Nitrite Accumulation in Activated Sludge Denitrification: Effects of Dissolved Oxygen, Substrate Limitation and pH

Nitrite accumulation in activated sludge denitrification was investigated with various factors; DO, substrate limitation and pH. When DO levels in mixed liquor were changed from 0.1mg/L to 2.0mg/L, nitrite was not accumulated during activated sludge denitrification showing that oxygen repressed both nitrate and nitrite reductases evenly. While carbon substrate (acetate) was reduced to the C:N ratio of 1.1:1, nitrite was accumulated during acetate-limited denitrification. This can be explained by nitrate respiration outcompeting nitrite respiration for limited acetate electrons. In contrast to neutral pH(=7) condition, alkalinity pH(=8) condition demonstrated considerable nitrite accumulation during denitrification, even though the specific denitrification rates in both pH conditions were similar.