Green Building Library
Waste Reduction and Recycling

What is a Composting Toilet System and How Does it Compost?

Composting toilet systems (sometimes called biological toilets, dry toilets and waterless toilets) contain and control the composting of excrement, toilet paper, carbon additive, and, optionally, food wastes. Unlike a septic system a composting toilet system relies on unsaturated conditions (material cannot be fully immersed in water), where aerobic bacteria and fungi break down wastes, just as they do in a yard waste composter. Sized and operated properly, a composting toilet breaks down waste to 10 to 30 percent of its original volume. The resulting end-product is a stable soil-like material called "humus," which legally must be either buried or removed by a licensed seepage hauler in accordance with state and local regulations in the United States. In other countries, humus is used as a soil conditioner on edible crops.

The primary objective of the composting toilet system is to contain, immobilize or destroy organisms that cause human disease (pathogens), thereby reducing the risk of human infection to acceptable levels without contaminating the immediate or distant environment and harming its inhabitants.

This should be accomplished in a manner that

  • is consistent with good sanitation (minimizing both human contact with unprocessed excrement and exposure to disease vectors, such as flies).
  • produces an inoffensive and reasonably dry end-product that can be handled with minimum risk.
  • minimizes odor.

A secondary objective is to transform the nutrients in human excrement into fully oxidized, stable plant-available forms that can be used as a soil conditioner for plants and trees.

The main components of a composting toilet are:

  • a composting reactor connected to one or more dry or micro-flush toilets;
  • a screened exhaust system (often fan-forced) to remove odors, carbon dioxide, water vapor, and the by-products of aerobic decomposition;
  • a means of ventilation to provide oxygen (aeration) for the aerobic organisms in the composter;
  • a means of draining and managing excess liquid and leachate;
  • process controls, such as mixers, to optimize and manage the process; and
  • an access door for removal of the end-product.

The composting reactor should be constructed to separate the solids from the liquids and produce a stable, humus material with less than 200 most probable number (MPN) per gram of fecal coliform.

General Types of Composting Toilet Systems

Composting toilet systems can be classified in several ways:

Self-Contained versus Centralized

Composting toilet systems are either self-contained, whereby the toilet seat and a small composting reactor are one unit (typically small cottage models), or centralized or remote, where the toilet connects to a composting reactor that is somewhere else.

Manufactured versus Site-Built

One can either purchase a manufactured composting toilet system or have a site-built composting toilet system constructed (however, the latter can be difficult to get permitted by local health agents).

Batch (Multiple-Chamber) versus Continuous (Single-Chamber)

Most composting toilet systems use one of two approaches to manage the composting process: either single-chamber continuous composting or multi-chamber batch composting processes

A continuous composter (including Clivus Minimuses and such brands as CTS, Clivus Multrum, Phoenix, BioLet, Sun-Mar) features a single chamber into which excrement is added to the top, and the end-product is removed from the bottom.

A batch composter (such as the EcoTech Carousel, all Vera systems, BioLet NE, and many site-built composters) utilizes two or more interchangeable composting reactors. One is filled at a time, then allowed to cure while another reactor fills, just as with two- and three-bin yard composters.

Proponents of continuous composting maintain that it is simple (takes place in one fixed reactor), allows urine to constantly moisten the process, and allows the center of the mass to heat up through uninterrupted microbial activity.

Advocates of the batch-composting approach say that by not continually adding fresh excrement and urine to older, more advance material, the material composts more thoroughly, uninterrupted by the added nutrients, pathogens, salts and ammonia in fresh excrement. Also, by dividing the material, it can have more surface area, and thus better aeration. Batch composting also offers an opportunity for unlimited capacity, as one simply adds compost reactors to the process to add capacity.

Batch systems require monitoring the level of the composter to determine when a chamber has filled and a new one must be moved into place. However, because there is more surface area and the material is divided, there is often less or no mixing and raking of the material.

What is true is that complete composting needs time to ecologically cascade all the by-products of the myriad organisms in the composting food web until all of the organic matter is finally transformed into safe, stable humus.

In continuous composters, urine or flush water can leach fresh excrement into the finished compost removal area. Segregating the material into batches reduces the risk of having living disease organisms in the finished products.

(One of the authors [Del Porto] has designed, sold and serviced both types of composters. He prefers batch composting-based systems, because they are more forgiving and users seem to be happier with them. However, he would never say that single-chamber continuous systems do not work. They do!

It is difficult to generalize about which process affords the greatest opportunity for complete processing and minimizes the potential for pathogen survival. In a batch system, a finite supply of nutrients is cycled and recycled through microbe populations until the nutrients, both the free ones and those bound in microbial protoplasm and cell walls, are ultimately converted to stable, fully oxidized forms, and the fungi have performed their work on the remaining lignin and cellulose compounds, releasing antibiotics in the process.

Definitive research is needed in this area.

Active versus Passive

As with solar systems, composting systems are usually either passive or active. Passive systems are usually simple moldering reactors in which ETPA (excrement, toilet paper and additive) is collected and allowed to decompose in cool environments without active process control (heat, mixing, aeration).

Active systems may feature automatic mixers, pile-leveling devices, tumbling drums, thermostat-controlled heaters, fans, and so forth. The trend in the composting of municipal solid waste (garbage and trash), sewage sludge and yard and agricultural residues is toward active systems. By making the process active, the size of the composter can be reduced, because composting efficiency is speeded up (and the volume of the material reduced faster).

Passive systems are designed to optimize the process by design, not mechanical action, allowing only time, gravity, ambient temperature and the shape of the container to control the process. Passive composters are often referred to as moldering toilets, as the process at work is natural uncontrolled decay at cool in-ground temperatures at or below 68° F. In this cool environment, molds (fungi and actinomycetes) are the primary biological decomposers, because it is a bit too cool for the faster-acting mesophilic and thermophilic bacteria.

composting toilet organisms

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composting toilet system book

This excerpt is presented with permission from the publisher


What is a Composting Toilet System?

What is Composting?

Pathogens and Composting Toilet Systems

The Basics of Vectors and Insects



composting toilet diagram composting toilet drawing