## 1 Introduction  
Currently, the activated sludge process is the most widely used process in wastewater treatment plants to reduce the biochemical oxygen demand (BOD), nutrients and to some extent other micro-pollutants such as pharmaceuticals, personal care products and other household chemicals. The concentration of dissolved oxygen (DO) in the aeration tank(s) in an activated sludge process is an important process control parameter that has a great effect on the treatment efficiency, operational cost and system stability. As the DO drops, the quantity of these filamentous microorganisms increases, adversely affecting the settle-ability of the activated sludge. It is important to recognize these early warning signs and make corrections to dissolved-oxygen levels before the quality of the effluent deteriorates. If dissolved oxygen continues to drop, even low dissolved-oxygen filamentous microorganisms will not be present in the mixed liquor, and treatment efficiencies will be seriously affected. At this point, effluent turbidity will increase and treatment will deteriorate rapidly. Higher dissolved oxygen is often a target, but in reality, this is for the assurance of mixing. If dissolved oxygen is 5.0 or higher there is a good chance that dead zones are minimal since normal currents and mixing will transport the oxygenated mixed liquor throughout the reactor. However, if the dissolved oxygen is excessive then there could be problems in the settling of sludge due to shearing of flocs and re-suspension of inert materials. A high DO concentration also makes the denitrification less efficient. Both the above-mentioned factors will lead to waste of energy. On the other hand, a low DO level cannot supply enough oxygen to the microorganisms in the sludge, so the efficiency of organic matter degradation is reduced [1; 2]. Therefore, the premise of how the wastewater treatment process can perform stably will depend on how effectively the concentration of DO is be maintained within a reasonable range [3]. Due to the complex nature of microbial activities that are present in an activated sludge process, even a small change introduced to the system (for example, change in flow rate, water quality of the influent, the temperature of the wastewater in the reactors and so on) can affect the concentration of DO. The air supplied to aeration tanks by blowers allows the oxygen to be transferred from the air to the liquid phase (wastewater). The oxygen transfer is a complex process characterized by large time-delays as well as strong nonlinearity, coupling and disturbance, which further increases the difficulty of controlling the concentration of DO [4; 5]. A large number of studies have been carried out and achievements have been made by researchers all over the world to control the concentration of DO level; a series of control methods to control the concentration of DO have been put into practice and they have achieved some good effects. Currently, the proportional-integral (PI) or proportional-integral-derivative (PID) control strategy is widely used in the process control of wastewater treatment plants. It is well known that the control effect might be affected by the unknown, unexpected disturbances and the great changes of operation conditions while using the PI or PID control strategy. In order to improve the dissolved oxygen control performance of the controller in the wastewater treatment process, various solutions are proposed, such as fuzzy adaptive PID, multivariable robust control and model predictive control (MPC) strategy [6; 7; 8; 9]. MPC [2] is an effective way to control DO, not only maintaining the DO concentration at a set value, but also catching up with the real-time changes that occur in the process. [PERSON] et al. Proposed an adaptive fuzzy control (AFC) strategy for tracking the DO set-points applied to the Benchmark Simulation Model No. 1 (BSM1) [10] that was proposed by International Water Association (IWA) [11].