Two decades of research on waste management in the circular economy: Insights from bibliometric, text mining, and content analyses

Abstract Achieving environmental sustainability and transition from a linear economy to a circular economy (CE) highly relies on effective waste management (WM) and how waste is treated as a potential future resource. This research aims to provide an inclusive map of the scientific background of WM in the CE context over the last two decades from 2001 to 2020 to identify its salient research themes and trends, main characteristics, evolution, and potentially valuable directions for future studies. To achieve that, the following research questions were addressed by applying a mixed-method approach including bibliometric, text mining, and content analyses: (i) how has the field of WM research evolved within the CE domain? (ii) what are the salient research themes and trends of WM in the CE? and (iii) what are the possible directions for future research on WM within the CE context? As a result, the synthesized bibliometric networks were constructed and analyzed for a total of 962 journal articles extracted from the Web of Science database to visualize the main body of literature. Consequently, the seven major research themes of WM in the CE context were identified as follows: (1) bio-based WM; (2) CE transition; (3) electronic waste; (4) municipal solid waste; (5) environmental impacts and lifecycle assessment; (6) plastic waste; and (7) construction and demolition WM. The provided inclusive research landscape of WM systems, and its prominent highlight patterns can serve as a base for a real-time guideline to lead further research areas and as a tool to support WM policy-makers and practitioners to support the CE transition (which aims to minimize the waste generation). Finally, the future research directions to better position WM research activities within the CE context as a waste minimization approach are provided.

[1]  Michael Nelles,et al.  Waste Management in Germany – Development to a Sustainable Circular Economy? , 2016 .

[2]  S. Hellweg,et al.  Do We Have the Right Performance Indicators for the Circular Economy?: Insight into the Swiss Waste Management System , 2017 .

[3]  G. Finnveden,et al.  Environmental Assessment of Possible Future Waste Management Scenarios , 2017 .

[4]  W. Tsai Turning Food Waste into Value-Added Resources: Current Status and Regulatory Promotion in Taiwan , 2020, Resources.

[5]  L. Agostinetto,et al.  Disposal of animal healthcare services waste in southern Brazil: One Health at risk , 2019 .

[6]  Matthias Zessner,et al.  Overview and description of technologies for recovering phosphorus from municipal wastewater , 2015 .

[7]  J. Merigó,et al.  A bibliometric overview of the Journal of Business Research , 2016 .

[8]  Tasbirul Islam,et al.  Reverse logistics and closed-loop supply chain of Waste Electrical and Electronic Equipment (WEEE)/E-waste: A comprehensive literature review , 2018, Resources, Conservation and Recycling.

[9]  Daniel C W Tsang,et al.  Extended theory of planned behaviour for promoting construction waste recycling in Hong Kong. , 2019, Waste management.

[10]  Teresa M Mata,et al.  Bio-refinery approach for spent coffee grounds valorization. , 2018, Bioresource technology.

[11]  Mohamad Mojarab Soufiyan,et al.  Exergy analysis of a lignocellulosic-based biorefinery annexed to a sugarcane mill for simultaneous lactic acid and electricity production , 2018 .

[12]  David Laner,et al.  Circular economy of plastic packaging: Current practice and perspectives in Austria. , 2018, Waste management.

[13]  Manu Sharma,et al.  Assessing enablers of e-waste management in circular economy using DEMATEL method: An Indian perspective , 2020, Environmental Science and Pollution Research.

[14]  Adisa Azapagic,et al.  Environmental assessment of microwaves and the effect of European energy efficiency and waste management legislation. , 2018, The Science of the total environment.

[15]  Matthew D. Jones,et al.  A circular economy approach to plastic waste , 2019, Polymer Degradation and Stability.

[16]  K. Van Acker,et al.  The environmental impacts of municipal solid waste landfills in Europe: A life cycle assessment of proper reference cases to support decision making. , 2020, Journal of environmental management.

[17]  E. Imbert Food waste valorization options: opportunities from the bioeconomy , 2017 .

[18]  Jonas Pagh Jensen,et al.  Evaluating the environmental impacts of recycling wind turbines , 2018, Wind Energy.

[19]  Reza Tavakkoli-Moghaddam,et al.  Reliable design of a closed loop supply chain network under uncertainty: An interval fuzzy possibilistic chance-constrained model , 2013 .

