Comparison of Composting Toilets: Towards a Global Commode

SUMMARY

Three leading brands of self-contained composting toilets are Sun-Mar, Envirolet, and Biolet. These claim to convert human waste (with peat moss/wood shavings added) to a product resembling dirt, suitable for putting on a compost pile or even to be used directly on flower beds or trees. A comparison of the mechanics of these three toilets, along with on-line user feedback, indicates that Sun-Mar toilets are more likely to perform as expected. The Sun-Mar rotating drum seems to give better control of liquid contents than seen with Envirolet and Biolet devices. Non-electric versions of these toilets often cannot keep up with liquid evaporation, so a functioning liquid overflow tube is essential.

Highest user satisfaction seemed highest with “urine-diverting” toilets, such as Nature’s Head, Air Head, C-Head, and Separett. Urine is directed to a storage container or a grey-water drain. Solids drop into a bucket where they are typically covered with peat moss. Disposing of the relatively dry solids perhaps once a month is straightforward. This type of toilet may be useful in regions of the world that cannot afford plumbing and sewage systems: urine is rich in nitrogen and phosphorus and is nearly sterile, and thus could be used directly as fertilizer, while disposal of the solids alone is easier than dealing with combined liquid and solids.

CONTENTS

Fully-Composting Small Toilets

– Sun-Mar, Envirolet, and Biolet

Customer Experiences with Biolet, Envirolet, and Sun-Mar Self-Contained Composting Toilets

Urine-Diverting Toilets

– Nature’s Head, Air Head, C-Head, and Separett

User Experiences with Urine-Diverting Toilets

Bucket/Compost System: Loveable Loo

Some Global Takeaways

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *

A key discovery of nineteenth century science was that diseases can be transmitted via pathogens in human waste.  In regions of high population density, this can lead to epidemics if adequate sanitation facilities are not available. A milestone in epidemiology was the 1854 cholera outbreak in London. A physician named John Snow analyzed the incidence of the disease and concluded that the Broad Street public water pump was the source of infection. Even though he had no explanation in terms of germ theory at that time, he persuaded the authorities to remove the handle of that pump. This stopped the cholera epidemic. The well from which this pump drew had been dug a few feet away from an infected cesspool. A replica of this pump still stands in London:

The replica Broad Street Pump in Soho. http://toilet-guru.com/cholera-pump.php

The replica Broad Street Pump in Soho. http://toilet-guru.com/cholera-pump.php

Improved sanitation in the West and in prosperous areas of the rest of the world led to a dramatic decrease in deaths by disease, especially among children. Using water to sluice wastes to a septic tank or to a central treatment plant has proven an effective means to handle these wastes, for single homes and for vast urban population centers.   However, an estimated 2.5 billion people, about a third of the world’s population, still lack access to basic sanitation. Those living in rural settings may cope by relieving themselves in the woods or fields, but many live in crowded urban slums and are too poor to install flush toilets with their requisite water supply and sewage piping and treatment facilities. Microsoft founder Bill Gates has attempted to address this problem. The Bill and Melinda Gates Foundation has initiated the Reinvent the Toilet Challenge, to support the development of a “next-generation” toilet which:

  • Removes germs from human waste and recovers valuable resources such as energy, clean water, and nutrients.
  • Operates “off the grid” without connections to water, sewer, or electrical lines.
  • Costs less than US$.05 cents per user per day.
  • Promotes sustainable and financially profitable sanitation services and businesses that operate in poor, urban settings.
  • Is a truly aspirational next-generation product that everyone will want to use—in developed as well as developing nations.

These are lofty goals for the humble commode. Some of the technologies put forward to meet this challenge involve high cost or high tech components whose maintenance could be problematic. For instance, here is one of the prototype sanitation units developed in response to the Gates challenge:

While I salute the enterprise and ingenuity embodied in this device, my guess is that any viable sanitation solution for the masses will likely involve some much lower-tech approach. Here we examine a suite of small composting toilets whose descriptions are available on the internet. In North America these devices are sold primarily for use in vacation homes or on boats. We will evaluate them for these purposes, based mainly on online comments. From an engineering point of view, we will examine the key materials-handling aspects of these different models, and attempt to predict which features might be most useful in a “global toilet”.

