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Initial Publication Date: November 1, 2016 | Fix Links: July 30, 2024 (see revision history: 2 events)

Student Reading: Composting Toilets

Composting Toilets: Alternative to flush toilets

Learning goals: Students will be able to:

Describe how a composting toilet functions in comparison to a flush toilet.
Describe how a septic system functions.
Evaluate water use differences between composting and flush toilets.

Introduction

Like all animals, humans eat food for energy to grow and maintain our bodies. We eliminate semisolid wastes in feces and liquid wastes in urine. The feces are rich in organic matter (which contains energy and matter used by microbial decomposers) and urine contains much nitrogen. When we are ill, the feces may contain pathogens (e.g., bacteria that cause dysentery). Throughout human evolution we have returned feces and urine to the earth, where decomposition releases the nutrients and recycles them back to the environment for use by other organisms. As human populations began to concentrate in towns and cities, there was simply not enough land available for people to just go outside and defecate or urinate into a hole in the ground. The lack of sanitary treatment of the feces also caused the spread of disease.

The Romans "solved" this problem by building "drop-in" toilets over streams that ran through the city. This approach was "industrialized" with the introduction of flush toilets in the 19^th century; however, urine and feces contaminated many of the rivers draining cities and towns. This approach was premised on the concept that the "solution to pollution was dilution."

By the beginning of the 20^th century the consequences of this approach let to epidemics of dysentery, the degradation of aquatic ecosystems due to "eutrophication," the closing of shellfish beds, and a reduction in other aquatic-based foods. This prompted the construction of wastewater treatment plants beginning in the early 20^th century. But it was not until passage of the Clean Water Act in the 1970s that most US cities moved to effective wastewater treatment.

Composting refers to the practice of allowing organic matter to decompose in the presence of oxygen. Composting toilets use this same approach to breakdown human fecal matter and recycle it. One of the first patents for a compost toilet was submitted by Henry Moule for his Earth Closet in the mid-1800s. The toilet used a lever to release dry earth to cover the feces which would then have to be taken outside for disposal. Today's composting toilets are more sophisticated and easy to use but they are not common in the US. They use little or no water and keep nutrients in a tight cycle by not allowing them to flow into adjacent waters. They also offer advantages in cases where it is more cost-effective to treat waste on-site. In the US, compost toilets are usually found in remote areas (e.g., at a trail head or in a cabin), aboard ships, recreational vehicles, tiny homes or in areas lacking access to water. However overseas there are projects that have attempted to use compost toilets on a large scale. For example, in Haiti project SOIL has successfully recycled material from composting toilets used by nearly 6,000 people.

Complete composting of the feces is necessary to kill any associated pathogens but it can be accomplished under the right conditions. Managing the wastes and creating the right conditions for decomposition require a watchful eye by the owner. This is one hurdle for their widespread use because in general we tend to avoid the subjects of defecation and urination. This aversion to human wastes and how it is managed has been used as a tool to disenfranchise people. There was a period in US history when access to public toilets played a central role (see Jim Crow (Acrobat (PDF) 497kB Aug15 17)) in denying the civil rights of African Americans.

The flush toilet

Most of the people living in the United States have access to a flush toilet but prior to 1982 most of these toilets used 11.4–19 liters of water per flush, now newer toilets (after 1994) may use as little as 6 liters of water. In 2016, a typical US household spent about 24% of its water budget on flushing toilets. That is a lot of perfectly good water flushed down the drain. Nonetheless, flush toilets are becoming more common in many parts of the world, but depending where you are, they may not be linked to an improved sanitation facility (i.e., septic systems, wastewater treatment plants, compost toilets, or pit latrines). in fact, there are still large segments of the world's population that lack access to improved sanitation facilities. In 2008, just over a billion (17%) of the world's people practiced open defecation where the raw sewage is thrown outside and where it can drain into nearby waters. Fortunately, in the United States most toilets are connected to a septic system or a wastewater treatment plant.

Septic Systems

Septic tank systems were first used by the French in the 1870s, and by the 1880s, septic tank systems similar to those used today were being installed in the United States. The term "septic" refers to the anaerobic bacterial environment that develops in the tank and breaks down the waste discharged into the tank. Septic tanks generally consist of a tank between the size of 1,000 and 2,000 gallons (4000–7500 liters) which is connected to an inlet wastewater pipe at one end and a septic drain field at the other. In the United States about 20% of all housing units use septic systems.

A typical septic system consists of:

A pipe through which the household wastewater exits the home to the septic tank.
The septic tank: A buried tank made of waterproof material (e.g., fiberglass). It holds the wastewater and allows the more dense material to sink (forming sludge) while the less dense material (e.g. grease) floats to the surface (as scum). It also allows partial decomposition of the solid materials.
The drain field: When the wastewater moves out of the septic tank, it goes into the drain field for further treatment by the soil. The drain field consists of pipes that are buried in trenches with gravel placed at the bottom of the trench.
The soil: The wastewater flows to the drain field, moves down through the soil (i.e., percolates) which removes bacteria and nutrients. Having the right kind of soil is necessary for the system to function properly, and soil suitability is determined using a percolation test.

