Comparative environmental assessment of alternative waste management strategies in developing regions: A case study in Kazakhstan

Abstract

The management of municipal solid waste in the Republic of Kazakhstan is still in its infancy. This situation poses a potential threat to the environment and public health and, therefore, it is necessary to introduce improved management schemes in the country. In this study, the life cycle assessment methodology was followed to evaluate the potential environmental benefits of implementing alternative management schemes based on low-waste generation and renewable energy production. The current situation of the capital city Astana was considered as the base case. Environmental results showed that air emissions in terms of landfill gases are the major contributor to climate change impacts, while landfill disposal of the non-recovered fraction of recyclable materials was responsible for the highest impacts in the other categories (especially land use). However, the reuse of recycled materials largely offsets the related environmental burdens, along with energy generation. In comparative terms, it was demonstrated that the proposed waste management scenarios are more environmentally friendly than current practices (S0), mainly owing to the credits associated with the valorisation of renewable energy (S2) and recovered materials (S3). Consequently, the evaluation showed that greater efforts should be made to exploit the energy potential of organic fraction, together with higher recycling rates, to move towards lower environmental impacts associated with municipal solid waste management.

Keywords

Municipal solid waste waste management life cycle assessment environmental profile developing countries Kazakhstan

Introduction

In recent decades, policies related to municipal solid waste (MSW) management have changed in response to social and environmental concerns (Su et al., 2007; Wilson et al., 2015). However, modern waste management practices have not kept pace with the continuous growth of waste generation in developing countries (Bezama et al., 2007; Inglezakis et al., 2017; Wilson et al., 2015). Inadequate collection systems and disposal facilities have led to potential risks to public health and environmental pollution (Bezama et al., 2007; Orazbayev et al., 2013). The lack of comprehensive and suitable waste management systems therefore affirms the importance of promoting a comprehensive evaluation of the state-of-the-art and what needs to be done to address major environmental problems (Inglezakis et al., 2017; Syrlybayew et al., 2016).

Landfilling remains the dominant alternative for MSW management in the Republic of Kazakhstan; however, it seems that increasing environmental concerns have forced authorities to move towards more environmental-friendly strategies for MSW treatment (Inglezakis et al., 2017). Waste management in this country has been regulated by the Environmental Code of the Republic of Kazakhstan in combination with numerous orders and resolutions on sanitary regulations (Ministry of Justice of the Republic of Kazakhstan, 2007). Among them, the Program of Modernisation of Municipal Solid Waste Management for the years 2014–2050 has recently been developed as a legal tool to help move towards a green economy by increasing the efficiency and environmental reliability of MSW management services in Kazakhstan (Ministry of Environment and Water Resources, 2014). It proposes the gradual implementation of measures to promote higher recycling rates and updated management technologies, such as anaerobic digestion, composting and biogas valorisation (Ministry of Environment and Water Resources, 2014).

Consequently, in recent years mechanical–biological treatment plants have been introduced for the recovery of recyclable materials, although the valorisation of the organic waste through the generation of renewable energy has not yet been established in Kazakhstan (Inglezakis et al., 2017). This approach is in line with other developing regions on waste management that apply similar technologies as a first step towards environmental sustainability (Abeliotis et al., 2012; Bezama et al., 2007; Othman et al., 2013; Yay, 2015). Some authors went beyond conventional practices and also referred to waste-to-energy plants in which thermal treatment (incineration, gasification) or anaerobic digestion schemes were prioritised to generate energy and organic fertilisers as by-products (Chaya and Gheewala, 2007; Othman et al., 2013; Panepinto et al., 2015; Song et al., 2017). In most cases, they were found as economically attractive alternatives as well as potential environmental-friendly technologies compared with conventional ones. Thus, the valorisation of valuable recovered resources and the generation of renewable electricity are responsible for a great reduction in environmental impact owing to the optimal exploitation of the different fractions of MSW. However, most of these schemes are still in their early stages, and preference for one or the other option may depend to a large extent on MSW composition, as well as the local income level (Chaya and Gheewala, 2007; Othman et al., 2013; Tock and Schummer, 2017).

