CU Boulder — EVEN 2909 — Circular Economy & Materials
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Circular Economy & Materials

EVEN 2909: Introduction to Sustainability Engineering — Week 10

University of Colorado Boulder

Linear vs. Circular Economy

The Linear Model: Take → Make → Waste

  • Extract raw materials from the earth
  • Manufacture products with planned obsolescence
  • Use briefly, then discard to landfill or incineration
  • This model has dominated since the Industrial Revolution
  • We currently extract 100 billion tonnes of materials per year — only 7.2% is cycled back
Extract
→
Produce
→
Consume
→
Dispose

The Circular Model: Reduce → Reuse → Recycle → Regenerate

  • Design out waste and pollution from the start
  • Keep products and materials in use at their highest value
  • Regenerate natural systems rather than deplete them
  • Shift from ownership to service models (product-as-a-service)
  • Estimated $4.5 trillion economic opportunity by 2030
“Waste is a design flaw.” — Kate Raworth, Doughnut Economics

Sources: Circle Economy Circularity Gap Report 2024; Ellen MacArthur Foundation

The Butterfly Diagram

The Ellen MacArthur Foundation’s butterfly diagram illustrates two distinct material cycles in a circular economy:

Biological Cycle (left wing)

Biological Nutrients
Materials that can safely re-enter the biosphere: food, natural fibers, wood. These are consumed, composted, and returned to soil to regenerate natural capital.
  • Cascading uses: cotton shirt → second-hand clothing → furniture stuffing → insulation → compost
  • Anaerobic digestion: organic waste → biogas + digestate fertilizer
  • Regeneration: nutrients return to soil, completing the loop

Technical Cycle (right wing)

Technical Nutrients
Materials that do not biodegrade: metals, plastics, synthetic chemicals. These must be maintained, reused, refurbished, remanufactured, or recycled — never mixed with biological flows.
  • Inner loops preferred: maintain > reuse > refurbish > remanufacture > recycle
  • Smaller loops = more value retained (less energy, less material loss)
  • Product-as-a-service: manufacturer retains ownership, incentivized to design for durability

Key principle: Never mix biological and technical nutrients. When you put plastic in compost or food in a landfill, both cycles break down.

Source: Ellen MacArthur Foundation, 2013

Life Cycle Assessment (LCA)

LCA is the systematic methodology for quantifying the environmental impacts of a product, process, or service across its entire life — from raw material extraction to final disposal.

Goal & Scope
Define boundaries
→
Inventory Analysis
Quantify inputs/outputs
→
Impact Assessment
Evaluate effects
→
Interpretation
Draw conclusions

Key Concepts

  • Functional unit: what are you comparing? (e.g., “one hand-drying event” for paper towels vs. air dryers)
  • System boundaries: cradle-to-gate, cradle-to-grave, or cradle-to-cradle
  • Allocation: how to divide impacts among co-products

Impact Categories

  • Global warming potential (kg CO₂e)
  • Acidification (kg SO₂e)
  • Eutrophication (kg PO₄e)
  • Ozone depletion (kg CFC-11e)
  • Water use (m³)
  • Resource depletion (kg Sbe)

Engineering tool: LCA prevents “burden shifting” — solving one environmental problem by creating another. It forces you to see the whole picture.

Sources: ISO 14040/14044; EPA LCA guidance

Embodied Carbon

Embodied carbon is the total greenhouse gas emissions from extracting, manufacturing, transporting, and installing a material or product — everything before it is used.

11%
of global GHG from building materials
8%
of global CO₂ from cement alone
7%
of global CO₂ from steel

Why It Matters Now

  • As buildings become more energy-efficient and grids get cleaner, operational carbon decreases but embodied carbon stays locked in
  • For a new high-performance building, embodied carbon can represent 50–70% of lifetime emissions
  • Embodied carbon is emitted upfront — during the critical window when we need to cut emissions fastest
  • Once a building is constructed, its embodied carbon cannot be recovered

Reducing Embodied Carbon

  • Material selection: mass timber instead of steel/concrete, recycled content, local sourcing
  • Design optimization: use less material through structural efficiency
  • Reuse: salvage and repurpose existing building materials
  • Low-carbon manufacturing: green hydrogen for steel, alternative cement chemistries

Sources: Architecture 2030; Global Alliance for Buildings and Construction

The Plastics Crisis

400M
Tonnes produced per year
8M
Tonnes enter oceans annually
9%
Of all plastic ever recycled
500 yr
Time to decompose

The Recycling Myth

  • Of all plastic ever produced (~9.2 billion tonnes), only 9% has been recycled, 12% incinerated, and 79% accumulated in landfills or the environment
  • Most “recycling” is actually downcycling — a bottle becomes a park bench, then landfill
  • Resin codes (1–7) mislead consumers; most municipalities only recycle #1 (PET) and #2 (HDPE)
  • China’s 2018 National Sword policy exposed the West’s dependence on exporting waste