How it functions: Wastewater flows into the tank at one end and leaves the tank at the other. Material that floats rises to the top and forms a layer known as the scum layer. More dense material will sink to the bottom and form the sludge layer. In-between the scum and sludge layer is a third layer of clarified effluent that contains bacteria and chemicals like nitrogen and phosphorus that act as fertilizers; this layer is largely free of solids. As new water enters the tank, it displaces the water that is already there, which forces a layer of clarified effluent out of the tank and into the drain field.

In the drain field, water is slowly absorbed and filtered by the soil. The size of the drain field is determined by how well the ground absorbs water. In places where the soil absorbs water slowly (e.g., clay) the drain field has to be larger.

Septic tanks are an efficient way of managing wastewater and can be long-lasting as long they are maintained properly. This includes hiring a licensed installer; pumping out the sludge every few years, and getting a septic tank that fits your water use. Be sure not to put grease or fats down the drain because they can clog the septic tank, and do not add chemicals into the system that can kill the bacteria in the tank. View the video How a septic system works and fails.

Wastewater Treatment Plants

These plants are used to treat the wastes of 80% of the housing units in the United States. Wastewater treatment plants are designed to remove organic wastes that would cause high biological oxygen demand in receiving waters, and to reduce the levels of inorganic nutrients (N and P) that stimulate the growth of algae. Most plants in the United States conduct primary and secondary treatment of the wastewater.

The first step in a wastewater treatment plant is the screening of the incoming wastewater (or influent) to remove large objects and grit (e.g., coffee grains or sand), which are taken to a landfill. Water then flows into a holding pond for primary treatment. In these holding ponds (i.e., primary clarifiers) the water slows down which allows heavier material (sludge) to settle to the bottom. The less dense material remains afloat (e.g., fats or oils) and is skimmed off the surface of the water. These floating materials are removed, and what is left flows into the aeration basins for the secondary treatment process.

The secondary treatment process involves managing bacteria that will break down feces, food particles, and other organic matter. An important aspect of this process is aeration because it promotes the growth of bacteria that consume the matter in the wastewater. After aeration, the water flows into the secondary clarifiers where the denser material settles to the bottom (i.e., secondary sludge). The water moves from the settling tanks for disinfection (tertiary treatment) by exposure to chlorine or ultraviolet lighting, which will kill any pathogens before the water (or effluent) is released into a local body of water.

The sludge material from the primary and secondary treatment processes is mostly water so the sludge is processed to remove the water in thickening tanks. From there the sludge moves to the digesters which heat it up to stimulate the growth of bacteria. They eat the sludge and in the process release methane which can be captured and used to generate electricity. The remaining material (called biosolids) is processed further to remove water. Final disposal of the biosolids is regulated by the Environmental Protection Agency, but depending on its quality, it may be used as fertilizer or taken to a landfill.

The Composting Toilet

A possible technology that can safely and efficiently manage human generated feces plus conserve water and energy is the composting toilet. Unlike flush toilets, little to no water is used and the fecal matter is treated nearby through the process of aerobic decomposition. With a compost toilet, the wastes are not flushed away but are recycled by generating the environmental conditions needed to breakdown the wastes and kill any pathogens.

Compost toilet structure

The basic components of a composting toilet are:

The compost chamber, where the fecal and composting additive mix together for decomposition
An exhaust system to remove odors, heat and gases from the decomposition process
Ventilation to provide aeration that will support the growth of aerobic organisms
A way of managing excess liquid and leachate (optional)
A way to withdraw the compost

Generally, compost toilets can be divided into two different types (although some may use electricity others may not, some flush with a small amount of water and others use no water (see Compost toilet designs (Acrobat (PDF) 324kB Aug15 17))). The first is the self-contained toilet and it has a composting reactor directly underneath the seat where the feces and composting material are mixed. It is one single unit and the initial composting occurs within it but is recommended that it be taken to a larger compost pile for further treatment. In the central or remote chamber design the toilet rests above a large composting reactor which could be located in a basement. The chamber serves as the central collection point and is able to decompose large amounts of feces from one or several toilets.

How it works

When one defecates, there is usually an unpleasant odor; as discussed in the biofuels module, a product of anaerobic decomposition is hydrogen sulfide. Since our intestines are anaerobic, we produce reduced sulfur substances that emit a smell as soon as our feces are exposed to air. Sewage in pipes or highly polluted rivers retain this odor, as they are also low-oxygen environments interacting with the oxygen in the atmosphere, but a functioning compost toilet shouldn't smell.

The optimal conditions for composting are developed with the right Carbon to Nitrogen ratio (C:N), moisture, aeration, pH and temperature. As a first step, the feces are covered and mixed with an additive or bulking agent composed of stable vegetable organic matter. This might be peat moss, coconut husks, shredded corn stalks, etc. The additive reduces the possibilities of flies or other insects laying their eggs in the mixture. It also creates a more porous material that encourages aeration and serves as a source of low-nitrogen organic matter. This is important because the organisms of the composting community do best with a C:N ratio of about 30:1.