The adoption of the life cycle assessment (LCA) has been recommended with the aim of evaluating alternative technologies to make the most of comprehensive MSW management (Ministry of Environment and Water Resources, 2014; Ministry of Justice of the Republic of Kazakhstan, 2007). Indeed, according to the literature, LCA has been widely accredited for its ability to assess the potential environmental damages and benefits associated with different waste treatment configurations in both developed and developing regions (Abeliotis et al., 2012; Bernstad and la Cour Jansen, 2012; Chaya and Gheewala, 2007; Clearly, 2009; Laurent et al., 2014a, 2014b; Othman et al., 2013; Song et al., 2017; Tock and Schummer, 2017; Vandermeersch et al., 2014; Yay, 2015). However, the lack of consistency in methodological choices, as well as particular local conditions, makes it more difficult to achieve an absolute ranking of alternative schemes for MSW management (Bernstad and la Cour Jansen, 2012; Clearly, 2009). Therefore, although these disparities are sometime not decisive in the order of the preferred option, it should be desirable to analyse each situation individually pending progress towards a common methodological framework among LCA practitioners (Clearly, 2009).

In this context, to the best of the authors’ knowledge, no studies are available to date focusing on the environmental evaluation of prevailing MSW management technologies becoming obsolete in Kazakhstan besides the potential alternatives to move towards a more sustainable framework in the country. On the basis of the LCA guidelines (ISO 14040, 2006), the present study aimed at estimating the environmental performance of the current situation of the capital city of the country (Astana) as the baseline case for MSW management in Kazakhstan. Subsequently, the potential environmental benefits from alternative improvement schemes were also evaluated in relation to critical issues on current practices in the country.

Materials and methods

As aforementioned, the LCA methodology was selected to perform the environmental analysis. According to the International Organization for Standardization (ISO) standards (ISO 14040, 2006), four main phases can be distinguished in a LCA study: goal and scope definition, inventory analysis, impact assessment and interpretation.

Goal and scope definition

The research area considered in this study was located in Astana, the current capital city of Kazakhstan, with an estimated population of 872,619 inhabitants and a MSW generation rate of approximately 1118 t day^-1 in 2013 (Inglezakis et al., 2015; NSC, 2016). However, owing to the absence of a suitable waste collection system, only around 800 t day^-1 (72% of the total MSW generated) is subjected to further processing (Inglezakis et al., 2014, 2015), while the rest is uncontrolled, managed at the source. The average composition of municipal waste in Astana (Table 1) is led by the organic waste (27.6%) – mostly composed by food waste – followed by plastics (15.5%), glass (14.9%) and paper and cardboard (11.2%) (Inglezakis et al., 2017).

Table 1.

Average MSW composition in Astana (Inglezakis et al., 2017).

Waste fraction	Percentage (%)	Mass flow (t day^-1)
Organics (food waste)	27.6	220.8
Plastics	15.5	124.0
Glass	14.9	119.2
Paper/cardboard	11.2	89.6
Garden	2.8	22.4
Metals	1.0	8.0
Wood	0.6	4.8
Others	26.4	211.2
Total	100.0	800.0

Currently, the waste management system in Astana is mainly based on the mechanical separation of only a small fraction of recyclable materials and organic waste (before landfill disposal), in detriment of landfilling practices applied in other regions of the country (Inglezakis et al., 2014). However, despite the effort, this waste management strategy is yet far from ideal, and further studies for the implementation of more sustainable treatment schemes are being developed in the city (Inglezakis et al., 2017).

