The use of jar-filtration tests to compare performances of coagulants in direct filtration

Modified jar tests were performed in order to compare performances of coagu lants treating, by direct filtration, raw water having a low turbidity and dissolved organic carbon. The object of this direct filtration is to produc e water with a turbidity equal to or less than 0.10 NTU. The tests comprise a coagulation step with a laboratory flocculator, followed by a filtration step on a granular medium. Two poly aluminium silicate sulphates (PASS 100 and PASS 55), poly aluminium chloride (PACl), aluminium sulphate (alum), F erriclear (prehydrolized ferric sulphate) and ferric chloride were studied. It was observed that all these coagulants are capable of lowering the turb idity of filtered water to 0.10 NTU or even less. Among the aluminium-based coagulants, PACl seems to be the most effective; while for iron-based coag ulants, ferric chloride performs best. Although the doses are higher, alum offers a commercial advantage over the other aluminium-based coagulants bec ause it costs less. When the turbidity objective of 0.10 NTU is reached, th e amount of dissolved organic carbon removed is small. The best removal rat e, which is 23%, is obtained with ferric chloride.