Study On Formation And Influencing Factors Of Disinfection By-products In Chlorine Contained Disinfection Process Of Drinking Water

Focussing on the situation of disinfection process of drinking water in small towns and recent development trend of this field, optimized combination of different disinfection processes for source water contaminated with different degrees was studied in this paper. These disinfection processes include three types of disinfectants that are chlorine dioxide, monochloramine and chlorine. Formation and influencing factors of disinfection by-products （DBPs） were studied by using modern analysis technologies. Health risk of correlative DBPs and disinfection process was evaluated, and optimized disinfection processes for water samples with various qualities were presented. Under the support of Key Project from Ministry of Science and Technology of China, the purpose of this research is to provide academic suggestion and technical guidance for drinking water works with various water qualities.The content and main results of this paper include the following eight parts:（1） The relationship of disinfectant dosages between various disinfection processes and contaminated degrees of water samples was investigated. The results showed that TOC, UV₂₅₄ and A₄₁₀ of humic acid can be used to denote concentrations of humic acid because of their good correlation. Formation of DBPs remarkably reduced when substituting ClO₂ for Cl₂ as disinfectant. In this study, seven disinfection processes, i.e. ClO₂, ClO₂+NH₂Cl, 90%ClO₂+NH₂Cl, Cl₂+NH₄Cl, ClO₂+Cl₂, 90%ClO₂+Cl₂ and Cl₂, were applied to investigate the disinfection processes for slight-polluted, low-polluted, medium-polluted and serious-polluted water samples simulated with humic acid solutions. Indicator microorganisms in water samples were inactivated by determined CT values of various disinfection processes, and the disinfected water can meet the need of national drinking water sanitation standard of China. Disinfectant concentration in water samples decreased according to second-order kinetic model after disinfectants were added to water samples for certain time.（2） Formation and influencing factors of DBPs using various disinfection processes were studied. In this part, a simple and rapid method for determination of five haloacetic acids （HAAs） in drinking water was developed using gas chromatography（GC） coupled with short capillary column and electron capture dectetor（ECD）. TCM, TCAA and DCAA were primary DBPs detected from disinfected water samples. Preferential order of various disinfection processes for DBPs formation control was determined. Bromide in water remarkably affected the speciation and quantity of trihalomethanes （THMs） and HAAs. Active carbon filtration can reduce the formation of DBPs efficiently.（3） By using gas chromatography and mass spectrometry method, it was investigated that semivolatile organic compounds （SVOCs） transformation in humic acid simulation water samples disinfected by various processes. It was found that some SVOCs disappeared, some SVOCs decreased, and others increased. Some new SVOCs produced when water sample was disinfected by various processes. Primary SVOCs in drinking water were determined in disinfected humic acid simulation water samples too. It was demonstrated that humic acid water solutions can be used to simulate source water in disinfection research processes.（4） It was studied that variation of water quality parameters of humic acid simulation water samples disinfected with different processes. The results showed that molecular weight of humic acid reduced 71.5%-92.0% and fluorescence intensity decreased 0.7% -27.6%, Substituent effect and weighty atom effect were the possible dominating organic structure factors that affected fluorescence intensity of humic acid water sample.（5） DBPs formation and influencing factor of humic acid in different molecular weight distribution were investigated. The results showed that the humic acid in the molecular weight range between 50-100ku was the most important precursor of THMs and HAAs, and the precursors of THMs and HAAs were not the same substances. Variation of UV₂₅₄, A₄₁₀ and A₄₆₅/A₆₅₆ in different molecular weight range indirectly reflected the reactive variation of humic acid in corresponding molecular weight range. （6） Research of aerobic degradation of HAAs indicated that HAAs spontaneously degradated according to first order kinetic model and HAAs degradation rate affected by temperature obviously. HAAs degradation rate order is: DBAA＞DCAA＞MBAA＞TCAA. The possible degradation mechanism of HAAs is hydrolysis-oxidation at 7-35℃and chemical degradation at 45℃respectively.（7） Study of reduction of HAAs by Zn⁰ and Fe⁰ indicated that HAAs was reduced by first-order kinetic rate under anoxic conditions. However, DBAA was reduced according to second-order and first-order kinetic rate by Zn⁰ and Fe⁰ respectively in aerobic condition. Zn⁰ reduced HAAs more rapidly than Fe⁰ did. Degradation rate of DBAA was significantly affected by metal dosage, pH value, coexistence of MBAA with high concentration and dissolved oxygen. MBAA and TCAA significantly reduced by Zn⁰ too, but no obvious reduction of DCAA by Zn⁰ was observed. Reduction of DBAA by Zn⁰ and Fe⁰ was controlled by a mixture of chemical reaction and mass transfer, but Reduction of MBAA by Zn⁰ was controlled only by chemical reaction. The reductive reaction proceeded via sequential hydrogenolysis. Water molecules participated in the reductive reaction by providing protons.（8） Health risk of drinking water disinfection was assessed by using multi-pathway risk assessment method. It was found that TCM and DCAA are the primary source of lifetime cancer risk and noncarcinogenic hazard risk in disinfected humic acid simulation water samples. TCM has higher cancer risk and noncarcinogenic hazard risk than DCAA does. Ingestion route is the main route of cancer risk and noncarcinogenic hazard risk. In the following disinfection process order, cancer risk and noncarcinogenic hazard risk ranged from high to low: Cl₂＞90% ClO₂+Cl₂＞ClO₂+Cl₂＞Cl₂+NH₄Cl＞90% ClO₂+NH₂Cl＞ClO₂+NH₂Cl＞ClO₂. Bromide in water changed the primary DBPs speciation which bring cancer risk and noncarcinogenic hazard risk. Active carbon filtration decreased the health risk of drinking water significantly.