In past years, we were able to find a product called chlorinated lime that made using the outhouse a reasonably pleasant experience. In the past few years, we've been unable to find the product. Most states have outlawed the sale of chlorinated lime, judging it to be a hazardous substance. There are a number of options available. Some models use no electricity, some are powered by propane gas, and some are powered by electricity or photovoltaic cells.
Models are available that can be installed either indoors or outdoors.