[20]  P. Pazienza,et al.  For a new plastics economy in agriculture: Policy reflections on the EU strategy from a local perspective , 2020 .

[21]  Kannan Govindan,et al.  Industry 4.0 based sustainable circular economy approach for smart waste management system to achieve sustainable development goals: A case study of Indonesia , 2020 .

[22]  Costas A. Velis,et al.  Metrics for optimising the multi-dimensional value of resources recovered from waste in a circular economy: A critical review , 2017 .

[23]  Bernard Yannou,et al.  How circular economy and industrial ecology concepts are intertwined? A bibliometric and text mining analysis , 2020, ArXiv.

[24]  Bernd Bilitewski,et al.  The circular economy and its risks. , 2012, Waste management.

[25]  Jiří Jaromír Klemeš,et al.  Minimising the present and future plastic waste, energy and environmental footprints related to COVID-19 , 2020, Renewable and Sustainable Energy Reviews.

[26]  Yang Liu,et al.  Barriers to smart waste management for a circular economy in China , 2019, Journal of Cleaner Production.

[27]  Zilong Zhang,et al.  An Integrated Measurement of the Efficiency of China’s Industrial Circular Economy and Associated Influencing Factors , 2020, Mathematics.

[28]  Anthony Halog,et al.  Developing strategies for managing construction and demolition wastes in Malaysia based on the concept of circular economy , 2017 .

[29]  N. Voća,et al.  Life-cycle assessment of solid-waste management in city of Zagreb, Croatia , 2018 .

[30]  E. Rutkowski,et al.  Waste management drivers towards a circular economy in the global south - The Colombian case. , 2020, Waste management.

[31]  H. Ng,et al.  Recent advances on the sustainable approaches for conversion and reutilization of food wastes to valuable bioproducts. , 2020, Bioresource technology.

[32]  Leonidas Milios,et al.  Plastic recycling in the Nordics: A value chain market analysis. , 2018, Waste management.

[33]  Amr Tolba,et al.  Internet of things-based urban waste management system for smart cities using a Cuckoo Search Algorithm , 2020, Cluster Computing.

[34]  Fu Jia,et al.  Sustainable global sourcing : A systematic literature review and bibliometric analysis , 2018 .

[35]  W. Vermeulen,et al.  The circular economy: New or Refurbished as CE 3.0? — Exploring Controversies in the Conceptualization of the Circular Economy through a Focus on History and Resource Value Retention Options , 2017, Resources, Conservation and Recycling.

[36]  Eren Özceylan,et al.  A closed-loop supply chain network design for automotive industry in Turkey , 2017, Comput. Ind. Eng..

[37]  Xiu-Hao Ding,et al.  Exploring the domain of open innovation: Bibliometric and content analyses , 2020 .

[38]  A. Azapagic,et al.  Environmental and economic implications of recovering resources from food waste in a circular economy. , 2019, The Science of the total environment.

[39]  F. Quatraro,et al.  Three pillars of sustainability in the wake of COVID-19: A systematic review and future research agenda for sustainable development , 2021, Journal of Cleaner Production.

[40]  Julia A. Stegemann,et al.  Limitations of the waste hierarchy for achieving absolute reductions in material throughput , 2016 .

[41]  V. Narayanamurti,et al.  Urban waste to energy recovery assessment simulations for developing countries , 2020 .

[42]  J. Fellner,et al.  Potentials for a circular economy of mineral construction materials and demolition waste in urban areas: a case study from Vienna , 2020 .

[43]  D. Fino,et al.  Eco-efficient waste glass recycling: Integrated waste management and green product development through LCA. , 2012, Waste management.

[44]  Atiq Uz Zaman,et al.  A comprehensive review of the development of zero waste management: lessons learned and guidelines , 2015 .

[45]  B. Singh,et al.  Sewage sludge disposal strategies for sustainable development , 2017, Environmental research.

[46]  Amirreza Mahpour Prioritizing barriers to adopt circular economy in construction and demolition waste management , 2018, Resources, Conservation and Recycling.

[47]  Jun,et al.  The challenges of water, waste and climate change in cities , 2017, Environment, Development and Sustainability.

[48]  Y. Xing,et al.  The bibliometric analysis and review of dioxin in waste incineration and steel sintering , 2019, Environmental Science and Pollution Research.

