In the pre-denitrification of anoxic-aerobic reaction process

nitrite accumulation and sludge performance at different phenol concentrations were investigated.The results showed that there were two obvious nitrite accumulations finally stabilized at 70%±5% when phenol varied from 0 to 90 mg·L-1.At low phenol concentration (0 to 30 mg·L-1)

nitrite accumulation and recovery were caused by microbial structure varieties and secreting extracellular polymer.At high phenol concentration (60—90 mg·L-1)

nitrite accumulation was caused by microbial population changes of ammonia-oxidizing bacteria and nitrite-oxidizing bacteria was caused by phenol inhibition.Ammonia oxidation rates and nitrogen oxide production rates varied from 10.85 mg N·(g MLSS)-1·h-1 and 10.12 mg N·(g MLSS)-1·h-1 to 2.79 mg N·(g MLSS)-1·h-1 and 2.32 mg N·(g MLSS)-1·h-1

respectively.Nitrite accumulation rate had a negative correlation with nitrogen oxide production rate

whereas a positive correlation to phenol concentration.Fluorescence in situ hybridization showed that nitrifying bacteria communities changed due to phenol inhibition

with AOB relative quantity increasing from 2.80% to 9.30%.As carbon source

phenol caused sludge concentration increase from 2500 mg·L-1 up to 5870 mg·L-1.When total extracellular polymeric substances declined from 67.20 mg·(g VSS)-1 to 32.10 mg·(g VSS)-1

sludge volume index (SVI)accordingly changed from 165 ml·g-1 to 50 ml·g-1.Nitrite accumulation

filamentous bacteria and extracellular polymer contributed to SVI changes in the activated sludge system

and nitrite accumulation was the main reason followed by filamentous bacteria.