The performance of flocculation in direct depth-filtration was assessed 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 shear rate in the filter during filtration were made to investigate the effects of chemical doses and grain sizes on the performance of direct filtration. Furthermore, a method was suggested to determine the porosity 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 was only slightly affected by the grain size used in the study. (C) 1997 Elsevier Science B.V.