Synergistic Effect of Cement and Cationic Polyacrylamide on Enhancing Dewaterability and Strength of Fe2+/S2O82− Oxidation-Pretreated Landfilled Municipal Sludge

Landfilled municipal sludge (LMS) poses great challenges for landfill stability and future expansion, because LMS has high water content (WC) and low strength. This study explores the feasibility of using cement and cationic polyacrylamide (CPAM) combining Fe2+/S2O82− oxidation to reduce the LMS volume and increase its strength. The results showed that the proposed approach was efficient in LMS volume reduction and strength improvement. The net dewatering efficiency (NDE) of LMS was as high as 71.6% (the corresponding WC was reduced from 79.6%–47.8%), and the unconfined compressive strength (UCS) of 28-day solidified sludge was 179.0 kPa, achieved under optimal Fe2+/S2O82− oxidation conditions, with only 10% cement (based on the dry sludge mass) and 0.15% CPAM. Microscopic analyses revealed the underlying mechanisms as: (1) Fe2+/S2O82− oxidation pretreatment disrupted the colloidal agglomerate in the sludge, releasing the bound water to become free water, which significantly improved the sludge’s dewaterability; and (2) the synergistic use of cement and CPAM enhanced the resistance of Fe2+/S2O82− oxidation-pretreated LMS to compression, facilitating sludge’s sufficient dewatering (i.e., volume reduction), and thus the dewatered denser sludge matrix allowed the subsequent cement’s chemical reactions to occur more efficiently, resulting in a high-strength microstructure in solidified sludge (i.e., strength improvement). The novelty is that the proposed approach is more effective in both dewatering and strength improvement while requiring a lower binder usage compared with existing methods.