Formation Of Disinfection By-products From Soluble Microbial Products In Wastewater Treatment

Due to the worldwide shortage of freshwater resources,wastewater reuse has become a promising way to conserve the limited freshwater resources.To inactivate the pathogenic microorganisms,disinfection must be conducted for wastewater effluents before their reuse.However,the oxidative disinfectants can also react with organic matter and some inorganic ions to form disinfection by-products(DBPs),which were associated with the teratogenic,carcinogenic and mutagenic risks.For the effluents of biological wastewater treatment plants,soluble microbial products(SMPs),which consist of macromolecules and cellular debris including proteins,polysaccharides,humic acids,and DNA,are usually the major components of soluble organic matter.SMPs could react with disinfectant and increase the DBPs formation in wastewater effluent,and thus increasing the ecotoxicity of reuse water and becoming an important constraint object in wastewater reuse.Therefore,based on the laboratory biological treatment of synthetic wastewater,this thesis investigated in-deep the characterization of SMPs,the DBPs species,the mutagenicity before and after chlorination,and speculated the formation pathways of several DBPs.The main conclusions are as follows:(1)The formation potential of disinfection by-products(DBPs),including trihalomethanes(THMs),haloacetic acids(HAAs),chlorinated solvents(CSs),haloketones(HKs),haloacetonitriles(HANs)and TCNM(chloropicrin),from soluble microbial products(SMPs)was investigated in this thesis.The formation of DBPs under different disinfection methods,including chlorination,chloramination and ozonation was examined,and the effects of temperature,pH,disinfectant dose,reaction time and bromide level on the formation of DBPs were also evaluated.The results showed that more carbonaceous DBPs(C-DBPs,including THMs,CSs,HKs and HAAs)and nitrogenous DBPs(N-DBPs,including HANs and TCNM)were formed in chlorination than chloramination.More dichloroacetic(DCAA),HANs and TCNM were generated by SMPs than natural organic matter(NOM).In addition,disinfection factors significantly affected the formation of DBPs,Higher temperature increased the formation potential of DBPs(except HAAs and CSs)during both chlorination and chloramination.During chlorination,an increase in pH led to higher formation potential of THMs,CSs and TCNM and lower formation potential of HAAs,HKs and HANs.During chloramination,DBPFP significantly decreased as pH increased.Except HKs and HANs in chlorination,higher disinfectant dose and longer reaction time increased DBPFP in chlorination and chloramination.Bromine-containing DBPs(Br-DBPs)formation potential increased with bromide level during chlorination,chloramination,and ozonation.Chloramination may be a good alternative to chlorination in reducing the formation of Br-DBPs from SMPs.Bromide level and pH were the key factors affecting the formation of DBPs in chlorination,chloramination and ozonation.(2)N-nitrosamines formation potential(NAsFP)during chlorination,chloramination and ozonation of SMPs were investigated in this thesis.The results showed that more NAs were formed in chloramination than chlorination and ozonation.Specially,the formation of NAs showed a good linear relationship with disinfectant dosage and bromide level,and increased with reaction time,but decreased at high temperature during chlorination,chloramination and ozonation.The effects of pH on the NAsFP were different for different disinfection methods.The pH values showed negative effects on the NAsFP in chlorination and positive effects on the NAsFP in ozonation.Yet in chloramination study,the NAsFP showed a first increasing and then a decreasing trend.Bromide level was the most important factor for NAs formation whether for chlorination,chloramination or ozonation.NDMA was the most NAs accounting for more than 50%.As for reducing NAs in the effluent,ozonation was the best alternative to chlorination and chloramination.(3)In this thesis,SMPs were prepared under different stressful conditions,including high ammonia content(HA),high salinity(HS),high levels of heavy metals(HM)and high temperature(HT),as well as a normal state(NS).The molecular weight(MW)distribution of SMPs was characterized using gel permeation chromatography(GPC).DBPs formation was investiged under different conditons.The mutagenicity of SMPs before and after chlorination was also evaluated using the SOS/umu test.The results showed that compared with NS,stressful conditions were induced to produce more low MW components of SMPs.Stressful conditions,had no significant effect on the species of DBPs,but played a crucial role in the levels of DBPs produced in SMPs solutions during chlorination.Among the stressful conditions tested,HT and HS resulted in higher levels of both C-DBPs(THMs,HAAs and HKs)and N-DBPs(HANs and TCNM).Levels of C-DBPs and N-DBPs were lower under HM condition.HA stimulated the production of N-DBPs,but had no impact on the levels of C-DBPs.The mutagenicity of SMPs solutions was higher under the stressful conditions than that under NS condition in both before and after chlorination.For each SMPs solution,the mutagenicity of SMPs increased after the chlorination,except for SMPs solution under HM and NS conditions.(4)Nucleobases are the basic unit of nucleic acids,and nucleic acid is an important component of SMPs.In this thesis,the formation and the mutagenicity of DBPs were investigated after chlorination of nucleobases including three pyrimidines(cytosine,uracil and thymine)and two purines(guanine and adenine).The results showed that generally pyrimidines generated higher levels of DBPs than purines.Uracil generated the highest level of DBPs,while cytosine produced the highest level of N-DBPs.The mutagenicity of chlorinated pyrimidines was higher than that of purines,and the mutagenicity of chlorinated uracil and cytosine was higher than that of glycine(the most common amino acid in wastewater).In addition,the formation pathways of TCNM,dichloroacetonitrile(DCAN),trichloroacetonitrile(TCAN),DCAA and trichloroacetic acid(TCAA)from cytosine during chlorination were tentatively proposed.The molecular formulas of four new intermediates generated during chlorination of cytosine were identified using an electrospray ionization time-of-flight mass spectrometer,and the addition site was calculated by the frontier electron density(FED).