By what I've seen in pictures and reading online the tank has 2 sections. The first tank takes waste in. The pump is in this section. The bubbles gets things rolling and the action helps break any matter down. Some of the waste settles to the bottom. The waste water then enters the second section were the matter settles and water drains off.
My pump locked up. When? Could have been last year. Average 3 to 24 months. You won't know it until it clogs or you have an alarm on the pump. The solids don't get broke down and clog the spillway to the second tank.
I looked today. The water at the end of the drain pipe is at a drip. The clean out cap is off, water is about 4" from the top. I removed the vent cap. The water is about 3 ft down. That tells me the spillway is clogged.