Fully-Composting Small Toilets

There are a number of composting toilets sold where the compositing unit is located on a floor below the actual throne. These “remote” composting toilets include the venerable Clivus Multrum and the newer Phoenix. These are large and fairly expensive (typically over $5000) units, sometimes servicing commercial establishments. Sun-Mar, Envirolet and EcoTech offer remote composting toilets for around $2500-3000 for full-time family use. (All prices here are approximate).

Several manufacturers offer compact or “self-contained” composting toilets where the composting chamber is built into the toilet itself. These toilets are the focus of this review. These units typically include apparatus for stirring the wastes to promote aeration. The higher-capacity models use electric heating and air flow to evaporate urine. To handle excess liquid accumulation, an emergency liquid overflow tube leads to some outside drainage spot such as a gravel-filled hole. These liquids are fecal-contaminated “black-water”, so they require careful disposition.

Forced-air ventilation to the outside serves to promote urine evaporation and mitigate odors. Final composted product is removed from a tray or drawer situated below the main chamber. This may or may not require further composting.

Some models are aimed at sites with only solar electricity. These toilets would have 12-volt motors for the ventilation fan, and no electric heating of the main toilet. Non-electric models are also made. These rely on natural convection up a vent stack to move air through the toilet.

Three leading manufacturers of these devices are Sun-Mar, Envirolet, and Biolet (Mulltoa). We will describe the workings of these devices, and note the positive and negative comments and reviews from the internet.

Sun-Mar Self-Contained Toilets ($1850 for Excel model)

The signature hardware feature of Sun-Mar units is their drum-shaped composting chamber, as shown in the figures below. Shown on the left is the Excel, which is their most popular model. This is claimed to service up to four people full-time. Like the other manufacturers, Sun-Mar makes units of varied sizes and capacities.

Exterior and innards of Sun-Mar composting toilet. http://toilet-composting.com/what-are-composting-toilets/

Exterior and innards of Sun-Mar composting toilet. http://toilet-composting.com/what-are-composting-toilets/

Components of Sun-Mar composting toilet. Source: http://sun-mar.com/tech_our.html

Components of Sun-Mar composting toilet. Source: http://sun-mar.com/tech_our.html

Waste drops into the drum through an inlet port on the top side. A bulking peat moss/ wood shaving mixture is also added daily. Liquid drains from the drum through a screen down to the evaporation tray, which can be electrically heated. Every few days the user turns a crank to rotate the drum, to aerate and distribute the contents. The unit is designed to maintain the optimal moisture content (40-60%) for composting. The door over the inlet port is automatically closed during rotation. Every few months, depending on usage, it is time to drop some material from the drum to the finishing tray. This is done by releasing a locking switch and rotating the drum backwards. The material then sits on the finishing tray for at least a month to finish composting and to dry off. If all goes well, the final product is dry and ready to be strewn upon flower beds:

Envirolet Self-Contained Toilets (e.g. MS-10 $2400)

Cutaway view of Envirolet composting toilet. http://www.envirolet.com/scienbehen.html

Cutaway view of Envirolet composting toilet. http://www.envirolet.com/scienbehen.html

Envirolet claims their main AC-powered MS-10 model can handle six full-time users. Their 12-volt and non-electric models have lower capacity, since they cannot evaporate liquids as fast.

With the Envirolet self-contained toilet, the user opens a trap door, does his business, and closes the trap. The waste drops onto a perforated holding tray. Warm air is blown across this tray to evaporate liquids. Peat moss (daily) and Compost Accelerator (weekly) are added to aid composting. About once a week the upper handle on the front is pulled back and forth to move “mulcherator” metal blades through the contents in the holding tray, for distribution and aeration. The contents are emptied several times a year. From the operation manual, the procedure for emptying is as follows: (a) Don’t use the toilet for 2-3 days. If contents still look wet, add dry peat moss and stir and wait. (b) Remove the bottom panel in front. (c) Work the lower handle (Rake Bar) back and forth a few inches to get material to drop down from the bottom of the main chamber to the emptying tray. Keep checking that the tray is not getting overfilled. (d) You may need to remove more than one tray-full. Ideally the product is dry and finished, similar to the Sun-Mar product. The material can be left for several weeks on the emptying tray within the toilet to dry further, as long as the material in the main chamber is not so wet that liquids drip onto the emptying tray below.

Biolet Composting Toilets ( $1800 for Biolet 10 [3 users],   $2800 for Biolet 65 [4 users] )

Figure of Biolet 10 or 20 toilet from owner’s manual. http://www.biolet.com/support/articles/BioLet-10-Standard-20-Deluxe-Manual.pdf

Figure of Biolet 10 or 20 toilet from owner’s manual. http://www.biolet.com/support/articles/BioLet-10-Standard-20-Deluxe-Manual.pdf

The Swedish Mulltoa toilets are sold as Biolet in the U.S. and Ecoethic in Canada. Waste drops into a main compost chamber. Warmed air is circulated across the top of the chamber to evaporate liquids. Mixing arms attached to a central shaft distribute and aerate the contents. The trapdoor sealing the opening down into this chamber is normally closed, but opens mechanically when the user sits down on the seat. Thus, the user typically does not see the contents of the chamber. The toilet is initially loaded with several gallons of a special mulch mixture sold by Biolet, and another half-cup is added after every fecal use. The user manual gives a recipe if you want to make your own mulch. It is mainly fine (peat moss) and coarse (wood shaving) fibrous material, plus a little soil, molasses, perlite, and grain hulls.

After each use of the toilet, the mixing shaft is rotated, by hand or automatically by electric power. With each mixing event, some material filters down through holes in the bottom of the composting chamber and onto the humus tray below. Excess liquid will also accumulate with the solid material on that tray. The user needs to monitor the dampness of the material in the chamber, and adjust the heater thermostat accordingly. If the heat setting is too low, the material will be too wet to compost well and be hard to stir, and fecal-contaminated liquids will make a sloppy mess on the bottom humus tray and possibly overflow the tray into the base of the toilet. If the heat setting is too high, the material will dry out and become too hard to turn the mixer.

Every six months, or sooner if needed, the humus tray is emptied. It is recommended to first spread out a plastic sheet on the floor. If the liquid level inside is high, the humus on the tray is likely to be wet, and it may be necessary to carry the toilet outside to empty it. Wet or merely moist, this product should be placed in a compost pile for further stabilizing.

Biolet also offers the BTS 33 ($1200), which seems to be the essentially the same as the EZ-Loo Air and the Biolet 30 NE. This has few moving parts, and is aimed at sites with no electricity. The human waste is deposited in a bin in the toilet. Mulch is added regularly. The bin is changed out or emptied when it gets full. Air is drawn through the unit and up a vent pipe by natural convection. Liquids that overflow the bin are drained through a drain nipple.

Customer Experiences with Biolet, Envirolet, and Sun-Mar Self-Contained Composting Toilets

The vendor websites provide plenty of testimonies from satisfied customers, so these devices do work for some people. However, there are a number of complaints to be found on more neutral sites like Amazon. (I will assume these comments are genuine, although they could be the work of trolls or shills).

A serious problem is that the liquid overflow drains are prone to plugging with goo and fine mulch material, which can lead to fecal liquids dripping out onto the floor: “When the drain clogs you never know it till you walk in the bathroom and the floor is covered with brown stinking pee/crap soup. Its horrible”.  The user might need to regularly reach deep inside the unit to proactively keep the entrance to the drain cleared. Also, it is important to ensure the drain tube is sloped downhill over its entire length.

