The mixed liquid from the aerobic tank is returned to the anoxic tank, affecting the anoxic condition
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Nitrification occurs in the aerobic tank, while denitrification takes place in the anoxic tank. In many processes, the nitrified liquid from the aerobic tank is recycled to the anoxic tank for nitrogen removal. The DO level in the aerobic tank is generally high, which results in a high DO level in the recycled nitrified liquid as well; this in turn raises the DO level in the anoxic tank, hindering the process of denitrification. How can we ensure that the anoxic tank remains in an anoxic state for denitrification? For example, in processes such as the intermittent feed pre-anoxic AO process, or when there is a high degree of reflux from the aerobic tank to the anoxic tank, can the anoxic tank still remain in an anoxic state to carry out denitrification?
The DO level in the anoxic tank is a crucial parameter for the efficiency of denitrification, as denitrifying bacteria are facultative organisms that can survive under anaerobic conditions as well as carry out metabolism under aerobic conditions. Moreover, since it is easier to obtain energy under aerobic conditions, the denitrification process carried out by these bacteria is inhibited in environments with high DO levels, resulting in a **reduction in denitrification efficiency**. Generally, the anoxic tank is located at the water inlet; the concentration of organic matter is relatively high there, and the amount of oxygen required for aerobic processes is substantial. It is not difficult to maintain a DO level of 0.5 mg/L in this area ； If the DO is greater than 0.5 mg/L, it is necessary to balance the relationship among the DO in the aerobic tank, the mixing return flow rate, and the external return flow rate. 1. Generally, the amount of dissolved oxygen entering the anoxic tank is reduced by lowering the DO at the outlet of the aerobic tank ； 2. Appropriately reduce the mixing return flow rate, provided that the total nitrogen level remains within acceptable limits ； 3. Reduce the external return flow and increase the retention time in the secondary sedimentation tank to lower the dissolved oxygen in the return flow ； 4. Increasing MLSS can also effectively reduce DO.