[Fast Start-up of Shortcut Nitrification in a CSTR and an MBR]

In order to achieve fast start-up of shortcut nitrification, a continuous stirred tank reactor (CSTR) and a membrane bioreactor (MBR) were selected for a comparative study about the start-up characteristic of shortcut nitrification. The differences in three aspects of the two reactors were investigated, namely start-up time, nitrogen transformation, and sludge performance. The results showed that the start-up of shortcut nitrification was successfully achieved in the CSTR and MBR after 44 and 56 days of operation, respectively, with influent C/N=1, temperature around 30℃±1℃, pH of 7.5-8.0, and dissolved oxygen (DO) of 0.6-1.0 mg·L^-1, as well as combining anoxic/aerobic ratio 1:3 (15 min:45 min) with shortened hydraulic retention time (HRT). The start-up period of the MBR was shorter. On the 14th, 28th, and 56th day, the average nitrite accumulation efficiencies in the CSTR were 51%, 66%, and 89%, respectively, and 50%, 71%, and 93% in the MBR, and nitrate formation rates of the CSTR was successively 7.4, 4.0, and 1.7 mg·(g·h)^-1 (NO₃^--N/MLVSS), respectively, and 7.6, 3.5, and 1.0 mg·(g·h)^-1 (NO₃^--N/MLVSS) for the MBR. A higher nitrite accumulation efficiency and a lower nitrate formation rate were detected in the MBR on the 28th and 56th day, respectively, which could be beneficial to the fast start-up of shortcut nitrification. During the operation period, the shortcut nitrification sludge in the two reactors was yellow, the sludge volume index (SVI) was between 55 and 110 mL·g^-1, and the mixed liquor volatile suspended solids (MLVSS)/mixed liquor suspended solids (MLSS) ratio was around 0.6-0.8. The good sludge performance created favorable conditions for the fast start-up of shortcut nitrification in both the CSTR and MBR. In conclusion, the MBR exhibited better performance in the fast start-up of shortcut nitrification.