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This paper describes an experimental investigationinto the effect of backwash rate, type of coagulant, degree of clogging and accumulation of residual deposits(not removed by backwash) on the efficiency of fluidized bed filter backwash in laboratory scale filters treatinga natural raw water coagulated with either alum or a cationic polymer blend. Backwash efficiency was definedin terms of the mass of floc retained in the filter after backwashing. Small variations in the raw watercharacteristics (manifested as variations in raw water turbidity, temperature, pH, rate of headloss developmentand turbidity removal efficiency) within each set of experiments appeared to affect the efficiency of backwash inaddition to the parameters varied deliberately. Stepwise linear regression was used to determine which of severalpossible filtration and backwash parameters were the best predictors of backwash performance.Backwash rate, filter run time, rate of headloss development and mass of residual deposits accumulated duringprevious runs were found to be the best predictors of backwash efficiency for any given filter cycle. Flocdeposits appeared to become more difficult to remove the longer they remained in the filter, while rate ofheadloss development appeared to provide some indication of floc adhesiveness for filter runs of similar length.The efficiency of detachment of freshly deposited floc appeared to increase as the mass of residual deposits and mudballs in the filter increased. The simple linear correlations developed here provide an important first step towards developing and calibratinga more comprehensive model of backwash efficiency that could be easily integrated with existing models offiltration. While this investigation was limited to fluidized bed backwash without air scour or surface wash, thegeneral approach is equally applicable to the more technologically relevant case of backwash with auxiliarywash. Includes 18 references, tables, figures.

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Edition: Vol. - No. Published: 06/17/2005 Number of Pages: 20File Size: 1 file , 1.9 MB