The Control Mechanisms Of Disinfection By-Products Produced By Chlorination Of Algogenic Organic Matter By Water Treatment Units

Controlling algae-derived disinfection by-products(DBPs)in drinking water is critical for ensuring the safety of the water supply.The most effective control measure for algae-derived DBPs is to regulate the water treatment process to reduce algogenic organic matter in water before disinfection,and understanding the role and mechanism of algogenic organic matter removal by water treatment processes is a prerequisite for the control of DBPs.To this end,algogenic organic matter of Microcystis aeruginosa,the most representative of cyanobacteria,was investigated in this thesis.The extracellular organic matter(EOM)and intracellular organic matter(IOM)of the algae were characterized using three-dimensional fluorescence excitation-emission matrix(EEM)spectroscopy,UV spectroscopy,Fourier transforms infrared(FT-IR)spectroscopy,organic carbon analysis,and molecular weight distribution.Based on the identification of the main precursor components of DBPs in algogenic organic matter that need to be focused on control,the removal of the precursor components in the water treatment processes(coagulation,ultrafiltration,coagulation-ultrafiltration)was investigated,and the control mechanism of DBPs by water treatment process was revealed.Meanwhile,in order to meet the demand for real-time monitoring of DBPs,a quantitative prediction model for DBPs was developed with the fluorescence regional integration(FRI)of the specialised EEM regions as the independent variable data.The following are the key research findings and conclusions:(1)Lipid,protein,amino acid,polysaccharide,humic acids-like substances,and fulvic acids-like substances were the main biochemical components of algogenic organic matter.The major components of IOM and EOM,based on dissolved organic carbon(DOC),were proteins(71.20±2.10%)and polysaccharides(53.02±2.12%),respectively.The corresponding surrogates(bovine serum protein,starch,fish oil,fulvic acid,and humic acid)of targeted biochemical components were selected for the chlorination experiments of DBPs formation potential(DBPFP).A prediction model for DBPFP of algogenic organic matter(4)=∑(4)×4)))was developed based on the percentage of DOC of each biochemical component in algogenic organic matter(4))and the DBPFP of the surrogates(4)),and the difference between predicted and experimentally detected values was within 15%.The protein-like and humic-like substances were eventually identified as the key biochemical components for the formation of algae-derived DBPs,with their cumulative contribution exceeding 80%,implying that the targeted removal of protein-like and humic-like substances before the chlorination process can be a feasible measure to effectively reduce the formation of DBPs.(2)The highest DOC removal rates for EOM and IOM were about52.0%and 31.2%after coagulation.25.1%and 27.1%of protein-like were found in the residual organic matter of IOM and EOM after coagulation.Trichloromethane(TCM),trichloroacetic acid(TCAA),and dichloroacetic acid(DCAA)remained the main DBPs species after coagulation.The variation of these DBPs was consistent with the changes trend of protein-like with coagulant dosage.In addition,it was found that TCM and TCAA yields from the residual EOM and IOM were significantly correlated with the FRI of the protein-like(r>0.8,p<0.05),indicating that the removal of protein-like was the key to the inhibition of the DBPs formation by coagulation.Meanwhile,it was shown that the combination of fluorescence regions of protein-like and humic-like((Ex/Em=(250～300nm)/(300～390 nm)and(250～400 nm)/(400～470 nm))could be used as an optical surrogate for DBPs.(3)The distribution characteristics of molecular weight,hydrophobicity,and biochemical components of the algogenic organic matter are directly linked to the formation of DBPs.Algogenic organic matter was divided into 12 groups with molecular weight>100 k Da,100～10 k Da,10～1 k Da,and<1 k Da for hydrophilic,transitional hydrophilic,and hydrophobic fractions.The results showed that the hydrophobic fraction of IOM with MW>100 k Da contributed 54%TCM,44%TCAA,and 44%DCAA for DBPs.The highest contribution to the TCM(21%)was in the hydrophobic fraction with molecular weights>100 k Da for EOM,the hydrophilic fraction with MW<1 k Da for EOM contributed 30%TCAA and 36%DCAA in DBPs.Ultrafiltration using 100 k Da membranes dramatically reduced IOM’s protein-like content and hindered the formation of TCM,TCAA,and DCAA,but had no significant effect on EOM.Compared with ultrafiltration or coagulation alone,coagulation-ultrafiltration increased the removal rate of EOM and IOM to 71.2%and98.1%respectively,with 87.3%protein-like and 44.1%humic-like substances removed from EOM,and 87.9%protein-like and 45.6%humic-like substances removed from IOM.Coagulation-ultrafiltration provided the best control of TCM,DCAA,and TCAA yields from IOM and EOM,with cumulative control effects exceeding 76%and 80%,respectively.(4)EEM of EOM and IOM have three positionally stable fluorescence signature regions:protein-like,fulvic acid-like,and humic acid-like regions.The FRI of these fluorescent regions correlates significantly with the DBPs generated.a model for predicting THE content of DBPs is established based on the FRI of three dimensional fluorescent as independent variables.Using the FRI of protein-like,fulvic acid and humic acid regions as the independent variable data set,the models were developed for predict the formation of DBPs.The coefficients of determination(R~2)than 0.91 for the models of TCAA,DCAA,and TCM,and the statistical test showed that the model was significant.The error between predicted value and measured value of actual sample is small.The This indicates that the FRI combined with multiple linear regression model developed by EEM could predict the formation of algal-derived DBPs.