Enhanced sludge solubilization and dewaterability by synergistic effects of nitrite and freezing

In this study, nitrite was added into sludge during freezing process to evaluate its role in waste activated sludge (WAS) solubilization and effect on sludge dewatering characteristics. The results showed that the introduction of 100 mg L⁻¹ of nitrite could increase dissolved organic carbon (DOC) concentration from 29.5 to 48.8 mg DOC g⁻¹ VSS under freezing conditions. More DOC was released with the increase of nitrite concentration. Freezing temperature, or freezing speed, also played a role in sludge solubilization. It was found that some readily-biodegradable low molecular weight (LMW) compounds, e.g. LMW protein, LMW polysaccharide, LMW neutrals, building blocks and LMW acids, were mainly released during the freezing process with the presence of nitrite. Interestingly, nitrite could also improve the sludge filterability at the lower nitrite concentration as a result of the increased sludge particle size. However, electrolytes (sodium nitrite) addition effects may mask such enhancement when nitrite concentration was high (800 mg L⁻¹). The rheological characteristics of sludge could be well modeled by Herchel-Bulkley model and the introduction of nitrite into freezing process further increased sludge flowability and decreased sludge viscosity. These results indicated that freezing with the presence of suitable concentration of nitrite could promote sludge solubilization and dewaterability. As such, good liquid and solid separation can be achieved with the recovery of liquid stream as carbon source.