Study on Influencing Factors and Chemical Kinetics in the High-Concentration Simultaneous Nitrification and Denitrification (SND) Process

High concentrations of activated sludge are an excellent biological treatment; in particular, simultaneous nitrification and denitrification play a huge role in nitrogen removal. However, the influencing factors of SND have not been fully elucidated. The effects of sludge concentration and dissolved oxygen (DO) concentration on the performance of SND in a high-concentration activated sludge reactor assisted by chemical agents were investigated, and the SND reaction effect was de-termined by analyzing the along-stream changes of elemental nitrogen in the reactor. The results showed that the SND phenomenon in the reactor was most obvious when the system activated sludge concentration (MLSS) was maintained at 7–9 g/L and DO concentration at 1–2 mg/L. When MLSS decreases within the range of 5–9 g/L, the nitrification reaction improves, but the SND phe-nomenon decreases or even disappears; the SND phenomenon diminishes with increasing DO con-centration. Thus, high sludge concentrations and low dissolved oxygen concentrations are im-portant influences associated with SND and promote unconventional nitrogen removal pathways. In addition, the average value of MLVSS/MLSS for the high-concentration activated sludge process was 0.586, which indicates that the system has a higher activated sludge volume and better sludge activity, which is very effective in enhancing SND. In addition, this study also further investigated the influencing factors of SND in the high-concentration method by exploring the kinetic modeling of the SND reaction in the high-concentration method.