Disinfection byproduct (DBP) precursors in wastewater during the reversed anaerobicanoxic-oxic (A2/O) process,as well as their molecular weight (MW) and polarity-based fractions,were characterized with UV scanning,fluorescence excitation emission matrix,Fourier transform infrared and nuclear magnetic resonance spectroscopy.Their DBP formation potentials (DBPFPs) after chlorination were further tested.Results indicated that the reversed A2/O process could not only effectively remove the dissolved organic carbon (DOC) and dissolved total nitrogen in the wastewater,but also affect the MW distribution and hydrophilic-hydrophobic properties of dissolved organic matter (DOM).The accumulation of low MW and hydrophobic (HPO) DOM was possibly due to the formation of soluble microbial product-like (SMP-like) matters in the reversed A2/O treatment,especially in the anoxic and aerobic processes.Moreover,DOM in the wastewater displayed a high carbonaceous disinfection byproduct formation potential (C-DBPFP) in the fractions of MW ＞ 100 kDa and MW ＜ 5 kDa,and revealed an increasing tendency of nitrogenous disinfection byproduct formation potential (N-DBPFP) with decrease of MW.For polarity-based fractions,the HPO fraction of wastewater showed significantly higher C-DBPFP and N-DBPFP than hydrophilic and transphilic fractions.Therefore,although the reversed A2/O process could remove most DBP precursors by DOG reduction,it led to the enhancement of DBPFP with the formation and accumulation of low MW and HPO DOM.In addition,strong correlations between C-DBPFPs and SUVA,and between N-DBPFPs and DON/DOC,were observed in the wastewater,which might be helpful for DBPFP prediction in wastewater and reclaimed water chlorination.