Paul_KD7HB:
Your situation calls for a "blanket heater" that wraps completely around the tank and heats evenly.
Indeed, that'd be best, but I guess it's too expensive and not too practical for a waste storage tank that's to be heated once a month (just so it's liquid so temperature doesn't matter much, and it may take a day to actually reach the liquid state). Bigger surface area of the element would definitely be an advantage, for sure.