Because, in a landfill, compostables don’t compost.

To make compost you need air, water, and the right carbon/nitrogen ratio in your organic waste. (While there are anaerobic composting methods out there, I’m sticking with the most commonly known/used method of making compost. That uses aerobic bacteria.)

A landfill is basically a giant, airless, almost perfectly preserved stash of humanity’s junk. Water doesn’t have much chance of penetrating, since everything is smushed in there as compactly as possible. (Sure, there will be moisture in some of the things that got added, but it won’t be enough and it definitely won’t be evenly distributed.)

There’s also the part where your compostables are mixed through with all sorts of non-organic material like plastics, old TVs, tyres, broken ceramics, possible industrial waste and hazardous chemicals, and who-knows-what else. That does not make for a good composting environment.

Now, why compost things in the first place?

• Most cities do, in fact, use the resulting compost on municipal gardens and other plantings. Depending on the city they may also sell it to the public to use on their private gardens as well.
• Diverting things out of the landfill - green waste, recyclables, whatever - makes the landfill site last longer before it needs to be closed down. Many municipalities are finding it harder and harder to find new sites to build landfills when the old ones are full.
• Compost is an excellent soil amendment and fertiliser. This makes it worth money. Have you ever walked down the compost/soil amendments aisle at a nursery or hardware store? Tiny little 25L bags of the stuff are being sold for $9 and up. This means that city councils can often make a profit from diverting the green waste, composting it, and selling it back to the public. Uno degli appaltatori locali del compostaggio vende la roba a 80 dollari/tonnellata (circa 1,5 metri cubi), o 10,50 dollari per un sacchetto da 30L.

Sì, l'idea è che il compost sarà usato per i giardini, sia dalla città che dai residenti o venduto alle aziende.

Un altro vantaggio è che la discarica non si riempie così rapidamente.