Metal retention from leachate using Industrial Waste Products
Abstract: Landfill leachate arises when water percolates through a landfill. The degree and type of pollution is dependent on the content of the landfill but also on a range of other factors, including the age of the landfill and the landfilling technique. In recent years, concerns about potentially hazardous compounds in landfill leachate have stimulated research into on-site treatment. Examples of new techniques for treatment are the sequent batch reactor technique (SBR), wetland systems and different forms of filter solutions.Lilla Nyby landfill is the municipal landfill in Eskilstuna, Sweden. Leachate streams have been found on site that are weaker than the main stream. One of these has been investigated in the present study. It contains heavy metals, especially zinc, copper and nickel, in concentrations which require treatment. Low strength landfill leachate streams require special treatment. There are various reasons for this, but examples can be geological reasons or dilution problems in the treatment facility for the main leachate.The technique chosen for the leachate stream, reactive filters, is rather new and few pilot scale experiments have been conducted. Therefore many questions still remain concerning the applicability of the method on site, and there was a need for more experimental data. Previous research has shown that many industrial by-products may be suitable to capture metals from leachate. Among the materials previously recommended, blast furnace slag and pine bark had advantages which were well suited for the present application. They were also available in large quantities near the site. A column experiment was set up at the landfill and three filter materials were chosen; pine bark and two types of blast furnace slag. In addition, one of the columns was filled with sand as a reference. As a complement to the column study, laboratory experiments were conducted at Mälardalen University. The aim of the laboratory experiments was to investigate factors that might limit metal retention by the filter materials. Different concentrations and contact times were tested as well as the presence of other substances (i.e. metals and organic carbon).The results from the first laboratory experiments (initial metal concentration, competing metal ions and contact time) and the on-site column study were contradictory. Metal uptake in the laboratory was very efficient, as has also been found by other researchers. On-site, many factors influenced sorption and uptake was very unstable, especially for the slags. It was found that pine bark is more effective and stable than the slags with respect to metal retention, especially at low concentrations. The contact time between the substrate and metals is important for effective retention. The final experiment, showed that dissolved organic carbon (DOC) can greatly inhibit metal retention. To further develop reactive filter technology, a method to reduce the DOC content may be necessary.In contrast to what has been previously assumed, leaching of COD from the pine bark column at Lilla Nyby landfill was negligible. Finally, some suggestions for future work on reactive filter technology are made. For instance, it could be interesting to develop an alternative technique for filtration. Packed columns have been addressed with a series of problems, related to hydraulic parameters. By using fluidised reactors, a comparison with batch experiments becomes possible.
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