Traditional A2 / O process is a single sludge system, in which functional microorganisms such as PAOs, denitrifying bacteria and nitrifying bacteria grow in the same system, while the sludge age required for various microorganisms to maximize their functions is different:

1) compared with common heterotrophic aerobic bacteria and denitrifying bacteria, autotrophic nitrifying bacteria have a longer generation cycle. In order to become the dominant bacteria group, it is necessary to control the operation of the system under the condition of long mud age. When the system has good nitrification effect in winter, the sludge age (SRT) should be controlled over 30 days; even in summer, if SRT is less than 5 days, the nitrification effect of the system will be extremely weak.

2) PAOs is a kind of short generational cycle microorganism, even its maximum generational cycle (Gmax) is smaller than the minimum generational cycle (gmin) of nitrobacteria.

From the perspective of biological phosphorus removal, the discharge of phosphorus rich sludge is the only way to reduce phosphorus in the system.

If the sludge is not discharged in time, on the one hand, due to the internal respiration of PAOs, the glycogen in the cell will be exhausted, which will affect the absorption of acetate and the storage of poly - β - HYDROXYALKANOIC acid (PHAs) in the anaerobic zone, the phosphorus removal rate of the system will decrease, and even lead to the secondary release of phosphorus from the phosphorus rich sludge in serious cases; on the other hand, SRT will also affect the dominant growth of PAOs and glycans in the system.

In the long sludge age (SRT ≈ 10d) anaerobic environment at 30 ℃, the absorption rate of GAOs to acetate is higher than that of PAOs, which makes it dominant in the system and affects the full play of the phosphorus release behavior of PAOs.