The resulting seven classes are pre-
sented in Fig. 1. Further detail on waste types under each class and
allocation percentages can be found on Supporting Information (SI)
section.
The conceptual model of the HWMS presented in Fig. 2 displays
the relationships among the system components. According to this
model, different types of hazardous wastes are collected at point of
origin some of which may be subject to waste prevention and
minimization practices on site. This fact makes them difficult to be
incorporated into transportation/location problems. Therefore, any
waste minimization, on-site recovery or on-site transfer of wastes is
omitted from system boundaries of the conceptual model.
Furthermore, non-hazardous portions separated from HWs and
non-hazardous residues are excluded from the HWMS system
boundary.
Upon collection at the source, hazardous wastes are transported
to the appropriate processing facility according to their type (blue
lines in Fig. 2). The model allows co-location or establishment of
integrated facilities at the same node. It is especially important that
incinerators and landfills be integrated since residues from haz-
ardous waste incineration are likely to be hazardous and must be
sent to a hazardous waste landfill.
The proportions of treatment and incineration residues with
respect to total amount of waste entering a process step are ob-
tained from the literature and current practices. They are incor-
porated into the model by means of mass reduction ratios (denoted
by upper case M in Fig. 1) provided in SI Table S1. These coefficients
represent the relation between the amount of waste and the
amount of residues entering the process and are needed for flow
balance constraints.