Lamella clarifier design sizes a stack of inclined plates so the projected settling area handles the design overflow rate. Because settling depends on the horizontally projected area of each plate, a lamella pack delivers the equivalent settling surface of a conventional clarifier in a fraction of the footprint. The governing criterion is surface loading, not tank depth.
What is a lamella clarifier and why does it save space?
A lamella (inclined-plate) clarifier is a gravity settler in which the clarification zone is filled with a stack of parallel plates set at a steep angle, typically 55–60° to the horizontal. Particles need only settle the short vertical distance between adjacent plates before landing on a plate surface, then slide down and out of the flow.
The reason for the space saving is subtle. In ideal-settling (Hazen) theory, whether a particle is captured depends only on its settling velocity relative to the overflow rate — the flow divided by the horizontal plan area — and is independent of tank depth. A plate inclined at angle θ projects a horizontal area of its own width × length × cosθ. Stack N plates above the same footprint and the effective settling area becomes the sum of every plate’s projection, so a compact pack replaces a large open tank. This is why inclined-plate packs are among the most footprint-efficient clarification equipment available.
What governs particle capture? Stokes' law and discrete settling
For dilute suspensions of discrete (non-flocculating) particles, each particle settles independently at its terminal velocity. Balancing gravity, buoyancy and Stokes drag on a small sphere in the laminar (creeping-flow) regime gives the settling velocity:
where vs = terminal settling velocity (m/s), g = 9.81 m/s2, ρp = particle density (kg/m3), ρ = liquid density (kg/m3), d = particle diameter (m) and μ = dynamic viscosity (Pa·s). Valid for particle Reynolds number Rep = ρ vs d / μ < 1.
The quadratic dependence on diameter is the crux: doubling particle size quadruples settling velocity. It is also why coagulation and flocculation upstream matter so much — growing 10 µm primaries into 100 µm flocs raises vs by a factor of roughly 100 (until the flocs become porous and Stokes no longer strictly applies). Design targets the slowest particle you still need to capture; anything settling faster than the overflow rate is removed.
A quick check fixes ideas. A 40 µm hydroxide floc of density 1,100 kg/m3 in water (ρ = 1,000 kg/m3, μ = 1.0 × 10−3 Pa·s) settles at vs = 9.81 × 100 × (40×10−6)2 / (18 × 10−3) ≈ 8.7 × 10−5 m/s, or about 0.31 m/h. That single figure — the target settling velocity — becomes the ceiling on the overflow rate. The particle Reynolds number here is Rep ≈ 3.5 × 10−3, comfortably inside the Stokes regime, so the equation is valid.
How does overflow rate set the settling area?
Hazen’s ideal-settling model treats the clarifier as an idealised rectangular basin with uniform horizontal flow and quiescent settling. A particle is captured if its downward settling velocity exceeds the overflow rate (also called surface loading rate or surface overflow rate):
where v0 = overflow rate (m/h or m3/m2·h), Q = design flow (m3/h) and Aproj = projected (effective) settling area (m2). Any particle with vs ≥ v0 is removed regardless of where it enters. Typical projected loading rates for lamella units are 0.3–1.5 m3/m2·h on the plate-projected area.
The design problem therefore reduces to one thing: provide enough projected area that the design overflow rate falls at or below the settling velocity of the target particle. Depth, retention time and tank volume follow from mechanical and sludge-storage constraints, not from capture efficiency.
How do you calculate the projected area of an inclined-plate pack?
Each plate of width w and length L, inclined at θ, contributes a horizontal projection of w·L·cosθ. For a pack of N plates the effective settling area is:
where N = number of plate gaps (channels), w = plate width (m), L = wetted plate length (m) and θ = inclination from horizontal (deg). Steeper plates (larger θ) reduce cosθ and thus projected area per plate — the price paid for reliable self-cleaning.
This exposes the central design tension. Lowering θ increases cosθ and therefore the projected area for a given number of plates — but below about 45–50° the settled solids no longer slide off reliably and the plates blind. The standard compromise, 55–60°, sacrifices a little projected area (cos 55° ≈ 0.57) to guarantee gravity-driven sludge removal.
Worked example: projected area and footprint versus a conventional clarifier
Size a lamella pack for a metal-finishing rinse water requiring clarification of hydroxide flocs.
Inputs: design flow Q = 40 m3/h; target projected overflow rate v0 = 0.8 m3/m2·h; plates 1.0 m wide × 2.0 m long, inclined at θ = 55° (cos 55° = 0.573).
- Required projected area: Aeff = Q / v0 = 40 / 0.8 = 50 m2.
- Projected area per plate channel: w · L · cosθ = 1.0 × 2.0 × 0.573 = 1.146 m2.
- Number of channels: N = 50 / 1.146 ≈ 44 channels.
- Physical footprint of the pack: with 50 mm plate spacing, the horizontal length occupied is roughly N × spacing / sinθ ≈ 44 × 0.05 / 0.819 ≈ 2.7 m along the flow, over the 1.0 m plate width — about 2.7 m2 of tank plan area (plus inlet/outlet zones).
Comparison: a conventional horizontal clarifier delivering the same 50 m2 of settling surface needs 50 m2 of tank plan area. The lamella pack achieves the equivalent capture in roughly 2.7 m2 of plan area — an order-of-magnitude (about 18×) reduction in footprint for the clarification zone, before allowing for sludge hopper and hydraulics. That density is exactly why lamella units dominate space-constrained retrofits.
What keeps flow laminar between the plates?
Ideal-settling theory assumes quiescent, non-turbulent flow. Between narrow plates this must be verified with two dimensionless criteria based on the channel hydraulic diameter dH (≈ 2× spacing for wide thin channels) and the mean velocity v between plates:
Good practice keeps the channel Reynolds number Re < 500 (well into laminar flow, avoiding resuspension) while the Froude number is kept high enough (Fr > 10−5) to ensure flow is stable and gravitationally self-correcting rather than short-circuiting. Low velocity satisfies Re; adequate velocity satisfies Fr.
These two pull in opposite directions and bracket the working velocity. Too fast and turbulence (high Re) scours settled solids off the plates; too slow and buoyant density currents (low Fr) let the flow short-circuit and channel unevenly. Plate spacing (typically 50–100 mm) is selected to land the design velocity inside this laminar, stable window. See our overview of clarifier hydraulic design for how these limits feed a full specification.
How does self-cleaning by sludge slide-off work?
Captured solids accumulate on the upward-facing side of each plate. For the deposit to slide down under gravity rather than build up and blind the channel, the gravitational component along the plate must overcome the deposit’s wall friction — broadly, when tanθ exceeds the effective friction coefficient of the sludge. Cohesive hydroxide and biological sludges need steeper angles; this is the physical reason the industry settled on 55–60°.
The counter-current arrangement (flow rising up the channel while sludge slides down) is the most common configuration because the descending sludge sheet leaves the underside relatively clear for clarified liquid to rise. Co-current and cross-flow layouts exist but counter-current dominates for its clean separation of the two flows. Smooth, low-friction plate surfaces (PVC, PP or coated steel) aid slide-off, and the sludge collects in a hopper below for periodic or continuous withdrawal to downstream sludge dewatering.
Because self-cleaning depends on gravity alone, lamella clarifiers have no submerged moving parts in the settling zone, which keeps maintenance low. The trade-off is sensitivity to fibrous or scaling solids that can bridge the narrow channels; where that risk exists, wider spacing, periodic flushing or upstream screening is specified.
Related upstream separation is covered in our guides to dissolved air flotation for low-density solids and oil and grease separators, which handle the buoyant fraction a gravity lamella cannot.
