It got me interested, so I googled "biodegradable vs compostable poop bags"
I found:
"While biodegradable items refer to just any material which breaks down and decomposes in the environment, compostable goods are
specifically organic matter which breaks down, the end product having many beneficial uses which include fertilizing and improving soil health"
"One 2019 study found that bags marked biodegradable lasted in soil, submerged in seawater, and out in the open air for three years or more. That's why researchers say compostable bags are a
more eco-friendly option. They're usually made of plant starch and contain no plastics, so they dissolve much more quickly"
"Even the very best, most scientifically-proven
biodegradable plastics won't degrade in a landfill, where compression and lack of oxygen lead to “mummification” of garbage. To get the best results from earth-friendly poop bags, you have to dispose of them correctly"
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Summary: biodegradable is kind of scam. Compostable poop bags are preferred (eco-friendly). Seems that even if buried in soil (without real composting) they will fully degrade, instead of just breaking down into microplastic pieces.
Thanks
@scaredycat72 for the link to the really compact foldable TRIPTIPS Portable Toilet