Single-stream recycling is a system in which recyclables of all kinds (including plastics, paper, metal, and glass) are placed in a single curbside bin by consumers. The recyclables are then collected and transported to a material recovery facility (MRF) where they are sorted and processed. This system is also called commingled or single-sort recycling.
A benefit of the single-stream approach is that consumers, or the depositors, don’t have to separate and sort the recyclables themselves. Rather, they're encouraged to put everything that's not trash into a single bin. This helps to increase the quantity of material recovered. However, there are also drawbacks to this system, as it's contributed to higher contamination rates, such as mixed cardboard items containing glues and inks.
The Environmental Protection Agency (EPA) began collecting and reporting data on the generation and disposition of waste in the U.S. more than 30 years ago. Single-stream recycling popped up in several California communities in the 1990s as a low-barrier entry into recycling. This trend was slowly adopted by communities across the U.S. looking for ways to deal with the onslaught of plastic and other packaging materials (for example, 2.8 billion plastic water bottles were sold in 1996, jumping to 42.6 billion by 2010).
The EPA reports that over time, recycling rates have increased from just over 6% of municipal solid waste (MSW) generated in 1960 to about 10% in 1980, to 16% in 1990, to about 29% in 2000, and to over 35% in 2017. The total MSW recycled in 2017 was 67.2 million tons, with paper and paperboard accounting for approximately 66% of that amount. Metals comprised about 12%, while glass, plastic, and wood made up approximately 5%.
For more than 25 years, the U.S. has been sending the majority of its plastic waste to other nations—especially China—to be broken down and recycled. In 2017, China announced it would no longer accept imports of plastic, textiles, unsorted paper, artificial fibers, various metals, and approximately 20 other types of solid waste. While American demand for plastic continues to grow, the future of harvesting and processing clean recyclables in the U.S. remains uncertain.
Once recyclables are put into curbside recycling bins, MRFs collect, sort, and process the recyclables. After processing, similar kinds of recyclables are baled and shipped to recyclers of specific materials, ultimately to be utilized in the production of new products.
The actual sorting process may vary with respect to the automation employed in the system, involving technologies such as conveyors, screens, forced air, magnets, optical material identification, and eddy current.
Single-stream recycling is a simple process for consumers, but it's costly for U.S. recycling businesses, who employ laborers and expensive automation to process waste.
The Sorting Process
The process for single-stream recycling is as follows:
- All material is unloaded and placed on a conveyor.
- Non-recyclable items are manually sorted and removed.
- The material moves to a triple-deck screen.
- Items too heavy or light, such as cardboard, containers, and paper, are removed.
- Heavier containers drop to the bottom level, while lighter items head to the second level.
- A screen breaks glass containers for the safety and convenience of the workers.
- The remaining material passes under a powerful magnet to remove tin and steel cans.
- MRF staff watch carefully for specific commodities that may still have inadvertently made it down the line.
- A reverse magnet called an "eddy current" causes aluminum cans to fly off the conveyor and into a bin.
- MRF workers separate cardboard, newsprint and office paper, and drop each piece into a bunker below. Once all the material is separated, it's baled and shipped to recycling companies for processing.
The entire process of single-stream recycling involves a combination of machines and human workers.
One of the most notable advantages of single-stream recycling is increased recycling rates. As individuals or consumers don’t have to do the sorting, they are more encouraged to participate in curbside recycling programs. Again, less space is required to store collection containers.
Regarding collection, costs for the hauling process are reduced versus separate pickups for different recycling streams, or the hauler having to place different materials into various truck compartments. This simple process receives greater public approval.
The most notable disadvantages of single-stream recycling is that it has led to a decrease in the quality of materials recovered. Putting all material into a single bin can increase the likelihood of contamination due to broken glass and the propensity to toss non-approved material into the recycling bin. This ultimately causes significant problems for MRF operators and communities.
Although consumers and depositors are not sorting the material themselves, someone ultimately has to sort, making the cost of recycling higher.
Looking Toward the Future
There are new processes being tested that can reduce the total emissions output of our current system for transporting and processing plastic and other recyclable materials. IBM claims that by 2050, there will be more plastic in the ocean than fish. This unsettling projection has inspired the development of their latest technology, the VolCat: a catalytic chemical process that digests certain plastics (called polyesters) into a substance which can be fed directly back into plastic manufacturing machines in order to make new products.
"In the coming years, advancements like VolCat will make plastics recycling more efficient and more versatile in treating more material types than its predecessors. Unlike traditional mechanical recycling, future plastics recycling will break down both colored and clear plastics, as well as dirty and clean containers, producing a high-quality final product that is 100% recyclable."
While recycling has come a long way in the last decade, it remains to be seen if new technological solutions can maximize consumer convenience, volume recovery, and material quality through a single-stream process.