In this context, this study has focused its attention on two primary objectives: (i) the assessment of the environmental impacts of the current priority waste management practice in Astana and (ii) the comparison with the environmental performance of alternative management schemes proposed as potential improvement configurations. A cradle-to-grave analysis was carried out covering all stages, from the collection and transport of waste to the final management and/or disposal of the different MSW fractions. Moreover, a system expansion approach was applied to avoid allocation issues, in accordance with ISO standards (ISO 14040, 2006); this means that the overall system must be expanded to include additional functions related to the co-products obtained. Therefore, since both recycled materials and renewable energy can be recovered as main outputs of the system, the environmental benefits from their further use as substitutes for non-recycled materials and fossil energy, respectively, were also considered. Similarly, related burdens should also be accounted for, since the subsequent usage stage may have consequences on the environment.

The functional unit

The functional unit (FU) can be defined as the quantified performance of a product system to be used as a reference unit in an LCA study (ISO 14040, 2006); therefore, it provides the reference necessary to ensure comparability of LCA results from different sources (ISO 14040, 2006). Waste mass-based FUs are frequently used in LCA studies of waste management systems (Clearly, 2009; Laurent et al., 2014a; Othman et al., 2013). Since the main objective of this study was to evaluate and compare the environmental profile of alternative strategies focusing on MSW management, a FU of 1 t of treated MSW was considered the best choice as a common basis for comparison in agreement with literature.

Description of alternative management scenarios

A total of four alternative scenarios were considered for MSW management: Mechanical treatment (MT) without landfill gases valorisation and minor material recycling (current scenario – S0), landfilling without landfill gases valorisation and material recycling (S1), MT with landfill gases valorisation and minor material recycling (S2) and MT with landfill gases valorisation and major material recycling (S3). The main stages of each management scheme are shown in Figures 1 –4. It should be noted that, as the selective collection of MSW at source is not yet available in Astana to date, MT facilities were designed to be able to manage mixed incoming waste, while integrating a modern sorting system to separate some fractions of recyclable materials. Moreover, a cogeneration (cogeneration/combined heat and power; CHP) unit was assumed to be used for energy generation from the combustion of landfill gases in hypothetical scenarios involving their valorisation (S2 and S3).

The same input flow and MSW composition was considered for all the scenarios (Table 1). In this regard, it is important to underline the limitations on the waste acceptance capacity of the MT plant, below 50% of the collection rates (≈340 t day^-1 vs. 800 t day^-1) in 2013. Consequently, it is assumed that about 340 t are treated daily at the MT plant in all the scenarios, while the remaining waste must be briquetted to be sent to landfill (Inglezakis et al., 2017). Accordingly, it was considered appropriate to include an additional scenario (S4) in the study, based on a reduction in waste generation at source to comply with the current capacity of the MT plant, i.e. around 340 t day^-1 (Figure 5); analogous to the other scenarios, the same MSW composition was assumed in S4 (Table 1).

However, different recycling rates were evaluated. Thus, a greater (20%) recyclability of the potentially recovered materials was assumed in S3, avoiding the production of their analogous selves in the market (system expansion approach), in accordance with the recovery capacity initially projected for the MT plant. By contrast, only a minor fraction (6%) was considered recyclable for subsequent use in S0, S2 and S4 in agreement with current practices. Finally, recovery of materials does not exist in S1 (landfilling). The following recovered materials were included in the calculations: Paper/cardboard (26.6%), plastics (35.7%), glass (35.4%) and metals (2.3%).

Life cycle inventory analysis

The inventory analysis involves data collection to quantify relevant input and output flows for the different scenarios evaluated on an annual basis (2013–2014). A standard approach for data collection was followed to ensure the reliability of the comparative results. The following inventory information, available for the different alternative scenarios, was considered (Table 2): MSW input flow and composition, transportation activities, electricity requirements and renewable energy generation (only in case of landfill gases valorisation), recovered materials (except for landfilling), land use and diffuse emissions in terms of methane (CH₄) and carbon monoxide (CO); biogenic carbon dioxide (CO₂) was assumed to be discharged to the atmosphere but without related environmental impacts.

