On-line control of polymer dosage in waterworks sludge conditioning was studied on a pilot-scale basis. As an on-line parameter, the streaming potential (SP), which arises in an electrophoretic field in sludge filtrate, was investigated in terms of the amount of polymer dosage savings and the feasibility regarding dewatering performance. Results show that the polymer dosage determined by the streaming potential corresponded to the range of 0. 1-0.3%/TS, which means that it was very similar to the amount of the dosage studied in the basic results of this study. Also, there was a SP set-point showing good dewatering ability in view of time-to-filter (TTF), at which the charge condition was identical with the charge neutralization determined by the zeta potential. Furthermore, the proper mixing (this study, mixing gradient (G) 7.5 s(-1)) leading to sludge floc agglomeration should be accompanied, because the TTF was changed by the flocculation mixing condition even though the polymer was optimally dosed by the streaming potential. This indicates that optimum polymer dosage in case of the control at the rapid mixing step should be taken in a broad range of conditions, including optimum flocculation mixing, for a better dewatering performance. (C) 2007 Published by Elsevier BN.