Study On The Formation And Transformation Mechanisms And Control Strategies Of Algae-derived Organic Chloramines In Drinking Water Treatment Processes

The recurrent incidence of algal blooms resulting from water eutrophication poses a substantial peril to the assurance of potable water safety.Upon ingress into water treatment facilities,algae profoundly impede their regular operations,thereby engendering predicaments such as contamination of sedimentation tanks and obstruction of filtration beds.Consequently,the effluent quality and production efficiency of these facilities are adversely impacted.Regarding the issue of treating algal-containing water,numerous water treatment technologies have been developed by researchers for the purpose of algal cell removal,including pre-oxidation and advanced treatment methods.Nevertheless,within the treatment process,the algal cells are susceptible to degradation through techniques such as pre-oxidation,consequently liberating algal organic matters（AOM）residing within the cells.The AOM exhibit a broad distribution of molecular weights and possess high hydrophilicity,thereby rendering them resistant to removal via conventional water treatment procedures encompassing coagulation,sedimentation,and filtration.In the disinfection,AOM are capable of undergoing reactions with chlorine-based disinfectants,giving rise to disinfection by-products（DBPs）that exhibit teratogenic,carcinogenic,and mutagenic propensities.The intricate nature of AOM renders the mechanistic elucidation of disinfection by-product formation arduous,while the control and regulation of disinfection by-product generation lack a firm theoretical foundation.During the disinfection,a critical class of intermediate compounds called algal-derived organic chloramines emerges as a result of the transformation of AOM.The study centers on algal-derived organic chloramines as the focal point and elucidates the generation and transformation mechanisms,ultimately focusing on the strategies for regulating these compounds.Considering the practical conditions encountered in water treatment plants,a multi-tiered barrier process is proposed to efficiently regulate algal-derived organic chloramines and subsequent disinfection by-products.This contribution provides theoretical guidance and technical support for the treatment of algal-contaminated water and the control of disinfection by-products.The main precursors of algae-derived organic chloramines were identified.By comparing the organic chloramines formation potential of two common organic substances in water,natural organic matter（NOM）and AOM,the more significant precursor was determined to be AOM.The typical precursors of organic chloramines were found to be hydrophilic and unsaturated by AOM fractionations and Fourier transform ion resonance cyclotron mass spectrometry（FT ICR-MS）data.The FT ICR-MS data and Spearman correlation analysis,combined with the improved Van Krevelen’s diagram region delineation,accurately identified that the amino acids,peptides and proteins were the main precursors of algal-derived organic chloramines.The formation and transformation mechanisms of algae-derived organic chloramines were studied.The formation mechanisms of organic chloramines were clarified through experiments and density function theory computations,and the reaction site and sequence were also determined.It was identified that temperature and copper ions catalysis could be two important factors that could affect the decomposition rate of organic chloramines.The transformation pathways of organic mono-and di-chloramines were determined by high performance liquid chromatography tandem quadrupole-electrostatic field orbital trap high resolution mass spectrometry,and it was found that the decomposition pathways and products of organic mono-and di-chloramines were significantly different.The decomposition of organic monochloramines resulted in the formation of ammonia nitrogen,while the decomposition of organic dichloramines led to the formation of the nitriles.This provided a solid foundation for controlling the transformation from organic chloramines to DBPs.The control strategies for organic chloramines formation were proposed based on pre-oxidation and nanofiltration.The performances of KMn O₄ pre-oxidation and preozonation on algae-containing water treatment were tested,and KMn O₄ pre-oxidation proved to be a better option.A covalent organic framework nanofiltration membrane was fabricated.The membrane could enhance the removal rate of organic chloramines and the water flux.KMn O₄ pre-oxidation and COF membrane nanofiltration were combined to treat algae-containing water,which showed good performance.Compared to preozonation coupled nanofiltration,KMn O₄ pre-oxidation coupled COF membrane nanofiltration could decrease the organic chloramines formation potential by 91.7%,and increase the water flux by 176.5%,which could reach 28.2 L m^-2 h^-1 bar^-1.A two-step chlorination strategy was proposed based on the transformation mechanisms of organic chloramines.This strategy,under the precondition of maintaining the disinfectant dosage unchanged,enables targeted interception of the conversion of organic chloramines into typical nitrogen-containing disinfection by-products.It was verified that the two-step chlorination strategy could significantly reduce the formation of organic chloramines by up to 47.5%in the algal-containing water treatment process under experimental conditions.The DBP formation was determined by gas chromatograph,which identified that two-step chlorination could block the transformation of organic chloramines to haloacetonitriles（HANs）.The two-step chlorination decreased the HANs formation potential by 92.0%.The sterilization experiments identified that the two-step chlorination could also enhance the disinfection effect.Finally,the strategies proposed in this study were proficiently amalgamated.The multi-stage barrier composed of KMn O₄ preoxidation-first chlorination-coagulation-sedimentation-microfiltration-nanofiltration-second chlorination（disinfection）was used for algae-containing water treatment.The results showed that the strategy could effectively control algae-derived organic chloramines and reduce the DBP formation in algae-containing water treatment.