[pushkins]

Septic systems are pretty basic in their design and functionality.
The tank holds the liquid and solids allowing the bacteria to eat away at the waste until it is more liquid than solid. The liquid passes out through the leach field pipes and is allowed to soak into the ground.
The soils permeability factor will dictate how fast the "soaking" process takes and usually how long the leach field needs to be.

The biggest reasons for septic failure are tree's/gardens roots etc..., disturbance of the leach field (a truck driving over the pipes and crushing them) and possibly the biggest reason, none biodegradable objects in the tank.

Many septic systems are put on mounds especially where virgin soil is less than ideal for permeability reasons, they build a mound install the tank and then backfill the entire area including trenches with good permeable soil and rock (in the trenches), this allows the leach fields to absorb the waste water into the new soil and help the transition to the original ground.

Systems like this that I've seen fail (only a few) usually come on mounds build over very heavy clay soil and extreme conditions that flood the leach lines , the water cannot soak into the ground fast enough and eventually the water builds up between the new soil and the clay original soil and this is the easiest way out for the water.

So in short contact your health department, get the OK to put in a new system and the area they are allowing it to be installed, then contract with a reputable septic company.