Spectral analysis of pressure variations during combined air and water backwash of rapid gravity filters

Abstract

Simultaneous water and air backwashing has been established as the most effective cleaning regime for rapid gravity filters. For certain air and water wash rates the bed can reach a state termed “collapse-pulsing”. This has been identified as the optimum cleaning regime for simultaneous air and water backwashing. An empirical formula exists to predict collapse-pulsing, this is given in terms of the water, air and minimum fluidising velocities. However, no suitable method exists to confirm that the collapse-pulsing condition has been achieved, other than by visually subjective observations of the bed. Visual observation is not always physically possible or practical on industrial size filters. This study examines pressure fluctuations, taken at various heights in the bed, and their associated frequency spectra as a means of identifying the collapse-pulsing condition. Preliminary results show that, particularly in the upper sections of the bed, distinct peaks occurs in the frequency spectrum at collapse-pulsing.