Neutral protease enhances sludge foaming and drying by tuning the hydrophilicity/hydrophobicity balance of proteins.

Sludge foaming can promote sludge drying efficiency. In the foaming process, sludge protein plays a critical role due to its amphiphilic properties. However, the existing alkaline hydrolysis can generate sufficient solubilized proteins but unable to adjust their properties suitable for subsequent foaming. In response to this issue, acid pretreatment, thermal pretreatment, and enzymes of α-amylase, neutral protease (NP), and alkaline protease were applied in combination with alkaline hydrolysis. Among all methods, the enzymatic hydrolysis of 10 ‰ NP (NP10) within 2.0 h optimally adjust the proteins hydrophilic/hydrophobic, reducing foaming time by 36.7 %, drying time by 19.7 %, and energy consumption by 21.1 %. The NP pretreatment followed by alkaline hydrolysis achieved a controllable hydrolysis of proteins, with NP10 reaching the highest ratio of polypeptides to proteins at 0.4 ± 0.02, which is desirable in sludge foaming. Meanwhile, the plastein reaction was observed in NP group, characterized by increased aggregated strands and decreased α-helix/(β-sheet + random coil) in protein secondary structures, which can improve the hydrophobicity of protein hydrolysate. Two-dimensional correlation spectroscopy analysis confirmed the highest reaction activity of aggregated strands with NP. In sludge drying, the NP10 enhanced the value of effective moisture diffusivity by 51.1 % during the second falling rate period, and significantly decreased the resistance to internal moisture migration in sludge matrix. This work offered a synergistic strategy for efficient sludge foaming and drying by tuning the hydrophilic/hydrophobic balance of protein hydrolysate, and provided a deep understanding on sludge protein properties in relevant treatment processes.