Precursors Of Nitrosamines Disinfection Byproducts And Its Removal Characteristics In A Lake

N-nitrosamines, a kind of emerging disinfection by-products formed during the chloramination process in drinking water treatment, have drawn much attention for their potential high carcinogenicity. The formation, precursors, removal characteristics and mechanism of their precursors in a lake in East China were systematically studied.It is the first time that polarity rapid assessment method(PRAM) was applied in the fractionation of nitrosamine precursors in drinking water source. In comparison with classical resin method, this method could give more information on the charge distribution of precursors. Meanwhile, the PRAM method was more simple and reliable.The nitrosamine formation potential(NAFP) was relatively high(175-330ng/L) and varied seasonally, higher in autumn and winter but lower in spring in the lake. The majority of precursors shared the characteristic of small molecular weight(MW<0.5kDa), non/weak polar and positively charged. These fractions contributed to 62.3%, 60% and 63.4% to the sum of NAFP. Unlike the typical nitrosamine precursors(such as dimethylamine and ranitidine), the dominance of precursors presented weak acidity in this lake. This was evidenced by that the precursors in the lake were much easier absorbed by C18 with the decrease of pH since more precursors deprotonated and lost the positive charge in acidic atmosphere.The treatment characteristics and mechanisms of nitrosamine precursors were investigated with the jar and pilot test. The removal mechanisms were further studied by the characterization of precursors before / after each process, including coagulation,sedimentation, ozonation and biological activated carbon(BAC) process. The removal efficiencies of nitrosamine precursors in coagulation/sedimentation process was among 10-21.5%, which mainly were non/weak polar components. The removal efficiencies with dosage of 2mg/L ozone(O3) was 34.1%, oxidation preferred to remove non/weak polar components and had no selectivity on charged components. High dosage of Powdered Activated Carbon(PAC) could remove nearly 100% of the precursors, PAC worked for all components but was not efficient for polar ones. The biomass could remove about 80% of them, which primarily were polar and positively charged components. As various fractions contributed to nitrosamines formation and the fraction of precursors removed by each process were different, the combined processes should be adopted to dramaticlly remove the precursors and reduce the risk of their formation. The conventional and O3/BAC processes could removed 94% of nitrosamine precursors at the initial operational stage,the NAFP in the finished water was <20ng/L and it was <2ng/L with actual disinfectant dosage. After 1 year operation, the removal efficiency declined to 78%. The decreased amounts in BAC(from 89% to 69%), were attributed to the decrease of adsorption capacity of AC. Among the fractions of nitrosamine precursors removed by long operated BAC filter, 58.5% of them was biodegraded by microorganism, only 14.4% was removed by AC adsorption, the residual was removed either by biodegradation or by adsorption. In addition, backwashing could enhance the removal efficiencies from 62% to 80%.The biodegradation mechanism of nitrosamine precursors was further investigated. Addition of carbon source can promote their removal while the impacts of nitrogen source addition turn out to be small. As the pre-oxidation process before BAC could increase the ratio of biodegradable organic matters, it could favor the precursors removal by co-metabolism of biomass in BAC. Moreover, the microbial population was explored with molecular biology method, the result indicated that proteobacteria, bacillus and sphingol bacteria were supposed to be the main bacteria of degrading the nitrosamine precursors.