Table 2.

Average annual inventory data (primary and secondary sources) for the different scenarios (Vassilis Inglezakis, April 2016, personal communication): S0 – MT plant; S1 – landfilling; S2 – MT + 100% landfill gases valorisation; S3 – MT + 100% landfill gases valorisation + major material recycling; S4 – MT + waste reduction.

Inputs/outputs	S0	S1	S2	S3	S4
Inputs
MSW collected (t)	292,000	292,000	292,000	292,000	124,100
Electricity (kWh)^a	320,178	0.00	320,178	1,067,260	320,178
Transportation (tkm)	1,606,000	1,606,000	1,606,000	1,606,000	682,550
Land use (ha)	1.22	1.25	1.22	1.15	0.50
Emissions
CH₄ (t) (air)^a	16,441	16,441	0.00	0.00	6987
CO (kg) (air)^b	524	524	524	524	223
Materials disposal
Paper/cardboard (t)	28,857	32,704	28,857	18,922	10,052
Plastics (t)	42,778	45,260	42,778	36,945	16,754
Glass (t)	42,763	43,508	42,763	41,026	17,746
Metals (t)	3997	2774	3997	1595	2402
Organics (t)	80,592	80,592	80,592	80,592	34,252
Others (t)	85,567	87,162	85,567	88,100	35,448
Total (t)	284,554	292,000	284,554	267,180	116,654
Renewable energy
CH₄ combustion (t)^a	—	—	16,441	16,441	—
Electricity (MWh)^c	—	—	87,279	87,279	—
Heat (MWh)^c	—	—	109,669	109,669	—
Materials recycling
Paper/cardboard (t)	3847	—	3847	12,782	3847
Plastics (t)	2482	—	2482	8315	2482
Glass (t)	745	—	745	2482	745
Metals (t)	372	—	372	1241	372
Total (t)	7446	—	7446	24,820	7446

Abeliotis et al. (2012), Bernstad and la Cour Jansen (2012).

Cherubini et al. (2009).

Pöschl et al. (2010).

MSW: municipal solid waste.

Primary inventory data regarding MSW flow and composition, transport distances and collection system (transport fleet, collection frequency) as well as recovered rates for recyclable materials (only current state) and final land use requirements was prioritised and collected based on personal communications from facilities managers.

However, it was also necessary to collect secondary from the literature to complete the inventory of the different systems in the absence of primary information. Greenhouse gas (GHG) emissions from landfilling (landfill gases) were estimated according to emissions factors provided by Abeliotis et al. (2012) and Bernstad and la Cour Jansen (2012) for CH₄ (around 300 m³ CH₄ t^-1 organic waste) and Cherubini et al. (2009) for CO (13 mg CO m^-3 landfill gas). Moreover, the energy potential of such landfill gases was also taken into consideration (only in case of their valorisation).

Thus, a calorific value of 9.45 kWh m^-3 CH₄ was assumed, resulting in a ratio of 5.67 kWh m^-3 landfill gas with a composition of 60% CH₄ (40% biogenic CO₂) (IDEA, 2014); note that total valorisation was assumed in the absence of landfill gas losses in relevant scenarios. Average rates of 40% and 48% were considered for electrical and thermal efficiency in the CHP unit, respectively (Pöschl et al., 2010); however, 4.5% of electricity produced was assumed to be consumed as input for CHP operation (Pöschl et al., 2010). Similarly, the electricity consumption rate (≈43 kWh t^-1 MSW recovered) of the MT facilities as a whole was also estimated according to the studies available in the literature (Abeliotis et al., 2012; Bernstad and la Cour Jansen, 2012); no energy requirements were assumed at landfill facilities (Abeliotis et al., 2012).