Biolet and Envirolet products seem to come in for the most criticism. There are many passionate and seemingly well-grounded complaints about these units, e.g. at Compare The Brands , at The Poop Report , and (regarding the Biolet 30 NE)  at Amazon.  For instance, a commenter at The Poop Report wrote:

I also have an envirolet self-contained electric model. Two of my neighbors do as well. We all have frustrations with these units. Just as poop’n steve mentioned, you need to use a stick to move the poop around. The mulcherator doesn’t work well. The unit needs to be emptied frequently, and because it’s a continuous composter, you always get fresh, uncomposted poop mixed in with the stuff you’re trying to get rid of. Flies get in, but if you screen the vent to keep flies out, the unit leaks water back inside and makes a mess. The rake bar usually gets stuck, so you can’t use the nifty emptying tray, and instead you need to empty the unit with a trowel, mixing fresh poop in with your composted poop. I have a PhD in ecology, and I’ve been composting out in the garden for 20 years. But I still find this toilet incredibly fussy. Either it gets too dry, and the poop turns into rocks and stops composting, or else it gets too wet, and turns anaerobic. Toilet paper doesn’t break down. On and on. Don’t get me wrong–I’m all for composting toilets. But get a remote model! And get a batch model, so that your poop can compost separately before you empty the unit. Continuous, compact models don’t work.

Liquids management seems to be a key challenge. It may be that the users are not following all the instructions carefully. Just pulling out a product tray without taking care and time (up to several days of avoiding toilet use) will likely result in wet, nasty product. However, the design of the Envirolet and Biolet toilets allows liquids to easily get out of balance. These systems rely on warm air circulation to evaporate moisture from the solids. Perhaps these vendors should devise an electronic sensor for the moisture level in the compost in the chamber, which would then automatically adjust the heater to maintain the desired moisture.  Another problem seems to be unrealistically high claims for the number of full-time users; probably these units cannot actually evaporate the liquid fast enough for four to six people, even with electric power. The capacity of the non-electric versions will be even lower, especially in humid regions where liquids will not naturally evaporate well. These versions will be especially dependent on functioning overflow drains.

User satisfaction seems higher for Sun-Mar toilets than for the other two vendors. The drum design gives good aeration without trying to force mixer blades though the solids, and allows excess liquids to immediately drain away from the solids. Having a tray dedicated to liquid evaporation allows its heater setting to be automatically controlled. The isolated product tray allows material to sit there for weeks while finishing and drying out. Also, Sun-Mar seems to rate the capacity of its units more realistically. Nevertheless, even Sun-Mar units require attention and adjustments, are subject to occasional mechanical failures and insect infestations, and have a few deeply-dissatisfied customers (especially for the 12V or non-electric models).

A remote composting toilet allows a much bigger waste-handling unit to be located on a floor below. Most of these remote units have similar moving parts as the compact versions, but with greater capacity to evaporate liquids. Ecotech offers a batch “Carousel” model, which has four separate compartments into which the waste drops. When a compartment is filled, it is rotated away to quietly compost for several months, and a new compartment is positioned beneath the throne.

Regarding the compact units in general, Toilet-Composting.com  opines:

Because of their small size, self-contained composting units only have a limited capacity. If there are more than two individuals using the toilet year round, a self-contained composting toilet system is not for you. In fact, even two individuals may overburden many models, and you need to choose your model carefully if for more than one individual. Self-contained composting toilets are probably most ideal for occasional usage in cottages or seasonal and vacation homes, or for year around usage by a single individual.

Urine-Diverting Toilets

Another class of small waterless toilets solves the problem of liquids by separating them at the source. Part of the waste opening is occupied by an inclined tray which directs the urine to a different destination than the solids. The solids typically drop into a mulch-filled container, which is stirred with a crank to mix the solids with the mulch. Four such toilets are discussed here. The first three units were initially developed for use on boats, so they are compact.

Nature’s Head ($950)

Two views of the Nature’s Head toilet are shown above. The lower chamber is initially filled with 2 gallons (1 U.S. gallon= 3.8 liters) of composting material, such as dampened peat moss or similar material. Urine runs to the front of the toilet, and drains into the liquid waste vessel. Reportedly, the urine-diverting feature is not hard for users (men or women) to get used to. Some mindfulness is required regarding one’s positioning.

The trap door is opened with a lever for solids deposition into the lower chamber. After use, the crank is turned several times to mix the solids into the peat. The unit comes with an electric exhaust fan installed. The user runs the vent line to the outside as part of the installation. Air flow through the unit flow prevents smells, and provides fresh air for the composting process. Below is a view down into the lower chamber, showing the agitator which is turned with the crank.

