Effects of pH on nitrite accumulation during wastewater denitrification

The characteristics of nitrite accumulation during denitrification in mixed liquors with different pH values were studied. The mixed liquors were prepared by adding acclimated denitrifying activated sludge to synthetic wastewater with methanol (CH3OH) as the carbon source (chemical oxygen demand = 600 mg/L) and sodium nitrate (NaNO3) as the nitrogen source (NO3(-)-N = 50 mg/L). The results showed that the pH during denitrification could be kept constant using buffer solutions composed of KH2PO4, Na2HPO4 x 12H2O and Na2B4O7 x 10H2O. Nitrite accumulation was more serious at low pH than at high pH, regardless of whether the pH of the mixed liquor was stabilized during denitrification. The specific rates of nitrate and nitrite reduction were both inhibited by increased pH. The specific rate of nitrate reduction was more vulnerable than that of nitrite reduction to higher pH. Nitrite accumulation during these experiments resulted from different reduction rates for nitrate and nitrite. Nitrite reduction rates were inhibited by the presence of nitrate, which stimulated nitrite accumulation during denitrification.