Toolkit: Complex Systems Toolkit.
Author: Milan Liu, Ph.D. Candidate (Cranfield University); Dr. Lampros Litos (Cranfield University).
Topic: Towards circular economy: development of systems-based interventions in complex systems.
Title: Improving metal recycling and recycled content intake.
Resource type: Guidance article.
Relevant disciplines: Any; Production and manufacturing engineering.
Keywords: Available soon.
Licensing: This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.
Downloads: Available soon.
Who is this article for?: This article should be read by educators at all levels of higher education looking to highlight the connection between complex systems and sustainability within engineering learning.
Related INCOSE Competencies: Toolkit resources are designed to be applicable to any engineering discipline, but educators might find it useful to understand their alignment to competencies outlined by the International Council on Systems Engineering (INCOSE). The INCOSE Competency Framework provides a set of 37 competencies for Systems Engineering within a tailorable framework that provides guidance for practitioners and stakeholders to identify knowledge, skills, abilities and behaviours crucial to Systems Engineering effectiveness. A free spreadsheet version of the framework can be downloaded.
This resource relates to the Systems Thinking, Life Cycles, Capability Engineering, Systems Modelling and Analysis, and Design INCOSE competencies.
AHEP mapping: This resource addresses several of the themes from the UK’s Accreditation of Higher Education Programmes fourth edition (AHEP4): Analytical Tools and Techniques (critical to the ability to model and solve problems), and Integrated / Systems Approach (essential to the solution of broadly-defined problems). In addition, this resource addresses AHEP themes of Materials, equipment, technologies and processes, and Sustainability.
Learning and teaching resources:
- The complexity and interconnectedness of circular cities and the circular economy for sustainability
| Resource | Type | Best for | Quick classroom use | URL |
| Insight Maker | Web-based modelling tool | Building stock-and-flow models and simple simulations | Convert the aluminium CLD into stocks/flows and run a scenario | https://insightmaker.com |
| Loopy | Interactive causal-loop diagram app | Fast, visual CLDs and in-class demonstration of loop behaviour | Live demo of reinforcing vs balancing loops; students toggle link polarities | https://ncase.me/loopy |
| Vensim PLE | Free desktop system-dynamics software | Introductory quantitative modelling and sensitivity runs | Short lab: implement simplified aluminium-recycling model and compare policy scenarios | https://vensim.com/free-download/ |
| Leverage Points (Meadows) | Concept primer on leverage points | Framing where to intervene in systems | Assign as required reading; students map which leverage points the CLD targets | https://donellameadows.org/archives/leverages-points-places-to-intervene-in-a-system/ |
| MIT System Dynamics materials | Course notes and lecture videos | Structured curriculum and worked examples for deeper study | Use selected lectures and problem sets for follow-up or flipped classroom | https://ocw.mit.edu/courses/15-871-introduction-to-system-dynamics-fall-2013/ |
Premise:
Several sustainability challenges, such as transitioning to a circular economy, are embedded in complex socio-technical systems. A circular economy is an economic model that replaces the linear take-make-dispose pattern with systems that keep materials and products in use for longer through designing for durability, reuse, remanufacturing, and recycling, while minimising waste and regenerating natural systems (Rizos, Tuokko, and Behrens, 2017).
Complex systems like these exhibit feedback loops, delays, non-linear change, path dependence and emergent behaviour (Sterman, 2000; Meadows, 2008). This article introduces the idea of systems-based interventions using the example of aluminium recycling systems. It is designed for engineering educators who plan to provide learners with a baseline understanding of complexity and practical entry points for designing and developing and evaluating interventions that can move a system towards sustainability.
Complexity of aluminium recycling systems:
Aluminium is infinitely recyclable, yet achieving truly closed material loops at scale remains a challenge. Most of today’s recycling occurs in situations where post-consumer scrap is collected from a wide variety of end-of-life products and the boundaries of the recycling system are difficult to define and control. This creates high variability in both the composition and the quality of recovered aluminium, since different products contain different alloys and levels of contamination (IRT M2P, 2023). At the same time, the volume of available scrap is difficult to predict, as it depends on product lifespans and consumer behaviour. These fluctuations make it harder for producers to plan and optimise secondary aluminium output, particularly when industries rely on consistent standards or just-in-time manufacturing.
The recycling system is also shaped by broader economic and regulatory forces. On the one hand, demand for low-carbon materials and the cost advantage of recycled over primary aluminium are powerful drivers of growth. On the other hand, the system faces constraints from volatile scrap prices and shifting global trade dynamics, such as U.S. tariffs on aluminium imports. Meanwhile, new policy instruments are adding further complexity. The EU’s Carbon Border Adjustment Mechanism (CBAM) is set to reshape trade flows and investment patterns, while the forthcoming Digital Product Passport (DPP) will transform how information is shared across the value chain. Together, these forces influence technologies, markets and business models, underscoring the dynamic and interconnected nature of aluminium recycling.
These interconnected factors highlight aluminium recycling as a complex socio-technical system, in which technological capabilities, market incentives, policy frameworks, and global trade are deeply interconnected. For educators, this makes aluminium an effective example for teaching students how multiple forces interact to create both opportunities and challenges for sustainable engineering.
Intervention from systems perspective:
System Dynamics (SD), first formalised by Forrester (1968), has proven to be a highly valuable approach for understanding and managing complex resource and recovery systems. SD is an interdisciplinary approach, drawing on insights from psychology, organisational theory, economics, and related fields (Sterman, 2000). More supporting information about SD pedagogical tools and techniques can be found through the System Dynamics Society and Insight Maker.
