This is something that homeowners should be concerned with. How are you going to get rid of your waste without being able to flush the toilet? Probably the best option for homesteaders and preppers alike is building a working outhouse. People used outhouses for years. They could be a good thing if the grid ever went down. To build an outhouse you first want to choose a location that is 50 - 150 ft. from your home. This is to keep the smell at a minimum. A hole should be dug around 5 ft. deep and up to 3 ft. in diameter. A small building should be constructed on top of the the hole. Vent pipes should be ran from the pit to on top of the roof to vent the methane gas that would build up.
It might seem pretty primitive but if the grid went down it's going to be one of your only options to get rid of your waste. A well built outhouse would last years and with a 5 ft. deep pit your going to always be in a constant cycle of decomposition which shouldn't ever fill up. The outhouse would need to be well built and sealed up good around the pit itself. Outhouse specific toilet seats can be purchased to seal the pit and keep bugs and the smell down as much as possible. When people used outhouses they would sprinkle lime into the pit to aid in decomposition along with keeping the smell down. If you are a homesteader, you might want to look into building one of these if you expect the grid to go down for any length of time. Below is a diagram of an outhouse to give you an idea of how to build one.