Choose Between Counter-Current and Cross-Flow Lamella Clarifiers

Choose between counter-current, co-current and cross-flow lamella clarifiers by where the feed enters relative to the sludge outlet. Counter-current units send water up while sludge slides down, and one study ranks them first on performance, with cross-flow next. Co-current layouts suit thickening. Match the layout to the job.

Define the Three Layouts

A Minerals Engineering review classifies inclined settlers as countercurrent, cocurrent or cross-flow by the position of the feed relative to the underflow. In cocurrent units the feed enters at the top or midway and flows down toward the underflow. In cross-flow units the flow crosses the settling elements sideways, with a countercurrent component because the feed sits below the overflow.

A lamella gravity separator patent gives the same three classes in terms of the sludge. Counter-current means suspension and liquid flow against the sludge, concurrent means with it, and crosscurrent means at right angles.

Lamella Clarifiers Are Often Counter-Current

The Minerals Engineering review shows a Nordic Water lamella separator designed for countercurrent operation, with a flocculant tank upstream. Wuxi Yosun’s product page describes the same pattern. Flocculated water is distributed to the tank bottom, rises slowly, meets the inclined plates, and the solids cut off by the plates sink into the sludge pit.

For lamella clarifiers of this kind, the sludge and water move in opposite directions along the plates. That keeps settled solids away from the rising stream, which is the point of the layout.

Read the Ranking With Care

A Wroclaw University of Science and Technology paper on cross-current lamella tanks presents three layouts, counter-current, cross-current and parallel flow. It ranks the counter-current system first for performance and the cross-current system next. It also notes that a parallel-flow system works well as a sludge thickener.

That is one study, with one test rig. I’d treat it as a prompt to ask a vendor why it chose its layout, not as a rule.

Cross-Flow Suits Plates in Rectangular Tanks

The Minerals Engineering review says plate-like settling elements are compatible with the cross-flow configuration. Wuxi Yosun’s catalog page describes a lateral-flow design where muddy water passes through the side of the plates, and calls the common advection lamella clarifier the usual pool type.

Cross-flow packs drop into rectangular concrete basins easily, since water enters on one side and leaves on the other. The cost is a less uniform velocity across the pack, which makes inlet baffles more important.

Co-Current Suits Thickening

The Wroclaw paper says a parallel-flow arrangement works well for sludge thickening. When the goal is dense underflow rather than clean overflow, letting solids and liquid move the same way along the plate can help sludge compact as it leaves.

Wuxi Yosun’s product page lists a concentration tank among its application fields, where the plate pack separates water from sludge to thicken it. For that duty, ask for the underflow concentration the design has reached on your type of solids.

Tube Arrays and Alternating Direction

The review notes that settling elements can be closed conduits, square, hexagonal, circular or chevron-shaped, and that packs need not point one way. Arrays with alternating direction sections can add stability to the settler, a point it attributes to Letterman and colleagues.

Plates are simpler to clean, and tubes can be easier to stack in odd tank shapes. The choice between them is the subject of another comparison, so here I’d only ask whether the vendor’s pack direction suits the tank you have.

Check Entry and Exit Geometry

A lamella patent says the design goal is to maximize use of the projected plate area while preventing settled sludge from being re-entrained in the liquid flow. Where water enters the pack and where it leaves decide both.

Ask for the inlet opening size, the velocity at each plate entrance and the outlet trough layout. A pack with fast entry jets will stir the sludge it is supposed to collect.

Ask for a Flow Sketch

Request a one-page sketch that shows feed point, plate direction, sludge outlet and clarified water outlet, with arrows for each stream. Then pick the layout by duty: counter-current for clarification, parallel flow for thickening, and cross-flow where the tank shape dictates. Wuxi Yosun’s lamella clarifier page shows its working process, and any bid for lamella clarifiers should show its own.

Frequently Asked Questions

Q: What is the difference between counter-current and cross-flow?
A: In counter-current units water rises while sludge slides down. In cross-flow units there is a sideways flow across the plates with a counter-current component.

Q: Which layout performs best?
A: A Wroclaw study ranks counter-current first and cross-current next, and says parallel flow works well as a thickener. It is one test rig, so confirm on your water.

Q: Which layout suits thickening?
A: Parallel flow, according to that study, although Wuxi Yosun also lists a concentration tank use for its plate pack.

Q: Can plates point in different directions?
A: Yes. Arrays of alternating direction sections can add stability, according to the review.