Acetate injection into anaerobic settled sludge for biological P-removal in an intermittently aerated reactor

Injecting acetate into the sludge layer during the settling and decanting p eriods was adopted to enhance phosphorus release inside the sludge layer du ring those periods and phosphorus uptake during the subsequent aeration per iod in a KIST Intermittently Decanted Extended Aeration (KIDEA) process. Th e relationship among nitrification, denitrification and phosphorus removal was investigated in detail and analyzed with a qualitative floc model. Depe ndencies of nitrification on the maximum DO level during the aerobic phase and phosphorus release on residual nitrate concentration during the settlin g phase were significant. High degree of nitrification resulted that phosph orus release inside the sludge layer was significantly interfered with nitr ate due to the limitation of available acetate and the carbon sources from influent. Such limitation was related to the primary utilization of organic substance for denitrification in the outer layer of the floc and the retar ded mass transfer into the inner layer of the floc. Nevertheless, effects o f acetate injection on both denitrification and phosphorus release during t he settling phase were significant. Denitrification rate after acetate inje ction was two times as high as that before acetate injection, and phosphoru s release reached about 14 mg PO43--P/g MLVSS/hr during the decanting phase after the termination of denitrification inside the sludge layer. Extremel y low level of maximum DO (around 0.5 mg/L) during the aerobic phase may in hibited nitrification, considerably, and thus nearly no nitrate was present . However, the absence of nitrate increased when the phosphorus release rat e was reached up to 33 mg PO43-P/g MLVSS/hr during the settling and decanti ng phase, and nearly all phosphorus was taken up during subsequent aerobic phase. Since the sludge layer could function as a blocking layer, phosphoru s concentrations in the supernatant was not influenced by the released phos phorus inside the sludge layer during the settling and decanting period. Ph osphorus removal was directly (for uptake) and indirectly (for release) dep endent on the median and maximum DO concentration during the aerobic phase, and those optimal values may exist within the range from 0.2 to 0.6 mg/L a nd 0.4 to 1.2 mg/L, respectively.