Lösungsprozesse und Transport leichtlöslicher Salze in Monodeponien für Rückstände aus der Müllverbrennung

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.