Conventional jar tests and on-line size monitoring were used to investigate the effects of slow-mixing intensity and duration on residual turbidity and floc size during charge neutralization coagulation and sweep flocculation with polyaluminum chloride. The compensatory effect of slow-mixing on coagulation performance following inadequate or excessive rapid-mixing was also examined. It is found that slow-mixing intensity has a more marked positive effect on charge neutralization coagulation than on sweep flocculation. The optimal root-mean- square velocity gradient

G

for slow-mixing is 15 s

-1

for both coagulation mechanisms

and charge neutralization coagulation requires a longer slow-mixing duration. The optimal slow-mixing duration

based on residual turbidity

is longer than the time to form the largest mean flocs. The optimal product of G and mixing duration

GT

for slow-mixing during charge neutralization coagulation (13500) are higher than that during sweep flocculation (4500) and both are less than the range of values recommended by the American Water Works Association (24000-84000). The optimal GT value under various slow-mixing conditions increases with G. Appropriate extension of slow-mixing duration during charge neutralizati

on coagulation can improve coagulation performance after an inadequate or excessive rapid-mixing duration

but during sweep flocculation

appropriate shortening of slow-mixing duration after an excessive rapid-mixing or appropriate extension of slow-mixing duration after an inadequate rapid-mixing is favorable.