Cleaning out a DIY septic tank

winniman is correct. Although it"s possible to not have 2 chambers (the vast majority do), you must have the outlet baffle. A barrel with holes drilled in it does not have the outlet baffle. And if the holes are drilled all the way (or most of the way) to the bottom of the barrel, it's useless. What the OP has is a rusted and probably broken and dilapidated "used to be barrel" full of ****. Full being the operative word. No baffle, no standing water, no organisms to breakdown the solids in the OP's barrel, hence, the solids didn't breakdown.
What he had was an outhouse and just like the old timers did when the hole got full, it's now time to dig a new hole.