A septic tank is always "full"..
It is full of water, and whatever stuff was in the water, that comes out of your house... When the water enters the tank, the solids settle to the bottom, and the light stuff (fat/oil, soap scum etc) floats to the top. This happens pretty well in a clean tank, since there is very little actual water movement in the tank, because it is large... think of it as a giant sediment-settling tank. Whenever water comes in one end of the tank, the same volume of water goes out the other end of the tank, into your drainage field where is is absorbed by the ground. It is very important that the water that goes out of the tank be "clear" water, ie no particles or suspended solids or anything. It is not clean water, but it needs to be only liquid.
The solid waste that enters the tank and settles to the bottom is worked on by bacterial action, and the total volume is reduced drastically, sometimes to as little as 1/100 of the original size. But, there is some leftover residue. This residue (or sludge) build up over time, and can significantly decrease the total volume of the tank, eventually. This is known as a "full septic tank" and it is very important to have the tank pumped to remove the sludge before this happens. The time between pumping depends on how much water and solids you put into the tank, depending on factors like how many people live there, what kind of clothes washer you use, if you have a separate greywater system (not common around here in MD) and how much you use a kitchen-sink garbage disposal.
Another important point is that on a septic tank you need to be careful about what you put into it... anti-bacterial soaps are the worst, as the triclosan in them stays active for a long time, killing bacteria... and as mentioned it is the bacteria that "eat" the solid wastes for you. Even a lot of toothpaste these days has triclosan in it, and anything labeled "anti-bacterial" is very bad for your septic system. As a general rule, you should only put stuff into the septic system that you have eaten first, or at least COULD have eaten first. That means using a lot of harsh household cleaners should be on your list of do-not-do as well. Anything that will kill/upset the bacterial action in the tank should be avoided if you can.. within reason of course. A little bit of ammonia or soap won't hurt it, and even bleach used sparingly can be OK.
What happens when there's too much sludge in the tank is that the free-water volume is reduced so that there's not enough "dwell time" for new water entering the tank to have all of the solids settle out (and lipids float up to the scum layer). That means that the water that goes OUT of the tank into your drainage field will have these particles in it. Those particles (especially the greasy ones) will rapidly "plug up" the soil around the pipes in your drainage field, making it unable to absorb water anymore. When that happens, you'll start to see water puddling on the ground over the drainage field after using the septic system, most often when there's large inputs of water like showering or using the clothes washer.
Unfortunately, that appears to be what's happened to your system..... the only fix is to install a new drainage field (if you have the space) or to get some earthmoving equipment in and replace all the clogged dirt/soil in your current drainfield and replace it.
Here's some further reading, and Google will give you lots and lots of information on septic systems.
Septic Systems A to Z Maintenance
Septic System Maintenance, AEX-740-01
good luck!
~aaron
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