The effects of operational conditions on nitrogen removal were investigated in the step feed anoxicoxic activated sludge process, which is one of the most practical methods for the nitrogen removal in existing plants. Laboratory-scale experiments were conducted under various operational conditions such as different values of the step ratio (influent flow ratio to the first anoxic tank), and the experimental results were discussed with the stoichiometrical analysis. The main results obtained are as follows: 1) Many operational parameters affect nitrogen removal efficiency correlatedly: 2) higher recycling ratio does not always give better nitrogen removal especially if DO in oxic tank is excessively high; 3) the nitrogen removal efficiency can be estimated by the stoichiometrical calculation among nitrogen, substrate, alkalinity and DO; and 4) the optimal step ratio for the sewage studied here is estimated to be 50%, and the recycle ratio is to be 150% if DO in oxic tanks is high.
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