Sludge dewaterability of a starch-based flocculant and its combined usage with ferric chloride

A series of cationized starch-based flocculants [i.e., starch-3-chloro-2-hydroxypropyl trimethylammonium chloride (St-WH)] with different charge densities (CDs) was synthesized by etherification and used as sludge dewatering conditioners. In this series of St-WH samples, St-WH4, with the highest CD, exhibited the optimal sludge dewaterability properties, including specific resistance of filtration, filter cake moisture content, and floc properties, because of the enhanced dual effects of charge neutralization and bridging flocculation. An inorganic coagulant, namely, FeCl3, was combined with the organic flocculant (St-WH) at different dosing sequences to improve the dewatering performance. In comparison with St-WH4, FeCl3, and polyacrylamide, used individually, as well as St-WH4 dosed before and at the same time as FeCl3, the combination of the starch-based flocculant fed after FeCl3 showed better sludge dewaterability and less compressible property because of the synergistic effects of FeCl3 followed by St-WH4. FeCl3 and St-WH4 exhibited good coagulation effects and functioned as skeleton builders. The Pearson correlation between various components of extracellular polymeric substances (EPS) and sludge dewaterability showed that FeCl3 could efficiently compress the protein-like components in EPS. The starch-based flocculant might exhibit not only effective bridging flocculation effects but also good affinity to polysaccharide components in EPS because of their similar structures besides to protein-like components. Thus, sludge dewaterability was substantially improved by using the combination of St-WH and FeCl3.