It is hard to accept you are ...

It is hard to accept you are sure the STP will accept this and that no pipe flow problems will result. You should identify the STP's capacity and willingness to accept your ground-up garbage. It is likely they will want you to separate solids before grinding and dispose of them separately. The sewer needs to be evaluated to ensure there is no blockage created. Talk to (1) STP owner, (2) sewer owner, and (3) grinder manufacturer. You will need data on the raw (in-ground) garbage such as contents of recyclables, etc.  A fair amount of effort to make this work, even though the quantity seems small.