Surface and membrane properties and morphological structure of microbial cells with potential implications on the dewaterability of waste-activated sludge by chemical conditioning

Chemical conditioning for sludge dewatering is effective and widely used in wastewater treatment plants. However, the cellular and molecular level mechanism of deep sludge dewatering by chemical conditioning is unclear. Herein, two typical bacteria, Pseudomonas laurentiana 1–1 and Exiguobacterium acetylicum 3–1, derived from the waste-activated sludge, were used to investigate the hydrophobicity of the bacterial surface and alterations in the membrane properties and morphological structure of bacterial cells under ferric chloride (FeCl 3 ) conditioning. The results showed that 0.5 mM/g dry bacteria (DB) of FeCl 3 degraded proteins and polysaccharides in Pseudomonas laurentiana 1–1 and Exiguobacterium acetylicum 3–1 extracellular polymeric substances (EPS) and released water from the EPS. FeCl 3 conditioning at this concentration increased the hydrophobicity of Gram-negative Pseudomonas laurentiana 1–1 by 1.9 times, and Gram-positive Exiguobacterium acetylicum 3–1 increased by 18.7 times. The Gram-positive Exiguobacterium acetylicum 3–1 showed greater sensitivity to FeCl 3 due to its ability to degrade peptidoglycan in the bacterial cell wall, peptidoglycan is much higher in Gram-positive bacteria than in Gram-negative bacteria. Gram-positive bacteria were abundant in sludge bacteria. The peptidoglycan in Gram-positive bacteria cell wall was degraded after treatment with the appropriate dosages of FeCl 3 , which significantly increased the hydrophobicity of the Gram-positive bacteria, thereby effectively improving sludge dewatering. Besides, the cell membrane permeability of two typical bacteria was greatly increased by 0.5 mM/g DB of FeCl 3 , and the intracellular gel structure was degraded, releasing the intracellular water. This study provides theoretical guidance at the cellular-molecular level for the mechanism exploration of chemically conditioned sludge deep dewatering.