Inclined Plate & Tube Settler Design Guide: Principles, Applications & Limitations in Wastewater Treatment

Inclined Plate (Tube) Settling Tank

What is an Inclined Plate (Tube) Settling Tank?

An inclined plate (tube) settling tank is a type of sedimentation tank designed based on the “shallow sedimentation” principle, where inclined plates or honeycomb tubes are installed to improve sedimentation efficiency. Based on the relative flow direction of water and sludge, inclined plate (tube) settling tanks can be categorized into three types: counter-current flow, co-current flow, and cross-flow. In Wastewater Treatment, theupflow counter-current inclined plate (tube) settling tank is predominantly used.

Inclined plate (tube) settling tanks offer advantages such as high sedimentation efficiency, short retention time, and small footprint. They are commonly used in primary sedimentation tanks for municipal wastewater and pretreatment processes (e.g., oil separation) for small-flow industrial wastewater. These tanks provide stable treatment performance and require minimal maintenance. However, they are rarely used in secondary sedimentation processes in wastewater treatment because:

• The mixed liquor after biological treatment contains high solid content, making inclined plate (tube) settling tanks less resistant to shock loads and resulting in unstable performance.

• The high dissolved oxygen content in the mixed liquor promotes algal growth and biofilm formation on the plates/tubes, which may clog the flow area over time and is difficult to clean.

The surface loading rate of inclined plate (tube) settling tanks is approximately twice that of conventional settling tanks. Therefore, they are suitable for optimizing the capacity of existing tanks or reducing the footprint of sedimentation facilities.

What are the basic requirements for inclined plate (tube) settling tanks?

In commonly used inclined plate (tube) settling tanks, influent enters from the lower section of the plate/tube layer and flows upward. Suspended particles settle on the bottom surface of the plates/tubes, accumulate, and then slide down into a sludge hopper, where they are discharged through perforated pipes. The clarified water is collected via perforated pipes or overflow weirs (e.g., triangular weirs) at the water surface. The basic requirements for inclined plate (tube) settling tanks are as follows:

Plate/Tube Specifications

• Vertical spacing between inclined plates: 80–120 mm.
• Diameter of inclined tubes: 50–80 mm.
• Length of plates/tubes: 1.0–1.2 m.
• Inclination angle: 60°.
• Water depth above plates/tubes and height of the bottom buffer layer: 0.5–1.0 m.

Installation

• The upper end of the inclined plates should tilt toward the inlet end of the tank.
• To prevent short-circuiting, baffles should be installed in the gaps between the tank walls and inclined plates.

Inlet and Outlet Design

• Inlet: Equipped with a distribution and flow-straightening device (e.g., perforated distribution plates or slit distribution plates). The flow velocity through distribution holes should be below 0.15 m/s.
• Outlet: Multiple collection channels with orifice or triangular weir arrangements are typically used on the water surface.

Sludge Discharge

• Sludge is collected in hoppers and discharged by gravity.
• Sludge discharge frequency: 1–2 times per day, or more frequently/continuously based on operational needs.

Hydraulic Retention Time (HRT)

• Primary sedimentation: ≤30 minutes.
• Secondary sedimentation: ≤60 minutes.

Cleaning Facilities

• Cleaning mechanisms must be installed. During maintenance or temporary shutdowns, the tank should be drained, and high-pressure water used to thoroughly flush accumulated sludge from the plates/tubes to prevent clogging.

Design Parameters

• Surface loading rate for upflow inclined plate (tube) settling tanks: 3–6 m³/(m²·h) (approximately twice that of conventional settling tanks).
• Hydraulic retention time: 30–60 minutes.

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