Two hydraulic concepts dominate primary clarifier exams.

Detention Time (Hydraulic Retention Time)

[ \mathrm{Detention\ time} = \frac{\mathrm{Tank\ volume}}{\mathrm{Flow\ rate}} ]

Express volume and flow in consistent units (e.g., gallons and gal/day → days, or convert to hours). Typical primary detention times are often on the order of 1.5–2.5 hours at average design flow (plant-specific). Too short → solids wash through. Too long → septic conditions, especially in warm Florida weather.

Surface Overflow Rate (SOR)

[ \mathrm{SOR} = \frac{\mathrm{Flow}}{\mathrm{Surface\ area}} ]

Often expressed as gpd/ft². SOR is a loading rate: higher SOR means less opportunity for a particle to settle before it exits over the weir. Design SOR values are set so discrete and flocculent particles can reach the sludge zone. Wet-weather peaks raise SOR; if peaks exceed design, expect TSS breakthrough unless flow is equalized or clarifiers are added.

Septic Sludge Risks

Septic sludge forms when settled solids remain too long without adequate withdrawal. Anaerobic bacteria produce gases (CO2, CH4, H2S) that buoy solids to the surface—looking like a failed clarifier even when the mechanism runs. Warm Florida temperatures accelerate septicity.

Symptoms and risks:

• Black, rising sludge clumps and odor
• Floating ash-like particles in effluent
• High primary effluent TSS despite low SOR
• Corrosion and H2S safety hazards near covered tanks
• Shock load of septic, soluble BOD/COD to secondary treatment when gas-lifted solids redisperse

Correction: increase sludge pumping (without stripping the blanket entirely if that upsets thickeners), restore collector mechanisms, check for industrial toxic discharges that killed normal settling, and verify that detention is not excessive at low seasonal flows.

Performance Indicators Operators Track

Primary clarifier “health” is not one number. Track:

1. Primary influent vs effluent TSS and BOD (or COD) and calculated % removal
2. Visual: clarity over the weir, floating solids, scum beach condition
3. Sludge blanket depth (core sampler or sonic tools where used)
4. Sludge pumping rate and solids concentration (too thin vs too thick)
5. Weir loading and cleanliness
6. Odor and gas bubbles indicating septicity
7. Hydraulic events: storm peaks, recycle streams dumped to headworks

Recycle streams (thickener supernatant, dewatering filtrate, digester supernatant) can return high solids and ammonia loads to primary tanks—sometimes mistaken for “collection system” problems. Coordinate with solids handling.

Primary Clarifiers in the Florida Train

Many Florida plants are secondary or AWT facilities where primary clarifiers reduce load to aeration and help digesters with high-energy sludge. Coastal tourism swings change both flow and strength; operators may need different sludge-pumping frequencies in peak season vs summer afternoon thunderstorm I/I days. Always link primary performance to secondary F/M and clarifier load: when primary is bypassed or washed out, secondary bears the full particulate and FOG burden.

Master classic BOD/TSS removal ranges, SOR and detention time meaning, scum/sludge duties, septic-sludge signs, and wet-weather washout. Those ideas dominate primary sedimentation questions.