[49]  Evina Katsou,et al.  Municipal solid waste management and waste-to-energy in the context of a circular economy and energy recycling in Europe , 2017 .

[50]  L. R. Luttenberger Waste management challenges in transition to circular economy – Case of Croatia , 2020 .

[51]  Ruoyu Jin,et al.  Science mapping approach to assisting the review of construction and demolition waste management research published between 2009 and 2018 , 2019, Resources, Conservation and Recycling.

[52]  Carmen Ruiz-Puente,et al.  Development of the Tool SymbioSyS to Support the Transition Towards a Circular Economy Based on Industrial Symbiosis Strategies , 2016, Waste and Biomass Valorization.

[53]  Reinout Heijungs,et al.  Electricity generation and GHG emission reduction potentials through different municipal solid waste management technologies: A comparative review , 2017 .

[54]  Alejandro Gallego-Schmid,et al.  Is Recycling Always the Best Option? Environmental Assessment of Recycling of Seashell as Aggregates in Noise Barriers , 2020, Processes.

[55]  C. Madu,et al.  Implementation of circular economy principles in management of end-of-life tyres in a developing country (Nigeria) , 2020, AIMS Environmental Science.

[56]  M. Smol,et al.  Transformation towards Circular Economy (CE) in Municipal Waste Management System: Model Solutions for Poland , 2020, Sustainability.

[57]  A. Fullana,et al.  Plastic flexible films waste management - A state of art review. , 2018, Waste management.

[58]  Maria Victoria Peeler,et al.  International comparative study of 3R and waste management policy developments , 2011 .

[59]  O. Ogunseitan,et al.  Medical waste: Current challenges and future opportunities for sustainable management , 2021, Critical Reviews in Environmental Science and Technology.

[60]  Laith A. Hadidi,et al.  Deploying Municipal Solid Waste Management 3R-WTE Framework in Saudi Arabia: Challenges and Future , 2020, Sustainability.

[61]  Vincenzo Torretta,et al.  Waste Mismanagement in Developing Countries: A Review of Global Issues , 2019, International journal of environmental research and public health.

[62]  Jian Zuo,et al.  Construction and demolition waste research: a bibliometric analysis , 2019, Architectural Science Review.

[63]  A. Rashid,et al.  Towards circular economy implementation: a comprehensive review in context of manufacturing industry , 2016 .

[64]  M. E. Correa-Cano,et al.  Potential trade-offs between eliminating plastics and mitigating climate change: An LCA perspective on Polyethylene Terephthalate (PET) bottles in Cornwall. , 2020, The Science of the total environment.

[65]  M. Agovino,et al.  The good and the bad: Identifying homogeneous groups of municipalities in terms of separate waste collection determinants in Italy , 2019, Ecological Indicators.

[66]  S. Fiore,et al.  Assessment of the Synergy between Recycling and Thermal Treatments in Municipal Solid Waste Management in Europe , 2020 .

[67]  M. Aghbashlo,et al.  Comprehensive exergoeconomic analysis of a municipal solid waste digestion plant equipped with a biogas genset. , 2019, Waste management.

[68]  S. Chauhan,et al.  Circular economy research: A bibliometric analysis (2000–2019) and future research insights , 2020 .

[69]  K. Winans,et al.  The history and current applications of the circular economy concept , 2017 .

[70]  A. Pires,et al.  Waste hierarchy index for circular economy in waste management. , 2019, Waste management.

[71]  S. Rafiee,et al.  Environmental life cycle assessment of different biorefinery platforms valorizing municipal solid waste to bioenergy, microbial protein, lactic and succinic acid , 2020, Renewable and Sustainable Energy Reviews.

[72]  P. Dias,et al.  A circular approach to the e-waste valorization through urban mining in Rio de Janeiro, Brazil , 2020 .

[73]  Kannan Govindan,et al.  A review of reverse logistics and closed-loop supply chains: a Journal of Cleaner Production focus , 2017 .

[74]  M. Loizidou,et al.  Utilization of Agricultural and Livestock Waste in Anaerobic Digestion (A.D): Applying the Biorefinery Concept in a Circular Economy , 2020, Energies.

[75]  A. Prieto,et al.  To be, or not to be biodegradable… that is the question for the bio‐based plastics , 2016, Microbial biotechnology.

