A biofilm sequencing batch pilot reactor was operated 400 days in order to determine the effects of different operation strategies on the capacity of the biofilm to remove nutrients (C, N, and P). The reactor was fed with wastewater from the main campus of the National University of Mexico. The wastewater was enriched with a molasses and phosphate solution when needed to vary the nutrients concentration. The operation was controlled with an industrial programmable timer. The treatment cycles were adjusted with four stages: filling, anaerobic phase, aerobic phase and draw of treated wastewater. Cycles of 8 and 12 were tested with different anaerobic/aerobic time ratios. The highest removal COD and PO4-P rates were obtained with 12-hour cycles and phases duration of 37/63 percent anaerobic/aerobic. Analyzing COD, TOC, PO4-P, NH4-N, and NO3-N, shifting of the different bacterial groups could be followed. When the organic loading rate was higher than 5 gCOD/m2·d the activity of the phosphate accumulating bacteria and nitrification could not be observed. The best results regarding phosphate removal and nitrification were obtained when the mean organic load was 3 gCOD/m2·d.

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