Rapid removal of bound water from dredged sediments using novel hybrid coagulants

Dredging operations produce sediment masses that require chemical conditioning and dewatering before disposing of dredged sediments. Conventional chemical conditioners have difficulty removing the bound water parceled in extracellular polymeric substances (EPS), and the removal of bound water determines the dewatering ratio and the rate of sedimentation. In this study, the conventional conditioners FeCl3, Al-2(SO4)(3), and cationic polyacrylamide (CPAM) and a new covalently-bound hybrid coagulant (CBHyC) were investigated for dewatering sediments in laboratory conditions that simulated real-world conditions. The water content of dewatered cakes was 61.4-68.3% for sediment conditioned with FeCl3, Al-2(SO4)(3), and CPAM, while the water content was 52.6% with CBHyC. CBHyC achieved a setting rate that was 66-359% faster than conventional conditioners, decreased specific resistance to filtration (SRF) by 72-86% compared with conventional conditioners. CBHyC achieved the best sediment dewaterability due to its structure, which consists of hydrophobic group of a long carbon chain and hydrophilic groups of Fe-O-Si complexes and quaternary ammonium. This structure increases electrical neutralization ability and bridging effects, allowing formation of large dense flocs in sediments. Moreover, CBHyC also functioned as a surfactant, dissolving EPS (approximately 59.7%), especially proteins (approximately 59.9%) and humic substances (approximately 25.4%), from the sediment into water. The result was approximately 37.1% bound water parceled in EPS was transformed into free water. Lastly, the sediment conditioned with CBHyC exhibited a discontinuous and porous structure; thus, the water was more likely to flow quickly outward from interior locations. These results demonstrate the potential application of CBHyC for high efficiency and rapid dewatering of dredged sediments.