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Composting Toilets 101
"Actually, all pollution is simply an unused resource. Garbage is the only raw material that we're too stupid to use."
– Arthur C. Clarke
The way humans dispose of their waste is a bellweather of civilized society. Does this society protect its members from the horrors of dysentery and even worse diseases? Over the past century sanitation in North America has gradually evolved from a system where almost every home had an outhouse in the backyard, and rivers in major cities were simply open sewers, to modern flush-and-forget-it systems, where everything seems to simply disappear, and rivers run cleaner.
Making distasteful items disappear and rivers run clean is certainly going to win public approval, and we won't be so foolish as to suggest that there was some mystical good in the old days. There wasn't. Outhouses smell bad in the summer, are too close to the house, and allow flies to spread filth and disease. In the winter they don't smell, but are too far away. However, in our rush to sanitize everything in sight, we end up throwing away a potentially valuable and money-saving resource (not to mention over-designing expensive and energy-intensive disposal systems that still may pollute groundwater). Think about the absurdity of mixing human waste with drinking water and giving wings to the bacteria and pathogens.
Composting toilets can close the nutrient cycle, turning a dangerous waste product into safe compost, without smell, hassle, or fly problems. They are usually less expensive than conventional septic systems and they will reduce household water consumption by at least 25 percent. But like the venerable outhouse, composting toilets only deal with human excreta. Unlike a modern septic system, they won't provide greywater treatment.
What Is a Composting Toilet?
A composting toilet is a treatment system for toilet wastes that does not use a conventional septic system. Composting toilets were originally developed in Scandinavia, where almost no topsoils suitable for conventional septic systems exist. A composting toilet is basically a warm, well-ventilated container with a diverse community of aerobic microbes living inside that break down the waste materials. The process creates a dry, fluffy, odorless compost, similar to what's in a well-maintained garden compost pile. Flowers and fruit trees love it — although we don't recommend using this compost on kitchen gardens, as some human pathogens could possibly survive the composting process.
The composting process in such a toilet does not smell. Rapid aerobic decomposition (active composting), which takes place in the presence of oxygen, is the opposite of the slow, smelly process that takes place in an outhouse, which works by anaerobic decomposition. Anaerobic microbes cannot survive in the presence of oxygen and the more energetic microbes that flourish in an oxygen-rich environment. If a composting toilet smells bad, it means something is wrong. Usually smells indicate pockets of anaerobic activity caused by lack of mixing.
How Do Composting Toilets Work?
A composting toilet has three basic elements: a place to sit, a composting chamber, and a drying tray. Most models combine all three elements in a single enclosure, although some models have separate seating, with the composting chamber installed in the basement or under the house. In either case, the drying tray is positioned under the composting chamber, and some sort of removable finishing drawer is supplied to carry off the finished and composted material.
Ninety percent of what goes into a composting toilet is water. Compost piles need to be damp to work well, but most composting toilets suffer from too much water. Evaporation is the primary way a composting toilet gets rid of excess water. If evaporation can't keep up, then many units have an overflow that is plumbed to the household greywater or septic system. Heat and air flowing through the unit assist the evaporation process. Every composting toilet has a vertical vent pipe to carry off moisture. Air flows across the drying trays, around and through the pile, then up the vent to the outside of the building.
The low-grade heat produced by composting is supposed to provide sufficient updraft to carry vapor up the vent. However, like any passive vent with minimal heat, these are subject to downdrafts. Electric composters use vent fans and a small heating element as standard equipment. Gaiam Real Goods offers an optional vent fan for nonelectric models that can be battery- or solar-driven. Adding a cup every day of high-carbon- content bulking agent, such as peat moss, wood chips, or dry-popped popcorn, helps soak up excess moisture, makes lots of little wicks to aid in evaporation, and creates air passages that prevent anaerobic pockets from forming.
The solids are treated with by a diverse microbiological community — in other words, composted. Keeping this biological community happy and working hard requires warmth and plenty of oxygen, the same things needed for speedy evaporation. Smaller composters usually employ some kind of mixing or stirring mechanism to ensure adequate oxygen to all parts of the pile, and faster composting action. Composting toilets work best at temperatures of 70°F or higher; at temperatures below 55°F the biological process slows to a crawl, and at temperatures below 45°F it comes to a stop. The composting action itself will provide some low-level heat, but not enough to keep the process going in a cold environment. It is okay to let a composting toilet freeze, although it shouldn't be used when cold or frozen. Normal biological activity will resume when the temperature rises again.
The earliest composter designs, such as the Clivus Multrum system, use a lower-temperature decomposition process known as moldering, which takes place slowly over several years. These composters have air channels and fan-driven vents, but they lack supplemental heat or the capability of mixing and stirring. Therefore, these very large composters don't promote highly active composting activity.
There are two other disadvantages to this early, low-temp design. Liquids often have to be manually removed or pumped out of these units because the lower temperatures' slow evaporation. And, because temperatures don't get very high, there's a greater chance of pathogens or parasite eggs surviving the composting process. This type of large, slow composter is most effective in public access sites, and many are currently in successful use at state or national parks. Their large bulk lets them both absorb sudden surges of use, and weather long periods of disuse without upset.