Single-Stream Recycling Faces Challenges as Plastic Waste Grows

Single-stream recycling, in which all items to be recycled are dropped into a single bin, has made recycling easier for Americans by the millions. Simple as it is, however, recycling is significantly hindered by contamination and inefficiency, most notably that of plastics. Mixed recyclables collected are delivered to materials recovery facilities where they are subjected to operations including shredding, magnetic separation, gravity screening, and vibration screening. They first sort the cardboard and paper, followed by metals, glass, and plastics. While metals and glass are in most cases likely to be well enough sorted, plastics are the major problem based on the incredibly large variety of plastics and composition of chemicals they can have. Plastics themselves, along with plastic film and other non-recyclable contents, are a problem, thus making the sorting process costly and leading to masses of material to be rejected.

Only 9% of the United States' plastic trash makes it to successful recycling, and there is so much of what is recyclable yet contributes to landfill fill. The remainder is burned or becomes part of plastic pollution problematics that increase year by year. Among the hardest plastics to recycle are PVC, LDPE, and PS, used to create widely used household items such as food wrappers, plastic bags, and foam boxes. They are physically difficult to process or hazardous to work with in traditional recycling streams since they are flexible, brittle, or toxic.

Better accepted recyclable plastics include polyethylene terephthalate (PET), high-density polyethylene (HDPE), and polypropylene (PP). PET, which is in bottles of soda, is usually easily sorted by water flotation techniques because it will sink due to the fact that it is heavier than water. HDPE and PP, which float and are chemically incompatible when they could be used in the milk jugs or yogurt cups where they could be used as a package, are thus not readily recyclable as a package. This incompatibility requires the use of advanced sorting technology such as infrared spectroscopy, increasing the cost of processing.

Chemically, PET can be dissolved back to its initial base monomers via the solvolysis process, so it can be recyclable in the manufacture of new plastics. It works by dissolving the bonds holding the monomer units in place, effectively reversing the plastic to its initial components prior to their recombining. Other operations like pyrolysis and gasification where plastics are combusted to break them down have come under health and environmental scrutiny due to the risk of the emission of toxic fumes.

Mechanical recycling also offers promising solutions, especially in the use of compatibilizers—chemical additives that enable blending incompatible plastics like HDPE and PP. The chemical structures of these compounds follow both types of plastic, enabling recycling production with the desired traits of both. Research on these compatibilizers is ongoing and can potentially increase the value of recycled plastics significantly.

But technology isn't enough. The efficiency of recycling also hinges on consumer habits. Rinsing food tins, not putting non-recyclable items like plastic bags in the bins, and following local recycling rules are the key to making materials reusable again. Various city governments have different ordinances, and knowing symbols and numbers marked on plastics can assist consumers with knowing what is acceptable.

A breakdown of how recycling in the U.S. is, shows that nearly two-thirds of paper and cardboard fall into the bucket of being recycled to the same fate, whereas a third of metals and a quarter of glass fall to the same fate when being recycled. Plastics trail far behind at less than 10%. It shows a shift in paradigm within recycling plants and among people alike.

A circular economy with repeated reuse of materials, rather than allowing them to waste away, depends on solutions to the failure of current recycling processes. Recycling plastic can be enhanced by enhanced sorting technology, chemical advances, and increased consumer understanding to reduce landfill waste and reduce environmental harm. The future of recycling requires an alliance between municipalities, industries, and households to make recycling greener and efficient.

Source & Credits: The Conversation, Alex Jordan, University of Wisconsin-Stout, Phys.org