Science Fair Project Encyclopedia
There are several challenges facing anyone designing a composting toilet. The toilet must control odours. This is achieved by ensuring adequate ventilation (sometimes simply by leaving a small gap between the top of the wall and the roof, more sophisticated systems may incorporate some kind of low voltage extractor fan ), by as far as possible ensuring that urine and faeces are kept separate, and by adding high carbon content 'soak' material (see below) to absorb excess liquid. It must also either heat the faeces to the point that pathogens are destroyed (a thermophilic process), or else allow sufficient time (up to a year) for such pathogens to break down and disappear naturally (a mesophillic process). The upside however is that they do not use any significant amount of water and they may produce fertilizer safe for small scale agricultural use.
Sometimes it is the case that composting toilets are more expensive and require more attention than traditional water closet style toilets, however it is also possible to build 'DIY' systems that require very little cost or maintenance.
There are two basic types of compost toilet, those that complete the composting process 'in situ' such as an outhouse, and those that are emptied to a separate compost pile remote from the toilet itself. The latter arrangement is sometimes referred to colloquially as a ‘bucket and chuck it’ system. This means that faeces is deposited into a plastic container to which soak material such as straw, sawdust, dry grass, etc, is added in order to absorb excess liquid, cover sewage solids, exclude flies, reduce smells and balance Carbon:Nitrogen ratios. When full the bucket is removed and emptied onto a composting pile that is kept separate from other composting materials such as kitchen or garden waste.
Some composting toilets use electricity, while others do not. Some electrical systems use fans to exhaust air and increase microbial activity . Other systems require the user to rotate a composting drum from time to time.
Some composting toilets are large with a significant space requirement in the room below the toilet. Others are not significantly larger than a traditional toilet.
All composting toilets need to be emptied, although some manufacturers claim as few as two to three times a year even with commercial use.
A related device, the incinerating toilet , uses natural gas or propane to reduce the waste material to ash in a process similar to a self cleaning oven.
Possible health risks and aesthetic issues
Human faeces can be far more hazardous than that of animals because they contain bacteria associated with human disease. For this reason, human waste should not be used as fertilizer without ensuring that it is composted thoroughly. The Centre for Alternative Technology (CAT) in Wales recommend that humanure should be allowed to break down for at least one year in a cool temperate climate such as that experienced in the UK, where true thermophilic decomposition cannot be guaranteed. They also advise that humanure should not be used as a fertiliser on crops that are directly handled and eaten, such as vegetables or salad leaves, but instead applied as a mulch around bush or tree fruits.
Many in the 'developed world' find the idea of a composting toilet to be aesthetically challenging, perhaps largely due to the health and hygeine issues raised above. However, as long as basic safety rules and common sense are used, the real risks associated with a compost toilet system should be no more significant than any other situation where their may be some level of feacal contamination (eg, using a WC style toilet, changing babies nappies, taking a bath, etc).
However, at the present time commercially produced composting toilets remain a niche market primarily sold for use in remote cabins, other places that traditional sewage treatment or septic systems are not practical, or where an overt display of environmentalism serves some need. Of course, water based toilets were originally viewed with the same type of suspicions when they replaced the chamber pot.
Many health departments will not approve composting toilets as an alternative to septic fields. A septic field may still be required for treatment of grey water even if a composting toilet is approved. Before making a significant investment, check with your local health department.
Ecologically, in the case of more complex systems it may be that the use of electricity should be weighed against the use of water within the context of a situation. In arid areas, water is probably more valuable than electricity while in wet areas, the opposite may be the case.
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