A composting toilet is not a toilet that composts. It is a container that holds material in conditions where composting can begin, and whether it finishes is almost entirely down to how you run it. That distinction is the reason most of what you read about these units is either oversold or unfairly dismissed.
What Actually Happens Inside a Composting Toilet?
The process is aerobic decomposition, the same microbial breakdown that runs a garden compost pile. Bacteria and fungi consume the carbon and nitrogen in the chamber, respire oxygen, and give off carbon dioxide, water vapour and heat. A fan pulls air up a vent stack, which supplies oxygen and carries odour outside instead of into the room.
The complication is that human waste arrives wet and nitrogen-rich. Left alone it goes anaerobic within days, which is what people mean when they say a composting toilet smells. Everything you add and every setting you adjust is aimed at preventing that.
What this article covers:
- The four variables that decide whether the chamber composts or rots
- Why urine diversion changes the whole system
- What the units cost to buy, run and empty, with real numbers
- Where they are legal in the US and what the codes actually require
- Whether the end product can go anywhere near your vegetables
Key Takeaway
Most household units are mesophilic, not thermophilic. They run too cool to kill pathogens by heat, so they rely on time, drying and later curing instead. That single fact drives every safety rule further down this page.
The Four Variables You Are Actually Managing
Carbon is the one you add by hand. Sawdust, wood shavings, coconut coir or shredded straw raise the carbon-to-nitrogen ratio, absorb liquid and hold open air spaces. A cup or so after each solid use is the standard instruction, and it does four jobs at once: covers the surface, absorbs odour compounds, deters flies and holds structure.
| Variable | Target | What Goes Wrong Outside It |
| Carbon to nitrogen ratio | 20:1 to 35:1 | Too little carbon means ammonia and odour; too much stalls the pile |
| Moisture (for active composting) | 40-60% by weight | Above 60% goes anaerobic and smells |
| Moisture (for die-off by drying) | Below 25% | Ascaris eggs may need as low as 5% |
| Temperature for reliable die-off | Above 122°F (50°C) | Most self-contained units never get there |
Sources: Mehl (2009), Pathogen Destruction and Aerobic Decomposition in Composting Toilets, WHO (2006) Guidelines for the Safe Use of Wastewater, Excreta and Greywater
Those two moisture rows are the crux. A chamber can be safe by getting hot or by getting dry, and household units mostly take the second route. A study of dry composting toilets in Mexico found the main mechanism reducing fecal coliforms was desiccation rather than biodegradation, and that units below 40 percent moisture were largely odour-free while those above 60 percent were not.
Urine diversion is what makes drying achievable. Urine carries most of the nitrogen and phosphorus a person excretes plus a lot of liquid, so routing it to a separate jug or infiltration bed removes the moisture load and the ammonia at the same time. That is why almost every modern unit separates at the bowl.
Why This Works: Closing the Loop
Permaculture treats waste as a yield in the wrong place. A flush toilet takes drinking water, mixes it with concentrated nutrients, and pays to separate them again downstream. A urine-diverting toilet does the separation at the source, before dilution, which is why it takes so little energy to do.
The Types, and What They Cost
Self-contained units put the bowl and the composting chamber in one fixture, which suits cabins, tiny houses and boats. Central or remote systems drop waste to a larger chamber below the floor, which handles more people but needs the space underneath. Sawdust bucket systems are the cheapest: a bucket, a seat and a covering material, emptied into an outdoor compost bay.
| System | Capacity | Power Draw | Price |
| Conventional flush toilet | Unlimited, 1.6 gal per flush | None | $150-$400 |
| WaterSense flush toilet | 1.28 gal or less per flush | None | $200-$500 |
| Nature's Head (self-contained) | 2 adults, empty every 1-2 months | 0.84 W fan | $950-$1,050 |
| Sun-Mar Excel (self-contained) | 3 adults, empty every 3-4 months | 250 W heater plus 35 W fan | $2,200-$2,500 |
Sources: EPA WaterSense, Residential Toilets, University of Arizona EcoCasa composting toilet report (2010)
Look at the power column before the price column. A heated unit averaging 150 W with a 35 W fan can pull 3 to 4 kWh a day under heavy use, which is a serious load on an off-grid solar system. A fan-only unit drawing under a watt uses about 20 Wh a day, which is nothing. The heater buys you faster drying and a smaller chamber, and it costs you electricity every hour.
On water, EPA puts US toilet use at 18.5 gallons per person per day, roughly 27 percent of indoor consumption. Arizona's EcoCasa project measured about 2,688 gallons saved per household per year across 35 installed toilets. A composting toilet uses none, which is the clearest argument for one in a place where water is scarce or a septic system is failing.
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Send Me the ChartWhere They Are Legal, and What Goes Wrong
NSF/ANSI 41 is the standard most US codes point to. It covers the fixture: construction, capacity, ventilation and, importantly, the quality of the end product a unit is expected to produce. It does not certify your installation, your maintenance, or that you may spread the output on a garden.
State positions vary and you have to check yours. Massachusetts handles composting toilets through its Title 5 innovative and alternative technology approvals. Vermont's Department of Environmental Conservation publishes a dedicated guide and treats the end product as regulated material. Oregon has explicit statutory standards for compost toilets in ORS 447.118, and Washington's Department of Health covers them under water-conserving onsite wastewater guidance. Almost universally, you still need a permitted way to handle greywater from sinks and showers, because a composting toilet solves the blackwater problem only.
