Composting Toilets
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A composting toilet is any system that converts human waste into an organic compost and usable soil, through the natural breakdown of organic matter into its essential minerals. Micro and macro organisms do this over time, working through various stages of oxidation and sometimes localized pockets of anaerobic breakdown.

Self-contained composting toilets complete the composting "in-situ,", while "central unit" ones flush waste to a remote composting unit below the toilet. Vacuum-flush systems can flush horizontally or upward.

Some composting toilets have a large compartment below the toilet (in another living space). A composting toilet with this feature and sloped composting room for the compartment is called a "Clivus Multrum" composting toilet. Most of these separate sloped compartments require a great deal of space (more than a traditional toilet). "In situ" and "central unit toilets" however can be little larger than a traditional toilet.

Finally, a special type of DIY-composting toilet is the double alternating pit compost toilet. Double alternating pit composting toilets or "fossa alterna" as they are sometimes called work similarly to regular composting toilets with separated compartment. However, the faeces compartment is just a pit (thus not contained in a bucket, ...) and after when the pit is full, another one is used and the contents of the first one is used for fertilisation purposes.

The arborloo does not make use of containers nor effective/optimal nutrient recycling. Instead, faeces falls into a pit untill it becomes full. At that moment, a new pit is dug and the old pit with faeces (nutrients) is foreseen with a tree. The tree gathers the nutrients and converts it to wood mass (and/or fruit if it''s a fruit tree). This wood mass and/or fruit can then be again used to supplement in the people''s food requirements and/or energy requirements.

Some composting toilets use electricity, and some electrical systems use fans to exhaust air and increase microbial activity. Others require the user to rotate a drum within the composting toilet to allow for a predominantly aerobic breakdown of waste.

In order to decrease smell, compost toilets need quick drying of its contents. In order to speed up this process, several special composting toilets have been made. UDD toilets (urine-diversion dehydrating) is one such special type. This type of toilet has 2 containers; 1 waterproof container for urine and another for faeces. UDD toilets can be bought or made DIY (see below for plans). UDD toilets have hardly any odour, because faeces can dry out better if not mixed with urine and water. Additional advantages are that urine collected pure can easily be reused as a fertiliser and faeces collected pure can (after drying) also be reused without much further processing in agriculture (see WHO guidelines from 2006 on reuse of excreta). They are reportedly also easier to operate and easier to build DIY than composting toilets, especially as much less knowledge is required.

Solar toilets are another special type of composting toilets. They are made as regular composting toilets where the faeces chamber is painted black so that one gets faster drying. Usually though, painting the faeces chamber black is normally not necessary because with a good vent pipe it dries fast enough anyway.

All composting toilets eventually need some end-product removal. A full-size composting toilet does not need to have solids removed for several decades if the active tank volume is at least three times the yearly addition. This is because the waste dramatically decreases in volume: after around 5 years only 1-2% of the original volume remains. It is then a mineralized soil, which will not decompose any further. Other smaller systems may need to remove solids a few times a year.