Enhancing Sedimentation and Trapping Sediment with a Bottom Grid Structure

Abstract

One of the great challenges of gravity settling is the difficulty of effectively removing suspended solids from thewater column and retaining the settled particles in the settling basin under high inflow rates and short resident times. To address these issues, a new concept of using a bottom grid structure (BGS) for enhancing suspended solids removal by introducing a downward vertical vortex force and reducing disturbances of the bottom settled sediment was proposed and tested in the laboratory. A series of comparative laboratory experiments, without and with BGS, were carried out in a 2.47 ðLÞ × 1.46 ðWÞ × 0.39-m (D) tank. The experimental results revealed that for the tested flow rates ranging from 4 to 8 L=s, the removals of test particles with the BGS in the tank were about 10 to 30% higher than those obtained for the same conditions, but with a smooth tank bottom. Thus, the newly proposed BGS acts as a structure trapping and storing solids. In general, the improvement of particle removal rates was proportional to the inflow rate (or flow speed along the BGS top surface), before reaching a maximum value. This indicated that particles settled in the BGS cell on the tank bottom were effectively protected from disturbances by the fast bottom flows or external forces. To further examine the retention of deposited sediment by the proposed BGS, comparative experiments were conducted by placing particles on the tank bottom before activating inflow, with and without the BGS present. The results showed that for an inflow rate of 6 L=s, 67% of the placed particles were retained in the settling tank with the BGS; without it, the corresponding percentage was just 26%. Thus, the BGS retained 41% more of the particles settled in the bottom. A computational fluid dynamics (CFD) model was also used to describe the hydraulic conditions in the settling tank under various inflow rates and such results were helpful for establishing the relationships between the improved removal of suspended solids and the bottom flow speed and flow patterns in general. BGS offers a great promise of improving the performance of engineering facilities (e.g., stormwater detention ponds) serving to remove suspended solids from environmental flows. DOI: 10.1061/(ASCE)EE.1943-7870.0000774. © 2014 American Society of Civil Engineers. Author keywords: Enhance sedimentation; Bottom grid structure; Computational fluid dynamics (CFD) hydrodynamic modeling; Retain sediment.