"Compostable" isn't a marketing adjective — it describes a specific chemical process. Here's what actually happens to a Greensack-style bag from the moment it's discarded to the moment it's gone.
What the bag is made from
Certified compostable bags are typically made from a blend of plant-based polymers rather than petroleum. The most common building blocks are:
- Cornstarch-based polymers (TPS): derived from corn, cassava, or similar starch crops, providing biodegradability and a renewable base.
- PLA (Polylactic Acid): a polymer made by fermenting plant sugars into lactic acid, then polymerizing it — chemically similar in structure to conventional plastic, but built from biological building blocks that microorganisms can break back down.
- PBAT (Polybutylene Adipate Terephthalate): a biodegradable co-polyester blended in to give the bag flexibility and strength, since pure PLA/starch blends can be brittle on their own.
The three stages of decomposition
1. Disintegration
In composting conditions — warm temperature (around 58°C), moisture, and oxygen — microorganisms begin colonizing the material's surface. The long polymer chains start breaking into shorter fragments. Within weeks, the bag loses structural integrity and visibly fragments.
2. Biodegradation
Bacteria and fungi metabolize the shorter polymer fragments as a food source, in the same way they'd break down fallen leaves or food waste. This is an enzymatic process — the microorganisms secrete enzymes that cleave the polymer chains at a molecular level.
3. Mineralization
The final products of that microbial digestion are carbon dioxide, water, and biomass (microbial cell material), which becomes part of the finished compost. Under EN 13432 / ASTM D6400 testing, at least 90% of the material must convert this way within 180 days — with no toxic residue and no lasting microplastic fragments.
The end result of full composting isn't "smaller pieces of plastic" — it's CO2, water, and organic biomass, chemically indistinguishable from any other compost input.
Why conditions matter
This process depends heavily on the right environment. A compostable bag sitting in a dry landfill with no oxygen, moisture, or microbial activity will decompose extremely slowly — landfills are actually a poor environment for any biodegradation, compostable or not. This is why certified compostable packaging is designed to be composted (industrially, or via municipal organic waste collection), not landfilled — and why proper waste segregation matters even when using compostable bags.
The practical takeaway
Compostable bags aren't magic — they're an engineered material designed to behave like a food scrap in the eyes of a microorganism, provided they end up somewhere that microorganism can reach them. Pairing compostable packaging with basic organic waste segregation is what actually delivers the environmental benefit.