3.2. WARM Scenario Analysis Results
WARM model results for the eleven scenarios described in Section 2.4
are shown on Figure 5
and Figure 6
, negative values indicate reductions compared to the base case, i.e., landfilling of all materials. The base case represents current conditions, in which all of the audited waste is disposed of at the Columbia bioreactor landfill, Figure 5
and Figure 6
. Disposal of the waste in the bioreactor landfill results in 11.1 metric tons of CO2
e and 3.45 gigajoules (GJ) of net energy as estimated using WARM. If the landfill did not have the capacity to generate electricity and flared the collected gas instead, which is more typical, the GHG emission estimate would be higher (21.5 mt of CO2
e) and require more energy (25.3 GJ) to manage this waste. Finally, if the landfill did not collect LFG, and instead released methane directly to the atmosphere the GHG emissions would be 60.2 mt CO2
e, with the same energy use as the flare scenario. Note that it is no longer legal to construct landfills without LFG capture systems, but some landfills have been exempted from this rule. This information is provided for context about the variability in emissions and energy use associated with possible landfill designs across different locations.
Overall, the two most effective approaches for reducing GHG emissions and energy use are recycling and source reduction of food waste. Recycling corrugated cardboard and mixed paper contributes the largest GHG savings in the recycling category, given the mix of recyclable waste found at the Stadium. Recycling achieves GHGs reductions of 25.4 mt GHGs and energy savings of 243.7 GJ. WARM estimates that GHG emissions are reduced by 3.45 mt CO2
e/mt of recycled corrugated cardboard and 3.91 mt CO2
e/mt for mixed paper; the majority of the savings are attributed to forest carbon storage [16
]. Recycling of aluminum results in the third largest GHG emissions reduction reflecting aluminum’s relatively large GHG reduction per unit of material, at 10.1 mt CO2
e per mt of aluminum. Source reduction of edible food waste results in a reduction of 103.1 mt of GHG emissions and 448.5 GJ of energy use or the largest opportunity for GHG reduction and energy savings over all scenarios. Note that edible beef food waste is responsible for the majority, 80%, of the CO2
e reductions associated with source reduction because beef is estimated to cause 30.05 mt CO2
e/mt beef in the WARM model, one to two orders of magnitude larger than any other food included in the model [16
]. Beef and poultry (which includes pork as described in the methods) result in 41% and 31%, respectively, of the overall energy use reductions associated with source reduction. Note that if all of the inedible food waste was also assumed to be source reduced WARM estimates an additional GHG reduction of 5 mt CO2
e and additional energy savings of 77.4 GJ. Scenario 5a
represents the case where all edible food waste is avoided, “source reduction,” and the remaining materials are landfilled as in the base case, resulting in −114.8 mt CO2
e, as compared to the base case. Net energy in Scenario 5a
is −448.8 GJ, as compared to the base case. These savings indicate the potential benefits of targeting only food waste reduction.
, recycling of all recyclable materials found in the waste stream currently destined for the landfill, would result in a reduction of 24.7 mt CO2
e and 243.8 GJ, as compared to the baseline, shown in Figure 5
and Figure 6
. The largest reduction in GHG emissions is due to recycling corrugated cardboard (11.6 mt CO2
e), followed by mixed paper (7.1 mt CO2
e) and aluminum (5.2 mt CO2
e). The largest source of energy reduction is associated with the recycling of aluminum (91.5 GJ), followed by corrugated cardboard (59.1 GJ), mixed plastics (46.8 GJ) and mixed paper (43.3 GJ). Scenario 1
requires no significant investment in waste management infrastructure and no replacement of materials. The major hurdle with regard to realizing Scenario 1
is the improvement of fan compliance with regard to sorting landfill waste and recyclables and placing them into the correct receptacles, this may require improvements in Stadium signage and receptacle type and placement.
In Scenario 2a, recyclables are recycled and all food waste is composted, the GHG emissions are further reduced from the baseline to a net reduction of 41.4 mt CO2e. The additional reductions occur because the CO2e associated with the food waste switches from a positive estimate to a negative estimate per unit when switching from landfilling to composting. The net energy associated with this scenario is also negative, as in Scenario 1, but slightly less so at −224.5 GJ, as compared to the baseline as the WARM model includes more energy use associated with operating an industrial compost operation than if the food waste were placed in the bioreactor landfill where methane generated due to anaerobic decomposition are used to generate electricity. Scenario 5b is the same as Scenario 2a, except that the edible fraction of the food waste is avoided, resulting in additional GHG and energy use savings for a total reduction of 143 mt CO2e and 688 GJ compared to the base case scenario.
Scenario 2b, in which paper and cardboard are composted (rather than recycled) along with food waste, using the proxy of Mixed Organics in WARM, and the remaining recyclables are recycled, is estimated to achieve a reduction of 23.5 mt CO2e compared to the base case. The net energy balance for Scenario 2b is considerably less than the 3.45 GJ in the base case with a value of −256.7 GJ. Scenario 5c is similar to Scenario 2b, except that the edible fraction of food waste is avoided, resulting in a large reduction in GHGs and energy use compared to the base case, −125.3 mt CO2e and −582.6 GJ, respectively, highlighting the benefits of reducing the wastage of food. Scenario 5c assumes that paper materials are composted rather than recycled, which results in less GHG and energy savings, largely due to carbon credits attributed to avoiding the logging of trees.
Scenario 3, in which recyclables are recycled, food waste is composted and non-recyclable materials are replaced with compostable PLA materials, GHGs are reduced 42.4 mt CO2e and energy use is reduced to −224.2 GJ compared to the base case. Scenario 5d, which is the same as Scenario 3 except for source reduction of edible food waste, results in the largest reduction of GHG emissions over all scenarios at 144.1 mt CO2e and the second largest reduction in net energy, at 688.0 GJ compared to the base case due to recycling and the source reduction of food waste. Scenario 5b has the most negative net energy at −688.3 GJ, but is effectively the same as Scenario 5d, the difference occurs between the net energy estimated to landfill plastics #3–6 in Scenario 5b compared to composting PLA in Scenario 5d.
In Scenario 4, all non-compostable materials are replaced with PLA and all materials are composted), this results in a GHG emissions value of −18.1 mt CO2e and net energy use of −24.7 GJ, as compared to the baseline. Scenario 5e is identical to Scenario 4 except that edible food waste is avoided. Replacing recyclables with PLA, as in Scenarios 4 and 5e, results in less net GHG and energy reductions than recycling of recyclable materials due to the large savings associated with recycling aluminum and paper products. The source reduction of edible food waste results in −101.3 mt CO2e savings and an overall reduction in net energy usage of 444.2 GJ.
Scenarios 3, 4, 5d and 5e would all achieve zero waste goals as all materials would either be composted or recycled, if not avoided altogether as with the source reduction of edible food waste in Scenarios 5d and 5e. Based on the weights observed in this study, Scenarios 2a and 2b would achieve 88% materials diversion from the landfill based on weight, or only 2% short of meeting the requirement to be considered zero waste. While Scenario 5b would result in a considerable reduction in the total volume of waste sent to the landfill, 21.6 mt compared to 47.2 mt, the diversion rate would only be 74%.