AP® Environmental Science · Unit 8 · Topic 8.10
Waste Reduction Methods
Waste reduction follows a hierarchy: not creating waste beats reusing it, which beats recycling or composting it, because each step down still costs energy and still produces some waste.
The waste hierarchy ranks strategies by how much material and energy they save. Source reduction, simply not creating the waste, sits at the top because it needs no collection, no transport, no processing. Reuse comes next: a glass jar washed and refilled skips manufacturing entirely. Recycling and composting sit lower because, even though they recover material, they still burn fuel to collect and haul waste, still run machinery, and still lose some material as unusable residue. Disposal in a landfill or incinerator sits at the bottom, since it recovers nothing.
Recycling a plastic bottle means sorting at a materials recovery facility: machines and workers separate plastic from glass, metal, and paper using magnets, screens, and optical sensors, then the plastic is shredded, washed, melted, and reformed into pellets for new products. Composting works biologically instead of mechanically. Microbes, mostly bacteria and fungi, break down food scraps and yard waste, respiring and generating heat that can push a pile past 55 degrees Celsius, fast enough to kill pathogens and weed seeds while converting the scraps into nutrient-rich humus. Both processes divert material from landfills, but composting returns nutrients to soil rather than making a new manufactured product.
A city of 200000 people generating 1.2 kg of waste per person per day produces 240000 kg total daily. If 30 percent of that stream is composted, the diversion is 240000 times 0.30, or 72000 kg per day kept out of landfills. That same arithmetic, population times per-person rate times fraction diverted, applies to recycling programs too, and connects directly to the next idea: not every diverted material is diverted equally well, since plastic degrades with each recycling pass in a way compost nutrients do not.
Key terms
- Waste hierarchy
- A ranked set of strategies, from source reduction down to disposal, ordered by how much material and energy each one saves.
- Source reduction
- Preventing waste by not creating it in the first place, such as using less packaging.
- Materials recovery facility
- A plant that sorts mixed recyclables into separate material streams using machines and manual labor.
- Downcycling
- Converting recycled material into a lower-quality product because repeated processing degrades it.
- Composting
- A microbial process that breaks down organic waste into nutrient-rich humus, often generating heat above 55 degrees Celsius.
Watch it explained
- How it worksWalk me through a plastic bottle's route through a materials recovery facility.
- How it worksHow does composting turn food scraps into soil using heat and microbes?
- Worked exampleFind the waste diverted when a city of 200000 composts 30 percent of a 1.2 kg per person daily stream.
- Common mistakeWhy isn't recycling a plastic bottle the same as making a new one out of it forever?
Check yourself
End of topic.