My wife and I were Peace Corps volunteers in Paraguay several years ago. In the three years we served there, we lived in three different houses. None of them had running water, and they all had horrific outhouses when we moved in.
To start with, the outhouses were all “squatters,” and we, like most Americans, prefer “sitters.” That was easily corrected with a few boards and a seat. The odor and fly situation required a bit more ingenuity. However, Peace Corps volunteers had been facing this problem for years, and the Peace Corps library in Asuncion, Paraguay, had a wealth of information on the subject, all filed under the initials VIP, which stood for “Ventilation Improved Privies.”
Armed with this information, I bought an 8-foot section of 11⁄2-inch plastic pipe and poked it down through the dirt, into the outhouse pit area from the outside, so it ended just under the floor on which the seat rested. I was careful not to plug the pipe with dirt. Working from the sunny, north side of the building (Paraguay is south of the equator), I fastened the pipe to the outside wall vertically, making sure not to crimp it. Then, I painted the pipe and the wall area just behind it flat black. As an indicator of airflow, I glued one end of a small piece of audio tape to the inside of the pipe outlet.
The next morning, I was outside early. As the sun heated the pipe, the piece of tape began to flutter and then stood straight out. While I watched, the flies left the pit and swarmed at the pipe outlet. After several days of this, all odor had disappeared, along with most of the flies. Later, as a further refinement, I wired a piece of screen over the pipe outlet.
This simple idea will work for squatters and sitters, as long as the seat or hole has a cover that’s kept closed when the seat is not in use. The cover should have a small opening of some sort just underneath it to permit air to enter the pit area, thus allowing convection flow up the pipe to sweep away any odors.