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.