[76]  Sarah J. Cowell,et al.  Extended Producer Responsibility for Waste Electronics: An Example of Printer Recycling in the United Kingdom , 2005 .

[77]  Bernard Yannou,et al.  A taxonomy of circular economy indicators , 2018, Journal of Cleaner Production.

[78]  S. Jakhar,et al.  The interplay of circular economy with industry 4.0 enabled smart city drivers of healthcare waste disposal , 2020, Journal of Cleaner Production.

[79]  Vincenza Faraco,et al.  Biological processes for advancing lignocellulosic waste biorefinery by advocating circular economy. , 2016, Bioresource technology.

[80]  Harold Krikke,et al.  Revealing an invisible giant: A comprehensive survey into return practices within original (closed-loop) supply chains , 2013 .

[81]  Qinghua Zhu,et al.  Corporate social responsibility for supply chain management: A literature review and bibliometric analysis , 2017 .

[82]  Wei Deng Solvang,et al.  Reverse Logistics Network Design for Effective Management of Medical Waste in Epidemic Outbreaks: Insights from the Coronavirus Disease 2019 (COVID-19) Outbreak in Wuhan (China) , 2020, International journal of environmental research and public health.

[83]  Yong Sik Ok,et al.  COVID-19's unsustainable waste management , 2020, Science.

[84]  Shu-Yuan Pan,et al.  Strategies on implementation of waste-to-energy (WTE) supply chain for circular economy system: a review , 2015 .

[85]  R Sarc,et al.  Dynamic visualisation of municipal waste management performance in the EU using Ternary Diagram method. , 2017, Waste management.

[86]  Kevin E. O’Connor,et al.  Bio-based and biodegradable polymers - State-of-the-art, challenges and emerging trends , 2020 .

[87]  Z. Harris,et al.  Don’t scrap the waste: The need for broader system boundaries in bioplastic food packaging life-cycle assessment – A critical review , 2020, Journal of Cleaner Production.

[88]  A. Heshmati,et al.  A review of the circular economy in China : Moving from rhetoric to implementation , 2013 .

[89]  R. Kemp,et al.  Circular Economy Policies in China and Europe , 2017 .

[90]  Bing Xiao,et al.  Research trend of the application of information technologies in construction and demolition waste management , 2020 .

[91]  Jagdeep Singh,et al.  Resource recovery from post-consumer waste : Important lessons for the upcoming circular economy , 2016 .

[92]  J. Jambeck,et al.  The Chinese import ban and its impact on global plastic waste trade , 2018, Science Advances.

[93]  F. Cecchi,et al.  Smart Approaches to Food Waste Final Disposal , 2019, International journal of environmental research and public health.

[94]  A. Jesus,et al.  Lost in Transition? Drivers and Barriers in the Eco-Innovation Road to the Circular Economy , 2017 .

[95]  M. Pérez-Camacho,et al.  Regional assessment of bioeconomy options using the anaerobic biorefinery concept , 2018, Proceedings of the Institution of Civil Engineers - Waste and Resource Management.

[96]  Pantelitsa Loizia,et al.  The concept of circular economy strategy in food waste management for the optimization of energy production through anaerobic digestion , 2018, Environmental Science and Pollution Research.

[97]  A. Kushairi,et al.  OIL PALM ECONOMIC PERFORMANCE IN MALAYSIA AND R&D PROGRESS IN 2017 – Review Article , 2018, Journal of Oil Palm Research.

[98]  Abu Bakar Mahat,et al.  Integrated forward and reverse supply chain: A tire case study. , 2017, Waste management.

[99]  A. Velenturf,et al.  Quality of resources: A typology for supporting transitions towards resource efficiency using the single-use plastic bottle as an example. , 2019, The Science of the total environment.

[100]  S. Ulgiati,et al.  A review on circular economy: the expected transition to a balanced interplay of environmental and economic systems , 2016 .

[101]  Tsuyoshi Fujita,et al.  Environmental and economic gains of industrial symbiosis for Chinese iron/steel industry: Kawasaki's experience and practice in Liuzhou and Jinan , 2013 .

[102]  Anastasia Zabaniotou,et al.  Food waste valorization advocating Circular Bioeconomy - A critical review of potentialities and perspectives of spent coffee grounds biorefinery , 2019, Journal of Cleaner Production.

