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The simultaneous nitrification and denitrification in step-feeding biological nitrogen removal process were investigated under different influent substrate concentrations and aeration flow rates. Biological occurrence of simultaneous nitrification and denitrification was verified in the aspect of nitrogen mass balance and alkalinity. The experimental results also showed that there was a distinct linear relationship between simultaneous nitrification and denitrification and DO concentration under the conditions of low and high aeration flow rate. In each experimental run the floc sizes of activated sludge were also measured and the results showed that simultaneous nitrification and denitrification could occur with very small size of floc.

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The step-feed anoxic/oxic biological nitrogen removal process has been proposed as an attractive alternative for the conventional biological nitrogen removal process for the purpose of enhanced nitrogen removal. For step-feed process, the biological nitrogen removal efficiency is a function of influent flow distribution. In this study, the effects of influent flow rate distribution on the performance of nitrogen removal process were investigated. The effects of influent flow rate distribution on COD, ammonia, total nitrogen removal efficiency, nitrification rate and sludge volume index value were also studied. According to the performance characteristic of the step-feed process, the concept of influent flow rate distribution ratio was firstly introduced. The maximum influent flow distribution ratios (λ max ) under the condition of different influent C/N ratios were determined, respectively, by trial and error method. The experimental results showed that high total nitrogen removal efficiency, higher than 95%, could be achieved under certain influent flow rate distribution ratio without internal nitrate cycle or addition of external carbon source. It was obvious that nitrification rate of each stage under different influent flow distributions decreased along with the decreasing of sludge loading and volume loading in each stage, and the degree of reduction rise gradually with the increasing of influent flow distribution ratio. The sludge volume index value would also increase along with the increase of influent flow distribution ratio.

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