The characteristic and comparison of denitrification potential in granular sequence batch reactor under different mixing conditions

• Mixing condition had a great impact on SDR of granules.
• The lowest SDR was achieved under intermittent aeration condition.
• The highest SDR was achieved under mixing by introducing nitrogen condition.
• Mixing under recycling supernatant did not benefit for continuous operation.
• Granules were maintained and high SDR was achieved under mechanical stir condition.

The aim of the present work was to characterize and compare the denitrification potential of aerobic granular sludge under four different mixing conditions in lab-scale granular sequence batch reactor (GSBR). Batch experimental results showed that the specific denitrification rate (SDR) reached 0.68 and 0.11 kg NOx–N/m3 d with and without mixing, respectively. The lowest SDR of 0.091 kg NOx–N/m3 d was achieved under intermittent aeration condition, which mainly resulted from the high dissolved oxygen (DO) concentration and poor mixing in denitrification process. The SDR and NOx–N removal efficiency was improved to 0.7 kg NOx–N/m3 d and 97% under mixing condition by introducing gaseous nitrogen to reactor during 60 min denitrification process. It was found that the mixture under recycling supernatant liquid condition did not benefit for the stability of granules due to the sludge floating problem in denitrification process. The mechanical stir had little impact on granules under oxic/anoxic combined with mixing by mechanical stir in anoxic phase operational strategy. When the total nitrogen (TN) and chemical oxygen demand (COD) were kept at 55 and 320 mg/L in the influent, the SDR, nitrogen removal efficiency and rate reached approximately 0.64 kg NOx–N/m3 d, 97.7% and 0.255 kg N/m3 d in 50 days continuous operation.