EME 807
Technologies for Sustainability Systems

5.2. Recycling: open-loop versus closed-loop thinking

As we can see from the previous page of this lesson, there are a number of conventional methods of waste treatment which depend on the system scale and type of waste. However, not all of them fit in the sustainability picture. For example, such common methods as incineration or landfilling are not sustainable solutions because, while eliminating problem in one zone (for example, human residence or industrial facility), they create additional pollution in the other (atmosphere, soil, aquifers, natural habitat). The purpose of recycling is to minimize or completely avoid sending waste to landfill or incinerator.

There are two major stages in recycling strategy: collection and processing. Both may consume resources and limit the process efficiency. The main recyclables are metals, plastics, glass, paper, and wood. Those materials are common in consumer products, so the public needs to be involved in the process. Public acceptance is important for the success of recollection of those recyclable materials (for example, public awareness and availability of collection points in public places plays a role – see image above). At the stage of processing, the question of recyclability is often related to the product design. How difficult and expensive is it to retrieve those materials from the product? You need to get those materials separated in a pure form in order to make them reusable in the same or new products.

The following reading material contains more information about what materials are recyclable and what happens to them afterwards. The article is not freely accessible online, so it is only included as supplemental reading. You may be able to check out a hardcover copy of the Encyclopedia from a local library.

Supplemental Reading:

Lawrence, S.R. et al., Recycling Technology, McGraw-Hill Encyclopedia of Science and Technology, 2007, 10th, ISBN 9780071441438, v. 15, pp. 262 - 270

This article provides quite a complete list of materials subject to recycling. It also gives you an idea what methods are used to process those materials and also what further manufacturing chains or markets they enter afterwards. Take a note of those connections.

Recycling indeed helps to save energy, resources, and prevent greenhouse gas emissions. How substantial are those benefits? Let us look at some numbers. The Popular Mechanics website compares recycling rates for aluminum, glass, newsprint, and some plastics, and links those data to market trends.

As seen from this information, an important factor responsible of viability of recycling business is the cost of the new material production. For example, production of virgin aluminum by bauxite mining is so energy-demanding that recycling of drink cans is very economically attractive. On the contrary, glass recycling, while technically simple, does not bring such high benefits, just because making new glass from silica sand is a relatively cheap technology.

Another factor that affects the viability of recycling system is collectability. Plastic recycling is quite profitable, with 76% energy savings compared to new plastic production. However, the case of polystyrene containers shows that if there is no technology to efficiently separate them from other plastics, process fails.

The bottom line here is that recycling is heavily relies on development of new advanced technologies and approaches for material processing (without quality loss), collection, and sorting recyclables.

Unfortunately, many cases of recycling only help postpone permanent waste generation. This happens if an original material gradually loses its quality while being recycled and cannot return to the same manufacturing process. It has to be reprocessed to a lower-grade products, which are not necessarily recyclable. For example, recycling of polyester soda bottles results in obtaining polymer fibers, which may be supplied to a carpet manufacturer. Carpet, however, is not easily recyclable since it is a more complex product. Polymeric fabrics are combined with other organic products and adhesives to make the final product. Separation of pure components after its use is not feasible; hence, the used carpet becomes a landfill material. This way of recycling, when a material lives a few lives but becomes less and less usable or pure or safe along its way to the landfill, is often termed "downcycling". In terms of sustainability, it means being "less bad", but still not good enough.

At this point, it would be appropriate to look at different concepts in material recycling.

Open-loop Recycling

Open-loop recycling basically means that a material is not recycled indefinitely and is eventually excluded from the utilization loop and becomes waste. The diagram in Figure 5.1. shows a material flow through the linear (open-loop) system. In this representation, stocks are shown with rectangular boxes, and transforming processes are shown by hexagon boxes.

In Figure 5.1. below, we see that natural resources extracted from the environment are transformed into a product via manufacturing process. After its use, the product may be discarded as one of the outputs: (a) whole product that is not needed anymore, (b) whole product that became obsolete (although still functional), (c) non-functional or old product because of its limited lifetime, (d) recyclable / reusable parts or scrapped materials, and (e) non-recyclable refuse. Those outputs enter one of the post-use channels – reuse, recycle, and garbage disposal, the latter contributing to the landfill. Reuse channel is usually limited, just postponing garbage disposal. Recycling loop results in producing another material, which is typically of lower grade and purity than the original material. It may be transformed further into a different product, which after use creates similar outputs. In the long run, a small part of the original resource may be stuck in the loop, but the majority of it becomes disposed of.

