CHAPTER 4: CASE STUDY OF CAPE TOWN'S WASTE STREAM
4.3 RESULTS AND DISCUSSION
Chapter 4: Case Study of Cape Town's Waste Stream
Chapter 4: Case Study of Cape Town's Waste Stream
they are sufficiently developed and hence the model later attempts to minimise the flow rate of the Recycling (Centres) stream - as this stream ultimately diverts recoverable material away from the income-generating MRFs. The Recycling (Centres) stream was given the constraint that the mass fraction of this stream cannot go below the initial value reported for 2005/2006 as a result of the fact that this sector represents private recycling operations that will not likely fall away. This is the reason that the mass fraction for this stream remains constant. It is interesting to note that the Recycling (Centres) stream was the favoured recovery mechanism for the early modelled years despite the airspace credit subsidies paid out to private recyclers being kept at 100%. This illustrates that for the current years of operation it is better to fully subsidise the Recycling (Centres) stream than to send this waste to the landfill. Another interesting trend shown in Table 4.4 is that the stream mass fraction of Household Composted Material is immediately given a zero value after the current year as a result of the fact that the costs associated with this stream are enormous. The Organics Collection stream, however, increases with time due to the fact that this stream helps to minimise the amount of putrescible waste in the commingled waste sent to MRFs, and hence ensures that a greater amount of commingled waste can be fed into the MRFs while still obeying the constraint of keeping the feed organics composition less than 25%. The Cape Town Municipal Council has developed several future landfill targets , which are highlighted in the following figure along with the model values.
2500000
M a>
* • >
CO
OH
s
U_ CO
1 1 11
S
c o
0
2000000
1500000
1000000
5 500000 c <0
0 4-
• Targets
• Model
2005 2010 2015 2020
Year
2025 2030
Figure 4.4: Model and target waste disposal flow rates for several future years.
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Chapter 4: Case Study of Cape Town's Waste Stream
Figure 4.4 above reveals that the target and model values are initially very similar, indicating that the targets set by the Cape Town City Council (reported in Mega-Tech Inc, 2004-1) can certainly be achieved at minimal cost to the Council. The targeted landfill waste becomes lower than the model values after 2020, which reveals that if the targets are to be met for the years following 2020, then the MSWMS has to be operated at a Overall Cost/Profit value lower than the maximum (at which the model values are set), or else alternative recovery schemes need to also be developed. The figure below plots the model and target generated waste recovery rates.
^ 70
2005
-*— Targets -•—Model
2010 2015 2020 2025 2030 Year
Figure 4.5: Model and target generated waste recovery rates for several future years.
As shown in Figure 4.5 the increases in the generated waste recovery rates for both the model and the target values are fairly parallel from 2005-2020. The plotted waste recovery target values exhibit a small initial five year lapse period, where the recovery rate remains fairly constant. According to the model it is favourable to start the commencement on recovery rate increases immediately, and the initial targets exhibit an unnecessary delay of the inevitable. The reason the model recovery rate starts to level off to the value of approximately 37.5% after 2015 is that this point represents the stage at which all of the economically recoverable material has been exploited and the remaining waste is either unrecoverable or too expensive to recover. To inform the decisions on how to properly increase the recovery rates of generated waste in line
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Chapter 4: Case Study of Cape Town's Waste Stream
with the target values it is imperative that the composition of the waste modelled to be sent to landfills be analysed. As a result the composition of landfilled material as well as the composition of recovered material for the model year 2030/2031 are displayed below.
2030/2031 10%
35%
35%
• Food Waste
• Plastic
• Metal
• Paper/Cardboard D Glass
• Garden Waste
2030/2031
24%
36%
31%
• Other l Organics
• Builders' Rubble • Recyclables Figure 4.6: Recovered material composition. Figure 4.7: Landfill material composition.
Figure 4.6 reveals that 45% of the recovered material is organic (food waste and garden waste), indicating the importance of organic material recovery from the perspective of ensuring a feed to the MRFs with an organic composition of <25%. Due to the fact that the composting facilities operating costs are higher than the generated income from them, the model only favours these recovery schemes because they allow for greater inputs of material to the income- generating MRFs. Aside from the above-mentioned reason it is also vital to recover organic waste if a significant amount of the generated waste stream is to be diverted from being sent to landfills. As is to be expected the recyclable material that exhibits the greatest recovery in Figure 4.6 is paper and cardboard. Figure 4.7 above shows that 36% of the material sent to landfills is material that is not readily recyclable (classed as Other). In order to reduce the amount of other material and hence the amount of waste sent to landfills the government has to develop laws that ensure manufacturers only use readily recyclable material to produce products as well as in packaging materials.
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Chapter 4: Case Study of Cape Town's Waste Stream
4.3.2 ECONOMIC RESULTS
The net cost/profit values for the various years modelled are plotted below. These values are the resultant objective function values determined by Solver to be the maximum Overall MSWMS Cost/Profit values for the particular year concerned, under the programmed constraints.
(A
•o
c (0
on H-
o c o
III
E
*-<
*-o k .
Q.
53 to
o
o
4-><D
z
600 500 400 300 200 100
o
2 -100 -200 -300
Figure 4.8: Plot of modelled net cost/profit (objective function values) for various years.
Figure 4.8 reveals that the payback period for the development of the proposed MSWMS is 18 years, meaning that if the MRF type MSWMS is chosen to be implemented in Cape Town then it will take 18 years for the income generated from the system to pay back the capital required for this MSWMS to be developed. The total calculated capital costs for the MRFs, Composting Facilities, Transfer Stations and Separate Collection Capital until 2030 are R2.36 billion, R0.61 billion, R0.76 billion, and R1.32 billion respectively. This indicates that the Cape Town Municipal Council requires a large amount of capital to start up this MSWMS, but when it is fully operational it will generate a large amount of income. After 2024, the Council will start to generate a net profit from the MSWMS, which will continue to increase. This profit could be used to implement further projects to increase the recovery of waste materials, and thereby decreasing the amount of waste sent to landfills.
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2030
Year
Chapter 4: Case Study of Cape Town's Waste Stream