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Environmental Science
The Science of Recycling
How Materials Are Sorted and Reborn
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Recycling is the science and engineering process of turning used materials into raw materials for new products. It matters because mining, logging, drilling, and manufacturing often use large amounts of energy, water, and land. When recycling works well, it reduces waste sent to landfills and lowers pollution from making materials from scratch. The benefits depend on correct sorting, clean materials, and real demand for the recycled material.
Key Facts
- Recycling rate = mass recycled / total mass generated x 100%.
- Energy saved = energy for virgin production - energy for recycled production.
- Aluminum is highly recyclable because it can be melted and reformed many times with little loss of quality.
- Magnetic sorting removes steel because steel contains iron and is attracted to magnets.
- Density sorting works because materials float or sink based on density, using the idea density = mass / volume.
- Plastic resin codes identify polymer type, but the code does not guarantee that the item is accepted or actually recycled.
Vocabulary
- Contamination
- Contamination is unwanted material in a recycling stream, such as food, liquid, plastic bags, or the wrong type of item.
- Materials recovery facility
- A materials recovery facility is a plant where mixed recyclables are sorted, cleaned, and prepared for sale to manufacturers.
- Optical sorting
- Optical sorting uses sensors and light to identify materials by color or polymer type and separate them with air jets.
- Closed-loop recycling
- Closed-loop recycling turns a used product back into the same type of product, such as an aluminum can becoming a new aluminum can.
- Downcycling
- Downcycling turns a material into a lower-value product because its quality or purity has decreased.
Common Mistakes to Avoid
- Putting plastic bags in curbside bins is wrong because they wrap around sorting equipment and can shut down the facility.
- Assuming every plastic with a recycling symbol is recyclable is wrong because many facilities mainly accept bottles and jugs made from PET 1 and HDPE 2.
- Leaving food or liquid in containers is wrong because contamination can spoil paper, attract pests, and cause whole batches to be rejected.
- Mixing broken glass with paper or plastic is wrong because small glass pieces can contaminate other streams and damage equipment.
Practice Questions
- 1 A town collects 12,000 kg of mixed recyclables in one day. After sorting, 9,000 kg are accepted for processing. What is the accepted recycling rate for that day?
- 2 Making 1 kg of aluminum from ore uses 200 MJ of energy, while making 1 kg from recycled aluminum uses 10 MJ. How much energy is saved by recycling 50 kg of aluminum?
- 3 A recycling facility receives clean aluminum cans, greasy pizza boxes, plastic film bags, clear glass bottles, and PET water bottles. Which items are most likely to move successfully through the recycling process, and which are likely to cause problems? Explain using sorting, contamination, and material value.