Environmental Engineering Reference
In-Depth Information
Similarly, the daily cover has a negative effect on the biodegradation of waste because it
decreases the O
2
diffusion into the waste which in turn diminishes the composting rate. If a
low permeability soil is used as a daily cover, it could create a barrier and may impact
leachate distribution and landfill gas flow into the collection system. A soil cover more
permeable than the waste can direct leachate to the sides. Use of alternative covers that do not
create such barriers can reduce these effects. On the other hand, positive effects of daily cover
soil may be expected if the soil supplies buffer to the landfill (e.g. contain lime) (Christensen
et al., 1992).
The cell with soil cover has a higher leachate concentration than the cell without it.
Initially, the cell without cover produced a high leachate concentration, but it was followed by
a rapid decrease (Ham et al., 1982). It appears that the lack of daily cover enhances the
aerobic activity and prevents a long acidogenic stage. The short acidogenic period leads to the
rapid establishment of the methanogenic stage. The landfill should utilize thin lifts and the
daily cover should not be used immediately.
Likewise, waste compaction has an adverse effect on the biodegradation of waste in the
landfill. Ehring et al. (1980) found that cells with low-density waste have shorter periods of
high leachate concentration. This means that there is an enhancement in the acidogenic stage.
Rees et al. (1982) concluded that by increasing the waste density from 0.2 to 0.47 t/m
3
, there
was a decrease in gas production due to acid accumulation, which in turn decreased the pH
and inhibited the methanogenic bacteria.
Waste compaction causes sudden decrease in the void space of the waste which in turn
decreases the moisture content dramatically. On the other side, the effect of load caused from
subsequent waste layers, which is examined in this thesis, causes a gradual decrease in waste
voids in a way that does not affect waste biodegradation.
7. Pre-treatment
The objective of the pre-treatment of MSW is to enhance the acidogenic stage and
decrease the accumulation of organic acids. This method is based on the stabilization of part
of the waste through aerobic processes which will dilute the organic acids and cause a balance
between the acidic phase and the methanogenic bacteria. This method was studied by Ham et
al. (1982); Beker (1987); and Stegmann (1983) and was reviewed by Komilis et al. (1999b).
Ham et al. (1982), Beker (1987) and Stegmann (1983) found that by placing fresh waste
on top of the composted waste layer caused a shorter acidogenic stage and enhanced the
methanogenic stage. This is due to the fact that the composted layer acts as an anaerobic filter
which has the ability to treat leachate as it passes through. The composted bottom layer can
be prepared by the following procedure: a layer (1.5m-2m) of waste is placed without
compaction, so that the easy degradable material can be decomposed aerobically with
leachate recycle. The leachate concentration and the temperature of the waste can be used as
indicators for the progress of the aerobic process. After one year of the placement, the waste
layer is compacted and an additional layer of fresh waste can be added on top (Stegmann,
1983). The author suggested that efforts should be made to reduce the time required for
aerobic decomposition of the first layer by injecting air through perforated pipes.
