A lamella clarifier is a mechanical separation stage in which suspended solids denser than water settle onto inclined plates and slide down them into a hopper. The plate pack provides a settling area many times greater than the floor area the unit stands on, so the same duty is performed in substantially less space than a horizontal settling tank requires.
Principle
The unit is a vessel with a conical hopper at the bottom and a pack of inclined plates in its middle section. Effluent enters beneath the pack, rises through the channels between the plates, and the clarified water is collected by overflow launders at the top, while the settled matter travels down the plates against the flow and accumulates in the hopper. There are no moving parts inside the vessel: separation is driven by gravity.
Removal is governed not by residence time but by the ratio of two quantities: the settling velocity of the particle — its hydraulic size — and the hydraulic loading, that is, the flow divided by the settling area. A particle is removed if it reaches the surface of a plate before the flow carries it out of the channel. The settling velocity itself follows Stokes' law: it is proportional to the square of the particle size and to the density difference between particle and water, and inversely proportional to the viscosity of the water.
The plate pack increases not the length of the water's path but the number of settling surfaces. Between adjacent plates a particle has to travel not the full depth of the vessel but a distance of a few centimetres, and the time it needs to reach a surface is shortened in the same proportion. The total projected area of the pack — the sum of the horizontal projections of all the plates — is several times the floor area the unit occupies; the exact ratio depends on the number of plates, their size and the angle of inclination.
Flow within the channels has to remain laminar: any mixing returns a settled particle to the flow. Hence the requirement that the flow be distributed evenly among all the channels of the pack: where some channels are overloaded and others underloaded, the area actually working is smaller than the design value.
The angle of the plates is chosen so that sludge slides down continuously rather than accumulating: it is normally 55–60° to the horizontal. The sludge slides into the hopper, thickens there and is withdrawn by pump, either on a timer or by level. Fine and colloidal solids are enlarged with reagents ahead of the clarifier: a coagulant neutralises the charge of the colloidal particles and a flocculant binds the microflocs into structures whose settling velocity suffices for the design regime.
Principle
Inclined plates work not by lengthening the path of the water but by shortening the height a particle has to travel to a settling surface. In a horizontal tank that is metres; in a plate pack it is centimetres between adjacent surfaces. The consequence runs against the usual expectation: increasing the volume of the unit achieves nothing in itself, whereas increasing the settling area has a directly proportional effect. That is why a lamella clarifier of a given capacity occupies several times less space than a horizontal tank, and why sizing it by residence time is pointless.
Sludge has to slide down off the plates continuously, and for that reason the angle of inclination and the properties of the sludge are bound together. Sticky, fibrous or poorly thickening sludge is retained between the plates, part of the channels drops out of service, and the unit loses settling area while remaining formally in order: the flow passes, there are no moving parts, there is nothing to fail. This shows only as a rise in suspended solids at the outlet, which is why a sample after the clarifier is not a reporting formality but the one direct indication of the state of the pack.
The same unit removes less in winter than in summer, and the equipment is not the reason. Settling velocity is inversely proportional to the viscosity of water, and the viscosity at 5 °C is roughly 1.7 times that at 25 °C. The settling velocity of the same particles falls in the same proportion, and part of the solids that reported to the sludge in warm weather passes straight through. Sizing on summer samples for an unheated site leads to exceedances in precisely the cold season.
Practice
The principal practical gain is the space occupied. A lamella clarifier fits where a horizontal tank does not fit at all: in the refurbishment of existing treatment works, inside an existing building, on a confined site. That is usually what decides the choice, since the separation principle of the two is the same.
In an effluent treatment scheme the stage sits after reagent treatment and ahead of polishing or biological treatment. It removes the bulk of the suspended solids and the share of COD bound to them; parameters relating to dissolved matter stay as they were, and where the exceedance concerns those, settling alone does not resolve it.
On mineral solids the stage works without reagents at all — this is the case in which operation costs least: what remains is the power for sludge withdrawal and the handling of the sludge itself. On colloidal and hydroxide solids a reagent side is mandatory, and maintaining it becomes the principal operating cost.
The achievable reduction is assessed from comparable samples at inlet and outlet, not from the appearance of the clarified water. On heavy mineral solids at the design hydraulic loading it is usually 70–90 % for suspended solids and can be higher on flocs produced by a matched reagent regime; on fine solids without coagulation the result may be close to zero with the unit entirely in order.
The unit is assembled from a vessel, a plate pack, the clarified water collection arrangement and a sludge hopper, and is supplied for a particular flow and connection. We do not manufacture it: our part of the work is the effluent analysis and the settling test, the determination of hydraulic size and of the settling area required, the selection of size and ancillaries, supply, commissioning and subsequent service.
Limits
Practice
The cases are typical examples, not site reports.
