What are the working principle, characteristics, structure, and design considerations of a Lamella clarifier?

 

 

 

 

 

1️⃣ It utilizes the principle of laminar flow.
The flow between the inclined plates or tubes has a very small hydraulic radius, resulting in a low Reynolds number, generally around Re ≈ 200. This keeps the flow in a laminar state, which is highly favorable for sedimentation. The Froude number inside the inclined tubes is about 1×10⁻³ to 1×10⁻⁴, indicating stable flow conditions.

 

2️⃣ It increases the effective settling area, thus improving sedimentation efficiency.
However, due to factors such as the specific arrangement of the inclined plates, the influence of inlet and outlet flows, and the flow conditions within the tubes or plates, the actual treatment capacity cannot reach the theoretical maximum. The ratio of actual sedimentation efficiency to theoretical efficiency is called the effective coefficient.

 

3️⃣ It shortens the settling distance of particles, greatly reducing the settling time.

 

4️⃣ Floc particles can collide and re-aggregate inside the inclined tube settler, promoting further particle growth and enhancing sedimentation efficiency.

 

 

 

The structure of a Lamella clarifier is similar to that of a conventional clarifier. It consists of four main parts: an inlet, a settling zone, an outlet, and a sludge collection zone. The difference is that the settling zone is equipped with multiple inclined tube settlers. Figure 1 shows a typical structure of a Lamella clarifier.

 

In a Lamella clarifier, according to the direction in which the water flows through the inclined plates, there are three types of flow: upward flow, downward flow, and horizontal flow, as shown in Figure 2. When the water flows upward through the inclined tube settlers and the sludge settles downward in the opposite direction, this is called upward flow (also known as counter-current flow). In downward flow clarifiers, the water flows downward through the inclined tubes or plates together with the settling particles.

 

 

When the flow direction of water and particles is the same, it is called downward flow (also known as co-current flow). When the water flows horizontally through the clarifier, it is called horizontal flow (also known as cross-flow), which is only applicable to inclined plates.

 

 

 

1. Inlet Zone
Water enters the clarifier horizontally. The inlet zone typically includes perforated walls, slotted walls, or downward flow inclined tubes, which help distribute the flow evenly across the width of the tank - similar to the design requirements for a conventional horizontal flow clarifier.
To ensure even flow distribution through the upward flow inclined tube settlers, it is necessary to maintain a certain height for the flow distribution zone below the tubes. The inlet flow velocity at the cross-section should not exceed 0.02–0.05 m/s.

 

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2. Inclination Angle of Inclined Tube Settlers
The angle between the inclined tubes (or plates) and the horizontal plane is called the inclination angle (α). A smaller inclination angle results in a lower particle settling velocity (u₀) and thus better settling performance.
However, to ensure that sludge can slide down automatically and be discharged smoothly, α should not be too small. For upward flow clarifiers, the inclination angle is generally not less than 55°–60°.
For downward flow clarifiers, since sludge discharge is easier, the angle is usually not less than 30°–40°.

 

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3. Shape and Material of Inclined Tube Settlers
To make full use of the clarifier's limited volume, inclined tube settlers are designed with compact cross-sectional shapes, such as square, rectangular, hexagonal, or corrugated forms.
For easier installation, several or even hundreds of tubes are grouped into one module, and multiple modules are installed in the settling zone.
Materials must be lightweight, strong, non-toxic, and economical. Common materials include honeycomb paper and thin plastic sheets. Honeycomb tubes are often made from impregnated paper cured with phenolic resin, generally formed into regular hexagons with an inscribed circle diameter of about 25 mm. Plastic sheets are typically 0.4 mm thick rigid PVC, formed by heat pressing.

 

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4. Length and Spacing of Inclined Tube Settlers
The longer the tubes, the higher the settling efficiency. However, excessively long tubes are difficult to fabricate and install, and extending the length beyond a certain point yields diminishing returns.
If the tubes are too short, the proportion of the transition zone (where the flow changes from turbulent to laminar) increases, reducing the effective settling length. The transition zone length is generally 100–200 mm.

Based on experience:

The length of upward flow inclined plates is usually 0.8–1.0 m, and should not be less than 0.5 m.

For downward flow, the length is about 2.5 m.

At a constant cross-sectional velocity, smaller spacing or tube diameter increases internal flow velocity and surface loading rate, allowing for a smaller clarifier volume. However, very small spacing or diameters increase fabrication difficulty and risk of clogging.
In water treatment practice:

The spacing or tube diameter for upward flow clarifiers is about 50–150 mm.

For downward flow clarifiers, the plate spacing is about 35 mm.

 

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5. Outlet Zone
To ensure even effluent flow from the inclined tube settlers, the arrangement of the effluent collection system is critical. The collection system consists of lateral collectors and main channels.
Lateral collectors may be perforated troughs, V-notch weirs, thin weirs, or perforated pipes.
The height from the inclined tube outlet to the collection holes (i.e., the clear water zone height) relates to the spacing of the lateral collectors and should meet the following:
h ≥ √3/2 × L, where:

h = clear water zone height (m)

L = spacing between lateral collectors (m)

Typically, L is 1.2–1.8 m, so h should be about 1.0–1.5 m.

 

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6. Settling Velocity (u₀) of Particles
The water velocity inside the inclined tubes is generally similar to the horizontal flow velocity of conventional clarifiers, about 10–20 mm/s.
When coagulation is used, the particle settling velocity u₀ is about 0.3–0.6 mm/s.

