Shortcut nitrification-denitrification and population dynamics of ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) were investigated in a continuous flow MUCT reactor treating real domestic wastewater. Shortcut nitrification-denitrification was achieved by controlling low dissolved oxygen (DO) concentration of 0.5 mg·L-1 and short hydraulic retention time (HRT) of 6 h. Nitrite accumulation ratios reached above 90%. TN removal in the phase of shortcut nitrification-denitrification was up to 90%

far higher than complete nitrification with 74% of TN removal. The results of quantitative real time PCR (QPCR) indicated that during complete nitrification-denitrification AOB abundance exhibited a decline tendency

decreasing from 1.69×109 cells·(g dried sludge)-1 to 3.76×107 cells·(g dried sludge)-1

and NOB abundance maintained at 108 cells·(g dried sludge)-1. During shortcut nitrification-denitrification

the abundance of AOB slightly increased from 3.17×106 cells·(g dried sludge)-1 to 1.32×107 cells·(g dried sludge)-1 accompanied with a slight increase of ratio of AOB to total bacteria. The abundance of NOB fluctuated in a range of 5.9×107-1.78×108 cells·(g dried sludge)-1. The ratio of NOB to total bacteria dropped from 1.44% to 0.47%. Therefore

the abundance decrease and bioactivities inhibition of NOB were the important factors to achieve shortcut nitrification-denitrification in MUCT process treating real domestic wastewater. During shortcut nitrification-denitrification due to low DO concentration and short HRT

AOB abundance and their relative distribution did not increase

even descended. But that did not influence the removal of ammonia and total nitrogen.