Impact of dosing order of the coagulant and flocculant on sludge dewatering performance during the conditioning process

The combined use of coagulant and flocculant can achieve excellent dewatering performance. In this study, we investigated the impact of dosing order of the coagulant and flocculant on sludge dewatering performance. The results showed that capillary suction time (CST) values during the coagulation-flocculation process decreased 20-25% compared to those during the flocculation-coagulation process using the same doses of additives. Moisture content of the sludge during the coagulation-flocculation process was lower. The dosing order of coagulants and flocculants during the conditioning process was clearly important for sludge dewatering, and the coagulant should be dosed before the flocculant. Furthermore, a mechanism for the different dewatering performance was proposed: larger agglomerated and destabilized colloid particles formed, and more bound water was released into the sludge bulk solution during the coagulation-flocculation process, compared with the flocculation-coagulation process, which resulted in better dewatering performance, as reflected in the CST value and moisture content of the sludge cake. These results enable a better understanding of combined conditioning with coagulants and flocculants on sludge dewatering.