4.5. Summary of discussion
The two approaches revealed similar results but the results of
the mixed integer model (second) are relatively better since the
cost is based on actual unit cost rather than the estimated cost calculated
based on the full loading capacity of truck. Both
approaches suggest buildings incinerators in Shek Kwu Chau and
Long Gang and one replacement truck warehouse in Yuen Long
and the objective value in the second approach (HK$247,144.5) is
better than that in the first approach (HK$248,883). The reason is
that the transportation cost is based on trips in the first approach,
assuming all trucks have full capacity each trip but the reality is
that some trucks maybe partially full, whereas the transportation
cost in the second approach is calculated based on the actual
amount carried in each trip. Similarly, the numbers of trucks
needed are different based on the same reason. The expected number
of trips is also shown to influence the number of trucks
required, especially in the first approach. In both approaches, the
results show two incinerators are at full capacity with 20% bottom
ash, and can incinerate 5000 tonnes daily in total. The landfill
amount is reduced by about 42% compared with the current plan.
Both results suggest building two incinerators and one replacement
truck warehouse, since the daily cost is not expensive and
the landfill rate decreases, significantly.
Sensitivity analysis has been done to determine how the changing
of certain parameters can influence the objective function and
more importantly, the preference of the building incinerator
plan can be illustrated by comparing different settings of the
parameters. In sensitivity analysis 1, it shows that the higher
fraction of bottom ash is, the cost for the municipal solid waste
management system will increase. It is less preferable to build
the incinerator. The revenue will be less and the landfill amount
will be correspondingly higher. However, it is the decision maker’s
choice since the minimal cost solution may not be suitable for the
current situation, such as in Hong Kong. With the help of these
approaches, the difference in objectives and landfill amounts in
the two conditions (build incinerators or not build incinerators)
can be quantified. Then it becomes a trade-off choice and the decision
makers can decide what is best for Hong Kong. The sensitivity
analyses 2 and 3 reveal that the breakpoint for building the
incinerators in terms of revenue and operational cost respectively,
these can help the decision maker control the cost when choosing
different incinerators.