[103]  M. Tiwari,et al.  Impact of COVID-19 on logistics systems and disruptions in food supply chain , 2020, Int. J. Prod. Res..

[104]  Andrea Thorenz,et al.  The environmental impacts of preparation for reuse: A case study of WEEE reuse in Germany , 2020, Journal of Cleaner Production.

[105]  Jenna Jambeck,et al.  Challenges and emerging solutions to the land-based plastic waste issue in Africa , 2017, Marine Policy.

[106]  T. M. Chan,et al.  Development of RFID-based Reverse Logistics System , 2009, Expert Syst. Appl..

[107]  G. Mckay,et al.  Food waste to biochars through pyrolysis: A review , 2019, Resources, Conservation and Recycling.

[108]  J. Marin,et al.  Circular economy as a COVID-19 cure? , 2020, Resources, Conservation and Recycling.

[109]  L. N. Van Wassenhove,et al.  Concurrent product and closed-loop supply chain design with an application to refrigerators , 2003 .

[110]  Yvonne Ryan-Fogarty,et al.  Challenges and opportunities for reduction of single use plastics in healthcare: A case study of single use infant formula bottles in two Irish maternity hospitals , 2019 .

[111]  R. Baumgartner,et al.  The narrative of sustainability and circular economy - A longitudinal review of two decades of research , 2020 .

[112]  Bong Gyou Lee,et al.  Research trends in text mining: Semantic network and main path analysis of selected journals , 2020, Expert Syst. Appl..

[113]  Christine Cole,et al.  An assessment of achievements of the WEEE Directive in promoting movement up the waste hierarchy: experiences in the UK. , 2019, Waste management.

[114]  G. Feijoo,et al.  Environmental assessment of viticulture waste valorisation through composting as a biofertilisation strategy for cereal and fruit crops. , 2020, Environmental pollution.

[115]  T. Astrup,et al.  Plastic waste from recycling centres: Characterisation and evaluation of plastic recyclability. , 2019, Waste management.

[116]  C. Chan,et al.  Reducing e-waste in China's mobile electronics industry: the application of the innovative circular business models , 2020 .

[117]  Ludo Waltman,et al.  Software survey: VOSviewer, a computer program for bibliometric mapping , 2009, Scientometrics.

[118]  V. Zeller,et al.  Assessing the environmental performance for more local and more circular biowaste management options at city-region level. , 2020, The Science of the total environment.

[119]  Simone Domenico Scagnelli,et al.  A big data approach to map the service quality of short-stay accommodation sharing , 2020 .

[120]  Dehu Chen,et al.  Evaluating critical barriers and pathways to implementation of e-waste formalization management systems in Ghana: a hybrid BWM and fuzzy TOPSIS approach , 2020, Environmental Science and Pollution Research.

[121]  Francesco Di Maio,et al.  A Robust Indicator for Promoting Circular Economy through Recycling , 2015 .

[122]  Michael Saidani,et al.  Towards a Smart E-Waste System Utilizing Supply Chain Participants and Interactive Online Maps , 2021, Recycling.

[123]  Jose-Luis Galvez-Martos,et al.  The Impact of Pay-As-You-Throw Schemes on Municipal Solid Waste Management: The Exemplar Case of the County of Aschaffenburg, Germany , 2017 .

[124]  V. Zeller,et al.  Urban waste flows and their potential for a circular economy model at city-region level. , 2019, Waste management.

[125]  R. Merli,et al.  How do scholars approach the circular economy? A systematic literature review , 2017 .

[126]  Jingzheng Ren,et al.  Construction and demolition waste management in China through the 3R principle , 2018 .

[127]  Martin Kurdve,et al.  Material efficiency in manufacturing : swedish evidence on potential, barriers and strategies , 2016 .

[128]  Silpa Kaza,et al.  What a Waste 2.0: A Global Snapshot of Solid Waste Management to 2050 , 2018 .

[129]  Claudia M Agudelo-Vera,et al.  Resource management as a key factor for sustainable urban planning. , 2011, Journal of environmental management.

[130]  N. Valenzuela-Levi Factors influencing municipal recycling in the Global South: The case of Chile , 2019, Resources, Conservation and Recycling.

[131]  Marianthi G. Ierapetritou,et al.  Optimal design of sustainable chemical processes and supply chains: A review , 2012, Comput. Chem. Eng..

