Aluminum speciation in polymerized aluminum chloride: Roles and chloride ion migration in sludge dewatering

Abstract

Chemical conditioning emerges as a pivotal determinant in augmenting sludge dewatering performance. In the realm of frequently employed aluminum chloride polymer conditioners, the impact of hydroxyaluminum is noteworthy. Beyond influencing dewatering efficacy, it introduces chloride ions, precipitating a cascade of subsequent treatment challenges. In this study, we investigated the dewatering efficiency of coagulants with varying aluminum speciation contents. Notably, the coagulant predominantly composed of medium polymeric aluminum (Alb) demonstrated superiority at a dosage of 0.08 g Al/g TSS, exhibiting favorable outcomes in terms of capillary suction time and specific resistance to filtration, measured at 22.3 s and 4.02 × 1012 m−1/kg−1, respectively. These values were notably lower compared to those obtained with low-polymerization state aluminum (Ala) at 25.2 s and 6.04 × 1012 m−1/kg−1, as well as high polymorphic aluminum (Alc) at 25.8 s and 4.69 × 1012 m−1/kg−1. It was attributed to multifactor (high charge density, high protein conversion efficiency, distinct pore drainage structures of microchannels, etc.). The investigation underscored that the predominant elevation in chloride ion content within the sludge system was primarily attributed to the dosing of polyaluminum chloride. Furthermore, the subsequent application of coagulant aids, such as cationic polyacrylamides and CaO, significantly influenced the chloride ion content within the sludge cake, thereby modulating dewatering efficiency and the subsequent disposal of sludge. Therefore, strategically managing aluminum forms and coagulant aids can optimize sludge dewatering efficiency and subsequent disposal processes.