Finally, the ecoinvent^® database (Classen et al., 2009; Dones et al., 2007; Hischier, 2007; Spielmann et al., 2007) was used for the background inventory regarding energy (electricity and heat) generation and diesel production (consumption of transport activities). The avoided manufacture of recovered materials (paper/cardboard, plastics, glass, metals) owing to recycling activities was also taken into account, as well as the generation of avoided fossil energy (avoided processes), accounted as possible environmental credits.

Life cycle impact assessment

The impact assessment of LCA involves associating inventory data with specific environmental impacts. To this aim, this evaluation phase includes three main stages: Selection of impact categories, classification and characterisation (ISO 14040, 2006). Classification consists of linking the inventory data to the different impact categories while characterisation focuses on calculating the final environmental results for each category taking into account the factors provided by the characterisation method selected (ISO 14040, 2006). The following impact categories were selected to evaluate the environmental profile of the different scenarios: Climate change (CC), terrestrial acidification (TA), freshwater eutrophication (FE), marine eutrophication (ME) and fossil depletion (FD). This set of categories is in line with related studies in literature, which report the impacts on global warming, acidification and eutrophication potential and resources depletion as the most relevant issues to be addressed in waste management systems (Clearly, 2009; Laurent et al., 2014b; Othman et al., 2013). Additionally, land use (LU) was also considered, taking into account the land occupied by waste management facilities (mainly owing to landfill surface).

The characterisation factors reported by the ReCiPe Midpoint (H) 1.12 method (Goedkoop et al., 2013a) were considered to estimate the environmental impacts in terms of terrestrial acidification, freshwater eutrophication, marine eutrophication and fossil depletion, while the effect on climate change was evaluated according to Intergovernmental Panel on Climate Change (IPCC) guidelines (IPCC, 2013). The SimaPro v8.2 (PRé Consultants, Amersfoot, Netherlands) software was used for the computational implementation of the inventory results (Goedkoop et al., 2013b). SimaPro can be defined as a professional tool to calculate the environmental impacts associated with a product throughout its life cycle, in line with the LCA perspective. It contains internationally renowned databases (such as ecoinvent), as well as the main impact evaluation methodologies, including the ReCiPe and IPCC methods (Goedkoop et al., 2013b). According to LCA experts, the former is the most updated alternative that provides a common framework in which both midpoint and endpoint indicators can be used, as opposed to similar methodologies to date (PRé Consultants, 2016). The IPCC characterisation factors were developed by the IPCC to focus particular attention on the direct global warming potential of air emissions (PRé Consultants, 2016).

Results and discussion

Environmental results of the current scenario (S0)

The environmental impacts associated with the current MSW management strategy in Astana (S0) are shown in Figure 6. Diffuse emissions refer to GHGs emitted directly into the atmosphere, while the disposal of materials includes the fraction of MSW discharged to landfill; the effect of the recovery of the different types of materials was also considered individually, according to the ratios reported in Table 2.

According to the results, the landfill of around 94% of the recyclable material was the main contributor to the environmental burdens in all the impact categories (except for climate change), especially in marine eutrophication owing to the effect of related nitrogen emissions. The emissions of GHGs (CH₄ and CO) derived from the degradation of organic waste in the landfill was responsible for the greatest impacts (around 73%) in climate change. Since energy requirements for the operation of the MT plant and transport activities from waste collection to plant location had minor influence on the overall results, both contributing factors were not included in the figure.

Conversely, the recovery and subsequent reuse of a small fraction (around 6%) of the recyclable material had a beneficial effect (below x-axis) except for climate change. This could mainly be attributed to recovered paper and cardboard (from 17% in fossil depletion to 98% in land use) followed by recovered plastics (up to 53% in fossil depletion). Special attention should be paid to the environmental-friendly contribution to land use from paper/cardboard recycling; it was directly linked to the arable land required for the cultivation of raw materials for the industrial production of both goods (paper and cardboard). Finally, no environmental credits were registered from power generation since no valorisation of landfill gases is available today in the MT plant.

