Explore the closed-loop disposal route of surplus sludge: Sludge self-circulation preparation of sludge-based biochar (SBB) to enhance sludge dewaterability

Sludge-based biochar (SBB) has been used to improve excess sludge dewaterability. However, the conditionings of sludge dewaterability using SBB prepared by different methods are discrepant. Herein three SBBs (SBB0, SBB1, SBB2) were prepared by direct carbonization, ZnCl2 pre-activation and carbonization-HCl post-activation method respectively.The feasibility of SBB as sludge dewatering conditioner and the mechanism were mainly explored. The results demonstrated that all three SBBs enhance sludge dewatering. SBB1 had the best conditioning effect among all the three methods at the same dosage. SBB particles significantly reduced the compressibility of sludge and improved the porosity of sludge cake. Zeta potential analysis showed that the positive charge on the surface of SBB could neutralize the sludge flocs with negative charges, reduce the electrostatic repulsion between sludge particles, make the sludge destabilize and aggregate. The SBB characteristics exhibited a coagulant aid effect. Meanwhile, SBB promotes the dissolution of inner EPS and effectively weakens the water holding capacity of sludge flocs by adsorbing viscous macromolecules through non-covalent interactions. Sludge colloid destabilization, EPS dissolution and removal of water-holding substances played a synergistic effect in the process of weakening the affinity between water molecules and solid phase. The specific surface area, pore volume, the number and type of surface functional groups, the content of silicon and aluminum compounds of SBB1 were better than those of SBB0 and SBB2 combined with the characterization results of three SBBs.Therefore SBB1 had the best effect on sludge dewatering. The proposed closed-loop sludge disposal route by sludge self-circulation using SBB derived from wastewater treatment plants (WWTPs) is a green sustainable insight and a promising research direction.