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Sager, Daniela (2007): Lösungsprozesse und Transport leichtlöslicher Salze in Monodeponien für Rückstände aus der Müllverbrennung. Dissertation, LMU München: Fakultät für Geowissenschaften



A homogeneous design for landfill bodies is generally agreed. However the manner in which waste is deposited may result in a sequence of layers that causes inhomogeneity. The goal of this research was to investigate the influences of such inhomogeneities on long-term leaching of highly soluble salts and ecotoxic species. This long-term leaching of was observed for 25 months. Laboratory column tests and field experiments (test bodies and lysimeters) were performed with municipal solid waste incineration residues. Laboratory columns were irrigated with tap water. Control field experiments were exposed to natural precipitation. Due to the fact that municipal solid waste incineration residues are continually landfilled on monofills, the percolating landfill-leachates are highly saline. Therefore extended experiments were carried out and these were treated with saline solutions: either solely or in addition to natural precipitation. Highly saline irrigation-media artificial leachates produced in the lab, actual landfill leachate or a concentrate from reverse osmosis was used, depending on the experiment. Besides irrigation, the application of residues, as the top or intermediate layers, provided continuous supply of highly saline solutions. Commonly, column experiments are conducted under water-saturated conditions. For this research however, unsaturated conditions were chosen to understand real landfill conditions. All experiments were performed as parallel experiments. One test had a uniform-grain setup and acted as a control. The application of a fine-grained bottom ash layer within a column filled mainly with coarse-grained bottom ash provided the inhomogeneous setup. Grain size changeover as a barrier layer within a column should change the permeability along the profile of the column. In some cases the barrier layer was compacted. The results of long-term the solid sample and leachate investigations pointed out that the application of a barrier layer by means of grain size changeover had mainly physical effects. With the control experiment, water contents of solid samples increased in general from the top of the experiments to the basis. In contrast, the following was observed for the application of a barrier layer:  impounding of leachate,  slowdown of leachate percolation time,  increased water retention capacity. Mainly the zone within the barrier layer showed the highest water content. A general observation was that the setting beneath the barrier layer was much drier. Another observation was that irrigation with saline water enhances water retention within the column. Additionally model experiments with plastic shots pointed out that the irrigation with small amount of water intensifies the physical effect of the barrier layer. Unsaturated water conditions which are common in landfills together with reduced permeability provide the basis for salt accumulations around the zone of the fine grained ash layer. An investigation on the distribution of the elements along the profile showed that impounding of the leachate at the barrier layer is linked to higher eluate soluble species concentrations: the tendency of accumulation was observable especially for the zone within and beneath the barrier layer and in the near-bottom samples for the control as well as extended experiments. Concerning a uniform setup the salt distribution along the profile was homogeneous up to gradual. In general the observations indicated that the application of a barrier layer by means of grain size changeover impounds the leachate. Thus, increased concentrations of different species of saline solutions in combination with changing water contents within the layers caused precipitation of the highly soluble salts out of the pore solution. The formation of salt accumulations is time-dependent. Most of the chemical and mineralogical progressions occurred only after a longer period:  tendencies for the leachate progression changed several times during the long term,  different processes depend on the pH-value (e.g. if the pH-value decreases Ca-concentrations will increase) and  this supports the vital importance of long term leaching. A main conclusion is that unsaturated conditions in combination with physical and chemical processes provide conditions conducive for salt accumulations around the zone of the barrier layer. The effects of inhomogeneities because of compaction and/or grain size changeover, as investigated in this research, furnish an explanation for actual observations at the Waldering monofill. In a particular filler part a salt horizon of several meters was encountered. This highly saline layer consists mainly of Halite and Sylvite. Hence there is the possibility that salt accumulations up to salt horizon are generated even in a humid climate. In principle the MSWI-residues afford high contents of saline species. Due to the application of a barrier layer the following advantages can be derived for landfill operations:  due to their high water retention capacity, salt accumulations cause little amounts of leachate,  high water retention capacity combined with the intensifying effect of salty crusts within the zone of the fine grained ash layer provides strong buffering concerning rainstorms.