Nitrogen removal via nitrite pathway was investigated in a lab-scale modified University of Cape Town（MUCT)process treating real domestic wastewater with low C/N ratio.The experimental results of 121 d showed that nitritation was achieved at hydraulic retention time（HRT)of 8 h

dissolved oxygen（DO)of 0.3—0.5 mg·L-1

sludge recycle ratio of 80%

anoxic recycle ratio of 120% and internal recycle ratio of 300% after startup period of 87 d.Nitritation was stabilized for 35 d with an average nitrite accumulation rate（NAR)of 62%

the highest NAR up to 80%

average ammonia removal efficiency of 65% and the highest of 87%.Mechanisms of nitritation indicated that the low DO concentrations and short HRT were really important to nitritation

whereas pH

free ammonia（FA)and free nitrous acid（FNA)had no effect on nitritation.And temperature and sludge retention time（SRT)possibly had a slightly positive effect on nitritation.The fluorescence in situ hybridization（FISH)assays demonstrated that the percentage of ammonia oxidizing bacteria（AOB)in biomass increased when the performance of MUCT reactor was converted from complete nitrification to nitritation

and the highest AOB percentage was up to 9.3%.Meanwhile

the percentage of nitrite oxidizing bacteria（NOB)decreased

and NOB were phylogenetically related to Nitrospira.Nitrogen removal via nitrite pathway saves the aeration costs and external carbon sources as compared to nitrate pathway.For the treatment of domestic wastewater

nitritation and denitritation are highly beneficial because the organic carbon source in it is typically limited.