The purpose of the tank is to separate the waste into three distinct layers:
- scum (top layer)
- liquid (middle layer)
- sludge (bottom layer)
ONLY the middle layer (liquid) is supposed to flow out the tank's effluent pipe and into your drain field. To make this happen, the tank has either a) baffles or b) downpipe at the effluent end. The baffles or downpipe prevent the floating scum from entering and blocking the effluent pipe.
You must pump your tank BEFORE the scum layer gets so thick that it gets into the effluent pipe. The other thing that can happen is that the sludge layer (bottom layer) builds up so high that it gets into the effluent pipe.
The scum layer is caused by grease, soap, hair, and other things. The sludge layer is mostly decomposing poop, and dirt.
When the guy comes to pump your tank, he should work the hose to remove the top scum layer while it is still floating, before sucking out the liquid and sludge. If he just sticks the hose all the way down and walks away while it is pumping, you're not getting a quality job. The scum will settle down into the sludge and won't get completely removed.