See text above for details.
Figure 5.1. Open-loop material flow (“cradle-to-grave”). A major bulk of materials sooner or later contributes to the landfill disposal. Only a fraction of material is recycled indefinitely.
Credit: Mark Fedkin

The bottom line is: even if recycling and reuse are involved, eventual down-grading renders material non-usable, and it contributes to waste generation in the end of the lifecycle. Open-loop recycling postpones disposal and slows down extraction of new natural resources, but does not provide ultimate solution to the problem.

Closed-loop Recycling

Closed-loop recycling is a more sustainable concept, which means that recycling of a material can be done indefinitely without degradation of properties. In this case, conversion of the used product back to raw material allows repeated making of the same product over and over again.

A few things to consider:

  • The recycled materials should provide the same quality of the product (no deterioration). For example, almost all recycled aluminum from soda cans is suitable to produce the same cans.
  • There should be no accumulation of contaminants or toxins in the multiple recycling loop, which can make the secondary product less safe.
  • The recycled material can also feed manufacturing process for a different product or industry, which may require different type of recycling.

The other part of closed-loop recycling concept is bio-degradable disposal. Everything that cannot be recycled or comes as a by-product in manufacturing process should return to the environment with no harm. Diagram in Figure 5.2 summarizes the above considerations. While starting from the same extraction, manufacturing, and use stages, the outputs in the closed-loop scheme become equally usable resource for the manufacturing chain. Greater fraction of materials should be designed for recycling and reuse. The refuse that is inevitable is biodegradable and brings no harm when returned to the environment.

See text above for more details
Figure 5.2. Closed-loop material flow. In ideal case no permanent solid waste is generated – all materials are returned as nutrients for natural or technical food chains.
Credit: Mark Fedkin

In any sustainability scenario, closed-loop approach is the goal. But it would take radical changes and innovative thinking at the level of product and process design.

To a greater extent, this closed loop thinking is advocated in the book of William McDonough and Michael Braungart “Cradle-to-Cradle”. The authors suggest that every product and all packaging should have a complete closed-loop cycle mapped out for each component, i.e., pathways should be identified for each component to either be recycled indefinitely or to return to the natural ecosystem.

Supplemental Reading:

McDonough, W. and Braungart, M., Cradle to Cradle. Remaking the Way We Make Things, North Point Press. New York, 2002. Chapter 4: Waste Equals Food, pp. 92-117.This document discusses the concept of sustainable development.

This is a really engaging reading that will make you understand the closed-loop philosophy better. A nice illustration of what design challenge may involve is described on p.105 in the DesignTex case.

Zero Waste Strategy

When closed-loop resource management is successfully implemented, we ideally should have zero waste produced, as all products in the end of their lifecycle become assimilated by either technical or natural systems to their benefit. In a wider context, zero waste thinking also covers zero emission and zero water pollution. Such targets seem ambitious and require careful life cycle analysis of all steps.

Zero waste concept responds to the principle #6 of sustainable design "Eliminate the concept of waste. Evaluate and optimize the full life-cycle of products and processes, to approach the state of natural systems, in which there is no waste" [The Hannover Principles., 1992]

"Zero waste philosophy encourages the redesign of resource life cycles so that all products are reused. No trash is sent to landfills and incinerators. The process recommended is one similar to the way that resources are reused in nature." [Source: Wikipedia Zero Waste Article - read this Wikipedia article to learn more about the historical development of this concept].

Zero-waste strategy supports sustainable development through the following pathways:

  • Environmental sustainability:
    • conservation of natural resources
    • minimization of non-degradable waste dumped to the natural ecosystems
  • Economic sustainability:
    • less waste = higher efficiency => lower cost
    • cost of compliance with regulations is reduced
  • Social sustainability:
    • generation of new jobs
    • more resources and energy become available for society

Source: Zero Waste Alliance

It should be noted, however, that zero-waste concept is not equivalent to closed-loop recycling in technical sense. It involves and relies heavily on the design of systems for reuse of products and resources without additional energy and labor expenditures which are usually required for classic recycling.