Engineering Measures to Optimize the Sedimentation Efficiency of Inclined Tube Settlers

Engineering Measures to Improve the Sedimentation Efficiency of Inclined Tube Settlers

Inclined tube settlers are widely used, have a mature technology, high sedimentation efficiency, and a small footprint. They are particularly suitable for northern cities that need to build factory for insulation. Therefore, the sedimentation process in water plants in northern cities often adopts inclined tube settlers. Although the main design parameters of inclined tube settlers, such as surface loading and tube dimensions, are clearly specified in the "Code for Design of Outdoor Water Supply Engineering" and the "Water Supply and Drainage Design Manual", there are still some detailed issues regarding water distribution and collection. If these issues are not properly handled, they will directly affect the sedimentation efficiency of inclined tube settlers. Here, the author shares some experience accumulated in engineering design for discussion.

1 Engineering measures to improve water distribution uniformity

1.1 Avoid strong hydraulic turbulence that breaks flocs

Due to their high surface loading and short sedimentation time, inclined tube settlers are less effective than horizontal flow sedimentation tanks in removing small flocs. Once flocs are broken, it is very difficult for them to re-agglomerate into large particles. Fine particles are easily carried by the flow into the clear water zone, increasing effluent turbidity and reducing sedimentation efficiency. Therefore, special attention should be paid to the hydraulic conditions at the inlet of the inclined tube settler to avoid breaking flocs.

From a plan view, the inlet of an inclined tube settler can be divided into two types: forward inlet and side inlet. Forward inlet means setting a transition section between the flocculation tank and the sedimentation tank, with flow rectification measures at the end of the transition section to make water enter the sedimentation tank uniformly, as shown in Figure 1. This water distribution method has good hydraulic conditions and is widely used. Side inlet is adopted when the layout of the flocculation-sedimentation tank is restricted by the water plant site and forward inlet cannot meet the requirement of distributing water along the long side. A distribution channel is set on one side of the sedimentation tank, and the effluent from the flocculation tank flows into the sedimentation tank from that side, as shown in Figure 2.

In a sedimentation tank with side inlet, a flow circulation phenomenon easily occurs at the end of the distribution channel. Due to hydraulic shear, flocs are broken, affecting sedimentation efficiency. Measures such as adding grids or gate plates are needed to reduce flow energy and control flow circulation. In one water plant, the sedimentation tank adopted side inlet (see Figure 2). Operation found that the effluent turbidity in zone II was 1–10 NTU, significantly higher than that in zone I (1–3 NTU). After retrofitting by adding adjustable grids in the distribution channel to increase flow resistance and artificially dissipate energy, the sedimentation effect in zone II improved significantly.

For the above reasons, when planning the layout of flocculation-sedimentation tanks, forward inlet should be used as much as possible to obtain better hydraulic conditions. The impact of side inlet along the short side on uniform water distribution is compensated by adjusting the collection troughs and controlling uniform outflow.

1.2 Avoid water flow stirring up settled sludge

To allow flow to enter the sedimentation tank uniformly along the tank width, a perforated wall is generally installed at the end of the transition section for flow rectification. Since the upper part of the inclined tube settler is occupied by tubes, the opening area of the perforated wall is limited to the space between the tube bottom and the sludge zone. This opening area is smaller than that in a horizontal flow sedimentation tank, often resulting in a higher orifice velocity, which can more easily stir up dead sludge deposited at the bottom of the distribution holes and carry it into the clear water zone, increasing effluent turbidity. To solve this problem, the author used a submerged weir for flow rectification in the design of the Wujintang water source project. The perforated wall was replaced by a full-width inlet opening of 1 m height on the end wall of the transition section. Because the opening area is large, the orifice velocity is low, preventing the flow from stirring up bottom sludge and ensuring effluent quality. To make the flow uniformly distributed along the tank width, a submerged weir was installed in the middle of the transition section, and the flocculation tank outlet was changed to bottom discharge. The effluent from the flocculation tank enters the sedimentation tank after being rectified by the submerged weir, as shown in Figure 3. After many years of operation, the sedimentation effect has been good.

2 Engineering measures to improve effluent uniformity

The uniformity of effluent from an inclined tube settler plays a decisive role in the uniform distribution of flow along the tank length. Therefore, appropriate measures should be taken in design to ensure uniform effluent.

2.1 Choose a collection method less affected by head variation

Common collection methods for inclined tube settlers include orifice collection troughs, thin-wall horizontal weir collection troughs, and triangular weir collection troughs. From the flow calculation formulas, the flow rate of these outlet types is a function of the head over the outlet. Theoretically, as long as the head is kept uniform, any collection method can achieve uniform outflow and thus uniform hydraulic load on each part of the tubes. However, due to construction errors, it is difficult to achieve perfectly uniform head in practice. Therefore, it is important to choose a collection method whose flow rate is less sensitive to head variation.

Orifice collection troughs generally have a row of horizontal holes (20 mm or 25 mm diameter) on each side wall, with a spacing of about 200 mm. The flow formula for a single orifice is given. The flow per meter length of a single side of the trough is also given. The flow formula for a thin-wall horizontal weir per meter length is given. The flow formula for a triangular weir is given.

For a triangular weir collection trough, typically five triangular weirs are set per two meters, so the flow per meter length is q = 3.5·h²·⁵. In water supply engineering, the collection flow per unit weir length for an inclined tube settler is about 1.3 L/s. The calculated heads over the weir and the derivatives of flow with respect to head for different collection methods are shown in Table 1.

From the data in Table 1, it can be seen that the thin-wall horizontal weir has the smallest head and the largest derivative. A construction error of ±1 mm will have a significant impact on effluent. If the weir plate is not installed level, it can easily cause short-circuiting in the tubes. Therefore, this collection method should be avoided as much as possible in design. Orifice outflow is the least affected by head variation and is most likely to achieve uniform effluent. It is an ideal collection method.

2.2 Use adjustable collection troughs

During operation, the tank may tilt to some extent due to various reasons. If fixed collection troughs are used, the effluent uniformity will be greatly affected. Adjustable collection troughs can compensate for this to some extent by adjusting the weir plate height. From the above analysis, the uniformity of triangular weir outflow is most affected by head variation, but adjustable triangular weir troughs are widely used in practical engineering, mainly because the adjustable weir plates are convenient and flexible. If we combine the advantages of adjustable triangular weirs and make adjustable orifice collection troughs, the flow will be least affected by head variation, and the orifice height can be flexibly adjusted. This would be an ideal collection method. This type of collection trough has been applied in projects designed by the author.

3 Conclusion

Inclined tube settlers have a small footprint and are particularly suitable for water plants in northern cities that need to build structures with insulation. The author has used this type of tank in several water supply projects, including those in Yingkou and Tieling. In the design, the main parameters were reasonably determined, and engineering measures were taken to ensure uniform flow distribution in the sedimentation zone. All these plants have achieved good sedimentation performance during operation.