DETACHMENT MECHANISMS OF DEPOSITED MATERIAL IN A BACKWASHED FILTER

Abstract

Theoretical and experimental studies were performed to evaluate the detachment mechanisms of deposited material by fluidizing the filter media. The hydrodynamic characteristics of a backwashed filter and the energy dissipation during backwashing processes were investigated. The existence of a viscous sublayer around each fluidized particle and laminar sublayer thickness at the wall were neglected. Direct measurements of the effluent turbidity Te and the total backwash water volume Vbt were made during backwashing in the laboratory experiments. Some relationships applicable to backwashing processes were developed with good accuracy. The turbulence parameter C0.5(a)/Re which characterizes the effect of turbulence fluctuations in a backwashed filter increased with increasing fraction solids (1-epsilon) and decreased with increasing the Reynolds number of the flow Re. The detachment rate of deposited material rd, was found to vary with hydrodynamic shear (taua), turbulence fluctuations (C0.5(a)/Re) and backwash water volume (Vb). Maximum shear stress caused a maximum detachment of the deposited material from the filter media. The detachment rate of deposited material rd had a minimum value at the turbulent parameter of 0.9 and fraction solids of 0.3. Also the detachment rate of the deposited material rd decreased with increasing the unit backwash water volume Vb which is the ratio of the total backwash water volume Vbt to the expanded bed volume Ve.