Using an extended activated sludge model to simulate nitrite and nitrate variations in TNCU2 process

In this study, an extended activated sludge model was established to describe the transformation of nitrite (SNO2)(SNO2), nitrate (SNO3)SNO3) and other components in TNCU2 process (National Central University of Taiwan No. 2) that consisted of anaerobic, oxic, anoxic, oxic zones in sequence. The significant differences between this extended model and other models were that two-stage nitrification, multi-stage denitrification, and phosphorus removal were taken into account simultaneously. The results indicated that the growth rate constants of XAOBXAOB and XNOBXNOB were 1.4 and 0.4 d−1, respectively. YAOBYAOB value was 0.14 and YNOBYNOB value was 0.04. According to model simulation, the heterotrophic microorganism (XH), phosphorus accumulating organism (XPAO), XAOBXAOB and XNOBXNOB concentrations were 1160–1322, 182–226, 21–26 and 13–17 mg l−1, respectively, in TNCU2 process. XH,XPAO,XAOBXH,XPAO,XAOB, and XNOBXNOB decreased in the anaerobic tanks because of the lysis reaction. Then XH,XPAO,XAOBXH,XPAO,XAOB, and XNOBXNOB increased in the aerobic tanks due to aerobic growth. XH,XPAO,XAOBXH,XPAO,XAOB, and XNOBXNOB increased in quantities by about 5%, 6%, 6% and 4% in the first aerobic tank and decreased in quantities by about 12%, 19%, 20% and 19% in the anoxic tank in which the step feeding influent flowed. The ratio of total nitrifying species to total active biomass was about 3% in each tank.