Garbage disposals are not good for septic system as stated above but more because the ground up food becomes suspended in the liquid layer of the tank and will migrate out to the EDA clogging the drain field. The treatment tank is made up of three layers, the bottom or sludge layer, the liquid layer in the middle and the floating scum mat on top. The problem with food particles from a disposer is they don’t sink and don’t float. This allows them to move with the effluent out into the EDA.
Here in NH, if you want a garbage disposer you need to increase the tank size by 50%, in Maine you have to install an effluent filter in the outlet baffle.
The problem with effluent filters and garbage disposers is they tend to clog very easy and have to be cleaned on a regular basis.
Normal pumping for treatment tanks is when the sludge layer makes up more than 1/3 the total volume of the tank. Usually every two years for a family of 4-5.