The Five Failures That Send Units to the Curb
Over-wetting is the most common, and it means odour within days. Not adding enough bulking material after each use runs a close second. Cold weather stalls decomposition entirely in unheated outbuildings. Fly and gnat problems follow exposed wet surfaces. And the emptying job is real physical work at a real interval, monthly for a two-person self-contained unit, and no marketing photograph shows it.
Can the End Product Go on Your Vegetables?
This is where the sources openly disagree, and you should know that before you decide.
The conservative US position is that composting toilet residuals are sewage sludge, governed by EPA 40 CFR Part 503. Washington DOH notes the material may be regulated as septage and that handling it as biosolids is impractical for a homeowner. Vermont allows direct land application only after solid waste certification with full testing for metals, pathogen reduction and vector attraction. Guidance summarising WA DOH practice restricts application to agricultural or forest land, not lawns or home vegetable beds.
Joseph Jenkins, whose Humanure Handbook shaped the practitioner side, argues that properly run thermophilic composting settles the question. He cites time and temperature pairs including 143°F (62°C) for one hour, 122°F (50°C) for a day, 115°F (46°C) for a week or 109°F (43°C) for a month. WHO's own guidelines are closer to Jenkins than to US state codes, endorsing reuse after 6 to 24 months of storage plus a withholding period of at least a month before harvest.
The honest reconciliation is that Jenkins is describing a well-managed outdoor hot compost heap, not a self-contained fixture. His system is closer to a small biosolids composting operation than to the chamber under a cabin seat. If you are running one, buy a long-stem thermometer and record temperatures, because the argument rests entirely on hitting those numbers.
What a Cautious US Gardener Should Do
Empty into a dedicated outdoor bay, add generous carbon cover, and leave it a further year minimum. Use the result on ornamentals, hedges and orchard trees rather than on salad or root crops. If you want it on food beds, run a monitored hot pile with logged temperatures, and check your state's rules first. Composting kitchen and garden waste separately, using standard hot composting, avoids all of this.
Why This Works: Obtain a Yield, Eventually
Permaculture's produce-no-waste principle is often read as permission to use everything immediately. The parallel with humanure composting is that the yield is real but arrives on a two-year timescale, through a second treatment stage you have to build. Systems that close loops honestly are slower than systems that pretend to.
Key Takeaway
A composting toilet is a first stage, not a finished process. Budget for the outdoor bay and the second year of curing before you buy the fixture, and the whole thing works. Skip it and you have an expensive bucket. For the wider design thinking behind this, see our guide to what permaculture actually is.
Frequently Asked Questions
How does a composting toilet work?
Solids drop into a chamber where you add a scoop of carbon material such as sawdust or coconut coir. A vent fan pulls air through the chamber, supplying oxygen to aerobic bacteria and carrying odour outside. Most units divert urine to a separate container, which removes the liquid and ammonia load. The material shrinks, dries and partly decomposes over weeks to months, then gets emptied into an outdoor compost bay to finish. The fixture starts the process; it rarely completes it.
Do composting toilets smell?
Run correctly, no. Odour is a symptom of two specific faults: moisture above roughly 60 percent, which turns the chamber anaerobic, and not adding enough carbon cover after each use. The Mexican field study found 64.5 percent of units had no detectable odour when contents stayed below 40 percent moisture. Check the fan is running whenever you smell something, since a stopped fan reverses the airflow that keeps odour out of the room.
How much does a composting toilet cost?
Self-contained fan-only units such as the Nature's Head retail around $950 to $1,050. Heated units such as the Sun-Mar Excel run $2,200 to $2,500. A sawdust bucket system costs under $100 in materials. Against that, a conventional toilet is $150 to $400 plus the septic or sewer connection behind it. Add the cost of an outdoor composting bay and bulking material, which is where the ongoing spending sits.
How often do you empty a composting toilet?
It depends on capacity and users. A Nature's Head serving two adults needs the solids bin emptied roughly every one to two months. A Sun-Mar Excel rated for three residential users goes about three to four months. The urine container is a different story: it fills in days, not months, and needs emptying far more often than the solids side. Plan the routine around the liquid, not the chamber.
Do composting toilets kill pathogens?
Not reliably on their own. Dependable die-off needs sustained temperatures above 122°F (50°C), and most self-contained household units run mesophilic, well below that. A study of dry composting toilets in Mexico found only 35.8 percent of units reached EPA Class A standard after six months, with drying rather than heat doing most of the work. Treat the fixture output as partly treated material needing further composting and long curing.
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Read the Free GuideResources
- WHO (2006): Guidelines for the Safe Use of Wastewater, Excreta and Greywater
- Mehl (2009): Pathogen Destruction and Aerobic Decomposition in Composting Toilets
- Systematic review and meta-analysis of pathogen reduction in onsite sanitation systems
- US EPA WaterSense: Residential Toilets
- EPA WaterSense: Single-Family New Home Specification, indoor water use figures
- University of Arizona EcoCasa: Composting Toilets and Water Harvesting final report
- Vermont DEC: Composting Toilet Guide
- Washington State DOH: Water Conserving On-Site Wastewater Treatment Systems
- Nebraska Extension: Water Wise, Water Conservation in the Home
