Deep physicochemical interaction between residual polyaluminum chloride and organic matrices triggered by anaerobic digestion-induced rearrangement for enhanced sludge dewatering.
Abstract
The sludge "water cage" formed by rigid cell membranes and the high affinity between organics and water constitutes a formidable barrier to efficient sludge dewatering. This study couples anaerobic digestion (AD) with the re-contact of residual polyaluminum chloride (PAC) adsorbed during upstream wastewater treatment to overcome this barrier via synergistic biological lysis, chemical complexation, and physical compression. Through this coupled approach, PAC reduced sludge bound water and total water content by 15.97% and 3.65%, respectively; conversely, AD without PAC deteriorated sludge dewaterability, with these parameters increasing by 8.51% and 4.14%. Further experimental results demonstrated that AD-induced biological hydrolysis triggered cell disruption, evidenced by a 227.44% surge in extracellular lactate dehydrogenase levels by day 3 at 30 mg Al/g TSS, exposing encapsulated hydrophilic groups and negative charges, while inducing protein structural rearrangement. Meanwhile, chemical complexation of aluminum ion with the exposed groups promoted surface hydrophobicity, which increased from 70.3° to 89.0°, while the interfacial free energy dropped from -27.76 to -42.98 mJ/m2. Simultaneously, physical compression resulting from charge neutralization and electrical double-layer compression reduced the median particle diameter from 45.14 μm to 40.05 μm, and increased the fractal dimension from 1.5688 to 1.9076, indicating the formation of more compact and rigid flocs. These microstructural shifts produced compact flocs and an eight-fold increase in the cake crack ratio, facilitating high-flux water evacuation through interconnected macropores. The participation of residual PAC converts AD from a dewatering-deterioration process into a deep-dewatering enhancement stage, establishing a novel paradigm for sludge stabilization and volume reduction.
