The aim of the present study was to investigate the effect of pre-treatment with humic acids on the activity of nitrifying bacteria (ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB)) in activated sludge processes. Varied concentration of HA (0

80

160

240 mg/L) were applied to pretreat (24 h) three different sludges

including complete nitrification activated sludge with low/high activity (Sc1/Sc2)

and the partial nitrification activated sludge (Sp). The results showed that the nitrate accumulation rate (RNA) of Sc1 increased significantly from 10.73 % to 53.93% after pretreatment with 80 mg/L of HA (50 mg/gMLVSS)

simultaneously the ammonia oxidation rates (RAO) changed insignificantly. In terms of Sc2

neither the RAO nor the RNA changed under each tested condition. For Sp

no NO3--N accumulation was observed after the pretrement

which demonstrated that the HA pretreatment did not destruct its partial nitrification performance. In conclusion

HA pretreatment improved the activity of NOB which with low overall nitrification activity

while exhibited insignificant improvement on AOB activity.