RESUMO
An anaerobic-anoxic/oxic (A2/O) multi-phased biological process called "phased isolation tank step feed technology (PITSF)" was developed to force the oscillation of organic and nutrient concentrations in process reactors. PITSF can be operated safely with a limited carbon source in terms of low carbon requirements and aeration costs whereas NAR was achieved over 95% in the last aerobic zone through a combination of short HRT and low DO levels. PCR assay was used for XAB quantification to correlate XAB numbers with nutrient removal. PCR assays showed, high NAR was achieved at XAB population 5.2 × 10(8) cells/g MLVSS in response to complete and partial nitrification process. It was exhibited that low DO with short HRT promoted XAB growth. Simultaneous nitrification and denitrification (SND) via nitrate were observed obviously, SND rate was between 69-72%, at a low DO level of 0.5 mg/l in the first aerobic tank during main phases and the removal efficiency of TN, [Formula: see text], COD, TP was 84.7 .97, 88.3 and 96% respectively. The removal efficiencies of TN, [Formula: see text], and TP at low C/N ratio and DO level were 84.2, 98.5 and 96.9% respectively which were approximately equal to the complete nitrification-denitrification with the addition of external carbon sources at a normal DO level of (1.5-2.5 mg/l).