Composting Toilet System

Composting toilets (also known as dry, biological, or waterless toilets) contains and processes excrement, toilet paper, carbon additive and sometimes food wastes. Unlike a septic system, a composting toilet relies on unsaturated conditions where aerobic bacteria break down wastes, just as they do in a yard waste composter. If sized and maintained properly, a composing toilet reduces its original volume by 70 to 90 percent. The end product is a soil-like material called humus. Although the humus is a very stable inert product it must be disposed of properly. Typically this is done by a licensed septage hauler in accordance with state and local regulations. Some areas allow for the disposal of the humus to be tilled into the ground in gardens growing non-edible plants.

Why Use a Composting Toilet

Design Criteria

Operation & Maintenance

Advantages & Disadvantages

        Why Use a Composting Toilet

The composting toilet is a non-water-carriage system that is well suited, but not limited to, remote areas where water is scarce, or areas with low percolation, high water tables, shallow soil, or rough terrain. The use of composting toilets in place of conventional flush toilets can reduce water use and allow a property owner to install an absorption field 35 to 40 percent smaller than a full-sized one.

Composting toilet systems are not flush-and-forget technology. They require a consciousness of what’s put into the toilet, some maintenance and well thought out siting and installation. They will also typically require electricity for operating fans and heaters.

Some people love composting toilets and others hate them. Usually peoples feelings towards them is based on prior experiences with a particular system. Some systems are just poorly installed. The most common installation mistakes are siting them in cold places, not draining away extra moisture (called leachate), and under-sizing the system. And most importantly, although composting toilet manufactures have come along way with the designs of the toilet itself, many people feel very uncomfortable on anything besides the standard porcelain stool.

Composting toilet systems range from little 7-gallon composters for boats to large aerated two-bin systems that serve more than 100 people daily. There are many different composting toilet designs, but they all have the same typical components:

        A waterless toilet stool or micro-flush toilet.

        A composter to which one or more dry or micro-flush toilets flow.

        A screened air inlet to provide air for the process.

        An exhaust system, often fan-forced, to remove odors, carbon dioxide, and water vapor.

        An access door to remove the end product.

Some systems may also have process controls, such as mixers and a means of draining and managing excess leachate.

        Design Criteria

A composting toilet is a well-ventilated container that provides the optimum environment for unsaturated, but moist, human excrement for biological and physical decomposition under sanitary, controlled aerobic conditions.

The main process variations are continuous or batch composting. Continuous composters are single chambers where excrement is added to the top, and the end product is removed from the bottom. Batch composters are actually two or more composters that are filled and then allowed to cure without the continuous addition of new potentially pathogen-contaminated excrement. Alternating concrete double-bins are the most common batch system.

The composting unit must be constructed to separate the solid fraction from the liquid fraction and produce a stable, humus material with less than 200 MPN per gram of fecal coliform. Once the leachate has been drained or evaporated out of the unit, the moist, unsaturated solids are decomposed by aerobic organisms using molecular oxygen.

Many compost systems have heated compost chambers to provide and maintain optimum temperature requirements for year-round usage.

        Operation & Maintenance

Operation and maintenance for composting toilet systems requires continuous attention. Composting is carried out by bacteria and fungi. These microorganisms thrive best in a warm, moist well-aerated environment. Composting is most efficient at temperatures of a least 65 F, the higher, the faster, up to 135 F. In cold climates, the temperature inside the composter must be monitored to assure it is operating at peak temperature. Microorganisms need 40 to 70 percent moisture content to optimally decompose organics. Composting microorganisms need air. Aeration can be improved by mixing the material, adding wood shavings or popcorn (to create air spaces) and by batching it.

For the composting microbes to fully transform the high nitrogen content of excrement to compost, they need an adequate amount of carbon, about 30 parts of carbon for each part of nitrogen. Good carbon sources are untreated bark mulch, wood shavings, rice hulls, oak leaves, etc. The carbon sources must be added based on the amount of system use.

As with all wastewater treatment systems, management is critical to the efficiency of the system. The level and consistency of the material in the pile must be monitored periodically.

The compost should be removed periodically, anywhere from every three months for a cottage system to every two years for a large central system. Finished humus has the consistency of composted leaves and should smell earthy but not offensive. The humus should be sent to a treatment facility or bury it under at least 12 inches of soil, preferably within the root zones of non-edible plants that can use the nutrients. Many users place the composted material outside to finish composting and add more mulch and yard waste.

If any leachate is drained from the composters it should either be disposed of in a septic tank, removed by a septage hauler, or taken to a treatment plant for further treatment.

        Advantages & Disadvantages

Some Advantages and Disadvantages are listed below.


        Composting toilet systems do not require water for flushing and thus reduce water consumption.

        These systems reduce the quantity and strength of the wastewater to be disposed of onsite.

        They are well suited for remote sites.

        Composting toilet systems have low power consumption.

        Composting human waste and burying it around tree roots and not edible plants keeps organic wastes productively cycling in the environment.

        Composting toilets systems can often accept kitchen wastes.

        Composting toilet systems divert nutrient and pathogen containing effluent from soil, surface and ground water.


        Maintenance of composting toilet systems requires more responsibility and commitment by users and owners than conventional wastewater systems.

        Removing the finished end product is an unpleasant job if the system is not properly installed or maintained.

        Composting toilet systems must be used in conjunction with a graywater system.

        Smaller units may have limited capacity for accepting peak loads.

        Improper maintenance makes cleaning difficult and may lead to health hazards and odor problems.

        Using an inadequately treated end product as a soil amendment may have possible health consequences.

        There may be aesthetic issues because excrement in some systems may be in sight.

        Too much liquid in the composter can disrupt the process if it is not drained and properly managed.

        Most composing systems require a power source.

        Improperly installed or maintained systems can produce odors and unprocessed material.