Structure of Inclined-Tube Sedimentation Tank
By: Kate Nana
Post Date: July 15th, 2025
Email:info@aquasust.com
Post Tags: Tube Settler，Environmental Tech , Filtration，Sustainable Water
The structure of an inclined-tube or inclined-plate sedimentation tank is similar to that of a conventional sedimentation tank, consisting of four main parts: the inlet, sedimentation zone, outlet, and sludge collection zone. The key difference lies in the sedimentation zone, which is equipped with numerous inclined tubes or plates. Figure 1 illustrates the typical structure of an inclined-tube sedimentation tank.
In inclined-tube/plate sedimentation tanks, the flow direction relative to the inclined plates can be categorized into three types: upward flow, downward flow, and horizontal flow, as shown in Figure 2.
- Upward flow (counter-current flow): Water flows upward through the tubes or plates, while settled solids move downward, opposing the flow direction.
- Downward flow (co-current flow): Water flows downward through the tubes or plates, moving in the same direction as the settled solids.
- Horizontal flow (cross-flow): Water flows horizontally, a configuration only applicable to inclined plates.
1. Inlet Zone
Water enters the sedimentation tank horizontally. The inlet zone typically includes perforated walls, slotted walls, or downward-flow tube settlers to ensure uniform water distribution across the tank width. The design requirements are similar to those of a horizontal-flow sedimentation tank.
For upward-flow tube settlers, a certain distribution zone height must be maintained below the tubes to ensure even water distribution. The inlet flow velocity should not exceed 0.02–0.05 m/s.
2. Inclination Angle of Tube Settlers/Plates
The angle between the inclined plates/tubes and the horizontal plane is called the inclination angle (α). A smaller α reduces the critical settling velocity (u₀), improving sedimentation efficiency. However, to ensure automatic sludge sliding and smooth discharge:
- For upward-flow tube settlers/plates: α ≥ 55°–60° (to prevent sludge accumulation).
- For downward-flow tube settlers/plates: α ≥ 30°–40° (easier sludge discharge).
3. Shape and Material of Tube Settlers/Plates
To maximize sedimentation efficiency within limited tank volume, tube settlers/plates are designed with dense geometric cross-sections, including:
- Square
- Rectangular
- Hexagonal
- Corrugated
For ease of installation, multiple tubes (or plates) are assembled into modules, which are then installed in the sedimentation zone.
Material requirements: Lightweight, durable, non-toxic, and cost-effective. Common materials include:
- Paper honeycomb tubes (phenolic resin-coated, typically hexagonal with a 25 mm inscribed circle diameter).
- Thin plastic sheets (e.g., 0.4 mm rigid PVC, thermally molded).
4. Length and Spacing of Tube Settlers/Plates
- Longer tubes/plates improve sedimentation efficiency but complicate fabrication and installation. Beyond a certain length, efficiency gains diminish.
- If too short, the inlet transition zone (where flow transitions from turbulent to laminar) occupies more space, reducing the effective settling length (~100–200 mm).
Recommended dimensions:
- Upward-flow tube settlers/plates: 0.8–1.0 m (minimum 0.5 m).
- Downward-flow plates: ~2.5 m.
Spacing/diameter:
- Smaller spacing increases surface loading, reducing tank volume but risking clogging.
- Upward-flow: 50–150 mm (tube diameter or plate spacing).
- Downward-flow: 35 mm (plate spacing).
5. Outlet Zone
Uniform effluent collection is critical. The outlet system consists of:
- Collection laterals (perforated channels, V-notched weirs, thin weirs, or perforated pipes).
- Main collection channel.
The clear water zone height (h) above the tube outlets must satisfy: h≥√3/2L
where:
- h = clear water zone height (m)
- L = spacing between collection laterals (typically 1.2–1.8 m), so h = 1.0–1.5 m.
6. Particle Settling Velocity (u₀)
The flow velocity inside tube settlers/plates is comparable to horizontal-flow sedimentation tanks (10–20 mm/s). For coagulation-assisted sedimentation, u₀ = 0.3–0.6 mm/s.
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