The mixture is kept moist (40% - 65% moisture content) but not wet. A moist mass is necessary for microbial growth. But a wet mass would slow the infiltration of oxygen, creating an anaerobic environment. It is essential to allow for the circulation of air to supply oxygen to the microbial community that is breaking down the feces and vegetable mater. Some designs use small electric fans (which may be solar powered) to assist aeration. Also, a mixing device is generally incorporated into the design. This keeps the compost loose to promote airflow, and is a way to stir the additive and the feces together.

For most composting toilets one problem is too much moisture that comes from urination. To solve this problem some designs include urine diverters to collect the liquid in bottles or send it directly to an adjacent plot of land. Land application of urine will fertilize the plants.

The pH of the compost pile usually ranges from 5.5 to 8.5. If the pH falls too low it signals that the system has become anaerobic and needs aeration.

The organisms that do the decomposition thrive at warmer temperatures which are crucial to this process because the heat helps to kill pathogens and increase the rate of decomposition. The first phase of the decomposition process is mesophilic (10-40 °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 reaches the thermophilic phase (40-60 °C) which can last several months under the right conditions. During this phase, thermophilic bacteria, actinomycetes and fungi are the dominant organisms and they breakdown fats, proteins and cellulose. Then the temperatures decline 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, it may be necessary to provide a source of heat to promote decomposition during the winter because the organisms doing the decomposition don't function well in cold weather. To maintain proper temperatures a heater system may be used, insulation provided or turning of the compost.

It is critical that the compost material reaches the thermophilic phase. The World Health Organization recommends that excreta be composted at 50 °C for two weeks and then composted in aerated piles at 55-60 °C for one month plus another 2 to 4 months of curing. After this the composted feces will be reduced to a mass of stable black organic matter (humus) that resembles a rich organic soil. The compost should be free of pathogens because they can't survive the composting environment. The product is rich in important plant nutrients (Nitrogen, Potassium and Phosphorous and many trace elements) and can be used to fertilize garden soils. If composting toilets replace the standard flush versions, the resulting compost could be moved to nearby farms to fertilize crops. This would close the nutrient cycle and save farmers from having to buy artificial fertilizers. However, the compost may be contaminated with pharmaceuticals that pass through the body and require further treatment. In addition, final disposal of compost from toilets is subject to regulations that vary by state.

Compost toilets have great potential to handle human wastes safely but the key is that the material undergoes thermophilic composting. Many of the commercial composting toilets do not generate the thermophilic conditions needed to create hygienically safe compost. This is why the material needs to undergo a secondary composting in a larger (> 1 m³) pile where those conditions can be generated and maintained. Another composting method is using worms to decompose the material (i.e., vermicomposting) and recent studies suggest that this approach may be better at converting human waste into safe humus.

There are compost toilets projects that have failed and others that have succeeded. These systems are not like flush toilets that allow for an out-of-sight-out-of-mind approach to human wastes. For this technology to succeed it will require committed community members who are knowledgeable in how to use and maintain the toilet plus manage the compost.

Collecting your thoughts: Systems thinking and reflection

You have just learned about the different approaches used to process human-produced urine and feces. Take a few moments to consider how this fits together. Think of the relationship between humans, their food, their excrement (urine and feces), soil, water, and air as a system. Industrialized countries mostly rely upon wastewater systems that drain into rivers or coastal habitats. What is the effect of this on the fertility of the soil that produces the food? What is the effect on the receiving waters (rivers, lakes, coastal oceans)? How would the switch to composting toilets influence the functioning of the human-food-environment system? How would your answers differ if you considered how most non-industrialized countries process human-produced urine and feces?

What would be the pros and cons of using composting toilets on your college campus?

To do before class:

Read the article "With 7 Billion People the World has a Poop Problem' and these papers (see References and Resources) - DeOreo et al (2016) plus Anand and Apul (2014).
Go to Turning Feces to Fuel in Kenya

Questions:

If homes in your neighborhood were equipped with composting toilets, what would be the environmental benefits or disadvantages?
What prevents the widespread adaptation of composting toilets in houses and other buildings?
Suppose a city with a population of 100,000 decided it would be the first in the United States to convert to all composting toilets for houses, public buildings, businesses, etc. Detail the steps required to obtain public acceptance, install, and maintain the composting toilets. Think through the entire process. Be sure to think about what new jobs this would create.

References and Resources

General:

Data on water use in the USA from the US EPA

Data on global access to improved sanitation from the World Bank's Indicators

A summary of the world-wide problem of sanitation: "With 7 Billion People, World Has a Poop Problem" from Livescience.com

DeOreo, W., P. Mayer, B. Dziegielewski, and J. Kiefer (2016) Residential End Uses of Water, Version 2 Project #4309. Denver, Colo.: Water Research Foundation.

Septic tanks: