A sedimentation tank which can remove fine sediment with low cost and high efficiency is of great significance for the wide application of drip irrigation techniques with the Yellow River water. In this study, the settling process of an inclined-tube gravity sedimentation tank which has high removal efficiency for fine particles in practice was thoroughly investigated. The sediment concentration distribution in the tank was measured by an optical back-scattering turbidimeter. The sediment thickness at the tank bottom was also measured. In addition, the size grading of sediment deposited at different positions on the tank bottom and at different heights in the inclined tubes was also measured by a laser particle size analyzer. It was found that the removal efficiency of fine sediment was 64.7–69.7% in the inclined-tube gravity sedimentation tank, which was higher than that of the sedimentation tank without inclined tubes (with a sediment removal rate of 20.7–32%). The sediment was mainly deposited in the flow adjustment area and settlement area with inclined tubes. A suitable height for the inclined tubes was 70–90 cm. In addition, the water inlet, baffle, and overflow weir in the tank negatively affected the fine sediment settling in two experiment cases. The experimental results enhance our understanding of the sedimentation characteristics in the tank, and indicate the direction for the subsequent structural optimization of the tank.

Clarifying the flow characteristics in the settling tank is the foundation for the design optimization and efficiency improvement of sediment removal. However, few research works have been undertaken on the precise observation of the flow movement in the tank. For this purpose, research into an inclined-tube settling tank for the water-sediment separation of the Yellow River was carried out. An acoustic Doppler velocimeter (ADV) was utilized to measure the velocity distribution in the tank. The dimensionless turbulence intensity and dimensionless Reynolds shear stress distribution were obtained subsequently. It was found that the velocity distribution in the stable settlement area (SSA) and the settlement area with inclined tubes (ISA) was uniform. The turbulence intensity and Reynolds shear stress in the tank were beneficial to the formation of the flocs and the subsequent acceleration of the sediment settling. The flow pattern could be significantly improved by the flow adjustment board and the inclined tubes. However, the inlet, the baffle and the overflow weir in the tank had negative effects on the flow pattern. The results enhanced our understanding of the flow characteristics in the inclined-tube settling tank and indicated a direction for optimizing the tank structure and improving the settling efficiency.