•The sludge produced from using composite coagulants has low volume and compact structure.•The discharge interval of sludge in enhanced coagulation process could be extended.•The sludge has high dewatering ability and can be dehydrated directly without additional polymer.•The above results may be caused by the strong enhancement in the charge neutralization and bridge.The characteristics and dewatering performance of sludge in sedimentation tanks, produced by enhanced coagulation process using composite coagulants were systemically studied in the treatment of micro-polluted raw water. Two processes, the conventional coagulation process using PAC and the enhanced coagulation process using PAC/PDMDAAC composite coagulants, were parallelly operated in 2 pilot-plants for each with production capability of 4 m3/h potable water. Then the volume, solid content, zeta potential and dewatering ability of the sludge from the two processes were compared. The results showed that the volume of sludge produced from enhanced coagulation process using composite coagulants was 30.06–38.63% less than that of using PAC, and then the discharge interval of sludge from using composite coagulants was 41.78–63.28% longer than that of using PAC. Besides, the sludge from the enhanced coagulation process using composite coagulant displayed a more compact structure, higher solid content and higher zeta potential values than that from conventional coagulation process using PAC, and the water content of filter cakes after vacuum filtration of the sludge could achieve to 80%, indicating an obvious feature of its dewatering ability directly. The reasons for the compact structure and low volume of the sludge might be the strong enhancement in the charge neutralization and bridge ability of PAC by composing PDMDAAC in the composite coagulants.

•The sludge produced from using composite coagulants has low volume and compact structure.•The discharge interval of sludge in enhanced coagulation process could be extended.•The sludge has high dewatering ability and can be dehydrated directly without additional polymer.•The above results may be caused by the strong enhancement in the charge neutralization and bridge.The characteristics and dewatering performance of sludge in sedimentation tanks, produced by enhanced coagulation process using composite coagulants were systemically studied in the treatment of micro-polluted raw water. Two processes, the conventional coagulation process using PAC and the enhanced coagulation process using PAC/PDMDAAC composite coagulants, were parallelly operated in 2 pilot-plants for each with production capability of 4 m3/h potable water. Then the volume, solid content, zeta potential and dewatering ability of the sludge from the two processes were compared. The results showed that the volume of sludge produced from enhanced coagulation process using composite coagulants was 30.06–38.63% less than that of using PAC, and then the discharge interval of sludge from using composite coagulants was 41.78–63.28% longer than that of using PAC. Besides, the sludge from the enhanced coagulation process using composite coagulant displayed a more compact structure, higher solid content and higher zeta potential values than that from conventional coagulation process using PAC, and the water content of filter cakes after vacuum filtration of the sludge could achieve to 80%, indicating an obvious feature of its dewatering ability directly. The reasons for the compact structure and low volume of the sludge might be the strong enhancement in the charge neutralization and bridge ability of PAC by composing PDMDAAC in the composite coagulants.