Optimal operational factors for nitrite accumulation in batch reactors

The environmental factors that affected the accumulation of nitrite in nitrifying reactors were investigated using a mixed culture. A batch reactor with 50 mg-N/l of ammonia was used. The pH, temperature and dissolved oxygen concentration were varied. The concentration of unionized free ammonia also changed with the oxidation of ammonia and the variation of pH and temperature. The accumulation of nitrite was affected sensitively by pH and temperature. A higher nitrite concentration was observed at pH 8-9 or temperature around 30 degrees C. The dissolved oxygen also affected, giving the highest nitrite accumulation at around 1.5 mg/l. These were the favored conditions for nitrite production. The free ammonia concentration influenced the nitrite accumulation also, by inhibiting nitrite oxidation. The inhibition became apparent at a concentration of approximately 4 mg/l or above, but insignificant at below 1 mg/l. Thus, simultaneously high free ammonia concentration and maximum specific ammonia-oxidation rate (above 15 x 10(-3) mg-N/mg-VSS x h) were needed for a significant nitrite accumulation. When the two conditions were met, then the highest accumulation was observed when the ratio of the maximum specific oxidation rate of ammonia to the maximum specific oxidation rate of nitrite (ka/kn) was highest. Under the optimal operating conditions of pH 8, 30 degrees C and 1.5 mg/l of dissolved oxygen, as much as 77% of the removed ammonia accumulated in nitrite.