How Does a Composting Toilet Work?

Composting toilets use aerobic decomposition to break down human waste instead of flushing it away in watery sewage. Most don't use water, so they don't need to be connected to a city wastewater system or septic tank.

These toilets are called "composting" because that is the process by which human waste is broken down. Like systems that compost kitchen scraps or animal manure, the result of the decomposition of organic matter turns human waste into a humus-like substance. It is dry and mostly odorless and can fortify soil when used as fertilizer (if local laws allow it).

Some composting toilets are set up to be portable, making them ideal for travelers and those who are in temporary living situations. Others are as permanent as a traditional system, meant to last for years. It's possible to DIY a compost toilet, also called the "humanure" system, but there are also a wide variety of composting toilets—both portable and permanent—available for sale.

What Is a Composting Toilet?

There are many systems used for composting toilets. Some have fans, some don't. Some need to be plugged in and others are for use off-grid and without power. Some have liners for easy cleaning and some separate urine.

There are two main types of compost toilets: slow and active. Slow compost toilets (sometimes called a moldering privy) are used infrequently or at remote locations. They are a box with a seat on the top. Underneath the box, a contained compost system holds the waste which slowly decomposes over time. This type of composting system can't be relied upon for pathogen elimination.

A slow compost toilet is separate from a pit toilet, found in similar places and locations where they are promoted out of necessity to reduce disease spread from people otherwise defecating in the open. A pit toilet isn't built to have a resulting compost product at the end of life. Instead, it has either partially or fully enclosed sides and when full, it is covered over and left behind and a new pit dug.

For remote or less-frequently used toilets, a composting toilet is preferable since over time it saves money and labor, and doesn't leave behind waste encased in a pit, which can contaminate groundwater.

Active composting toilets keep everything enclosed in a toilet unit typically bigger than a flush toilet. Like a slow compost toilet, you need to add absorbent material to help aerate waste and add carbon to the system, but that's where the similarities end.

Active toilets are so-named because they often have fans to keep oxygen flowing into the system, which speeds up composting. Some units have a heater which keeps the system at an optimal temperature for quick degradation of waste materials. A fan and heater require a connection to electrical power.

Some active composting toilet systems include a starter culture to ensure bacteria can make the compost work efficiently. Because these systems are more carefully controlled and calibrated, as long as you follow instructions to keep the elements balanced properly, there's a much higher likelihood you will get usable, pathogen-free compost from your toilet.

The frequency of use and its location will affect which kind of composting toilet you install. Frequency of use is important—is this a toilet at a remote campsite that will be used by a couple for two weeks per year? Or is this a system for six or more people year-round? Is the location inside or outside? Does it have access to electricity?

How Compost Toilet Systems Work

The use of a compost toilet system is mostly the same as a flush toilet. Most are designed to be sat on so urine is properly directed to the toilet.

Most composting toilets work similarly. You urinate or defecate into the toilet, then add a carbon-rich material (sawdust is common) to help add the proper mix of basic chemicals to break down the waste material. This broken-down inert material can be used as compost to fortify the soil.

From that basic function, composting toilets vary tremendously. The capacity for storage of the composting waste, the speed at which composting occurs, and the quantity of waste material all depend on the composting toilet design, ambient air temperature, number of users, and size of the unit.

A slow system might look and feel much like an old-fashioned outhouse, where you will sit on a toilet seat or bench above a hole or space below—but you will add sawdust, coconut coir, or another dry, carbon-rich material after use. There's no flush.

In an active system, you will be sitting on an enclosed toilet, and depending on what style you get it might be smaller or larger. You use it and then, like the slow system, add sawdust or another carbon-rich material. In both cases, this material both reduces odors immediately and also creates space for oxygen to get to the waste.

After you've finished evacuating your waste into the toilet and sprinkling in the sawdust or coir, the differences begin.

In most composting toilets (both active and slow), the actual composting process happens when microorganisms like bacteria and fungi consume and digest the materials. They do this both physically and biochemically.

In a slow system, these organisms may come and go to deal with the waste, which is why it takes time.

In an active system, organisms function best in a composting system with a controlled balance of heat, moisture, carbon, and nitrogen inputs. Composting depends on mesophilic organisms that function best at 68 F to 113 F (20 C to 45 C)—so the temperature is an important consideration. This is how some units have heating components and why composting toilets without heaters can be affected by ambient temperatures.

In non-tropical areas, the temperature will frequently drop below the ideal temperatures for mesophilic organisms and the composting process will slow considerably or even stop. When the system works effectively, it kills pathogens in human waste, and this temperature happens on its own.

Too much water in the system can unbalance it, so some composting toilets divert urine away from the compost.

Composting Toilet Maintenance

Composting toilets—unlike conventional water-based systems—are "not flush and forget," says Lloyd Alter, a composting toilet user and design editor at Treehugger. They "require work and maintenance and some discipline," he says.

There are several maintenance tasks associated with compost toilets. The first is that compost must be removed from the system regularly. Some people bag it and put it in the garbage, while others use it as fertilizer.

Urine removal might be required if it's a urine-separating active compost toilet. That unit will be separate from the solid waste and should be emptied and washed more often. How often depends on the size of the unit and frequency of use. Other maintenance includes exterior cleaning (like any toilet) and any service or replacement of fans or heaters.

Which Composting Toilet System Is Right for You?

When deciding on which composting toilet is best for you, research is your friend. Read descriptions and reviews of other users. Independent testing on compost toilets can be checked via the NSF International certification, which sets standards for composting toilets and verifies the products meet them.

The most important consideration for a composting toilet is unit size. Simply put, the bigger the system, the less work. "I would say the bigger the system the happier you are and the fewer times you have to empty it," says Alter.

If you are interested in a composting toilet for an RV—a popular choice as it allows for longer off-grid adventures—you will likely want to opt for something smaller.

In addition to not using valuable freshwater, composting toilets don't create tanks of waste that get full and need to be emptied. For example, in smaller RVs, waste tanks need to be changed every week or less with full-time use, according to RVing enthusiasts.

Other considerations for a composting toilet have to do with the design: Do you want a sleek, fully contained, comes-with-biodegradable-bags composting toilet with an automatic fan that feels like a regular flush toilet? Or do you want something super simple with no moving parts completely disconnected from the grid for seasonal use like the Loveable Loo?

Something in-between is the fan-less model like the Kildwick may be preferable if you are living off-grid or otherwise are watching your power consumption closely.

Environmental Benefits of Composting Toilets

A key advantage of composting toilets is that they don't use water (or little compared with flush toilets). In places where water is a concern and droughts are common, this is a huge advantage since a flush toilet is the primary user of water in a home, accounting for 30% of water consumption.

Flush toilets also require connection to a city sewer system or a septic tank, which have significant environmental impacts, and construction and maintenance costs.

A compost toilet generates a potentially usable material at the end of the compost cycle instead of using freshwater resources and potentially creating pollution.