Nitrate shock loading experiments were conducted in a bench scale SBR to investigate the effect of nitrate on phosphorus removal. After achieving satisfactory phosphorus removal under steady state operation, initial NO3-N concentration amounting to 10 and 20 mg /L was fed at the beginning of the cycle. It was observed that, 10 mg/L of NO3-N suppressed phosphorus release during the feed and mix phases. Organic consumption for denitrification lead to limited PHA storage by phosphorus removing bacteria, resulting in less PO4-P removal. For 20 mg/L, influent organic substrate was not sufficient even for complete denitrification, thus leading to the presence of higher NO3-N and PO4-P in effluent. To explain the dynamics of the nutrient removal system under the transient loading, a SBR model based on IAWQ ASM2 was implemented. After adjusting PHA contents, model simulations well predicted dynamic changes of nitrate and phosphate concentrations during a cycle. Based on the model simulations, competition of COD substrate among denitrification, fermentation and oxygen respiration were investigated by calculating their consumption rates during mixing phase. In addition, a nitrate disappearance model was proposed and implemented in conjunction with a settling model to predict remaining and effluent nitrate in a cycle of SBR. Furthermore, integrated model simulations highlighted the effect of remaining nitrate on phosphorus release considering different options of reactions in settling phase.

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