Comparative environmental results (S0–S4)

Characterisation results for the different scenarios and impact categories are reported and compared with the current situation (S0) in Table 3. In view of the results, the scenarios in which the valorisation of landfill gases could potentially take place (i.e. S2 and S3) showed the best environmental performances. The production of renewable energy from the use of the CH₄ generated in the landfill was the main one responsible for such desirable results regarding climate change mitigation. CH₄ is not released to the atmosphere, so that GHG emissions are deducted at the source; this is the rationale behind the significantly lower balance of S2 and S3 in climate change. On the other hand, the greater fraction of recovered materials made the difference between S2 and S3; while major rates (20%) of recyclable materials were assumed to be recovered in S3, only 6% was considered in S2. Raising the fraction of recovered materials resulted in lower production of its analogous selves in the market and, therefore, higher environmental credits owing to avoided processes. Accordingly, the environmental impacts resulting from the increased energy requirements of S3 were fully offset by the favourable contribution of its greater recycling capacity.

Table 3.

Comparative environmental results for the different scenarios per ton of MSW treated (FU): S0 – MT plant; S1 – landfilling; S2 – MT + 100% landfill gases valorisation; S3 – MT + 100% landfill gases valorisation + major material recycling.

Impact category	Units	S0	S1	S2	S3	S4
CC	kg CO₂ eq	1859.9	1910.8	318.2	121.8	761.2
TA	kg SO₂ eq	−0.1	0.1	−0.9	−1.4	−0.1
FE	g P eq	1.5	14.0	−36.1	−56.4	−6.5
ME	kg N eq	2.7	2.8	2.7	0.2	1.1
LU	m²	−91.3	1.3	−102.4	−318.1	−92.0
FD	kg oil eq	−12.6	6.6	−50.1	−125.4	−16.4

CC: climate change; TA: terrestrial acidification; FE: freshwater eutrophication; ME: marine eutrophication; LU: land use; FD: fossil depletion.

When land use is evaluated individually, environmental-friendly (negative) results can be found in all scenarios, except S1. Renewable energy generation did not make a relevant contribution in this category, while recovered materials exerted the greatest influence, according to previous results (S0 – Figure 6). Thus, while S0, S2 and S4 shared close results based on the same recovered fraction, again S3 showed to be the best scenario with greater recyclability.

Similarly, significant environmental advantages were identified when lower generation ratios were assumed in the city (S4). As aforementioned, the volumes of waste generated (and collected) are far above the capacity of the treatment facilities available, so that a large fraction must be sent directly to landfill. This causes significant impacts on the environment, especially owing to diffuse emissions and their consequent influence on climate change and eutrophication problems. Therefore, when lower volumes of waste are assumed to be generated, the related impacts are considerably minimised, even below the values associated with the scenarios focusing on energy recovery (S2 and S3) in terms of eutrophication potential. Climate change continued to be penalised by carbon emissions in S4 owing to the degradation of the remaining organic fraction in the landfill. However, similar to S2 and S3 strategies, the valorisation of landfill gases could partially offset the related impacts, resulting in a more favourable profile compared with the other alternatives.

Therefore, the environmental results would be in line with some innovative proposals promoted by authorities in Astana. Thus, modern technologies have been recently projected in the city involving the collection and valorisation of CH₄ as an energy source, seeking the avoidance of related climate change impacts (Inglezakis et al., 2017). However, the techno-economic feasibility of such scenarios should be also demonstrated before establishing the priority ranking. In fact, the integration of the economic approach into the sustainable design of management alternatives has become a key complementary competence in making sustainable decisions on waste management, which could lead to alternative outcomes, despite using the life cycle thinking as a reference framework.

