Abstract: The combined technology of chemical flocculation and vacuum preloading was proposed to achieve efficient and rapid dewatering of dredged sludge. Five representative flocculants were selected to facilitate the dewatering process using a self-made vacuum preloading filter device. The sedimentation and deep dewatering process of dredged sludge were comprehensively evaluated using a set of designed parameters, including supernatant height, mud-water interface height, settling rate and water content of bottom sludge. The experimental results indicate that the optimal amount of flocculants is 1 500 mg/L for Ca(OH)2, 200 mg/L for PAFSI, 200 mg/L for PAC, 50 mg/L for HCA and 500 mg/L for APAM, respectively. Compared with natural sedimentation process (17.14 cm in height and 96.8% of water content of bottom sludge), the effect of vacuum preloading can accelerate the consolidation and reduce effectively the sludge volume, i.e. the water content of bottom sludge decreases from 96.8% to 53.5% and the volume is compressed further by 20.48%?36.99%. The settling rate of sludge after vacuum preloading reaches its peak within 50 min, and the effect of flocculants associated with the mud-water separation degree plays a dominated role within 120 min. Compared with the original sludge subjected to vacuum preloading, the combination of flocculation-vacuum preloading significantly improves the settling rate of sludge and effectively shortens the required time to reach the peak settling rate. For the optimal flocculant (APAM), the time needed to reach the peak settling rate of bottom sludge and the peak settling rate calculated from total height of sludge are shortened by 87.5% and 83.33%, and the peak settling rate is increased by 3.56 and 5.18 times, respectively. The combined technology of chemical flocculation and vacuum preloading can effectively improve the dewatering performance of flocculated sludge, increase the particle size to prevent blockage, accelerate drainage, promote the sludge sedimentation and improve the mud-water separation efficiency. The combined technique of flocculation-vacuum preloading, contributing to decrease sludge volume, shorten construction period, speed up construction progress and reduce the floor area, can play an important role if it is applied to engineering practice.

Key words: sludge, flocculants, vacuum preloading, mud-water separation, settling rate, water content of bottom sludge