Environmental Engineering Reference
In-Depth Information
Apart from R, which represents the total daily MSW production, all symbols represent
flow percentages. More specifically, for every branching point, the following convention is
adopted: the symbol associated with an outgoing link represents the percentage of the flow
corresponding to the unique incoming link.
The total waste flow is partly gathered (percentage
α
) by a separate collection and then
sent to recycling. Note that recycling is not possible for three kinds of the above materials,
that is, heavy plastics, scraps and inert matter, whereas the other eight materials can be
separately collected by different methods. Besides to separate collection, material recovery is
also possible by dividing the various materials in a separator plant. From such a plant, three
flows may come out:
•
the metals that can be sent to recycling;
•
the organic material that must be sent to a treatment plant (humid material);
a fraction of material, with low humidity and high heating value (dry material), that can be
burnt (percentage ψ
I
), or sent to the plant for Refuse Derived Fuel (RDF) production
(percentage ψ
C
), or disposed in the sanitary landfill (percentage ψ
L
).
The RDF plant produces fuel, which can be sold to industries (percentage θ
M
) or burnt in
the incinerator (percentage θ
I
), and scraps, which can be sent either to the incinerator
(percentage λ
I
) or to the landfill (percentage λ
L
).
The organic material collected for recycling can be directly sent to a composting plant
because it is pure enough to produce compost for agricultural use. The humid material is
treated in the organic material treatment plant, which produces Stabilized Organic Material
(SOM). SOM can be sold (percentage γ
M
), burnt in the incinerator (percentage γ
I
), or sent to
the landfill (percentage γ
L
). Clearly, material recovery takes place not only through recycling
but also through the various treatment plants which provide SOM, RDF and metals. Energy
recovery by MSW combustion has to be taken into account as well. As recycling modifies the
composition of the refuse sent to incineration, it influences the heating value of the refuse that
has to be burnt, and hence energy recovery. The purpose of the DM is to determine the
various flows of the different materials in the whole MSW management system in order to
satisfy a number of technological and normative constraints and minimizing four main
objectives: the economic cost of material treatment, the quantity of unrecycled waste, the
quantity of waste sent to the landfill, and the emissions of the incinerator. In the following
sections the details of the mathematical formulation and of the approach of the multi-
objective decision problem (MODM) are illustrated.
3.3. The MODM Approach
In the considered context, the multi-objective problem can be in general expressed as a
vector optimization problem (VOP):
min
F
(
x
)
x
∈
X
(3.1)
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