[132]  James Sherwood Closed-Loop Recycling of Polymers Using Solvents : Remaking plastics for a circular economy , 2020 .

[133]  Giacomo Di Foggia,et al.  Designing Waste Management Systems to Meet Circular Economy Goals: The Italian Case , 2021, Sustainable Production and Consumption.

[134]  P. Abhilash,et al.  Circular economy practices within energy and waste management sectors of India: A meta-analysis. , 2020, Bioresource technology.

[135]  Gustav A Sandin,et al.  Environmental impact of textile reuse and recycling – A review , 2018 .

[136]  V. Torretta,et al.  Introduction of the circular economy within developing regions: A comparative analysis of advantages and opportunities for waste valorization. , 2019, Journal of environmental management.

[137]  Zbigniew Wzorek,et al.  The possible use of sewage sludge ash (SSA) in the construction industry as a way towards a circular economy , 2015 .

[138]  Agis M. Papadopoulos,et al.  Smart technologies for promotion of energy efficiency, utilization of sustainable resources and waste management , 2019, Journal of Cleaner Production.

[139]  T. Astrup,et al.  Contamination in plastic recycling: Influence of metals on the quality of reprocessed plastic. , 2018, Waste management.

[140]  N. Johansson,et al.  Circular economy running in circles? A discourse analysis of shifts in ideas of circularity in Swedish environmental policy , 2020 .

[141]  Evangelos A Voudrias,et al.  Healthcare waste management from the point of view of circular economy. , 2018, Waste management.

[142]  W. Sroka,et al.  Implementation of a Circular Economy in Ukraine: The Context of European Integration , 2020 .

[143]  C. Frazzoli,et al.  Editorial: The Environment-Animal-Human Web: A “One Health” View of Toxicological Risk Analysis , 2018, Front. Public Health.

[144]  A. Bonoli,et al.  The Commitment of Packaging Industry in the Framework of the European Strategy for Plastics in a Circular Economy , 2019, Administrative Sciences.

[145]  Michele Seghetta,et al.  Environmental-Economic Analysis of Integrated Organic Waste and Wastewater Management Systems: A Case Study from Aarhus City (Denmark) , 2018, Sustainability.

[146]  Fu Gu,et al.  Exploring "Internet+Recycling": Mass balance and life cycle assessment of a waste management system associated with a mobile application. , 2019, The Science of the total environment.

[147]  J. Millward-Hopkins,et al.  A pathway to circular economy: Developing a conceptual framework for complex value assessment of resources recovered from waste , 2017 .

[148]  O. Romero-Hernández,et al.  Maximizing the value of waste: From waste management to the circular economy , 2018 .

[149]  Maurie J. Cohen,et al.  A brave new world: Lessons from the COVID-19 pandemic for transitioning to sustainable supply and production , 2020, Resources, Conservation and Recycling.

[150]  Vivian W. Y. Tam,et al.  Construction and demolition waste management contributing factors coupled with reduce, reuse, and recycle strategies for effective waste management: A review , 2020 .

[151]  Paul J Lioy,et al.  Will the Circle Be Unbroken: A History of the U.S. National Ambient Air Quality Standards , 2007, Journal of the Air & Waste Management Association.

[152]  H. Schoenberger,et al.  Construction and demolition waste best management practice in Europe , 2018, Resources, Conservation and Recycling.

[153]  David Simchi-Levi,et al.  Sustainable supply chain design: a closed-loop formulation and sensitivity analysis , 2012 .

[154]  Ming-Lang Tseng,et al.  Municipal solid waste management in a circular economy: A data-driven bibliometric analysis , 2020 .

[155]  M. Aghbashlo,et al.  Biopower and biofertilizer production from organic municipal solid waste: An exergoenvironmental analysis , 2019 .

[156]  T. Astrup,et al.  Extended producer responsibility: How to unlock the environmental and economic potential of plastic packaging waste? , 2020 .

[157]  Helen S. Du,et al.  Bibliometric mapping on sustainable development at the base-of-the-pyramid , 2020 .

[158]  T. Astrup,et al.  Quality Assessment and Circularity Potential of Recovery Systems for Household Plastic Waste , 2018, Journal of Industrial Ecology.

[159]  Yong Geng,et al.  An overview of e-waste management in China , 2015 .