No matter how you treat sewage, sludge is a solid material that remains after the sewage has been digested. It is absolutely necessary, as Steve M. points out, to have the sludge removed every couple of years. Even if your system doesn't back up, once the tank is full of sludge, it no longer is doing its job and you have raw sewage entering the drain field, contaminating both ground water and the surface of the drain field with very dangerous toxins.
Individuals coming in contact with contaminated water can contract illnesses such as typhoid, tuberculosis, dysentery, cholera, tetanus, hepatitis and several types of gastroenteritis.
In many areas, the soil is not able to properly leech the output from a septic tank but permits are granted anyway. In these areas it is especially important that the septic tank (s) are able to properly digest the sewage before the liquid stream enters the septic drain field.