Enhancing Sedimentation Using Newly Proposed Virtual Bed Concept

Abstract

One of the greatest challenges faced when using gravity settling is achieving efficient particle removal and retention rates under high inflow rates. This paper proposes a new method for enhancing sedimentation and protecting the settled particle. This is accomplished by placing one layer of the virtual bed above the real bed to divide the water body into hydraulically different upper and lower regions. The normal structure of the newly-proposed virtual bed is a flat plate with many small perforations. When the water flows along the top surface of the virtual bed, it creates two effects: (1) it isolates and protects water in the area below the virtual bed from being disturbed by the fast and turbulent flows above, which enhances sedimentation of the particles in the lower region and protects the sediment which could be eroded; and (2) the vertical vortex generated by the surficial flows passing over the openings helps the nearby suspended particles to enter the quiescent water region below. To assess the performance of the newly-proposed virtual bed, a rectangular settling tank was used to conduct comparison tests of particle removal with three particle size ranges, four inflow rates, five virtual bed structural designs, and multiple experimental conditions. The results clearly showed that, compared with a traditional settling tank (without the virtual bed), the addition of the proposed settling structure notably enhanced the particle settling rate by 10%–15% for the particles tested and experimental conditions assessed. The virtual bed method easily can be applied to various water treatment devices and facilities to enhance the suspended particle removal efficiencies in treatments of storm runoff, wastewater, and many other kinds of water without the need for chemical additions or energy-intensive processes. This paper proposes and tested an innovative method for increasing the removal of suspended particles using a virtual bed. A virtual bed is composed of a plate with numerous holes through it suspended above particle-settling facilities. It enhances particle settling by providing a less disturbed region for settling particles, protecting already-settled particles, and encouraging the suspended particles to move toward the real bed. Laboratory tests in a rectangular tank with different particle sizes and flow rates showed that the virtual bed effectively improved particle removal rates. Various possible virtual bed structure designs are proposed and were examined. The best-performing virtual bed was identified, and possible reasons for its performance were discussed based on hydraulics and physics principles. Experiments with different tank configurations and virtual bed position arrangements were conducted, and based on the experimental results, general suggestions for using the virtual bed in practice more effectively are given. In principle, the proposed virtual bed can be applied to any particle-separation facilities.