What's my Tank for?
The Septic Tank acts like a filter for your leech field. Every toilet flush and washed dish adds to your system. The solids get to tank and over time break down into sludge and scum. What's left is the effluent water. The tank holds all of the heavy stuff and allows effluent to the leech field.
Why pump it?
Just like other filters in our world - it needs to be cleaned in order to work. Pumping the tank gets all of the solids that build up out to allow the tank to function at its best. When the solids build up too much, they can clog the outlet, create an issue at the inlet, or even worse, allow solids to the leech field and ruin its ability to drain effluent water.
when should i pump
There are quite a few variables for this answer. For a very informative read and easy to follow table click here.
We say that an average household of 4 with a standard 1000 gallon tank can go 3-4 years between pumping. But this varies wildly due to usage habits.
my tank is full already
"You guys pumped my tank last week and it's full already!"
Your septic tank will "fill up" within a week of normal use, however this is mostly water and does not need to be pumped. When we say that your tank is FULL; we mean full of scum and sludge. If you look closely at the cross section photo above, you'll notice where waste comes in and effluent go out. Those inverts (bottom of tank to bottom of pipe measurement) are only 3" apart. Having liquid is a crucial part of breaking down the waste into manageable effluent.