Table 5 gives an overview of energy production within
the different scenarios. Total electricity consumption
in MRS was about 17 TWh. Therefore, the energy generation
from MSW is only of minor importance (about
6% compared to electricity consumption in the same
year). On the other side, taking into consideration
only the amount of energy produced in MRS (25%
of total energy consumption, which means 4.25 TWh),
energy generation from MSW might contribute with
nearly 25%.
Worldwide, there is a general agreement that global
climate change is, to a large extent, caused by anthropogenic
CO2 emissions. It is necessary to reduce the CO2
footprint of human activities. Therefore, even though
the contribution to total energy consumption is relatively
low in the three scenarios, it is important to consider that landfill gas will form in any case; therefore,
it does make sense to use it as a renewable source of energy,
instead of releasing it into the atmosphere. The latter
would increase the emissions of greenhouse gases
due to the CH4 produced being 21 times more potent
than CO2. On the other side, the production of RDF and
its use as a secondary fuel in cement kilns might
contribute to about 25% to total energy consumption for
cement production in MRS (for the year 2007).
Evaluation of MSW management with respect to
sustainability
The management of MSW in MRS in the different scenarios
was evaluated on the basis of different sustainability indicators and on their associated target
values, as explained in the ‘Sustainability evaluation’ section
(see Table 6). These target values represent a commonly
agreed goal, which should be achieved in the
future in order to fulfill the principles of sustainability.
Indicators and target values were defined together with
Chilean investigators, consultants, and government
experts during several workshops within this study.
The improvement shown in the BAU scenario for the
amount of waste recovered (31%) is attributed to the installation
of mechanical sorting plants and segregated
collection of biodegradable waste and recyclables
through organized primary waste collectors, in addition
to energy recovery from landfill gas and biogas. The
organization of the informal workers is also reflected in
an improvement of their income level.
In the CR scenario, the recovered quantity of MSW
(43%) is higher than the target (36%) because the implemented
measures are based on several different collection
and treatment systems: segregated collection
of biodegradable waste, inclusion of primary collectors
into collection systems, participation of citizens in dropoff
systems, and recovery of the energy from MSW
and from landfill gas (Figure 4). Nevertheless, greenhouse
gas emissions are still far away from the target value. This
fact can be mainly attributed to the still large amount of
organics being disposed of at landfill sites.
The MI scenario shows large deficits in almost all the
indicators. It must be noticed that even though this scenario
presents the largest processing capacity for the
mechanical sorting plants, the recovery value in this scenario
is the lowest. One of the goals of MSW management
should be the conservation of resources [16],
which in general is more related with the recovery of
materials (mechanical sorting plants in this case), but of
equal importance should be energy recovery from waste.
However, the intrinsic characteristics of this scenario
[18] did not allow the implementation of these techniques
because all environmental measures are moved
by private markets and there are no laws forcing the
implementation of energy recovery. Neither the informal
waste pickers is included into the management system.
Therefore, they were not able to improve their working
capacity and working conditions.
In the three scenarios, there is still a large fraction of
mixed MSW disposed of at landfills. This is related with
high emissions of greenhouse gases due to the decomposition
of the biogenic fraction. The emissions of
greenhouse gases contribute to climate change. Energy
recovery from the biogenic fraction of the MSW might
contribute to mitigate this phenomenon because the
CO2 emitted from the thermal conversion of this fraction
is neutral for the climate. Additionally, it contributes
to the conservation of resources by substitution of other combustibles. If the landfill gas is not used to produce
electricity, but only collected and flared, the GHG
emissions in per capita terms increases by 13%, 17%, and
11% in the BAU, CR, and MI scenarios, respectively.
Therefore, the substitution of fossil fuels by landfill gas in
electricity generation plays a significant role in the reduction
of GHG emission, in particular in the CR scenario.
