According to the traditional biological denitrification theory, denitrification generally includes two stages of nitrification and denitrification. Nitrification and denitrification need to be carried out in two isolated reactors, or in the same reactor which causes alternating anoxic and aerobic environment in time or space. In fact, in the earlier period, in some activated sludge processes without obvious anoxic and anaerobic stages, The phenomenon of non assimilation loss of nitrogen has been observed for many times, and the disappearance of nitrogen has also been observed for many times in the aeration system. In these treatment systems, nitrification and denitrification often take place in the same treatment conditions and the same treatment space. Therefore, these phenomena are called simultaneous nitrification / denitrification (SND).

There are a lot of reports about snd phenomenon in various treatment processes, including biological turntable, continuous flow reactor and sequencing batch SBR reactor. Compared with the traditional nitrification denitrification process, snd can effectively maintain the pH stability in the reactor, reduce or cancel the addition of alkalinity; reduce the volume of the traditional reactor, and save the cost of infrastructure construction; for the sequencing reactor composed of only one reactor, snd can reduce the time required to realize nitrification denitrification; save the amount of aeration, and further reduce the energy consumption.

Therefore, snd system provides the possibility of reducing investment and simplifying biological nitrogen removal technology in the future.