The modern flush toilet is very wasteful of water. The alternative, of course, is a way to dispose of human waste that does not use up resources.
There are many models on the market that do not use water such as chemical (very smelly and messy), or burning it by means of gas or electric which means being dependent on the grid, or having big holding tanks under your house. The one resource that all of these systems use is great amounts of your hard earned money.
We have a composting toilet that did not take a lot of money, nor uses water, chemicals, or heat. It is simply a box and a bucket.
I built a box to hold a five gallon plastic bucket out of ½" plywood. It is 15"w x 16"d x 16"h. The front panel is hinged to make a door. I made a lid out of the same material and hinged it at the back. After cutting a hole alá an outhouse, I painted it with some gloss enamel paint so it would be easy to clean.
"Surely" you say, "You don't use it in the house?" Yep, sure do. "What about the smell? Doesn't it attract flies?" Nope! The secret to this system is rotten sawdust. After every use, you sprinkle enough sawdust to cover the deposit. When the bucket is full, you empty it place where it's out of the way and cover with more sawdust. The bucket is washed out, a layer of sawdust sprinkled in to thinly cover the bottom and placed back in the box.
We've been using this system for a year and the pile is about 4' long by 2' wide by 2' high. This spring we will start a second pile and let the first one compost for a year. By that time it will have heated up enough to kill any pathogens and broken down into rich black compost that is safe to use in your garden or around fruit trees.
I learned about this system from The Humanure Handbook, which can be ordered from Lehman Hardware, Kidron, Ohio. I suggest that you get a copy of the book. It's informative and well written with a sense of humor.
We have never had a problem with odor or flies, not even tile composting pile seems to attract flies. The rotten sawdust must have something in it that repels insects that usually are attracted to rotting organic matter.