That’s a great question, @Oshenlynx! There are many different types of wax materials that candles are made out of (beeswax, paraffin, soy, synthetic materials, etc). I’m not a disposal expert, but I imagine that different types of wax should be disposed of in different ways.
Something all natural, such as beeswax, could definitely be returned to nature. Heck, if you leave beeswax candle remains (or honey) outside, the bees will come take it back! 
After a honey harvest, some beekeepers leave their messy equipment outside and the bees reclaim any honey and wax leftovers.
For more ideas, the discussions How to Dispose of Spell Materials and Disposal of Spell Remains may have what you are searching for 
When casting positive spellwork (not Baneful Magick and nothing that involved drawing negative energy into the items- these spell materials should be safely disposed of), why not cleanse and then reuse the wax? 
It is both earth-friendly and wallet-friendly! 
I found a guide here that might be able to help you:
Just a few things you might consider! I think you are wonderful for being mindful of the Earth when disposing of your spell remains 
Good luck with your spellwork and wax disposal- I hope you’re able to find some methods that work for you!
Blessed be!