Typical lamella clarifier design values
The table gives defensible starting ranges for a first-pass design; confirm against settling-column tests on the actual suspension.
| Parameter | Typical range | Notes |
|---|---|---|
| Plate inclination θ | 55–60° | Steep enough for reliable self-cleaning slide-off. |
| Plate spacing | 50–100 mm | Perpendicular gap; narrower for well-flocculated feeds. |
| Projected loading rate v0 | 0.3–1.5 m3/m2·h | On projected area; lower for fine/weak flocs. |
| Channel Reynolds number | < 500 | Keeps flow laminar; avoids resuspension. |
| Plate length L | 1–2.5 m | Longer plates increase projected area per channel. |
| Plate material | PP / PVC / coated steel | Smooth, low-friction, corrosion-resistant. |
For difficult or variable suspensions, size the projected area against the slowest particle you must remove and add a coagulation/flocculation stage upstream to shift the particle-size distribution before the plates.
Lamella clarifier design sequence
- Characterise the suspension. Run a settling-column test to find the design settling velocity v_s of the slowest particle you must capture, after any coagulation/flocculation.
- Set the projected overflow rate. Choose v0 at or below the target v_s, typically 0.3-1.5 m3/m2.h on projected area, lower for fine or weak flocs.
- Calculate required projected area. Compute A_eff = Q / v0 from the design flow.
- Fix plate geometry and count. Select angle (55-60 deg), width and length, then N = A_eff / (w.L.cos theta) gives the number of channels.
- Check laminar-flow criteria. Verify channel Reynolds number < 500 and adequate Froude number by adjusting plate spacing (50-100 mm).
- Size sludge hopper and hydraulics. Add inlet distribution, launder/weir collection and a hopper sized for the solids load and withdrawal frequency.
Frequently asked questions
Why are lamella plates inclined at 55 to 60 degrees?
The angle is a compromise between projected settling area and self-cleaning. Shallower plates give more projected area (larger cosθ) but settled sludge no longer slides off and the channels blind. Above about 55° the gravitational component along the plate reliably overcomes sludge friction, keeping the surfaces clear at a modest cost in area.
How is the settling area of a lamella clarifier calculated?
Use the projected area Aeff = N · w · L · cosθ, the sum of each plate’s horizontal projection. Capture depends on this projected area versus the overflow rate v0 = Q / Aeff, not on the sloping plate area or tank depth, which is why a compact pack replaces a large open basin.
How much smaller is a lamella clarifier than a conventional one?
Because a stack of inclined plates multiplies the projected settling area within one footprint, lamella units typically occupy 10–20× less plan area than a conventional horizontal clarifier of the same capacity. The exact ratio depends on plate number, length and angle, plus the space needed for inlet, sludge hopper and outlet zones.
What overflow rate should I use for lamella clarifier design?
Projected overflow rates of 0.3–1.5 m3/m2·h are typical, applied to the plate-projected area. Use the lower end for fine, low-density or weakly flocculated particles and the upper end for dense, well-conditioned flocs. Always confirm against a settling-column test on the real suspension rather than a textbook value alone.
Does Stokes' law always apply to lamella clarifier design?
Stokes’ law holds for discrete spherical particles in laminar flow with particle Reynolds number below 1, which covers most fine mineral and chemical flocs. Large, porous or flocculating aggregates deviate because they are not solid spheres and their density falls as they grow, so settling-column testing is used to fix the true design settling velocity.
What keeps the flow between the plates laminar?
Low velocity in narrow channels keeps the channel Reynolds number below about 500, well inside laminar flow, so settled solids are not resuspended. Plate spacing (50–100 mm) is chosen so the design velocity also satisfies a minimum Froude number, preventing short-circuiting and density currents. The two criteria bracket the working velocity.
Sources & further reading
- Metcalf & Eddy / Tchobanoglous, Wastewater Engineering: Treatment and Resource Recovery — sedimentation and inclined-plate settling
- Crittenden et al., MWH’s Water Treatment: Principles and Design (Wiley) — discrete settling and overflow rate
- US EPA, Wastewater Technology Fact Sheet — sedimentation/clarification
- WEF Manual of Practice — clarifier design