The side-stream generated recycled water in the wastewater treatment plants is characterized as high in both nitrogen concentration and fluctuation of load. Thus, the design approach for recycle water management became an important part of the biological nutrient removal system design. The application of nitrogen removal from recycled water using the nitritation process has recently been increased due to economical reasons associated with an effective carbon allocation as well as the minimization of aeration cost. The concept of nitritation reactor design related to an inhibition of nitrite oxidizer has not been fully practiced in reality because of the limited field experiences. In this paper, a new approach has been proposed to determine reactor size and oxygen requirement in nitritation process design. The SRT-based design approach has been examined based on a series of laboratory experiments.