Control Technology Research Of The Precursors Of Disinfection By-products In Algae-laden Water

Cyanobacteria bloom is the prominent problem caused by lake eutrophication.Algal organic matter（AOM）are the important precursor of disinfection by-products（DBPs）.Furthermore,due to the complexity and diversity of AOM’s composition,they are difficult to be removed by the traditional drinking water treatment process of coagulation-sedimentation-filtration.Considering the high amounts of algal cells and AOM in the source water from eutrophic lake,efficient physical and chemical technology was commonly applied to remove the target contaminants and ensure the safety of drinking water.In this study,typical DBPs precursors in algae-laden water were characterized and chlorinated DBPs formation rule were further investigated.In addition,coagulation treatment were combined with Mn（Ⅶ）and Mn（Ⅶ）/Fe（Ⅱ）per-oxidations to remove DBPs precursors（including algae cells and AOM）in algae-laden water.Finally,the new technology composed with pre-oxidation,conventional process,in-situ ozone-ceramic membrane（O₃-CMF）was constructed for drinking water advanced treatment.DBP precursors removal mechanism and DBPs variation characteristics were analyzed in this treatment process.The main research results are as follows:（1）The physicochemical characteristics of extracellular organic matter（EOM）and intracellular organic matter（IOM）of cyanobacteria were significantly different.Fluorescence excitation emission matrix spectroscopy showed that humic acid-like and soluble microbial byproduct-like（SMPs）were the major components in EOM,accounting for 56.06%and 25.38%respectively.However,aromatic protein（AP）and SMPs contributed the chief components of39.29%and 38.92%respectively in IOM.Higher humic acid-like compounds led to higher specific UV absorbance（SUVA）in EOM.In addition,size-exclusion chromatography with organic carbon as well as organic nitrogen detection（LC-OCD-OND）characterization showed that biopolymers and humics（HS）were the main DBP precursors in EOM,contributing 35.50%to the TOC contents.Building blocks was the main DBP precursors in IOM,accounting for41.13%in the TOC contents.Carbonaceous DBPs（C-DBPs）were the primary DBPs produced by EOM and IOM after chlorination.IOM had higher DBP formation potential（DBPFP）.The substances with molecular weight（MW）less than 1 k Da both in IOM and EOM had higher C-DBPs production.The nitrogenous disinfection byproduct formation potential（N-DBPFP）of IOM was 2.06 times than that of EOM,mainly due to the high proportion of AP in IOM.Correlation analyses revealed that the SUVA values of EOM and IOM were significantly correlated with trihalomethane formation potentials（THMFPs）,but showed no obvious correlation with C-DBP formation optional（C-DBPFP）.Dissolved organic nitrogen（DON）in EOM had significant correlation with N-DBPFP.DON（sum of biopolymers and HS）in EOM and IOM were well correlated with N-DBPFPs.In general,C-DBP formation was majorly related to the properties of total organic carbon（TOC）,while N-DBP formation was more affected by DON contents.（2）Pre-oxidation with potassium permanganate（KMnO₄）was applied to enhance the coagulation（by poly aluminum chloride（PAC））effect,and further improve the removal effect of algal cells and AOM in algae-laden water.The removal effect of TOC and C-DBPs were39.91%and 42.01%respectively,under condition of KMnO₄ dosage 1.0 mg·L^-1.However,excessive addition of KMnO₄ could damage algal cells and lead to the release of IOM,which increased the risk of N-DBPs formation during chlorination of water.When KMnO₄ dosage was increased to 2.0 mg·L^-1,haloacetonitriles（HANs）content in the chlorinated effluent was increased by 211.64%.Hence,reductant such as Fe（Ⅱ）was used to alleviate the oxidation of algae cells by KMnO₄.A combined pre-oxidation process with KMnO₄,ferrisulphas（Fe SO₄）and PAC was established to control DBPs precursors in algae-laden water.Under the optimized condition of KMnO₄ dosage 1.0 mg·L^-1,Fe SO₄ dosage 2.7 mg·L^-1 and PAC dosage 12.0 mg·L^-1,the maximum removal rates of C-DBPs and N-DBPs were 46.40%and 30.14%,respectively.（3）A long process of drinking water treatment combined pre-oxidation,coagulating,sedimentation,sand filtering and in-situ O₃-CMF was established for further reducing the precursors of DBPs in algae-laden water.O₃-CMF showed excellent performance on TOC,DON,SUVA and AP removal in drinking water.Moreover,the removal rate of C-DBP precursors was increased from 33.05%to 64.66%by O₃-CMF,and the removal rate of N-DBP precursors increased from 33.47%to 56.63%.Besides the physical retention effect by CMF,ozonation could also mineralize TOC in the water,which contributed to the effective removal of DBPs precursors.However,it is worth noting that ozonation would bring about the increase of low molecular organic matter（DBP precursors）,which was difficult to be retained by CMF.Hence,specific DBPs such as haloketones and trihalonitromethane increased by 169.57%and84.36%in the chlorinated O₃-CMF effluent.