Free ammonia pretreatment deteriorates sludge dewaterability during anaerobic fermentation of waste activated sludge: An overlooked concern

Free ammonia (FA) pretreatment has been reported as an effective pretreatment method for waste activated sludge (WAS) fermentation to produce volatile fatty acid. However, its impact on sludge dewaterability, a critical factor determining the final WAS processing cost, remains unexplored. This study systematically evaluated fermented sludge dewatering performance with FA pretreatment through semi-continuous anaerobic fermentation (SCAF), overcoming batch test limitations. The results showed a significant deterioration of sludge dewatering performance after SCAF with FA pretreatment (180 mg/L), where capillary suction time (CST) and specific resistance to filtration (SRF) increased from 972.9 ± 28.9 s and 4.3 ± 0.1 × 10¹¹ m/kg (blank) to 1687.9 ± 54.6 s and 9.4 ± 0.2 × 10¹¹ m/kg ( p < 0.05), respectively. Mechanistic analyses displayed that FA pretreatment reduced the particle size distribution, Zeta potential and water contact angle, which decreased the relative hydrophobicity (70.5 ± 1.2 % to 61.8 ± 1.1 %, p < 0.05). FA pretreatment could also enhance the contents of PO₄³⁻, proteins, polysaccharides, and humic acid in the liquid phase through sludge disintegration, thereby raising the sludge hydrophilicity. Sludge pretreatment and conditioning tests demonstrated that FA pretreatment (including the roles of NH₃ and pH) exerted significantly stronger deterioration impact on sludge dewaterability than pH alone, which should be considered as the complete contributor. This study helps to fill the research gap of FA pretreatment in sludge dewatering during SCAF and provide data support and guidance for the further improvement of FA pretreatment technology before practical application.