View down into lower half of Nature’s Head, showing agitator and the composting mixture. http://natureshead.net/user_guide.html

View down into lower half of Nature’s Head, showing agitator and the composting mixture. http://natureshead.net/user_guide.html

For two people with full-time use, the liquid waste vessel (2.2 gallon capacity) is emptied about every two days and the solid waste needs to be emptied every 1-2 months. As feces dry down and decompose, their volume decreases considerably. After waiting at least 6 hours from the latest use of the toilet for solids, the top half of the toilet is removed and set aside. The bottom half is carried outside and emptied by inverting with a plastic bag over it. In use, the agitator mashes the solid waste in with the peat moss, so it is aerated, partly dried, and gets a good start on the composting process. The material inside the lower half of the unit appears much like plain peat moss, with an “earthy” smell. It should be dumped into an outdoors composting bin to finish it. On a boat, the product can be stored until landfall, then thrown in a dumpster, as one might dispose of soiled diapers. After emptying, the toilet is put back in its place (no need to specially clean it) and refilled with peat moss.

Air Head Toilet ($1000)

Air Head Urine-Diverting Toilet http://airheadtoilet.com/

Air Head Urine-Diverting Toilet http://airheadtoilet.com/

The Air Head toilet is very similar to Nature’s Head. The urine container is easier to remove. To prevent soiling the toilet bowl, many users put down a disk of paper (shaped like a coffee filter but flimsier material), make their solids deposit on that, and then open the trap door to let it all drop into the lower compartment.   A comparison of Air Head and Nature’s Head features appears on the Wooden Boat Forum . There is an Air Head vs. Nature’s Head vs. C-Head discussion at Cruiser Forums.

C-Head Toilet (About $600)

The C-Head has operational similarities to Nature’s Head and Air Head, but is constructed differently. It looks more like a piece of furniture and less like a marine head, and comes in several versions. Pictures of the C-Head are shown below. The black crank handle shown lying on the floor is inserted through the hole at the back of the toilet seat to rotate the compost agitator.

C-Head Urine-Diverting Toilet. Photos by Sandy Graves.

C-Head Urine-Diverting Toilet. Photos by Sandy Graves.

Urine is collected in a standard one-gallon plastic jug. Solids go into a 5-gallon bucket, partly filled with peat moss or similar material. The C-head is stirred after use, but the agitator action is different from the other two toilets. The agitator is a small, single-bladed sloped paddle rotating on a vertically-oriented shaft. Rather than mashing the feces, this agitator mainly stirs the peat moss and turns it over, coating the feces and forming them into 2-3 inch spheres whose surface quickly dries out to minimize odors. This form-factor of the solid waste can be advantageous in disposal. Unlike any of the other composting toilets discussed here, the C-Head does not need a ventilation hose hook-up. Thus, it can be easily moved around.

The liquid capacity is about half that of Nature’s Head or Air Head toilets, so for two people it may need emptying every day. This involves raising the top of the unit and lifting out the one-gallon jug. Filled jugs can be stored or emptied as appropriate. An external urine diverter can be installed to drain the urine to an external tank or to a gravel-filled pit, obviating the need to swap out jugs. For emptying the solids, the internal 5-gallon bucket is removed and dumped into a composting area or into a longer-term storage bucket.

Separett ($1400)

The Swedish-made Separett sends the urine to a drain tube. Forced ventilation of the toilet is required to eliminate smells and to aid solids drying, using AC or 12-volt power.

Normally a trap door covers the opening to the solid waste container, so the user is spared the sight of its contents. When the user sits down on the seat, this flap is automatically opened and the solid waste container below is incrementally rotated so it fills evenly. The user experience is like using a conventional toilet. There is no regular mulch addition and no working of levers before, or stirring of contents afterward.

The solid waste container is a bucket, lined with a biodegradable plastic bag. After perhaps two months (for two full-time users), the toilet is opened and the waste container is lifted out. It is taken outdoors, some dirt is added on top of the waste, and it sits for six months with a loose lid on it to inactivate any human pathogens. Then it can be added to a mulch pile for final composting or discarded elsewhere. Thus, there will always be several containers aging in the back yard.