 

 

 

Some data of counter-current and co-current inclined tube/plate settlers

 

Parameter	Counter-current (Upward Flow)	Co-current (Downward Flow)
Inclination Angle of Plates/Tubes	55° – 60°	30° – 40°
Plate Length	0.8 – 1.0 m	About 2.5 m
Plate/Tube Spacing	50 – 150 mm	About 35 mm
Inlet Flow Velocity	≤ 0.02 – 0.05 m/s	Similar or slightly higher
Transition Length (Tube Inlet)	100 – 200 mm	Similar
Reynolds Number of Flow	Around 200 (Laminar flow)	Possibly slightly higher
Particle Settling Velocity (u₀)	0.3 – 0.6 mm/s (with coagulation)	Similar or slightly higher

 

Design Considerations for Counter-current (Upward Flow) Lamella Clarifier:

 

The raw water turbidity should be maintained below 1000 NTU (Nephelometric Turbidity Units) over the long term.

The surface loading rate in the inclined tube settling zone can be set between 9.0 to 11.0 m³/(h·m²).

Tube diameter should be 25 to 35 mm, with a tube length of 1 m.

The inclination angle of the tubes should be 60°.

The clear water protection zone above the inclined tubes should be no less than 1.5 m

 

Design Considerations for Co-current (Downward Flow) Lamella Clarifier:

 

Suitable for raw water with turbidity consistently below 200 NTU.

The surface loading rate in the inclined plate settling zone should be determined based on raw water conditions and operational experience or test data from similar water treatment plants; generally, it ranges from 30 to 40 m³/(h·m²).

Plate spacing should be 35 mm.

Plate length in the settling zone should be 2.0 to 2.5 m, with the plate length in the sludge discharge zone not less than 0.5 m.

Inclination angle of plates in the settling zone is 40°, and in the sludge discharge zone is 60°.

 

 

 

The Lamella clarifier is currently a widely used physicochemical treatment process for wastewater. Our HENANECO technical team has analyzed practical issues encountered in the field-such as uneven flow distribution at the clarifier inlet, clogging of the sludge hopper, and flotation of flocs-that lead to deterioration of effluent quality. Based on these analyses, we have developed corresponding solutions to address these problems.

 

1,Factors Affecting the Performance of a Lamella Clarifier:

 

The central section of the Inclined Tube Settler maintains laminar flow, while the inlet and outlet sections are affected by inflow and outflow, causing some flow disturbances.

 

The water flow inside the Inclined Tube Settler is relatively stable, which helps improve sedimentation efficiency.

 

Since the settling distance and settling time are very short, adequate coagulation and flocculation must occur before water enters the clarifier.

Stratified flow of turbid water has the least impact on upward flow clarifiers; thus, upward flow designs are suitable for high turbidity water, while downward flow designs are better for very low turbidity water.

 

2, Excessive Effluent Turbidity

 

Cause Analysis:

 

Uneven flow distribution at the clarifier inlet causes severe turbulence or high inlet velocity near the inlet. This results in locally high flow velocities, which can resuspend sludge previously settled on the inclined tubes.

 

Localized short-circuiting ("short flow") disrupts floc stability, causing previously formed flocs to break apart into smaller particles.

 

To achieve uniform flow distribution, the perforated baffle (flower wall) openings in the Lamella clarifier are relatively small. This often leads to higher flow velocities through the openings compared to conventional horizontal clarifiers, causing secondary breakage of formed flocs and resuspension of settled sludge at the bottom of the distribution holes, thereby increasing turbidity in the effluent.

 

Solutions:

Install the inclined plates at a 60° angle to the horizontal, and add a row of wing plates below each inclined plate, also at a 60° angle. The added wing plates significantly reduce the Reynolds number of the flow, enhancing viscous forces during water movement, which favors sedimentation.

 

Additionally, the settling path of particles is shortened, benefiting the deposition of denser particles.

 

Ensure uniform flow distribution by using perforated baffles for water distribution. The horizontal flow velocity at the inlet of the distribution zone should be controlled between 0.010 and 0.018 m/s.

 

Add a horizontal flow rectification section before the settling zone, so the water does not flow directly from the outlet weir into the inclined tube settler. This horizontal flow section (about one-third of the total clarifier length) improves impact resistance, reduces horizontal flow velocity, and serves as a flow straightener. It also decreases upward flow velocity within the inclined tubes, improving sedimentation efficiency and increasing tolerance to shock loads. Additionally, install flow guide baffles between the horizontal and inclined tube sections to increase upward velocity inside the inclined tubes and further enhance settling efficiency.

 

3,Sludge Hopper Clogging and Poor Sludge Discharge in the Clarifier

 

Cause Analysis:

 

Mechanical sludge removal in the Lamella clarifier can easily create dead zones at the edges and ends of the clarifier where sludge accumulates, leading to excessive sludge buildup in these areas.

 

The design of the sludge discharge pipes is often inadequate or improper, which further contributes to poor sludge removal.

 

Solutions：

 

Modify the tank design to reduce sludge scraper dead zones. Use a large sludge hopper with gravity drainage instead of mechanical scraping. This reduces local flow disturbances and is less prone to clogging. The larger hopper slope improves sludge sliding, ensuring more complete sludge discharge.

 

Use a scraper mechanism for sludge removal and increase the number of sludge discharge channels on the tank bottom to improve sludge removal efficiency.

 

 

The HENANECO Water Treatment Technical Team specializes in the wastewater treatment industry. We provide comprehensive services including process design, equipment manufacturing, sales, and upgrade/retrofit solutions for water treatment projects.

 

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