Impact of pre-treatment on the stability and leachability of three different wastes

Abstract: The effect from mechanical-biological pre-treatment (MBT) of wastes in Sweden and Germany has been studied. The organic matter and biological activities were studied and compared. Other parameters such as heavy metals and nutrients were examined and related to the organic matter. The effect of low water content on degradation was investigated and the relevant legislation in the two countries was studied and compared. Three different waste materials were used; two from Germany (MBA 1 and MBA 2) and one from Trelleborg, Sweden. The German materials were municipal wastes while the Swedish one was a mixture of building waste with compost and sewage sludge. MBT means that the material is first sorted and crushed and then treated biologically, mainly through composting, to prepare the waste for landfilling, or use within the landfill. The solid waste and the eluate (leachate) were studied. The eluate was filtrated through four different membranes (cut-offs 0.45 ?m, 100 kDa, 30 kDa and 5 kDa) to learn more about the size of the organic matter and its relation to other pollutants. Four bioreactors with different conditions were used to simulate landfilling of the output material. The organic content decreased during treatment. In MBA 1 DOC decreased more (95%) than TOC (38%); i.e., the organic matter became harder to leach. The biological stability increased; the respiration activity and the gas formation potential decreased with up to more than 90%. This was also shown by parameters such BOD5/COD ratio, humic acids formation, C/N ratio, FT-IR spectrums and thermal methods. Pollutants such as most heavy metals and nitrogen became less leachable after treatment, thus contributing to a decreased emission potential. The metals studied were cadmium, chromium, cupper, lead, nickel and zinc. They seemed to be bound to the organic matter to some extent, perhaps with the exception of cadmium. In most cases less than one percent of the metals were eluted. In the cases where the initial leachability of metals was high, it decreased considerably (up to 99%) during treatment. Considering only the parameters analysed in this study, MBA 1 and the material from Trelleborg could be landfilled in Germany after treatment, and Trelleborg even before. Even though landfilling is allowed, this does not mean that the emission potential is zero. MBA 2 had too high DOC content and could not be landfilled. The German output materials could not be landfilled in Sweden and Trelleborg can only be landfilled if it can be classified as non-hazardous waste. The Swedish legislation is, unlike the German, not adapted for MBT-treated waste to be landfilled. The Swedish limits are generally stricter than the German ones and the most limiting criterion is TOC. TOC is however not the most relevant parameter since it is not directly associated with the emission potential. It could be replaced by activity measurements like RA4 or GP21 together with DOC. The Swedish limit values are strict but exemptions in the legislation enable waste with high emission potential to be landfilled anyway. The legislation could be better adapted to the actual situation. The landfill simulation showed that aerobic degradation is more efficient than anaerobic in reducing the respiration activity. It also showed that low water content will slow down the degradation considerably. A slower degradation will cause the degradable material to have a longer lifetime in the landfill. There is more to learn about the dissolved organic matter and also about its association with heavy metals. If the goal is to study changes during treatment, it is better to follow one batch of material through the whole process than to sample batches at different stages the same day.