We have been in the construction business for 5 generations. When building a new home if a garbage disposal is going to be installed it takes a larger septic field and often a larger size tank to meet building codes. Many older septic systems were not designed to handle waste from a disposal. The material put in a garbage disposal can and often does take much longer to break down. The volume of material put in a disposal varies with each user, some put everything down them others just scraps off of plates before washing. When running a disposal you are supposed to run a considerable amount of water at the same time to help liquify and flush the material down the drain lines and help disperse it into the tank. Many people do not do this and that can cause plugging in the drain lines and cause buildup of materail in one spot in the tank. In areas with building codes and code enforcement of any type you would need a permit to add a disposal to any residence. When getting the plumbing permit they would tell you then if are able to install one on your system.
If you are installing yourself many people will skip the permit process. Sometimes that can come back to bite you in the long run. If you have been having any problems with your septic system adding a disposal will only make things worse.
Mark