Simultaneous DBO removal and nitrification in WWTP

Hello community!
I want to know the general requirements for nitrification and DBO removal in an aerobic reactor in a wastewater treatment plant. How common is it for having both processes in the same reactor? what are typical hydraulic retention times for industrial wastewaters? what should the sludge age be so that nitrifying bacteria can work efficiently? Would it be the same requirements on an normal activated-sludge aerobic reactor and a MBBR aerobic reactor? 
Thanks so much in advance