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Inhibition of denitrification in activated sludge by nitrite

Nitrite inhibition of denitrification was investigated using activated sludge that had been adapted to a wastewater containing moderately high levels of nitrate (1 350 mg/L as N) and salinity (40 000 mg/L total dissolved solids). After adaptation, the activated sludge was used to study pH effects on nitrite inhibition of denitrification over a range of 15 to 2 100 mg/L nitrite nitrogen (N02–N). A pH range of 6 to 8 was used. Using a zero‐order model, it was found that the denitrification rate for 250 mg/L N02–N increased from 0.001 to 0.02 mg N/mg mixed liquor suspended solids (MLSS)·h as pH increased from 6 to 8. Denitrification rate reduction at the lower pH was probably caused by the accumulation of the species nitrous acid (HNO2), and the threshold inhibitory concentration for HNO2 was estimated to be low: less than 0.02 mg/L HNO2–N at pH 6. Mixed liquor suspended solids concentration appeared to have no effect on nitrite inhibition of denitrification.