From a systems perspective, interventions are not isolated events but strategic effort to influence system behaviour by targeting its structure and dynamics. A key concept here is leverage points – places within a complex system where small changes can lead to significant, systemic effects (Meadows, 1999). Meadows identified twelve types of leverage points, ranging from adjusting parameters to transforming the system’s underlying goals and paradigms, proving a conceptual framework for identifying impactful intervention.
Figure 1. Donella Meadows’ leverage points (Source: based on Meadows (1999); credit: UNDP/Carlotta Cataldi; reproduced from Bovarnick and Cooper (2021))
Exploration of potential leverage points:
System Dynamics (SD) tools such as Causal Loop Diagrams (CLDs) can help explore leverage points. CLDs can help visualise main components of a system and their interdependencies, making complex dynamics easier to understand. Besides, the process of building a CLD or more computational SD model encourages practitioners to clarify system boundaries, relationships, and drivers, laying the foundation for identifying leverage points.
For example, a CLD of aluminium recycling might capture how classification and sorting processes influence scrap quality, which then affects remelting efficiency and ultimately market uptake of recycled alloys (see Figure 2 below).
Figure 2. The causal loop diagram for auto aluminium recycling (Liu et al., 2025)
By tracing these circular cause-and-effect relationships, learners can see where interventions may ripple through the system. Highlighting reinforcing loops, balancing loops, and delays also shows why some interventions produce limited short-term results but more substantial long-term effects.
Leverage points can also be examined through the lens of information, rules, and goals. Improved information flows, such as those enabled by the Digital Product Passport, could reshape how scrap is sorted and valued. Rules, such as alloy specifications or trade tariffs, determine what types of recycled material can enter the market. At a deeper level, the goals of the system, whether to maximise throughput or to retain material value, fundamentally shape behaviour. Here too, CLDs are valuable because they allow users to visualise how changes to information, rules, or goals can shift system dynamics, providing a clearer picture of where interventions might be most effective.
Implication for educators:
This article equips educators with a focused, practical pathway to teach systems thinking through the example of aluminium recycling. Students can gain both conceptual understanding and hands-on skills to map feedback loops, identify delays, and design interventions that account for short-term trade-offs and long-term system behaviour. Teaching a single clear CLD followed by one modelling or scenario activity produces measurable learning gains while keeping the task accessible for beginners.
Educational approach:
- Prioritise structure before solutions: have students map feedback loops and delays before proposing fixes.
- Use one classroom-ready CLD as the anchor activity and one hands-on modelling task to test interventions.
- Emphasise leverage thinking: move from parameter tweaks to information, rules, goals and paradigms as students mature.
- Keep language simple and concrete: avoid jargon, introduce terms with examples, and reuse the same CLD across activities.
- Use open-access tools (Insight Maker, Loopy, Vensim PLE) so students can visualise and experiment without software barriers.
- Focus assessment on reasoning about system behaviour and predicted long-term effects rather than exact numerical answers.
Potential related learning outcomes within this topic:
- Define stocks, flows, feedback loops, delays, reinforcing and balancing loops.
- Explain why aluminium recycling is a complex socio-technical system influenced by technology, markets, policy, and information.
- Construct a simple CLD for an aluminium recycling pathway and identify at least two reinforcing and one balancing loop.
- Identify two leverage points and justify which one to prioritise, citing anticipated short- and long-term system effects.
- Translate the CLD into a basic stock-and-flow sketch in an open-access tool and run one scenario to compare outcomes.
Further resources:
- European Commission: Joint Research Centre, Environmental and socio-economic impacts of the circular economy transition in the EU cement and concrete sector – Analysing plastics material flows with life cycle-based and macroeconomic assessment models, Publications Office of the European Union, 2025, https://data.europa.eu/doi/10.2760/6579506
- The Complexity and Interconnectedness of Circular Cities and the Circular Economy for Sustainability — analysis of research themes and networked interactions relevant for urban/material systems; useful for teaching complexity and cross-sector links. https://onlinelibrary.wiley.com/doi/pdf/10.1002/sd.2766
References
- Bovarnick, A. and Cooper, S. (2021) “From what to how: rethinking food systems interventions,” Agriculture for Development. Edited by K. Hussein, 22 April, pp. 49–53.
- Forrester, J.W. (1968) “Industrial Dynamics—After the First Decade,” Management Science, 14(7), pp. 398–415. Available at: https://doi.org/10.1287/mnsc.14.7.398.
- IRT M2P (2023) Market Development for Secondary Casting Alloys beyond Motor Blocks – Study on casting alloy market and recycling, International Aluminium Institute. Available at: https://international-aluminium.org/resource/current-and-future-secondary-casting-alloy-market-in-europe/ (Accessed: February 27, 2024).
- Liu, M., Schneider, K., Litos, L., Salonitis, K., 2025. Enhancing Secondary Aluminium Supply: Optimising Urban Mining Through a Systems Thinking Approach, in: Edwards, L. (Ed.), Light Metals 2025. Springer Nature Switzerland, Cham, pp. 1273–1279.
- Meadows, D.H. (1999) Leverage Points – Places to Intervene in a System, The Sustainability Institute.
- Meadows, D.H. (2008) Thinking in systems: A primer. White River Junction, VT: Chelsea Green Publishing Company.
- Rizos, V., Tuokko, K., and Behrens A. (2017). The Circular Economy: A Review of definitions, processes, and impacts. CEPS Energy Climate House. https://circulareconomy.europa.eu/platform/sites/default/files/rr2017-08_circulareconomy_0.pdf
- Sterman, J. (2000) “Business Dynamics, System Thinking and Modeling for a Complex World.” Available at: http://hdl.handle.net/1721.1/102741 (Accessed: September 4, 2025).
Any views, thoughts, and opinions expressed herein are solely that of the author(s) and do not necessarily reflect the views, opinions, policies, or position of the Engineering Professors’ Council or the Toolkit sponsors and supporters.
