We don't recommend attempting to sterilize the media; it's impractical in this situation, as you noted, and it won't stay sterile for long, as airborne fungal spores and other microbes can always recolonize it once it's put back into use. (If you had experienced problems with suspected root or wilt diseases last year, then we'd suggest emptying the pot instead, cleaning it, and using fresh mix for new plants.)

To reuse potting mix, you can add compost to it (as much as 50% by volume) or add fresh potting mix in the same proportions. This will help increase porosity for good drainage, and compost in particular can reduce the risk of soilborne diseases by introducing beneficial microorganisms that help roots resist infection. This coming season, fertilize the new plants regularly as the old potting soil will have been largely depleted of nutrients, and nutrients from compost are slow to be released and will not be sufficient by themselves.

Miri

Where do I dispose. Of the half  un- 

Used  in a responsible way?

You can compost it, and if you don't have a home compost pile, you can ask a local landfill/municipal waste facility that does compost if they accept potting mix. It's also fine to use as "fill dirt" (so to speak) if you have divots in the yard (like low spots in a lawn that you will be reseeding) or other gullies that need to be topped-off with soil. Even though raised bed mix isn't true soil in most cases, it's fine to use in cases like that.

Miri