5. Discussion 87
5.1.6 Publication VI: Recovery of the mineral fraction from BA
apart from the high uncertainty incorporated in the determination of the leaching contents in thermal residues, which was studied in Publication V, special attention should be paid to the uncertainty associated with the characterization models used for the impact assessment, since the toxic categories should be improved and used with caution (Hauschild et al. 2013).
5.1.6 Publication VI: Recovery of the mineral fraction from BA
In the scenarios of Publication VI, the importance of PT widely varied between the impact categories, from the low importance for the HTPC to the significant importance for the GWP and HTPNON-C. Allegrini et al. (2015) performed a similar study incorporating MSWA BA pretreatment while accounting for the recovery of mineral, ferrous and non-ferrous fractions. As a result of the inclusion of ferrous and non-ferrous scrap recycling, the relative share of PT was negligible in the study by Allegrini et al. (2015) and was, therefore, outshone by the significant savings originating from aluminium and ferrous scrap recovery at the level of 170 kg CO2-eq and 110 kg CO2-eq/t of pre-treated ash, while the GWP of PT was around 6 kg CO2-eq/t of pre-treated ash. For comparison, the GWP of PT in the scenarios of Publication VI ranged from 3.4-5.8 kg CO2-eq/t of pre-treated ash. Similarly to the GWP, the impact of PT on the ADP was outshone by the significant impact originating from the recycling of ferrous and non-ferrous scrap in the study by Allegrini et al. (2015).
The additional impact on the GWP and the ADP observed in Scenario BA:S2-GSP was strongly related to the need for additional cement in the garden stone production process, where the mineral fraction obtained during ash pretreatment was utilised; however, the break-even point was not achieved during the sensitivity analysis, which accounted for the consumption of cement at a conventional level. Similarly, Allegrini et al. (2015) acknowledged the significant impact associated with the consumption of additional cement during the concrete production process incorporating the minerals obtained during the BA pretreatment process, which accounted for 13% of the total GWP of the alternative scenario, which included recycling of the mineral, aluminium, and ferrous fractions. In terms of the HTPC and HTPNON-C, an induced impact was recorded by Allegrini et al. (2015), which was primarily associated with the recycling of the ferrous scrap.
Impact of the system boundaries
As was previously discussed, the relative importance of the PT category was strongly affected by the system boundaries of the study. This will be discussed within the context of the example of the GWP for the scenarios in which the PT had a significant impact on the GWP, namely SS:S1-PR, SS:S1-NR, and BA: (S1-RC, S2-GSP). In all the scenarios, the system boundaries were strongly limited to only include the processes that were potentially affected by the recycling activities. In the SS:S2-NR scenario, only the impacts from the actual nitrogen recovery process and the avoided PS were considered. These correspond to System boundary 1 in Figure 5.1. However, accounting for the fact that sewage sludge could have been landfilled in the baseline scenario, SS:S0-DI, the avoided impact from the CD would account for a higher share, thereby decreasing the relative importance of PT, which corresponds to the System boundary 2 in Figure 5.1. The importance of PT would decrease even further if the primary waste management system was included; e.g., anaerobic digestion of sewage sludge, which corresponded to System boundary 4 in Figure 5.1. Under such conditions, the increased impact from the FRP, which would include incineration of biogas, would decrease the share of PT. A similar decrease in the relative importance of PT in Scenario SS:S1-PR could be expected.
Figure 5.1: A range of possible system boundaries in the LCA studies.
System boundary 5 System boundary 4 System boundary 3 System boundary 2 System boundary 1
Primary waste management
CD PT Recoveredmaterial Replacedmaterial PS
PS Other
possibly recoverable
material
Possibly replaced material Waste
studied
Primary recovered
product replacedPrimary PS
material PT
Use Waste
FRP
FRP
FRP
Production and distribution Product
5.3 Limitations of the Current Research 93 A slightly different situation was observed in the scenarios presented in Publication VI: (S1-RC and S2-GSP), in which the reduction in the impact of the PT category could first be associated with the additional impact of the recovery of other fractions contained in BA, such as non-ferrous and ferrous scraps (corresponding to System boundary 3 in Figure 5.1).
Finally, the smallest relative share of PT could be expected when conducting an attributional LCA study with the system boundaries set equivalently to System boundary 5 in Figure 5.1. In such a case, all the impacts originating from the entire life cycle of a product would be included in the study, thus outshining the relative importance of the waste pretreatment activities.
Limitations of the Current Research
Since each of the LCA studies was conducted based on the specific LCI data, the results of the study strongly rely on the data choices made. The break-even points were not achieved in most of the scenarios studied during the sensitivity analysis, which incorporated the variation of the parameters that had the most significant impact on the PT category.
However, numerous alternative parameters might vary significantly and have a strong impact on the relative importance of the other waste management categories; i.e., conventional disposal, final recovery process, and product substitution.
Another point of uncertainty lays in the use of different methodologies to calculate the inventory data. For example, in Publication I, the release of the landfill gas and the operational impact of the landfill were calculated using a methodology that is valid in the Russian Federation. On the other hand, the operational impact of the landfill in Publications V and VI was primarily modelled using the parameters from other studies that were performed in the context of Finland. The impact of the trucks was usually modelled using the database embedded in the software used, yet the emissions from the working machinery were calculated using a Finnish emissions database Lipasto.21
Finally, uncertainty could be attributed to the LCIA stage of the studies. Despite the fact that a strong consensus has been achieved on the assessment of the environmental impact caused by greenhouse gases and the creation of robust characterization models, some improvements are still required for the ADP impact category, while the toxicity-related impact categories should even be used with caution (Hauschild et al. 2013). Furthermore,
21 More information on the database could be found here: http://lipasto.vtt.fi/.
Furthermore, the average global values that are employed to model the fate of chemical substances in the environment are utilised in the characterization models, thereby disregarding the impact of local parameters, such as depth of groundwater, type of soil, or ambient temperature, when modelling the environmental impact. Also, the inclusion of a limited number of impact categories could bias the results since the impact on the rest of the impact categories available remains unknown.