Microplastics

  • Particles <5mm found in oceans, soil, air, drinking water, and human blood
  • Sources: tire wear, synthetic clothing fibers, packaging degradation
  • Health effects still being studied; linked to endocrine disruption and inflammation

Alternatives & Solutions

  • Bioplastics: PLA, PHA — but require industrial composting; not a silver bullet
  • Chemical recycling: pyrolysis, depolymerization — promising but energy-intensive
  • Reduce first: eliminate unnecessary single-use plastics at the design stage

Sources: Geyer et al., Science Advances 2017; UNEP Global Plastics Treaty negotiations

E-Waste: The Fastest-Growing Waste Stream

62M
Tonnes of e-waste in 2022
22%
Formally recycled
$62B
Value of recoverable materials

What’s Inside

  • Precious metals: gold, silver, platinum, palladium — higher concentration in e-waste than in ore
  • Rare earth elements: neodymium, dysprosium — critical for magnets, screens, batteries
  • Hazardous materials: lead, mercury, cadmium, brominated flame retardants
  • Conflict minerals: cobalt (DRC), tantalum, tin — ethical supply chain concerns

The Informal Recycling Problem

  • Millions of workers in Accra (Ghana), Guiyu (China), and Delhi (India) dismantle e-waste by hand
  • Open burning of circuit boards to recover copper releases dioxins and heavy metals
  • Workers — often children — exposed to neurotoxins without protective equipment
  • Basel Convention restricts hazardous waste exports, but enforcement is weak

Solutions

  • Extended Producer Responsibility (EPR): manufacturers fund end-of-life collection and recycling
  • Urban mining: recovering metals from e-waste is more efficient than mining virgin ore
  • Modular design: repairable, upgradeable devices (e.g., Framework laptop)

Sources: UN Global E-Waste Monitor 2024; ITU

Design for Disassembly

If products are designed to be taken apart, materials can flow back into the economy instead of into landfills. This requires a fundamental shift in how engineers think about the end of life at the beginning of design.

Modular Design
Components snap together instead of being glued or welded. Failed parts can be replaced individually. Example: Fairphone — a modular smartphone where the camera, battery, and screen are user-replaceable.
Material Passports
Digital records documenting every material in a product or building — what it is, where it came from, how to recover it. Think of it as a “nutrition label” for buildings. Madaster platform is leading this effort.
Right to Repair
Legislative movement requiring manufacturers to provide spare parts, repair manuals, and diagnostic tools. EU and several US states have passed right-to-repair laws. Fights planned obsolescence directly.

Design rule of thumb: If you can’t take it apart in under 30 seconds with common tools, it won’t be recycled. Snap fits > screws > adhesives > welding. Mono-material > multi-material. Labeled > unlabeled.

Sources: Bakker et al., Journal of Cleaner Production 2014; European Right to Repair directive

Cradle to Cradle

“Don’t be less bad. Be more good.” — William McDonough & Michael Braungart, Cradle to Cradle: Remaking the Way We Make Things, 2002

Cradle to Cradle (C2C) is a design philosophy that reimagines the industrial system as two distinct metabolisms:

Biological Nutrients

  • Materials designed to safely biodegrade and return to the soil
  • No toxic dyes, no persistent chemicals
  • Example: a T-shirt you could literally compost after wearing out
  • Think: food packaging, textiles, cosmetics

Technical Nutrients

  • Materials designed to circulate indefinitely at high quality
  • Never downcycled — aluminum stays aluminum, not aluminum filler
  • Products designed as “material banks” for future recovery
  • Think: electronics, vehicles, building components

C2C Certification Criteria

  • Material health: no carcinogens, mutagens, or endocrine disruptors
  • Material reutilization: designed for circular recovery
  • Renewable energy: manufacturing powered by clean energy
  • Water stewardship: clean water as output, not polluted effluent
  • Social fairness: labor rights and community impact

Source: McDonough & Braungart, 2002; Cradle to Cradle Products Innovation Institute

Industrial Ecology

Industrial ecology studies material and energy flows through industrial systems, treating the economy as an ecosystem where one firm’s waste becomes another’s feedstock.

Kalundborg Symbiosis, Denmark
The world’s first and most famous example of industrial symbiosis. Since the 1970s, a network of companies has exchanged waste streams: a power plant sends excess steam to a pharmaceutical factory and a fish farm; fly ash goes to a cement company; gypsum from desulfurization goes to a wallboard maker; sludge from the fish farm fertilizes local farms.

