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In order to reveal the behavior of heavy metals in urban wastewater treating process, water samples were collected from Xi’an Third Wastewater Treatment Plant that takes the Orbal oxidation ditch as the main unit for secondary treating. Sampling locations were chosen mostly at the outlets of the fine-screen, sand chamber, oxidation ditch, and secondary settler. The filtrate of water sample, which pass through 0.45μm membrane was taken as liquid phase. Sediments of the water were dried, grinded and sifted then take it as sludge phase. While doing our experiments, we have adopted the atomic absorption methods to the analysis of copper, zinc, manganese, and chromium concentrations in the solutions and sediments. Then analysis of migrating and removing properties of these four kinds of heavy metals, the main results as follow:We have found the main metal pollutant in urban wastewater was chromium, the other heavy metal pollution was not serious, and chromium may be mainly derived from industrial wastewater. In the wastewater treating process, an apparent tendency can be detected of these four heavy metals, migrating from the liquid phase to the solid phase. As there was the largest amount of sludge in the oxidation ditch, the concentration of these four metals was increased biggest in the oxidation ditch activated sludge. Effluents of these four heavy metals were able to achieve the water quality standard for urban recycling water.Furthermore, by membrane filtration method, the heavy metals in the water sample were divided into dissolved fractions (those in the solute) and particulate fractions (those attached to or combined with the particles). In the influent, the order was Cr>Mn>Zn>Cu for the dissolved fractions of such heavy metals, while they were mainly existed as particulate form, except for chromium. In the effluent from secondary settler, four kinds of metals except for copper were mainly existed as dissolved form, while the highest proportion of dissolved fractions was chromium. In the urban wastewater treating process, the morphology of heavy metals was changed with treating process. Heavy metals in the water from the fine-screen, sand chamber to the oxidation ditch, the dissolved fractions would be found decreasing, indicating a transformation of the dissolved metals into particulate metals in these treating units. However, in the secondary settler, an apparent tendency of some particulate heavy metals translate into dissolved form, thereby were stayed in the effluent.By the secondary treatment process, high removal efficiency can be achieved for copper as 77.7%, while manganese, chromium, and zinc removal efficiency were relatively low, reaching 34.0%,31.5% and 30.2%, respectively. The fine-screen, sand chamber oxidation ditch, and secondary settler have a certain degree of removal ability for the four kinds of metals. Different treating units have different contribution rate of heavy metal removing, copper was mainly removed by the sand chamber; most of manganese was removed by the oxidation ditch and secondary settler; the contribution rate of chromium removing in sand chamber was the largest than other structure; and zinc was mainly removed by the sand chamber and the secondary settler. Copper, zinc and chromium were removed mainly through physical role, and the manganese was removed by the physical and biochemical role. At last, proportion of particulate fractions for heavy metals in secondary settler can affect the removal efficiency, and the dissolved metals was difficult to remove than the particulate metals. Re-dissolution of the particulate metals in the secondary settler may be the main reasons for the removal efficiency of zinc, manganese, and chromium lower than copper in this study.
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