In any case, these measures would be insufficient to solve the problems arising from waste generation that exceeds treatment capacity. It has been demonstrated that the direct disposal of large volumes of waste is linked to large landfill sites and, consequently, to extensive land requirements (land use impact). Moreover, it also contributes indirectly to a greater impact on terrestrial resources, as well as to most impact categories as a whole. This situation enhances ongoing initiatives on waste reduction promoted by relevant authorities in the country, as a priority measure to move towards sustainability (Ministry of Justice of the Republic of Kazakhstan, 2007). It would also be associated with the absence of selective MSW collection systems at source, and it would be desirable to focus a more targeted effort on the advanced design of the primary treatment stages – collection and conditioning – in combination with incentives for reuse and reduction of the amount of waste disposed of by society. Finally, the implementation of advanced mechanical sorting units, which also involve the recovery of organic waste, as well as the recovery of recyclable materials, could lead to the most promising results from an environmental point of view.

Conclusions

The principles of the LCA methodology were followed in this study to analyse the potential environmental credits of implementing alternative MSW management strategies in Kazakhstan compared with the current situation. Primary data from actual MSW management in Astana (capital of the country) was used as the base scenario; landfilling as the worst-case scenario was also considered for comparison. The findings of this research reported that the scenarios in which the generation of renewable energy from landfill gases combustion is prioritised (in detriment of GHGs released into the atmosphere) show the best environmental performance in terms of climate change mitigation. Moreover, the higher the recovered materials fraction, the most environmental-friendly results were reported in the other impact categories evaluated.

Therefore, in view of the environmental results, the collection and subsequent valorisation of landfill gases could play a key role in improvement strategies, especially on energy terms. In this regard, the pending projects for the modernisation of new landfill facilities would make it possible to achieve more energy-efficient solutions and reduce the burdens derived from GHGs released to air that have a critical effect on climate change concerns. Similarly, technologies focusing on the use of energy from organic waste (such as anaerobic digestion and thermal treatment) should be also prioritised over other potential alternatives. In addition, special attention should also be paid to the recovery of materials owing to its relevant influence on most impact categories (including land use). This issue could be directly related to the promotion of actions for selective separation at source in households. Indeed, progress in this context could lead to a significant increase in recovery rates; recyclable materials and organic waste could be collected separately, so that they could also be treated in an individualised, targeted and effective manner.

However, in addition to the above strategies, the reduction of waste generation was also considered to be responsible for significant advantages in most categories, with the exception of climate change impacts, compared with the other alternatives. This demonstrates the potential benefits of improving waste reduction (and proper collection) as a first step towards building an integrated system for sustainable waste management. In this way, it could be understood as a complementary solution to waste recycling and bio-waste valorisation, leading to the most promising results in the most relevant environmental indicators.

These major findings are expected to help to continue promoting the development of more sustainable waste management schemes that comply with future environmental regulations in Kazakhstan and other developing countries in an analogous situation. However, sustainability has become a key issue in alternative decision-making situations involving waste management, so that a harmonious co-existence of environmental, economic and social perspectives, as well as technical issues, should be always promoted to provide valuable information on potential scenarios. In this regard, further studies on techno-economic feasibility should be carried out to fully demonstrate the wider suitability of such scenarios.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was financially supported by the Spanish Ministry of Economy and Competitiveness [project ref. CTQ2016-75136-P] and by Xunta de Galicia [project ref. ED431F 2016/001]. Also, this work was partly supported by the internal fund for research of Nazarbayev University [ORAU], project title ‘Development of municipal solid waste combustion and incineration technology for Astana (Kazakhstan) and investigation of municipal solid waste blending effects on reactivity of coals in CFB combustion and gasification processes’ [Research Council Decision No. 98 of 04.04.2017]. The authors (I Noya, S González-García, G Feijoo and MT Moreira) belong to CRETUS [AGRUP2015/02] and the Galician Competitive Research Group GRC 2013-032, co-funded by Xunta de Galicia and FEDER. Dr S González-Garcia would like to express her gratitude to the Spanish Ministry of Economy and Competitivity for financial support [Grant reference RYC-2014-14984].

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