Optimum Backwashing of Filters with Air Scour: A Review

Abstract

The problems occurring in the filtration facilities of water treatment plants, such as mudballs and poor initial filtrate quality are associated with the effectiveness of backwashing. Backwashing with water alone to fluidize the media is an inherently weak cleaning process. The effectiveness of backwashing is significantly improved by the use of air scour simultaneously with subfluidization water wash. When air and subfluidization water flowed concurrently through porous media, a condition termed “collapse- pulsing” occurred in which air pockets formed and collapsed within the bed. A theory has been developed that predicted the combinations of air and water flows that caused collapse-pulsing to occur. Model filter systems have shown that the fluid mechanical condition termed “collapse-pulsing” is also the condition of optimum removal of particles from the media grains during backwashing. The theoretical aspects of collapse-pulsing have been confirmed with an experimental filter system having an endoscope, high speed video system with freeze frame capability for visual observations and an XY Coordinator with computer hardware and software which enabled the determination of velocities of filter grains during air scour. Additional studies on optimum removal of particles from media during air scour have also demonstrated that the results may be extended to dual media filters and are valid at the plant scale too.

Hydraulic Behavior of Quasi‐Saturated Soils in the Presence of Entrapped Air: Laboratory Experiments

“…During the initial saturation, entrapped air is accumulated in the dead-end and smaller pores. When the percolation begins, under the influence of capillary forces, water is absorbed into the smaller pores, and entrapped air is displaced into the larger pores, in which air cavities [Amirtharajah, 1993] are most likely created and block the pores. As a result, entrapped air blocks the largest pores, so their contribution to the water flux is diminished.…”

Biological filtration for BOM and particle removal: a critical review

“…For n on biological filters, backwash in g with water alon e h as been sh own to be an in h eren tly in effective process becau se of th e lim ited collision s an d abrasion s am on g flu idized particles. 82 More recen t research on n on biological filters h as establish ed th at th e best rem oval of particles du rin g backwash in g is ach ieved by sim u ltan eou s u se of air an d water at su bflu idization velocities to ach ieve collapse pu lsin g con dition s. 83,84 Ah m ad an d Am irth arajah 85 in vestigated th e stren gth of attach m en t of differen t particles in lab-scale biofil-ters by in clu din g h ydroph obic an d h ydration forces ( Table 2). Th eir data in dicated th at biological particles (m easu red as h eterotroph ic plate cou n ts an d cellu lar aden osin e triph osph ate) are h eld with greater force th an n on biological particles (m easu red as tu rbidity).…”

Effects of Media, Backwash, and Temperature on Full‐Scale Biological Filtration

“…The use of appropriate backwashing strategies to ensure long‐term performance of biofilters has been emphasized in several reviews and investigations of biological filtration processes (Bablon et al, 1988; Bouwer & Crowe, 1988; Camper et al, 1987; Graese et al, 1987). In nonbiological filters, processes such as air scour improve cleaning efficacy by increasing media collisions and abrasion (Amirtharajah, 1993). The simultaneous use of air and water at subfluidization velocities to achieve collapse‐pulsing conditions has been reported as an optimal backwashing protocol for nonbiological filters (Amirthara‐jah et al, 1991) and has not demonstrated adverse effects on assimilable organic carbon removal by biological filters (Ahmad et al, 1998).…”