95
6. Conclusions
Quantification of the environmental impact of varying waste pretreatment methods along with the quantification of the overall impact of the waste management system revealed that the PT activities themselves, the type of waste being recycled, their conventional disposal methods and the type of product being substituted are of interest for the identification of a relative importance of the pretreatment activities on the environmental sustainability of waste management. However, a lack of a scientific method by which it is possible to compare several impact categories makes it impossible to generalise the results of this dissertation across the four impact categories studied, thus leading to separate conclusions for each impact category studied.
The PT activities were of significant importance for the GWP in the waste management systems that incorporated advanced waste treatment methods, such as those of Scenario SS:S1-PR, which involved thermochemical P recovery from sludge ash (Publication III);
SS:S1-NR, which involved N recovery from exhaust fumes during the process of thermal drying sewage sludge (Publication II); and BA: (S1-RC and S2-GSP), in which an advanced treatment method was used to segregate MSWI BA into several fractions (Publication VI).
The impact that the PT activities had on the overall GWP ranged from 29-64%.
Furthermore, the significance of the PT activities was further highlighted by the low impact of the avoided CD, which was landfilling of inorganic materials, and PS.
On the contrary, the low impact of PT activities on the GWP of 0.3-3.7% can be expected when the conventional disposal processes that have a high impact on GWP are avoided.
For example, landfilling of organic materials in a landfill without the landfill gas collection system has a significant impact on GWP, as studied in the scenarios presented in Publication I. Furthermore, it is reasonable to expect that PT processes will have a low impact on the GWP when waste recycling results in the substitution of materials that have substantial carbon footprints; e.g., burned lime as in the scenario TR:S4-RS (Publication V) or cement (Publication I).
Otherwise, the moderate importance of the PT could be expected in situations in which the GWP of PT is balanced with the avoided GWP of CD and PS, as in the scenarios of Publication IV. The sensitivity analysis of the studies in which the PT activities had a significant impact on the GWP revealed that the break-even point was only achieved in Scenario SS:S1-NR, in which N was recovered during the process of thermally drying sewage sludge (Publication II).
The sensitivity analysis revealed that the change in the GWP from the avoided impact of -18% to the induced impact of 2.6% could occur when doubling the electricity requirement for the process, variating the consumption of sodium hydroxide and taking the need for additional thermal energy into consideration. Finally, despite the fact that the PT processes had a significant impact on the GWP in the SS:S1-PR and BA:S2-GSP scenarios, the additional GWP of 28 and 59%, respectively did not transform into avoided GWP under the varying conditions studied in the sensitivity analysis.
The PT activities were of significant importance for the ADP in the waste management systems in which the PT activities required comparatively high amount of fuels, while having a low impact on CD and PS, as was the case in Scenarios DS: (S1-CEMFI, S2-LWA, and S3-CEMRU) outlined in Publication I, in which the share of the PT within the overall results ranged from 21-36%. On the contrary, the low importance of the PT could be anticipated when waste recycling results in the substitution of materials or fuels that have a high impact on the ADP; e.g., phosphorous, as in Scenarios SS:S1-PS (Publication II) and TR:S1-FF (Publication V); or cement, as in Scenario DS:S4-SW (Publication I), in which the share of PT ranged from 0.24-1.2%.
The sensitivity analysis performed for the ADP did not reveal any conditions for the achievement of the break-even points, thereby avoiding the impact on ADP. As outlined previously, cement has significant ADP, which was the reason why there was an additional ADP of 82% in Scenario BA:S2-GSP (Publication VI), in which additional cement was required when recycling the mineral fraction.
Straightforward results were achieved for the carcinogenic HTPc, where only a low (0.44-0.52%) and a low-to-moderate (3.7-5.0%) share of the overall impact was associated with the PT activities since the toxicity was mainly related to the release of heavy metals during the process of recycling the thermal residues. On the contrary, a moderate (1.9-9.2%) and significant (12-41%) share of the non-carcinogenic HTPNON-C was associated with the PT activities.
Unlike the rest of the impact categories, the major drivers for the high relative share, in this case, were the consumption of fuels required for the transportation and incorporation of the waste in the FRP. The impact of all scenarios on the HTPNON-C was positive under the default life cycle inventory. However, the sensitivity analysis identified a break-even point in the BA:S2-GSP scenario that involved recycling the mineral fraction obtained during the treatment of MSWI BA in the garden stone production process (Publication VI). The avoided HTPNON-C of 14% transformed into an additional impact of 17% when larger transportation distances and the higher diesel consumption of the pretreatment installation were considered.
Conclusions 97 Despite the significant variation in the relative importance of the PT activities across the studies and even controversial results for a certain scenario within different impact categories, an improved environmental situation was achieved in the majority of the scenarios studied. Therefore, the application of specific waste treatment, including advanced methods, is not expected to hinder the environmental sustainability of varying waste management systems, yet the execution of LCAs is recommended in the waste management systems similar to those that had a significant impact of the PT activities in the present dissertation.
99
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