It’s always worth checking your municipality’s composting rules. But frustratingly, you’ll probably need to throw compostable poop bags in the trash after using them, even if you have curbside compost pickup.
In a large-scale industrial composting facility, a certified compostable bag is designed to successfully biodegrade into carbon dioxide, water, inorganic compounds, and biomass. However, compostable poop bags are prohibited at virtually every industrial composting facility in the US.
One reason for that is the poop itself. As Mel Plaut, Wirecutter’s senior staff writer of pets coverage, reported in our guide to the best dog poop bags, “Most municipal composting programs in the US don’t accept pet waste because it can contain pathogens as well as the residues from antiparasitic medications.”
Another reason compostable plastic bags are not welcome at composting facilities is that they look too similar to non-compostable plastic bags.
As former EPA press secretary Remmington Belford told Mel, “Most sorting machines and human operators at recycling or composting facilities find it difficult to differentiate between the two—compostable and not.” Since it’s time-consuming and labor-intensive to identify petroleum-based plastic bags and then remove them, many composting facilities simply don’t take any bags that look like plastic.
Meanwhile, certified compostable plastic isn’t designed to break down in home-composting setups, since backyard bins and tumblers can never reach those very high, industrial-level temperatures necessary for biodegradation of the bioplastic to occur. As Bryson explained, industrial composting facilities sustain those high temperatures “for days or even weeks,” allowing the material to break down.
Bryson, who has studied home-composting setups, said that creating enough heat in such a small system “is just not realistic,” since “most home-composting systems don’t reach or sustain such high temperatures, making it difficult for PLA to break down effectively.”
You can find dog poop bags that claim to be suitable for home-composting environments, some of which we’ve tested, but Bryson said they may not live up to their claims. “They’re tested under controlled lab conditions, where temperatures remain constant,” she stated. “In real-world, outdoor, home composts, temperatures rise and fall over hours, days, and seasons. This natural variability means a certified home-compostable bag may not break down as well or as quickly as expected.”
One more bummer: You can’t put a compostable bag into your plastic recycling, even if it’s free of animal waste, because it’s made from different polymers that would contaminate the recycling stream.
