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Composting Toilets - Composting process and products
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Composting process and products

The process of converting human excrement is done through adding "cover materials" (see below) to the feces and letting it age. Additionally, kitchen (food) scraps may be added to the pile (eg meats, bones, fats, vegetable peels, liquids such as spoiled milk or stale beer, tea bags, egg shells, hair, ...), as well as animal manure and garden and yard materials. Compostation into safe and usable compost material can take 3 months to a few years depending on climate, temperature, and the composting system. The composting of the humanan feces is usually done by letting it age in containers or open boxes. Several separate spaces are required to accommodate feces from different age as it composts. Generally, 3 spaces are used to accommodate a pile of feces of 1 year and a pile of feces from 2 years age. The third space is used to place feces which is soon to be dispersed on the land. In 4 to 6 years there will be highly mineralized soil.

Some composting-toilet models concomitantly turn urine into an odor-free, pathogen-free organic liquid fertilizer. Some countries, for example Sweden, allow this liquid to be used in agriculture after it is stored for 6 months. In full-size composting toilets, urine goes through a process called nitrification, resulting in an odor-free and practically bacteria-free liquid fertilizer.

Typically, the waste breaks down to 10% of its original volume. Most toilet-composting systems are mouldering, or low-temperature, toilets, where the waste is left for long enough that pathogens break down naturally, but there are also hot, or thermophilic, toilets, which heat the waste material high enough that pathogens are destroyed.

Some composting toilets separate the urine and the feces. Others mix the two, with the process requiring either evaporation of the liquid or the addition of "cover materials" such as sawdust, or alternatively, another organic material such as peat moss, coco coir, rotted leaves, leaf mould, straw, or grass clippings to soak up the liquid. The organic material also eliminates (a great deal of) the odour and prepares the feces for compostation. The type of material best used depends on its local availability, the make-up of the composting loo itself, personal preference, ...

Urine, rather than feces, contains the major bulk of plant nutrients worth recovering for reuse, including 90% of the nitrogen and 70% of the phosphorus. One advantage of modern composting toilets over conventional outhouses is that the latter leaches most nutrients into the groundwater, instead of saving them to be reused in agriculture or spread on the land.