Enhanced dewaterability of anaerobically digested sludge by in-situ free nitrous acid treatment

As the protonated form of nitrite, free nitrous acid (FNA) is a renewable chemical that can be produced on site from the anaerobic digestion liquor by nitritation, and has been widely employed to improve the fermentation of waste activated sludge (WAS). However, it is not clear whether and how FNA improves the dewaterability of anaerobically digested sludge (ADS). This work therefore aims to provide such supports through comparing the dewaterability of ADS treated by nitrite at different concentrations (0-250 mg/L) under three pH values (5.5, 6.3, or 7.2). Environmental results showed that nitrite was completely denitrified within 12 h, and its addition improved the dewaterability of ADS in all the cases. The optimal normalized capillary suction time of 18.0 ± 0.4 s L/g VSS was obtained at nitrite 50 mg/L and pH 5.5 (equivalent of 0.35 mg/L FNA) in comparison with corresponding value of 23.2 ± 0.4 s L/g·VSS at pH 5.5 (equivalent of 0 mg/L FNA). Under this scenario, 80.5% ± 2.0% of water content was obtained in the FNA-treated sample after press filtration while the corresponding value was 88.5% ± 1.7% in the control. The mechanism investigations showed that FNA treatment reduced surface negative charge of ADS flocs and caused disruption of extracellular polymeric substances and release of intracellular substances, which enhanced the flocculability, hydrophobicity, and flowability, but decreased the bound water content, fractal dimension, and viscosity of ADS. Additionally, FNA treatment altered the secondary structure of proteins through destroying the hydrogen bond, which led to a loose structure of protein, benefiting the exposure of hydrophobic sites or groups in EPS proteins. The findings obtained deepen our understanding of FNA affecting sludge dewatering and provide strong supports to sustainable operation of wastewater treatment plants.