Insight into a new two-step approach of ozonation and chitosan conditioning for sludge deep-dewatering.

Sludge deep-dewatering, capable of reducing water content (Wc) of sludge cake under 60%, is one of the current hot topics in sludge treatment. In this study, a new environmentally friendly two-step approach of ozonation and chitosan (CT) conditioning was proposed and examined to be practicable for sludge deep-dewatering. With 60 mg/gTS ozone and 20 mg/gTS CT conditioning, sludge capillary suction time (CST) and Wc of the dewatered sludge cake decreased from 196.3 s and 84.7% of raw sludge to 15.8 s and 57.5%, respectively. Ozonation treatment could efficiently crack sludge flocs and cells, and release biopolymers, causing the decreases in viscosity, zeta potential and particle size. Subsequently, CT ameliorated the sludge dewaterability successfully by neutralizing negative charges and flocculating colloids to promote the spread of interstitial water. Furthermore, the contents of heavy metals (As, Cd, Cr, Ni, Pb and Zn) in conditioned sludge cake decreased obviously except Cu, but all detected heavy metals contents satisfied the A level of agricultural land (GB4284-2018). For chemical speciation of heavy metals, the proportions of the acid soluble/exchangeable state and the reductive state increased apparently, implying higher toxicity and bioavailability, except Pb. Hence, pretreatments were required to reduce the environmental risk of heavy metals in conditioned sludge cake prior to a further utilization.