城市污水处理中典型重金属离子去除与迁移规律研究

为了揭示典型重金属离子在污水处理过程中的去除与迁移规律,以某城市污水厂二级处理流程为对象,进行了各处理单元出水和沉泥的定期采样,分析了水相和泥相中6种重金属离子的含量.结果表明,以生活污水为主的城市污水受重金属离子污染不严重,且二级处理工艺能很好地去除水中的Cu、Hg和As,去除率分别为77.1%、85.8%和97.0%,而对Mn、Zn和Cr的去除率相对偏低,分别为40.0%、33.3%和30.8%.在污水处理过程中,多数金属离子有逐渐从水相迁移到泥相的趋势.通过离心分离法对水中的金属离子按溶解态和颗粒态进行了分类,发现各种金属离子在生物处理单元中有从溶解态转化为颗粒态的趋势,但在二沉池中某些金属离子又有从颗粒态转化为溶解态的现象.污水处理厂出水中Cu、Hg、As主要以颗粒态为主,而Mn、Zn、Cr主要以溶解态为主.颗粒态金属的溶出可能是这3种金属离子去除率较低的原因.

Investigation of typical heavy metals in domestic sewage treating process and their removing and migrating properties

The present article is inclined to present our study results of the behavior of typical heavy metals left-over in domestic sewage treating process. For this purpose, samples for the sewage and sludge were collected from a sewage treating plant which takes the oxidation ditch as the main unit for secondary treating. Sampling locations were chosen mostly at the outlet of the fine-screen, sand chamber, oxidation ditch, and the secondary sewage-settler. While doing our experiments, we have adopted the atomic absorption method to the analysis of Cu, Zn, Mn, Cr, Hg and As concentrations in the sedimentary solutions. As a result, we have found the sewage slightly contaminated by some heavy metals. In doing so, we have also measured the highest concentration for Mn in the actual sewage after the fine screen which is about 110 μg/L on average, while much lower concentrations were measured for other metals in the range of 10~(-1) to 10~2 μg/L. By the secondary treating process, high removal efficiency can be achieved for As and Hg as 97.0% and 85.8%, respectively, and that for Cu, Mn, Zn and Cr reaching 77.1%, 40.0%, 33.3% and 30.8%. Thus, an apparent tendency can be detected of the most of these heavy metals, except for Mn and Hg, migrating from the liquid phase to the solid phase. Furthermore, high speed centrifuging, the liquid-state heavy metals could be found to be divided into dissolved fractions (those in the solute) and particulate fractions (those attached to or combined with the particles). In the raw sewage, the dissolved fractions of such heavy metals usually range from 10% to 50%, while in the water from the oxidation ditch, the dissolved fractions would be found much decreasing or disappearing, indicating a transformation of the dissolved metals into metal particles in the biological treating units. However, in the effluent from the secondary settler, an apparent tendency of increase in the dissolved fractions can especially observed for Mn, Cr and Zn, which would reach 95%, 88% and 70%, respectively, in their dissolved fractions. Re-dissolution of the particulate metals in the secondary settler may be a reason for the lower removal (less than 40%) of Mn, Cr and Zn in this study.