Per capita consumption of water in the US is three times that of the average European and fifteen times that of the typical person residing in a developing country. Of all the potable water that comes into the average American household, fully a third gets flushed down the toilet. With worldwide average consumption rates rising twice as fast as population, and population projected to double in forty years, it’s plain to see that flushing will be a luxury we can no longer afford. Water is essential to life, but with this gift comes the responsibility to promote sustainable use of this precious resource. Composting toilets save water and provide an environmentally compatible alternative to flush toilets.
Supplies may be increased by dowsing for water, rainwater harvesting, or desalinization. However, it is better to conserve water in the first place. Agricultural practices may lose up to half of the irrigation water in the process of distribution to evaporation and runoff. More efficient methods such as drip irrigation can make a big difference. Industry can potentially save a lot through water reuse. Within the home, more efficient washers, dishwashers and low-flush toilets can reduce water usage significantly. The practice of “cascading”, or reusing dishpan or shower warm-up water in the toilet bowl or on outside plants can save more. But, the biggest savings can be realized by eliminating flushing altogether.
Composting toilets and latrine systems are a viable, environmentally compatible alternative to flush toilets. This technology may enable you to avoid building an expensive mound system while enriching your soils.
Design Decisions
Once you decide to install a composting toilet, there are several design decisions that must be considered to determine the best solution for your particular circumstances. The elements of a composting toilet must be designed to either passively or actively manage oxygen levels, temperature, moisture, carbon to nitrogen ratios, and pathogen levels. You will also need to determine whether to purchase a pre-manufactured model or to custom-design a site-built unit,
Pre-manufactured models
Pre-manufactured, self-contained units come with a variety of features, needing only to be hooked up to a ventilation system. Some designs may be fully adequate for year-round continual use. They may be single or multi-chambered, require electricity for heating and aeration, or be totally non-electric. Models are available that dehydrate or incinerate the contents, but these are not truly composting.
The centralized Clivis Multrum model, features a large continuous-use system. Centralized or remote means that the bowl is on the main floor and the actual composting unit is in the basement. In this design, fresh material is deposited on top and finished compost is removed from the bottom of an inclined chamber. Material going in includes both feces and urine since it is not urine-separating. The Phoenix system features a vertical chamber, and the manufacturer claims its design efficiently prevent contamination of lower layers with fresh deposits. If the unit is to be used continuously, especially by a large family, a multi-chambered model may be recommended. However, most pre-manufactured remote systems are only offered in a single vault option.
Refer to our article titled Composting Toilets – Decisions, Decisions for links to some popular manufacturers of pre-manufactured models.
Homemade models
A pre-manufactured, self-contained composting toilet costs at least $1,400. However, a homemade unit can be built for much less. Site-built systems are only as expensive as materials and construction costs. If recycled materials are used, a composting toilet can be built for minimal cost other than the time it takes to build and install the unit.
Double vault composting toilets
All of the composting latrines I designed and built in Africa and Latin America are double-vault, fixed-batch systems. The larger-sized vault, was made out of rammed-earth, ferro-cement, or brick. It had one and a half cubic meter capacity which was enough for a large extended family or small school. A smaller version for an average-sized family featured a one cubic meter vault. In both styles, a stool or stoop plate was mounted over each vault. It might take a year to fill up one vault, at which time the use is switched to the second, leaving the contents of the first to “cure” undisturbed. By the time the second vault is filled at the end of the following year, the first is ready to be emptied and reused.
There are alternatives, however, to dual vaults built permanently side by side. Another variation is a composting unit with multiple chambers which can rotate each time one is filled. Also, a single stationary unit may be designed to accept removable bins.
Determining the size of a composting toilet
If the composting toilet is operating ideally, the contents should be fully composted with all pathogens destroyed by the end of six weeks. To calculate the capacity of each vault, allow for twice as much time. Thus, allow at least three months.
