A practical guide for determining appropriate chemical dosages for direct filtration

Abstract

The tests conducted in this study have made it possible to propose a rapid and simple laboratory method for determining the appropriate dosages of chemicals according to filtering materials effective sizes (ES), for direct filtration applications, and for adjusting the dosages according to raw water quality changes. Application of the proposed procedure requires but simple laboratory equipment: an Ives' filterability index measuring device, a filtration system operating under a constant vacuum, and 0.45 and 8 µm membrane filters. The use of 0.45 µm membrane filtration allows one to determine the best dosages for fine material with an effective size of 0.4 mm, whereas Ives' filterability index and 8 µm membrane filtration help determine the best dosage applicable to the 1.2 mm ES. The results obtained showed that for other effective sizes in the 0.4–1.2 mm range, the best dosages of alum and polymer can be estimated by linear interpolation. This laboratory procedure is a useful tool for quickly determining the best chemical dosages versus filtering media ES for a given water quality. It should be applied to raw waters with unknown characteristics prior to carrying out a more accurate full-scale validation, if necessary.Key words: direct filtration, coagulation, flocculation, alum, effective size, Ives' filterability index.