The results also imply that the informal waste pickers
should be integrated in separate collection of recyclables
(BAU and CR scenarios). For biodegradable waste, a
good treatment option is anaerobic digestion with
utilization of the biogas produced (BAU and CR scenarios)
for energy generation.
In the three scenarios, incineration of MSW was not
taken into consideration because during debates with
Chilean experts, it was concluded that this will not be an
option for the next 20 years due to high costs of incineration
(US$110 to US$160 Mg−1) [41] compared with
current costs of waste management in MRS (collection
costs US$26 Mg−1, landfilling costs US$11 Mg−1) [43].
The results of the sustainability evaluation show that
each explorative scenario presents deficits; even in the
CR scenario, not all the targets can be reached. It is clear
that the integration of other factors is still required in
order to improve the sustainability of the MSW management
system. Technology is only one part of the whole
structure. A sustainable system requires the incorporation
of government policy and regulations, responsible consumption
patterns, and adequate cost calculations and
education, in addition to technological developments.
Moreover, it is necessary to take advantage of the subsystems
already working within the whole system. Two
important examples in this case are the integration of
the informal waste pickers, which already have an effective
network in MRS, and the use of landfill gas as a renewable
energy source for energy generation.
The three scenarios did not look to identify a best solution
but investigated possible futures in MRS. The
results should help decision makers to visualize how the
future might look like and take appropriate measures in
time. The authorities of MRS should start now to plan
and construct the appropriate MSW treatment plants
and start to implement the respective measures for the
MSW management that they want to achieve in 2030.
The model presented here could also be used, with the
respective adaptations, in other Latin American megacities
in order to determine the adequacy of the MSW
management there.
Conclusions
In this study, two key tools were used in order to evaluate
the sustainability of different MSW management systems
and to describe its development in the next 20 years.
On the first place, the ISHC was used as a tool to evaluate
the sustainability of the MSW management of Santiago de
Chile in the future. This concept, which has also been applied
to other fields of application within the Risk Habitat
Megacity Project (such as water management and energy
demand and supply), proved to be adequate because it provides
a methodology to select indicators, determine their
current (and future) value, and define desirable target
values, and by comparison of both (actual vs. desired), it
was possible to evaluate the actual MSW system as well as
future MSW management options with respect to whether
there will be an increase in sustainability or not.
On the other hand, in order to define how the MSW
management of Santiago de Chile will look like in the future,
the scenario technique was used. Explorative scenarios
were developed, giving an insight about which
probable MSW management trends will follow in accordance
with possible political, economical, and environmental
decisions taken today.
The BAU scenario was characterized by current trends
and policies. The CR scenario was characterized by stronger
emphasis on social values and implementation of
tougher environmental regulations. The MI scenario was
characterized by a materialistic culture, with a strong private
influence in all political and economic areas. The results
showed that MSW generation increased in total and
in per capita terms in the three scenarios, exceeding the
target value chosen (1.6 kg (person·day)−1). In the CR scenario,
a recovery value of 43% was obtained (target value
was 36%) through public-private partnerships (drop-off systems),
private investments (as in the case of sorting plants
at transfer stations), and organizational improvements of
the informal waste sector. In addition, there is a large contribution
to sustainability by the recovery of biowaste and
subsequent energy generation from biogas and RDF.
In all three scenarios, landfill sites contribute to methane
emissions in MRS and thus have a share in climate
change. The improvement in efficiency of landfill gas
collection systems results in a reduction of these emissions
in all three cases. The landfill gas collected is used
as a renewable energy source for electricity generation,
thereby reducing CO2 emissions from conventional
power stations fired by fossil fuels.
Even though the share of electricity production by
MSW fractions and biogas in the three scenarios is relatively
low, they are accompanied by positive aspects such
as the reduction of methane emissions on one side and
favoring energy supply within the MRS from renewable
energies on the other side. All three scenarios show
some sustainability deficits. Furthermore, the results
obtained show that an integration of several factors is
required in waste management systems. Technology is
only one part of the whole solid waste management
structure, a