The main danager of a garbage disposal is that the excess food can overload the ability of the septic tank to seperate and digest the waste. The additional solids may pass through the tank and can cause overloading of the drainfield by increasing the oxygen demand to digest the waste above the ability to supply oxygen in the drainfield. This can cause premature conversion of the drainfield to anaerobic conditions which can help the field to fail. 

In addition additional solids can themselves phisically clog the laterals and the stone preventing the waste water from flowing into the ground.

The expensive part of the septic system is the drainfield. With proper maintenance, for example pumping and inspection of the septic tank about every three years, the septic system can last many years. However care should be taken not to increase the amount of solids or dissolved solids in the effuent from the tank to the drainfield. This can help cause the field to fail and this can lead to expensive repairs and landscaping costs. See ourWEB page about preventative maintenance for the drainfield. 

I would not recommend the addition of a garbage disposal qwhich is connected to a septic tank.