Why is it bad when compostables land in a landfill?

Question

Don't they help with composting the mixed garbage in a landfill?

Answer 1

most landfills are so compact that there is no oxygen mixed in

this can cause say- a christmas tree to never, ever decompose because it uses oxygen to compost

Answer 2

in the old landfills, there was no aerobic activity, or anearobic, whatever, things didnt decompose that well.  If they did, methane gas is produced, which leaks into the atmosphere.  I dont know whether it is bad or not that compostable end up in a landfill, except that they take up extra space, and there are better places for them.  Some "waste transfer stations",  or whatever the newspeak is, have seperate areas for compostables. recyclables, burnables, etc.  A little extra effort on our behalf to seperate what we can may add up to a big difference eventually.