How Does a Compost Toilet Work? PROS & CONS

Typically, we are hesitant to talk about subjects such as defecation and urination. This hesitation in discussing human waste has allowed governments to establish waste management systems without any objections.

The main tenet of composting toilets is ecological responsibility, a behavior option fast becoming non-negotiable. Ecological sanitation (eco-san) is a strategy that tries to conserve water, reduce pollution, and return consumed nutrients to the soil.

Our Current Sanitation Process is Unsustainable

The foundation of modern sanitation practices requires vast amounts of potable water, piping, centralized collection, and processing. This strategy wastes water resources and is costly, ineffective, and energy intensive.

A quick look at the USGS National Water Dashboard will give any person interested the insights into the increases in all-time low water levels. Fresh water reserves are fast diminishing, and water shortage is not some distant problem; it’s already critical.

Even aquifers are rapidly depleting, and while we imagine we can desalinate the sea’s waters, at what point will we stop plundering and change the unsustainable practices that brought us to where we are? Our sanitation system is a case in point.

Most Americans have access to flush toilets; however, those installed after 1994 may only use 6 liters of water per flush, compared to most toilets installed before 1982, which typically consumed 11.4 to 19 liters.

Approximately 24% of the water used by the average US household in 2016 was used for toilet flushing. That is, almost one in four gallons of all potable water is flushed down the toilet.

What is a composting toilet system?

The composting toilet is one potential technology that can safely and effectively manage human waste while saving water and energy. In contrast to flush toilets, very little water is utilized, and adjacent aerobic decomposition is used to treat the feces.

The wastes from a compost toilet are recycled rather than flushed away, and this is done by creating the necessary environmental conditions to break down the wastes and eradicate pathogens.

How Does a Compost Toilet Work?

A person’s defecation typically has a bad stench because of the methane and hydrogen sulfide produced by the anaerobic digestive tract. Our compost toilet is odorless since it composts using aerobic principles.

The ideal carbon-to-nitrogen ratio (C: N), moisture, aeration, pH, and temperature are designed to produce anaerobic composting. The feces are first coated and combined with a stable vegetable organic matter bulking agent.

This could include sawdust, shredded maize stalks, coconut husks, or other carbon-rich materials. The ingredient lessens the likelihood of flies or insects laying eggs in the mixture.

Additionally, it produces a more porous substance that promotes aeration and acts as a source of organic matter with low nitrogen content. This is crucial because the composting community’s organisms thrive best when the C: N ratio is around 30:1.

The mixture is kept moist (moisture content range between 40 and 60%) but not wet. Although microbial growth requires a moist mass, a wet mass would hinder oxygen uptake, resulting in an anaerobic environment.

Aeration provides oxygen to the microbial community and is essential to decomposing excrement and vegetable matter.

Some designs use small electric fans that might be driven by solar energy to help with aeration. Additionally, a mixing mechanism is typically included in the design to maintain the compost’s looseness, encourage air inclusion, and improve the absorption of the additives into the feces.


A common problem for most composting toilets is too much moisture from urination. Separating urine from feces is done by including urine diverters in the design that collect the liquid and channel it directly to an adjacent plot of land. Urine is high in nitrogen, and land application will fertilize plants.

The pH of the compost pile usually ranges from 5.5 to 8.5. A drop in pH indicates that the system has become anaerobic and needs aeration.

The organisms that do the decomposition thrive at warmer temperatures, which is crucial to this process because the heat helps kill pathogens and increase the decomposition rate.

The first phase of the decomposition process is mesophilic (50 to 104⁰F (10 to 40°C)) and lasts for a few days. During this time, the mesophilic bacteria and fungi populations grow rapidly. The heat created by their actions raises the temperature until it reaches the thermophilic phase (104 – 160⁰F (40 to 60°C), which can last several months under the right conditions.

During this phase, thermophilic bacteria, actinomycetes, and fungi are the dominant organisms that break down fats, proteins, and cellulose.

Once the temperature peaks at about temperatures high enough to kill all pathogens (160⁰F (40⁰C)), the temperatures drop back to the mesophilic phase, and decomposition continues on the more resistant material (e.g., lignin).

During the third phase, in an outdoor compost pile, insects and other invertebrates can enter the compost and help consume the remaining organic matter.

In cold climates, providing a heat source may be necessary to promote decomposition during the winter because the organisms doing the decomposition don’t function well in cold weather.

Heating systems and insulation can help maintain proper temperatures and turning and aeration increases microbial activity and causes temperatures to rise.

Phases of Aerobic Composting

The compost material must enter the thermophilic phase to function properly. The World Health Organization advises composting excreta at 50 °C for two weeks, followed by one month in aerated heaps at 55 °C or higher, followed by a further two to four months of curing.

The resulting mass of stable, black organic matter (humus), which resembles fertile organic soil, will be made from composted feces. Pathogens shouldn’t be present in the compost because they can’t endure the thermophilic composting conditions.

The product, which may be used to feed garden soil, is rich in vital plant nutrients like nitrogen, potassium, phosphorus, and many trace elements. The resulting compost might be sent to neighboring farms to fertilize crops if composting toilets replace conventional flush toilets.

This would stop the cycle of nutrients and spare farmers from purchasing synthetic fertilizers. However, the compost might contain pharmaceutical contaminants that need additional treatment because they travel through the body. Additionally, different state restrictions apply to the final disposal of toilet compost.

Composting toilets have great potential for safely handling human waste, but the process must be thermophilic.

Several commercially available composting toilets do not produce the thermophilic conditions necessary to produce hygienically safe compost.

Due to the necessity for a larger pile (a cubic yard) where the necessary conditions can be created and sustained, the material must go through a secondary composting process. Vermicomposting, another composting technique, uses worms to break down the material, and new research indicates that this method may be more effective at turning human waste into safe humus.

In Closing

As I have shown, there are several reasons why exploring alternative methods of managing human waste is needed. While we’re culturally averse to discussing topics on defecation and urination, the need to get over ourselves is fast becoming urgent.

Like any efforts, there have been some compost toilet initiatives that were a success and others that were a failure. These technologies do not allow for out-of-sight, out-of-mind handling of human waste like flush toilets do.

For our children’s quality of life, we must challenge our approach to human excrement management and influence others to do the same.

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