Innovations
Scheme
A lamella clarifier hardly ever works on its own. Ahead of it a screen or sieve and a grit trap are needed: coarse matter enters the channels of the pack and lodges between the plates, while abrasive sand wears them. A balancing tank evens out the flow and with it the hydraulic loading, on which removal directly depends.
Where the solids are fine, colloidal or hydroxide, a dosing station and a flocculation chamber are placed ahead of the clarifier. What matters is not only the dosing itself but the stretch between the chamber and the unit: flocs already formed are broken up by a pump, by an abrupt reduction in section and by a long pressure line, and what reaches the plates is matter that no longer settles. The flocculation chamber is therefore placed immediately adjacent and the effluent brought in by gravity where the hydraulic levels allow.
Downstream of the clarifier the scheme divides. Clarified water goes to biological treatment, to polishing by filtration or to discharge, depending on which parameters remain out of limits. Sludge from the hopper goes to dewatering: hauling it at high moisture is not economically justified, and its volume is set by the mass of solids removed rather than by the size of the unit.
Hydraulic levels are fixed at the scheme stage, not during installation. The unit works by gravity if the difference between inlet and outlet levels covers the head loss in the pack and the launders; otherwise a pump is added to the scheme, and with it the breaking up of flocs.
Scheme
Operation
Energy
The stage has virtually no energy consumption of its own: separation is driven by gravity, and where the difference in hydraulic levels is sufficient the effluent passes through the unit by gravity too. Power is consumed by the ancillaries — the feed pump where gravity flow is impossible, the sludge pump, the flocculation chamber stirrer and the dosing station. With gravity feed and intermittent sludge withdrawal the total specific consumption stays within hundredths of a kilowatt-hour per cubic metre of effluent, appreciably below that of dissolved air flotation of the same capacity. Where the effluent has to be pumped, the governing item is not the stage itself but the head that has to be generated.
Consumables
What goes to drain
What leaves the water is sludge — the whole mass of retained solids together with reagent flocs where reagents have been used. Its quantity follows from the mass of suspended solids removed, not from the size of the unit: at a concentration of 500 mg/l and a flow of 10 m³/h some 5 kg of dry solids an hour leave the water. The moisture of the withdrawn sludge depends on its nature and on how often it is withdrawn: mineral sludge thickens in the hopper appreciably better than hydroxide sludge, which holds water and almost always calls for dewatering on a screw or belt press. The hazard class and the permitted route are determined by the composition of the sludge rather than by the type of equipment.
Sizing
What we account for
What a wrong choice costs
Sizing
Questions
The separation principle is the same: both settle whatever is heavier than water. The difference lies in where the settling surface is. A horizontal tank has one — the floor — and its area equals the area of the structure. A lamella clarifier has as many surfaces as there are plates. Hence at equal capacity the lamella unit occupies several times less space, and it is that, rather than the quality of clarification, which usually decides the choice.
It depends on the nature of the solids. Mineral solids — sand, machining particles — settle without reagents, and the scheme is then the simplest to operate. Fine and colloidal solids have a settling velocity close to zero and pass through the unit whatever its size unless a coagulant and a flocculant are used. Metal hydroxides are by definition formed by reagents, so there a reagent side exists in any case. What a particular effluent requires is shown by a settling test, not by the industry it comes from.
A clarifier settles what is heavier than water: mineral solids, sand, hydroxide flocs formed by reagent treatment. A flotation unit lifts what is lighter than water or can be made lighter by attaching an air bubble to the particle: fats, oils, petroleum products, light organics, surplus activated sludge. All they share is the reagent preparation ahead of the unit. Where the effluent carries both, the stages are applied in sequence rather than chosen between.
A lamella clarifier is engineered equipment. The settling area, the number and material of the plates, the hopper volume and the make-up of the dosing and sludge withdrawal arrangements follow from a calculation for the particular effluent, not from the flow alone: two units rated at 10 m³/h may differ both in pack area and in ancillaries. The models in the catalogue therefore read "on request", and a price is quoted once the effluent composition and the outlet requirements are known.
The working settling area shrinks and suspended solids begin to pass to the outlet. The unit meanwhile remains formally in order, and there is one indication — a rise in turbidity and in suspended solids in the clarified water. The remedy depends on the cause: a change in the frequency of sludge withdrawal, an adjustment of the reagent regime or, where blinding persists, withdrawal of the pack sections and washing. Sludges liable to blind the pack are better identified before ordering, during the settling test.
A margin on settling area acts directly: it lowers the hydraulic loading and raises the share of solids removed, and there is practically no lower bound on flow of the kind dissolved air flotation has. A margin does not, however, replace reagent preparation: particles whose settling velocity is close to zero are not removed at any area.
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