Since it's cold as a witch's tit up there right now, my solution for you won't work this time of year, most likely, as it involves digging the tanks out and dipping the crap out by hand using 5 gallon buckets and rubber gloves, dumping the buckets of crap into a "compost pile" of leaves, twigs, stick and wood chips and letting every thing compost to nothing.
Kinda like what the city sewage plants do with the crap they dig out of the bottom of their sewage treatment settling ponds. They they sell it as "compost", e.g., what the city of Austin Texas does - they sell it as
Dillo Dirt. Great for flowers, but I sure wouldn't use it in my veggies.
Another option is to buy a "solids" pump ($200 to $400) and pump the stuff out yourself. The problems with this solution are 1) you need a water source to flush the crap around after the pump has removed the initial slug of liquid
and
2) there's a lot more liquid involved in the disposal methods with this way.
You could get a couple of empty 55 gallon barrels and haul water from the river below your property to augment the pump, but again it's kinda cold out there right now to be dipping and hauling water. Anything you pump out this time of year will freeze up there in MO, so it wouldn't run too far from where you pumped it to, but....
Illegal would be dumping it on someone else's property or dumping it on yours and allowing it to run directly into a waterway.
Putting organic material directly into a compost pit? No problem there, most likely. Especially if it is a dedicated composting area, say like you have a "pit" of stacked concrete blocks that you routinely dump non-meat scraps, leaves, twigs etc into for composting purposes. I say non-meat scraps because meat scraps stink while "composting", i.e., rotting and they draw critters which dig through and/or destroy your pit.