Certain factors should be considered when making these calculations. Will the use be seasonal or year-round? Will it mainly be day-use only or continuous use? (Day use tends to accumulate a much higher percentage of urine by comparison.) Here are the relevant statistics you will need. One person produces 40.6 fluid oz.s (1.2 liters) of urine per day. The same person also produces 20.3 fluid oz.s (.61 liters) per day of feces. Over a full year, that amounts to 155.8 gallons of urine and 57.9 gallons of feces, or a total of 1,300 lbs. of excrement. In terms of volume, that represents 20.8 cubic feet (.6 m3) of urine and 7.7 cubic feet (.2 m3) of feces per average person per year. Keep in mind that the volume of the contents will constantly decrease during the composting process. In fact, the volume will decrease to as little as ten to thirty percent of its original volume by the end of the incubation period.
Controlling odor
In tropical climates, the design may be able to operate passively. In temperate climates, the design may need to be more of an active one. This means a small electric fan installed in the vent pipe is needed to increase the efficiency of aeration and an artificial heat source may be required to keep the contents above “biological zero” (42 degrees Fahrenheit) to speed composting. For every ten degrees Celcius rise in temperature, the composting rate doubles. This is known as the Q10 temperature coefficient. Different microorganisms operate at different temperatures. From 42 to 67 degrees Fahrenheit, actinomycetes and fungi dominate in psychrophilic or mouldering processing. At 68 to 112 degrees Fahrenheit, mesophilic bacteria operate under most typical conditions. Between 113 to 160 degrees Fahrenheit, thermophilic bacteria take precedence.
The reason for aerating is partly to evaporate liquids, but most importantly to encourage aerobic decomposition. It is the anaerobic bacteria which are mainly the pathogenic and odor-causing microorganisms. Beneficial aerobic bacteria thrive in the higher oxygen and temperature levels which destroy the anaerobic organisms. If a composting unit is operating efficiently and is well ventilated, there should be no noticeable odor detectable from above.
Urine Separation
Another design decision is whether to separate out urine or not. There are several advantages to separation. First, it reduces moisture levels in the compost, hopefully to the ideal consistency of a well wrung out sponge. Second, it prevents the ratio of nitrogen to carbon from becoming too high. This is another cause of odors. Third, it will minimize the amount of effluent you need to deal with. Urine separated out may be directed to an outside charcoal or limestone-filled soak-pit. It may also be directed to a conventional septic system or stored in a tank. The tank may be emptied periodically by a collection service, or used on trees, flowers or other non-food plants. Human urine is relatively sterile and actually has more nutrients than the feces, but it also has a high salt content.
Urine-separating seats may be purchased to mount on your custom-made bench, or the entire bowl can be ordered if preferred. When seated, the urine is directed forward to be captured and drained separately. If there is enough room, a separate waterless urinal can be installed to supplement the composting toilet.
Composting Chamber
Finally, it is advisable to construct the inside of the composting chamber with a slightly sloped floor so that the minimal leachate produced may drain toward a collection device. Radiant heat may be designed into this floor. Otherwise, a submersible aquarium heater, a light bulb in a fire-proof box, a heat tape, or waste heat from a dryer duct may be installed to warm the contents. Build the unit a foot or so off the floor in case of flooding, and naturally, install an access port for removing contents and another for inspection or turning contents.
Conclusion – composting toilets save water
Considering an alternative to the flush toilet is not just a good idea to make us less vulnerable to infrastructure collapse, it is also an environmental advantage with regard to water conservation and soil enhancement. Composting toilets are an environmentally friendly solution for water scarce locations. They are a sanitary alternative to pit latrines, are more economical than mound systems, and produce garden quality compost for soil enrichment. Dry composting toilets save water and improve the environment by providing an alternative to flush toilets.
Additional Articles:
Composting Toilets – Decisions, Decisions published April 1, 2017
You’ve Build Your Composting Toilet – What Do You Do Now? published December 13, 2020
© 2017 Dreama Brower, updated December 13, 2020 (added links to additional articles)
submitted by Steven Herbert, Water Resource Consultant
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