EFFECT OF COAGULANT DOSAGE AND GRAIN-SIZE ON THE PERFORMANCE OF DIRECT-FILTRATION

The performance of flocculation in direct depth-filtration was assesse d in relation to the shear rate and the floc size in the filter pore. Quantitative measurements of the pressure drop of bed, as well as the turbidity and the flee size in the filtrate, and estimation of the she ar rate in the filter during filtration were made to investigate the e ffects of chemical doses and grain sizes on the performance of direct filtration. Furthermore, a method was suggested to determine the poros ity in the deposited layer. In this study, the G (t) over bar product was estimated to be two to three times that in the initially clean bed after a 6 h operation of the filter using 50-100 ppm of alum. It was also found that the porosity of the deposited layer is very high up to a range of 0.93-0.99. The porosity decreased with the filtering time and filtration rate, and increased with the alum dosage. However, it w as only slightly affected by the grain size used in the study. (C) 199 7 Elsevier Science B.V.