Effect of High Oxygen Concentrations on Nitrification and Performance of High-Purity Oxygen A/O Biofilm Process

A respirometric method has shown that the dissolved oxygen (DO) concentration has an effect on the growth rate of nitrifying micro-organisms. Oxygen uptake rates were normal at DO concentrations of 5 to 15 mg/L, but at 15 mg/L and greater, oxygen uptake was reduced, and these DO levels may be toxic to the nitrifying micro-organisms. The anoxic/aerobic (A/O)-high purity oxygen submerged biofilm process was tested using synthetic wastewater influent. Four different internal recycle ratios (R), that is, R = 0.5, 1.0, 1.5, and 2.0, were tested at 0.64 kg COD/m³-day and 0.14 kg NH₄ ⁺-N/m³-day for the aerobic reactor. When R was increased, the removal efficiency of COD and total Kjeldhal nitrogen (TKN) also increased to greater than 90%. The efficiency of total nitrogen (TN) removal, however, was not proportional to the R. DO concentrations in the aerobic reactor were set at 5 and 10 mg/L to evaluate the effect of DO concentrations on the nitrification reaction. A COD removal efficiency of greaer than 90% was always achieved regardless of the DO concentration when the process aerobic reactor was operated in the range of 1.12 kg COD/m³-day and 0.21 kg NH ₄⁺-N/m³-day. The nitrification rate, however, changed significantly according to the DO concentration.