Yes, there are public waste bins with built-in sand urns specifically designed for dog waste, though they're not as common as regular trash cans. These bins are usually part of a "pet waste station" or "dog waste disposal system" found in parks, dog runs, and some urban sidewalks.
The built-in sand urn is a small, separate compartment or insert at the top or side of the bin. It's filled with coarse sand or a similar absorbent material. The idea is that after you pick up your dog's waste with a bag, you place the bag into the urn. The sand helps to weigh down the bag, absorb excess moisture, and reduce odors, making disposal cleaner and more contained.
In my experience, these bins are most often spotted in dedicated off-leash dog parks, trailheads with heavy dog traffic, and some well-maintained community parks in cities like Portland, Seattle, or Boulder. Many newer "smart" bins also incorporate a charcoal filter or even a small vent to manage smell, but the sand urn version is a simpler, low-tech solution.
That said, they are not the norm everywhere. Most public waste bins are just standard metal or plastic cans without a separate urn. If you're looking for one, I'd suggest checking local park department websites or apps for "dog waste stations." Alternatively, you can always carry a portable bag dispenser and use a general public bin—just be sure to tie the bag securely.