Research On Emerging Contaminants Removal From Drinking Water By UV-MicroO₃ Process

Emerging contaminants have posed a major threat to drinking water safety. A variety of Disinfection by-products and Endocrine Disrupting Chemicals have been detected at a range of concentrations through water bodies in China. It was known that many of these compounds are recognized as carcinogenicity, teratogenicity, and mutagenicity contaminants, which have been accumulating and enriching in natural environment, imposing negative even harmful impact on health of human as well as other species. What make it worse is that these organic matters can hardly degraded by conventional treatment technologies. Recently, the come up of a new type of organic pollutions called Emergency contaminants dramatically raises the demand on research about organic contaminants removal technologies and mechanisms. Among these technologies, UV/O3 process has been wildly researched and applied because this technology has fast reaction rate, mild reaction condition and high mineralization ability. However, the advantages of utilizing UV/O3 treatment process are accompanied by the oxidation of bromide to bromate, which is classified as a potential human carcinogen by the International Agency for the Research on Cancer in 1990. The bromate problem has been wildly concerned and the application of UV/O3 process is hence seriously hindered.In this research, a new technology called UV-microO3 process was introduced for Emergency contaminants removal. In this technology, ozone is generated not by a typical ozone generator but produced during the photolysis of oxygen in air inside the quartz sleeve that encases the low-pressure mercury lamp (emitting at 254 nm and 185nm), and the generated ozone is thus induced in the UV reactor in which the water is additionally irradiated by UV irradiation at 254 nm. The development of multiple-tube pilot-plant was obtained base on single-tube photoreactor, and optimize of ozone generation in UV-microO3 process were achieved. The contribution of hydroxyl radical (·OH) on organic matters was calculated using kinetic model. The degradation mechanisms of two Emerging Contaminants within UV-microO3 process were proposed after analyzing by-products which were detected by GC-MS. Furthermore, the comparison of organic pollutions removal and bromate control within UV/O3 and UV-microO3 was drawn.The invention of multiple-tube photoreactor and the optimization of ozone generation in UV-microO3 process:it was shown that the spatial arrangement mode of UV lamp could be determined by model calculation to achieve the best distribution level of UV irradiation intensity. And the generation of ozone could be optimized by adjusting the flow rate, humidity and pressure of inlet air and the distance between the UV lamps. The ozone concentration increased at first followed by a decline as the air flow rate increased within the range of experiments. For obtaining a high concentration of ozone generated by the UV irradiation, the humidity and pressure of inlet air should be reduced as much as possible, and the UV lamps should be placed to keep the distance beyond a certain level to avoid the photolysis of ozone. Ozone at concentration of 0.79 mg L-1 was generated in the pilot-plant when the pressure, dew point and rate of inlet air flow were 14.0 kPa,-24℃ and 3.0 L min-1, respectively. It should be noted that the ozone at relatively low concentration in UV-microO3 process can not affect the UV irradiation intensity remarkably even though ozone was proved to could absorb UV irradiation at 254nm.The generation of ·OH and its contribution to the removal of a variety organic contaminants:the results showed that there was ·OH generated and contributing to organic matter removal in UV-microO3 process, which hence could be recognized as an Advanced Oxidation Process. UV-microO3 process was shown to be more efficient on aniline, chlorobenzene and MTBE removal than UV/air process, because the greater ability of ·OH generation in UV-mciroO3 process:the concentrations of ·OH in UV-air and UV-microO3 process were 1.2×10-14mol L-1å'Œ2.95×10-14mol L-1. Direct photodecomposition by UV irradiation and oxidation by ·OH contributed to the degradation of organic pollutants. The contribution rates of ·OH in a variety of organic compounds degradation by UV-air and UV-microO3 process were different depend on the characters of the organic compounds. The introduction of air could enhance the organic contaminants degradation ability of UV irradiation, however photodecomposition is still the main way for degradation of organic matter in UV-air process, while organic compounds were degraded mainly by ·OH in UV-microO3 process.The factors and mechanism of Estradiol (E2) and 17a-ethinylestradiol (EE2) degradation by UV-microO3 process:UV-microO3 process was shown to be efficient on estrogen (E2 and EE2) and estrogenic activity removal. Humic acid and common ions in water impose complex influence on UV-microO3 process, via absorbing UV irradiation or accelerating or inhibiting ·OH reaction. The intermediate products of E2 and EE2 presented estrogenic activity, and hence the removal of estrogenic activity was shown to be lagging behind of the removal of estrogens. According to the results obtained by GC-MS analysis, ·OH could attack hydrogen atom in benzene ring or in the cyclic hydrocarbonfirstly when E2 was degraded by UV-microO3 process, generating hydroxyl followed by carbonyl and carboxyl, which were further degraded to CO2 and H2O.When EE2 was degraded by UV-microO3 process, hydrogen atom and hydroxyl on the benzene ring and acetenyl were firstly attacked by ·OH, and hence carbonyl and carboxyl were produced and degraded completely.The generation and control of bromate in UV-ozone system:the comparison on bromate formation in the process of organic compounds removal within UV-microO3 and UV/O3 processes showed that the removal rate of organic contaminants in UV-microO3 process was slightly lower than that in UV/O3 process. However, UV-microO3 process presented a remarkable advantage on bromate control. It was believed that the reduction of ozone dosage would lead to a more dramatically decline on bromate formation than on organic removal, because ·OH played the dominated role in organic matters removal in the UV-ozone system, while the bromate formation could be effected by both ozone and ·OH. Reducing ozone concentration and extending run time represented a feasible operation mode for simultaneous removal of organic contaminants and bromate minimization. In this operation mode, high ·OH exposure was obtained to achieve high removal rate of organic pollutants, while low ozone CT value and long-time exposure to UV irradiation were obtained to control the formation of bromate. Considering UV-microO3 process could obtain a high removal level of organic pollutants as well as performance well on bromate control, this technology was more predominant than UV/O3 process, especially for the water containing bromide at high concentration.To sum up, as one of AOPs, UV-microO3 process could remove organic pollutants efficiently from water via photolysis and ·OH oxidation. UV-microO3 process was believed to be more simple and practical for application compare to UV/O3 process, because the ozone was not generated by a typical ozone generator but produced by UV irradiation instead in UV-microO3 process, which hence could be recognized as a feasible alternative.