Nitrite production in a partial denitrifying upflow sludge bed (USB) reactor equipped with gas automatic circulation...

Nitrite production in a partial denitrifying (NO3(-)-N→NO2(-)-N) upflow sludge bed (USB) reactor equipped with gas automatic circulation (GAC) was investigated at ambient temperature (28.8-14.1 °C). The nitrite production rate (NPR) increased with the nitrate loading rate (NLR). Average NPR of 6.63 kgN m(-3) d(-1) was obtained at 28.0 °C with the organic loading rate (OLR) and NLR of 25.38 KgCOD∙m(-3)∙d(-1) and 10.82 kgN m(-3) d(-1), respectively. However, serious sludge floatation was observed when the NLR increased to 13.18 kgN m(-3) d(-1), which might be attributed to sludge bulking at high NLR. The USB reactor recovered rapidly when seeded with the sludge discharged before the deteriorated period, and a stable NPR of ∼4.35 kgN m(-3) d(-1) was achieved at 14.1-15.7 °C in the following 100-day operation, during which the maximum nitrate-to-nitrite transformation ratio (NTR) of 81.4% was achieved at the GAC rate of 1.08 L h(-1). The application of GAC in the partial denitrifying USB reactor enhanced mass transfer, which effectively avoided the channel and dead space, and improved the nitrate transform to nitrite. Moreover, it was found that the GAC system played an important role in promoting the stability of the USB reactor by preventing the sludge floatation. The Illumina high-throughput sequencing analysis revealed that the genus of Thauera was dominate in the USB reactor (67.2-50.2%), which may be responsible for the high nitrite accumulation. Results in this study have an important application in treating nitrate wastewater with an economic and efficient way by combining with ANAMMOX process.