Principles

  • Waste = food: every output stream is a potential input for another process
  • Closed loops: minimize material leaving the industrial system
  • Cascade energy: use high-grade energy first, then capture waste heat for lower-grade uses
  • Material flow analysis (MFA): map where materials go to find symbiosis opportunities

Modern Examples

  • Eco-industrial parks: co-located businesses designed for material exchange (China has 100+)
  • Brewery waste: spent grain → animal feed → biogas → fertilizer
  • Steel slag: used as aggregate in road construction and cement
  • CO₂ utilization: captured carbon used to grow algae, make building materials, or carbonate beverages

Sources: Frosch & Gallopoulos, Scientific American 1989; Kalundborg Symbiosis Center

Critical Minerals

The clean energy transition requires massive quantities of minerals that are geographically concentrated, hard to extract, and have their own environmental and social costs.

6x
Mineral demand increase by 2040
70%
of cobalt from DRC
60%
of rare earths from China
Lithium
Essential for batteries (EVs, grid storage). Extracted from brine (Chile, Argentina) or hard rock (Australia). Water-intensive brine extraction threatens Atacama Desert ecosystems. Demand projected to grow 40x by 2040.
Cobalt
Critical for battery cathodes. 70% comes from DRC, often through artisanal mining with child labor. Battery chemistries are shifting (LFP) to reduce cobalt dependence, but it remains important for high-energy-density applications.
Copper
The backbone of electrification: wiring, motors, transformers. An EV uses 4x the copper of an ICE vehicle. Ore grades are declining, meaning more rock must be mined per tonne of copper. Recycling rates are relatively high (~30%).

The paradox: Solving climate change requires mining on an unprecedented scale, which creates its own environmental damage. Circular economy strategies — recycling, substitution, dematerialization — are essential to mitigate this.

Sources: IEA Critical Minerals Report 2023; USGS

The Waste Hierarchy

Not all waste management strategies are equal. The hierarchy prioritizes actions by environmental benefit — from most preferred (top) to least preferred (bottom):

PREVENT
Avoid creating waste in the first place. Rethink whether you need the product.
REDUCE
Minimize material use. Lightweight packaging, concentrated products, sharing economy.
REUSE
Use again for the same or different purpose. Refillable containers, second-hand markets.
RECYCLE
Process into new materials. Better than disposal but requires energy and often degrades quality.
RECOVER
Extract energy via incineration or anaerobic digestion. Better than landfill, but still destructive.
DISPOSE
Landfill. Last resort. Generates methane, leachate, and occupies land indefinitely.

Most sustainability efforts focus on the bottom of the hierarchy. Real impact comes from moving up: prevent and reduce before you recycle.

Source: EU Waste Framework Directive; EPA

Colorado & CU Examples

Eco-Cycle Boulder

A National Model
Founded in 1976, Eco-Cycle is one of the oldest and largest nonprofit recyclers in the US. It operates Boulder County’s recycling center, the Center for Hard-to-Recycle Materials (CHaRM), and advocates for zero-waste policy at local and state levels.
  • Boulder’s diversion rate: ~40% (goal: 85% by 2025)
  • CHaRM accepts electronics, textiles, Styrofoam, mattresses, and other hard-to-recycle items
  • Eco-Cycle helped pass Colorado’s Producer Responsibility for Recycling Act (2022)

CU Boulder Zero-Waste Goals

Campus Sustainability
CU has committed to becoming a zero-waste campus as part of its Climate Action Plan. Current campus diversion rate is approximately 50%. Efforts include compost collection in dining halls, e-waste drives, and reuse programs.
  • Buffs Swap Shop: free exchange of furniture, supplies, and household goods during move-out
  • Game Day Zero Waste: compost and recycling stations at Folsom Field
  • Green Labs program: reducing waste from research operations
  • Colorado’s EPR law will shift recycling costs to producers starting 2025

Sources: Eco-Cycle.org; CU Environmental Center; Colorado Dept. of Public Health & Environment

Discussion

Your Material World

Pick one object you use every day (phone, water bottle, shoes, backpack). Think through its material flows:

Where did it come from?
What raw materials? Where were they extracted? How many countries were involved in manufacturing?
Where will it go?
When you’re done with it, what happens? Can it be repaired? Recycled? Or will it sit in a landfill for centuries?
How would you redesign it?
Using circular economy principles, how would you change the materials, design, or business model to close the loop?

Key Takeaways

1. Linear economies are a dead end
We extract 100 billion tonnes of materials per year and cycle back only 7%. On a finite planet, this is mathematically unsustainable.
2. LCA reveals hidden trade-offs
Without life cycle thinking, you risk solving one problem while creating another. Always ask: what are the full system impacts?
3. Recycling is necessary but insufficient
The waste hierarchy is clear: prevent and reduce first. Recycling is near the bottom — it should be a last resort, not a first strategy.
4. Design determines destiny
80% of a product’s environmental impact is locked in at the design stage. Design for disassembly, material health, and circularity from day one.
5. The clean energy transition has its own material costs
Critical minerals for batteries and renewables must be sourced responsibly and recycled aggressively to avoid replacing one extractive system with another.

Further Reading

Next week: Sustainable Buildings & Transportation — Where do we live, and how do we move?