Contamination is an issue - another comment mentioned beer in the paper, and that's pretty accurate. You get paid based on the quality of material you can send to your downstream buyers, single stream stock has significantly more contamination from poorly cleaned containers on average than multi-stream.
But another part is just the sheer volume of material. There's no time to do manual sorting other than basic visual inspection to make sure things like bowling balls and concrete get to the sorters.
Optical sorters are incredibly cool, but only just now getting to a place where they can work fast enough and accurate enough to make a difference [3]. Separating the material apart gives the machines the best shot at sorting; less work they'll need to do. Even if humans only get 80% of the sorting right at collection, it's significantly better than no sorting at all, as it gives the machines time to pick and blow the material to proper channels for recovery and resale.
See [1] [2] for videos of NYC recycling plants to see the scale. Videos don't do it justice, and when you tour different plants, it puts the complexity of the problem into perspective.
But another part is just the sheer volume of material. There's no time to do manual sorting other than basic visual inspection to make sure things like bowling balls and concrete get to the sorters.
Optical sorters are incredibly cool, but only just now getting to a place where they can work fast enough and accurate enough to make a difference [3]. Separating the material apart gives the machines the best shot at sorting; less work they'll need to do. Even if humans only get 80% of the sorting right at collection, it's significantly better than no sorting at all, as it gives the machines time to pick and blow the material to proper channels for recovery and resale.
See [1] [2] for videos of NYC recycling plants to see the scale. Videos don't do it justice, and when you tour different plants, it puts the complexity of the problem into perspective.
1 - https://www.youtube.com/watch?v=Ivu0HCdqEHs 2 - https://www.youtube.com/watch?v=HIHGKZL8_6Q 3 - https://www.recyclingproductnews.com/article/35841/the-lates...