Left: Looking down at Separatt. Liquid drain hole is in front. Blue trap door covers solid waste opening. Right: Removing solid waste container, after installing lid on it. http://www.separett.eu/villa-9000-en

Left: Looking down at Separatt. Liquid drain hole is in front. Blue trap door covers solid waste opening. Right: Removing solid waste container, after installing lid on it. http://www.separett.eu/villa-9000-en

User Experiences with Urine-Diverting Toilets

In on-line forums and reviews, user reports on all four of these devices have been overwhelmingly positive. They perform exactly as they claim. The handling of a clear liquid stream and of a fairly dry mulch/solids accumulation is straightforward. These toilets can handle visits from a beer-swilling crowd, as long as the host keeps up with emptying the urine collection vessel. For a fixed application like a vacation home, the chore of emptying the urine can be eliminated by installing a drain tube for the liquids. Because this stream is clear liquid with few pathogens, it can be hooked up to a gray-water drain system, or drained to a small gravel-filled pit. A heater for the outside portion of the drain line might be needed to prevent freezing in winter.

A fold-up urine-diverting toilet is available for camping or emergencies. With this Rescue Kit, liquids go into a tube which can drain into a jug or a hole in the ground, while solids go into compostable bags. A do-it-yourself kit is also available:

Rescue Kit folding urine-diverting toilet ($129) and Separett Privy Kit ($129) from Ecovita. http://www.ecovita.net/products.html

Rescue Kit folding urine-diverting toilet ($129) and Separett Privy Kit ($129) from Ecovita. http://www.ecovita.net/products.html

Urine-diverting toilets are also made for use in a squatting position, which is common in southeast Asia.

2-hole (pink) and 3-hole (blue) urine diversion ceramic squatting pan from Indian NGO EEDS. http://en.wikipedia.org/wiki/Urine-diverting_dry_toilet

2-hole (pink) and 3-hole (blue) urine diversion ceramic squatting pan from Indian NGO EEDS. http://en.wikipedia.org/wiki/Urine-diverting_dry_toilet

Bucket/Compost System: Loveable Loo ($50 – $300)

The “Loveable Loo” has a following among the ecologically conscious. Popularized by “humanure” apostle Joseph Jenkins, it consists of a wooden box with a toilet seat on top, and a large (e.g. five-gallon) plastic bucket within. There is a bin of sawdust nearby, with a scoop in it. After you do your business, you throw a scoop of sawdust on top of it. There is reportedly almost no odor, if sufficient sawdust is used. The use of peat moss is frowned upon, since that is not a sustainable material (peat is cut from bogs far faster than it can regrow).

About once a week, a new bucket is swapped in. The full bucket is covered with a lid and carried to an on-site composting station. A typical composting station might occupy about 4 feet by 13 feet, consisting of two composting bins (one for this year’s waste, the other digesting last year’s waste) with a roofed sawdust pile in the middle. The bucket’s contents are dumped on the active pile, and more sawdust is added on top. The bucket is then rinsed out. After sitting for a year, the humanure is transformed to clean, fertile compost.

For $299, including shipping, all this can be yours:

Many users make their own toilet box, and just purchase a toilet seat and some buckets. Most user comments for this toilet are enthusiastic. The folks who install this system are typically self-reliant and are proud of living out a commitment to sustainability. The danger of spreading infections is low if humanure compost is used only on one’s own family garden.

However, the carrying and emptying of the filled buckets is not a task that a small or frail person can do comfortably and safely. Many people are averse to handling the waste of non-family members. A writer at Toilet-Composting.com recounts his experience with this system in an eco-village in Missouri:

A rotational system was put in place whereby each member had a shift for emptying and cleaning the five gallon buckets into the humanure compost bins. It is probably no surprise that this rotation was not very popular in the village. Many members came up with often quite elaborate excuses to get out of the humanure rotation when their turn came up, and it was a source of considerable tension in the village…During the hot summer months, the compost buckets became quite foul smelling and also very liquid in nature, so it was difficult to empty them into the bins without having quite a bit of fecal matter splashing onto your clothes and body.

