That's an excellent and very important question! I learned this lesson the hard way years ago, so I'm happy to share what works. The absolute golden rule is this: Never, ever put hot or warm ashes directly into any trash bin. It might seem harmless, but ashes can retain intense heat for days, just waiting to ignite paper, plastic, and other trash.
Here is the only safe method for urban living:
Step 1: Cool Completely in a Metal Container
After you're done with your fireplace, grill, or fire pit, carefully transfer the ashes using a metal shovel into a dedicated, metal container with a tight-fitting metal lid. A metal bucket or an old metal galvanized trash can works perfectly. Never use plastic, wood, or cardboard containers for this—they can melt or burn.
Step 2: The Water Test
Once the ashes are in the metal container, take them outside, away from your home and any flammable structures. Slowly pour water over them, stirring gently with the metal shovel until you are absolutely sure every ember is out. You should see no steam or hear any hissing sounds. Let this mixture sit for at least several hours, or ideally, for 24-48 hours.
Step 3: The Hand Test
Before you even think about disposal, do the "hand test." Carefully place the back of your hand just above the cooled ashes in the metal container. If you feel any warmth at all, they are not ready. Wait longer.
Step 4: Secure Disposal
Only after the ashes are stone-cold should you double-bag them in sturdy trash bags and then place them in your urban trash bin. It's also a good practice to place the bag on top of your other trash, not buried underneath, just to be extra safe.
A quick but crucial reminder: Always keep your ash-collecting metal container stored on a non-flammable surface like concrete or dirt, never on a wooden deck. Following these steps might seem like a hassle, but it's a small price to pay for preventing a dangerous and destructive fire. Stay safe