Glossary
Sedimentation & Clarification Terms
Key terms used in plate settler and lamella clarifier design, operation and performance — in plain language for engineers and operators.
Sedimentation
The settling of suspended solids out of water under gravity. Plate settlers accelerate sedimentation by providing a large inclined settling area in a small footprint.
Clarification
The overall process of removing suspended solids to produce a clear liquid (clarified overflow). A clarifier is the vessel in which this happens.
Lamella
An inclined plate in a clarifier. A stack of lamellae (a lamella pack) provides the settling surfaces that give a lamella clarifier its compact footprint.
Plate Pack
The assembly of parallel inclined plates inside a plate settler. Its total projected area determines the settling capacity of the unit.
Projected Area
The horizontal-plane area of the inclined plates. Settling capacity depends on projected area, which is the plate area multiplied by the cosine of the inclination angle.
Surface Loading Rate
The design flow divided by the projected settling area, expressed in m/h (or m³/m²·h). Typical plate settler values are 1.5–3.5 m/h. Also called overflow rate or rise rate.
Overflow Rate
Another name for surface loading rate — the upward velocity of water relative to the settling area. Particles settle if their settling velocity exceeds the overflow rate.
Settling Velocity
The speed at which a particle falls through water under gravity. Particles with a settling velocity greater than the design overflow rate are captured.
TSS (Total Suspended Solids)
The concentration of undissolved solids in water, measured in mg/L. Reducing TSS is the primary purpose of a clarifier; plate settlers commonly achieve 90–95%+ removal.
Turbidity
A measure of water cloudiness caused by suspended particles, measured in NTU. It is a fast proxy for solids content and clarifier performance.
Coagulation
The addition of a chemical coagulant to neutralise the charge on fine particles so they can come together. It precedes flocculation and settling.
Flocculation
Gentle mixing that lets coagulated particles collide and grow into larger, settleable flocs ahead of the plate settler.
Floc
An aggregate of fine particles formed during coagulation and flocculation, large enough to settle in the plate settler.
Sludge Blanket
The layer of settled solids accumulating in the hopper below the plate pack. It must be kept below the plates to prevent carryover into the effluent.
Underflow
The concentrated sludge stream withdrawn from the bottom of a clarifier. Plate settlers achieve partial thickening in the underflow.
Plate Angle
The inclination of the plates from horizontal, typically 55–60°, chosen so settled solids slide down freely while water flows up between the plates.
Plate Spacing
The perpendicular gap between adjacent plates, typically 50–100 mm. Closer spacing adds area but requires cleaner water; wider spacing suits higher solids loads.
Hydraulic Loading
The volumetric flow rate applied to the clarifier. Together with projected area it sets the surface loading rate.
Short-Circuiting
Uneven flow that lets some water pass through the unit faster than the design residence time, reducing settling efficiency. Good feed distribution prevents it.
Retrofit
Installing lamella plate packs into an existing tank to increase its settling area and capacity while reusing the civil structure.
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