Lamella Clarifiers and Plate Settlers: Design and Applications

Lamella clarifiers use inclined plates or tubes to multiply the effective (projected) settling area within a compact footprint, typically achieving clarification rates several times higher than a conventional sedimentation basin of the same plan area — the exact factor depends on plate length, spacing, angle, and the projected-area basis used. Inclined plate clarifier technology has become a standard solution for water and wastewater treatment facilities facing capacity limitations, plant expansions, and high-rate treatment requirements.

Settling Theory: Why Inclined Plates Work

The performance of a sedimentation basin depends on its surface area, not its depth or volume. A particle settles out of suspension when its settling velocity exceeds the upward velocity of the water, which is determined by the overflow rate (flow divided by surface area).

Lamella clarifiers exploit this principle by installing a series of inclined plates (typically at 45–60° from horizontal) inside a tank. Each plate acts as a separate shallow settling surface, so the effective (projected) settling area is many times the plan area of the tank itself. The exact multiplier depends on plate length, spacing, and angle — the relevant comparison is the total horizontal-projected plate area, not the tank footprint — so published "multiplication factors" vary widely with the specific geometry and the basis on which they are calculated.

The result: for a given duty, a lamella unit can occupy a fraction of the footprint of a conventional rectangular or circular clarifier, with the realized footprint savings depending on the same geometric factors and the overflow rate selected.

Types of Lamella Systems

Plate Settlers (Retrofit)

Plate settler modules are sets of inclined PVC or stainless steel plates that can be retrofitted into existing sedimentation basins or clarifiers. They are the most common application of lamella technology in North America.

Typical configuration:

• Plate spacing: 2–4 inches
• Plate angle: 55–60° from horizontal
• Plate length: 4–8 feet along the incline
• Material: PVC (most common), stainless steel (high-temperature or chemical environments)
• Projected surface loading rate: 0.3–0.7 gpm/ft² (based on plate-projected area)

Plate settlers are widely used to increase the capacity of existing clarifiers by 50–100% without expanding the tank footprint.

Lamella Clarifiers (Purpose-Built)

Purpose-built lamella clarifiers integrate inclined plates into a tank designed specifically for high-rate settling. They include integral flocculation zones, sludge collection hoppers, and effluent launders.

Design features:

• Influent enters below the plate pack after flocculation
• Flow passes upward between the inclined plates
• Settled solids slide down the plate surfaces into collection hoppers
• Clarified water exits above the plate pack through effluent launders
• Counter-current flow configuration (most common): influent flows upward, solids slide downward

Tube Settlers

An alternative geometry using 2-inch hexagonal PVC tube modules instead of flat plates. Tube settlers provide similar settling area multiplication and are sometimes preferred for their self-supporting structure. Tube angle is typically 60° from horizontal.

Design Parameters

The projected surface loading rate is calculated on the horizontal projection of the total plate area. The effective overflow rate accounts for the multiplication factor of the inclined plates.

Applications in Water Treatment

Drinking water clarification: Lamella clarifiers paired with coagulation/flocculation treat surface water at significantly higher rates than conventional sedimentation. Many water treatment plants have retrofitted plate settlers into existing basins to avoid constructing new settling infrastructure during capacity upgrades.

Lime softening: Plate settlers increase throughput in lime softening clarifiers while maintaining effluent hardness targets. The higher solids loading and potential for calcium carbonate scaling require periodic plate cleaning.

Filter backwash recycling: Spent filter backwash water contains high TSS that must be removed before recycle. Lamella systems provide compact, reliable treatment for this sidestream.

Applications in Wastewater Treatment

Primary clarification: Lamella primary clarifiers achieve 60–80% TSS removal and 30–40% BOD removal at loading rates 5–8 times higher than conventional primary clarifiers. Particularly valuable for plant expansions within existing site boundaries.

Secondary clarifier augmentation: Plate settlers retrofitted into secondary clarifiers increase solids capture during peak flow events, reducing effluent TSS excursions during wet weather.

Tertiary clarification: Following chemical phosphorus precipitation, lamella clarifiers provide rapid solids separation for phosphorus removal to <0.1 mg/L.

Industrial pretreatment: Metal finishing, mining, food processing, and oil/gas produced water applications use lamella clarifiers for compact, high-rate solids removal.

CSO/SSO treatment: Combined sewer overflow and sanitary sewer overflow treatment facilities use lamella systems for rapid, space-efficient primary treatment during wet weather events.

Advantages

• Footprint: substantially smaller than conventional clarifiers for equivalent capacity (the realized reduction depends on plate length, spacing, angle, and the projected-area basis used)
• Capital cost: Lower than building new conventional clarifiers, particularly for retrofits
• Fast installation: Plate settler modules can be installed in existing tanks during a shutdown
• Scalability: Additional plate modules can be added as demand increases
• Performance: Consistently achieves effluent turbidity <1 NTU in drinking water applications

Limitations and Maintenance

Solids handling: The concentrated sludge blanket below the plates must be removed continuously or frequently to prevent blanket rise into the plate pack. Sludge collection and removal design is critical.

Plate fouling: Biological growth, grease, and precipitate accumulation on plate surfaces reduce effective settling area. Periodic cleaning is required—typically by draining and pressure washing, or through automated cleaning systems.

Algae growth: In open-top installations receiving su