Try dropping it off at the Salvation Army on 28th Street, I haven't been there in about 3 years but it's worth giving them a call. I had worked at the store when I was there for their rehabilitation program and they took mattresses all the time as long as it wasn't damaged. You can even call them for a pick up if need be and as long as they're still doing that
Goodwill doesn't accept mattresses & I can't think of anywhere that does anymore. You might list it in free items on 5miles if you don't want to just throw it out.
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