Environmental and Economic analysis of mechanical biological pretreatment of MSW: an LCA approach
In the next future it couldn't be possible to send in landfills Municipal
Solid Wastes (MSW) not jet treated. Up to now, unfortunately, there are a small
number of mechanical biological pretreatment plants, due to high costs and
management problems. Nevertheless in this kind of plants there is a non
negligible advantage due to the reduction of emissions and of leachate
production.
The present work uses the Life Cycle Assessment (LCA) method as a tool for
decisional support. The purpose of such work has been to put in evidence,
through the application of LCA method, the economic and environmental advantages
that a refuse pretreatment plant, just before landfill, can provide within an
integrated solid waste management. Particularly, it has examined a landfill of
first category located in Reggio Emilia Province.
The evaluation on the best solution has been carried out considering different
alternatives: mechanical pretreatment, biological or mixed type. Evaluations
have been based on time series and forcast data, both displayed in the Refuse
Disposal Infraregional Plan (PISR) of Reggio Emilia Province.
The aim of the MSW pretreatment is to improve the environmental impact due to
emissions and leachate that represent a great problem in landfills. Mechanical
biological pretreatment plants, allow to accelerate fermentation and
decomposition processes of the damp fraction with the advantage of a great
reduction of refuse weight and volume (up to 30¸50%). Of greater benefit is the
cutting down of emissions and leachate production both in terms of quantity and
time of production.
The analyzed plant is of mechanic-biological mixed type for the treatment of
45.000 t/year of municipal solid wastes. Actually the plant is able to receive
up to 60.000 t/year, foreseeing possible increases of refuse production or an
adhesion to the consortium of other neighboring municipalities. In this plant,
not selected waste is immediately separated mechanically in two fractions: dry
and damp. This phase is very important because the dry part is sent to the
incinerator for the power recovery, while the damp one needs a biological
treatment to make it stable and therefore suited for the definitive laying in
landfill. The required treatment to get stabilized soil has a duration of 90
days, during which the fermentation index is reduced by 99%. All the sheds, with
the only exception of the zone of maturation of the organic fraction, are closed
and include, an humidifier and a ventilation system to carry all exhausted air
to a biofilter. Besides the treatment of the air there is also treatment plant
for first rain waters collected from the squares and from the sheds coverage.
Since the objective of the present work is to evaluate the potential
environmental and economic advantage of the pretreatment, a comparison between
two landfills for MSW and assimilated has been done. In one case the not
selected waste is supplied after pretreatments, in the other case the MSW is
supplied as is. As Functional Unity has been assumed 1 ton of not selected waste
sent in the landfill of Poiatica of Reggio Emilia and there lying for 30 years.
For the LCA study of the pretreatment plant emissions have also been considered
and has been taken into account also the life cycle of the pretreatment plant.
As a method for damage characterization and evaluation Eco-Indicator 99 has been
adopted. Modification has been done in order to consider water consumption.
Related data to the plant of purification and to the landfill (quantity of
leachate and biogas emission, efficiency of harvest, combustion of the gas and
treatment of the leachate) have been given by AGAC, whereas data related to the
pretreatment plant have been given by UNIECO.
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