Just a few thoughts on septic tanks.
Tom covered this one about grease going down the drain. I use an empty coffee can, pour all grease into it, then wipe out the pan, etc, with a paper towel, before going into the dishwasher. The can of grease gets put into the garbage before completely full.
We have a garbage disposal, but nothing with grease goes into it. Anything greasy goes into the garbage.
My septic tank is a very old one, concrete. Had to "drill" and put a hole in the top, so I could put some concrete blocks over it, with the "holes" being used to put a pumping trucks' hose into, to pump out the tank. The top of the tank itself is about 3 ft. Under the ground. The top of the concrete blocks have a small 1 in. Think concrete block, with grass over it, about 4 in. Beneath the ground; easy to find using a screwdriver into the ground.
I agree with about 3 yrs to pump one out. I have a stick, actually an old fishing pole, marked with where the water is just below the drain pipe into the tank. I can check the water level anytime, looking at the mark. If it starts being above the mark, then it's time to pump it. Been about 4 yrs now, since last time.
Cost of pumping here in the Great Smokey Mountain area of VA is about $200 now.