Fitzpatrick, CSB; (1991) Filter backwashing mechanisms. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

An investigation of filter backwashing mechanisms was undertaken. High speed video recording equipment, operating at 200 frames/s, in conjunction with a rigid endoscope (borescope) has been used to obtain detailed visual information from inside the filter bed during backwash. In addition, experimental measurements of mass balance and backwash water turbidity have provided efficiency information. Experiments were performed using standard filter sand, either clean or clogged with a suspension of kaolin clay, or kaolin clay flocculated with alum and polymer in London tap water. Backwashing with water or water and air scour was recorded on video for subsequent analysis. A total of 30 experiments were performed backwashing with water only and 38 using combined air and water wash of the clogged bed. Using the XY Coordinator and a PC velocities of the sand grains were obtained from the video tapes for different backwash regimes. The results indicate that the majority of detachment is due to the fluid shear forces acting on deposits on the grain surfaces, particularly when a water only wash is employed. Kaolin and flocculated kaolin deposits are easily detached without fluidising the bed, but some remain trapped in areas where there is no flow, or, where the shear forces are insufficient for detachment. Fluidisation serves: (i) to mobilise grains and expose new faces to the shearing effects of the flow, and (ii) to allow flushing out of the resuspended deposits. Grain collisions and abrasion do take place, particularly as the bed undergoes expansion, but these were not the major cause of deposit detachment in the experiments described. When simultaneous air and subfluidising water flow are used, then the bed behaves in a way described as collapse-pulsing. A high degree of bed circulation is created and higher grain velocities with consequent shear stress leads to rapid deposit detachment. Following air scour it is necessary to fluidise the bed with water in order to flush out deposits and trapped air. The velocity data correlates well with the backwashing efficiency data, i.e. higher grain velocities result in better cleaning. This thesis includes a video recording of selected representative experiments and copies of publications from this work.

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