The basic bucket can be used with an even smaller footprint, by placing a toilet seat directly atop it. For short-term use, the Luggable Loo  (about $30) gets favorable reviews. Users typically line it with a plastic bag and add material which absorbs essentially all the liquids. The used bags are often thrown in the trash, although they could be emptied onto a compost heap. This is not meant for extended use by large families.

Luggable Loo

Luggable Loo

Some Global Takeaways

Even gadget-savvy North Americans can be challenged to make in-situ composting toilets work for them. This suggests that these devices would not translate well to more traditional cultures. It remains to be seen whether a novel, more fool-proof small device can be developed which economically performs desired transformations on human waste within a user’s home. Positive results for Latin America are claimed for larger, remote-type composting toilets built to serve large families or small neighborhoods, and serviced by professionals.

Collection of combined (liquid and solid) waste by low-wage workers, and careful composting at central locations by highly-trained workers, seems like a viable model for sanitation in very low-income areas. To kill off all pathogens in human waste requires certain combinations of temperature and time for the compost, and care to not mix fresh waste in with aged material. Because of the high nitrogen content of urine, additional cellulosic mulch must be added to achieve the optimal carbon:nitrogen ratio for composting.

If the users can adapt to them, urine-diverting toilets offer some advantages. This Wikipedia article describes projects using urine-diverting toilets in Haiti, South Africa, and elsewhere. The urine is typically directed to a pit, leaving a greatly reduced volume of the solid waste to deal with. The solids are collected to be composted, or to be dried down to kill pathogens.

The major nutrients in plant fertilizer components are nitrogen, phosphorus, and potassium. Urine contains most of the nitrogen and potassium, and about half of the phosphorus, produced in human waste. Urine is relatively sterile, although this is not guaranteed. With proper precautions, the separated urine can be directly applied as fertilizer, as described in Scientific American (“Gee Whiz: Human Urine Is Shown to Be an Effective Agricultural Fertilizer”). This usage would accomplish at least some of the goals set out in the Gates Toilet Challenge.

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About ScottBuchanan

Ph D chemical engineer, interested in intersection of science with my evangelical Christian faith. This intersection includes creation(ism) and miracles. I also write on random topics of interest, such as economics, folding scooters, and composting toilets. Background: B.A. in Near Eastern Studies, a year at Gordon-Conwell Theological Seminary and a year working as a plumber and a lab technician. Then a B.S.E. and a Ph.D. in chemical engineering. Since then, conducted research in an industrial laboratory. Published a number of papers on heterogeneous catalysis, and an inventor on over 80 U.S. patents in diverse technical areas.
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6 Responses to Comparison of Composting Toilets: Towards a Global Commode

  1. Syed Azvi says:

    Very good.

  2. Keith Sager says:

    Thanks so much for such a thorough article on a rather crappy topic (sorry… couldn’t resist ;>) Seriously though, while researching alternatives to septic tank systems, I found your article, and it appears that, at least for the situation(s) I am envisioning, the C-Head unit from your article will be ideal for me.
    Thanks so much for saving me, what would have been, another several hours of research. 🙂

    • Keith, glad this was useful to you. It was kind of rewarding (for an engineer who appreciates the solving of technical problemes) to come across the urine-diverting models after reading all the discouraging reports on standard small composting toilets. Good luck in your installation….

  3. James Richy says:

    You say the C Head “does not need” ventilation. It would be more correct to say “does not have ventilation”. There is stinky, moist air in a toilet. Without a fan, that air is entering your bathroom area – no way around it. You might get away with burying it, but it’s an inferior solution to venting the moist, stinky air outside.

    • James, the claim for C Head is that the vertical agitator, rather than mashing the feces, mainly stirs the peat moss and turns it over, coating the feces and forming them into 2-3 inch spheres whose surface quickly dries out to minimize odors. But I have not used it. You are probably right, that there can’t help being some odor. And I don’t know how much smell wafts back up from the urine bottle. Maybe this would be less objectionable in a drafty cabin than in a well-sealed home. Thanks for the insight.

  4. josdesouzaJ says:

    These model/designs from the Norwegian maker of the “Carousel” are also worth considering: https://www.hyttetorget.no/content/mma/publish/02/233/23384/Monteringsanvisninger_biologiske_toaletter.pdf

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