Comprehensive evaluation of the long-term effect of Cu2+ on denitrifying granular sludge and feasibility of in situ recovery by phosphate

With increased industrial development, vast heavy metals are inevitably discharged into wastewater. Cu2+ is one of the most hazardous heavy metals in biotreatment. However, the potential effect of Cu2+ on denitrifying granular sludge is still unknown. This work assesses the response of denitrifying granular sludge to Cu2+ stress from multiple aspects. The denitrifying granular sludge could tolerate 5 mg L-1 Cu2+, while the nitrogen removal efficiency decreased to 48.5% under 10 mg L-1 Cu2+. Enzyme activity and carbohydrate metabolism were inhibited, and the denitrifying bacteria were washed out under Cu2+ stress. The resulting deteriorated state was reversed by phosphate. The nitrogen removal efficiency recovered to 99% after 10 days, and the enzyme activity also recovered to the original level. Membrane transport, transcription and cellular processes were promoted. Overall, the results of this work provide a feasible strategy to rapidly restore the metabolic activity of denitrifying granular sludge under Cu2+ stress.