An Introduction To Composting Toilets: How And Why They Work With Example


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Categories: Building Methods

Composting toilets are simple, low-tech, waterless toilets. They are designed to provide favorable habitat for biological agents of decomposition such as bacteria, mold and fungi which break down feces and urine into compost. This miraculously transformative process encourages us to move beyond the concept of mere "waste disposal". It opens an opportunity to use our own compost to grow food for ourselves and thus to close the nutrient cycle. As such, using composting toilets can be a step toward relearning our place in the natural order.

We have been using composting toilets since 1990, using the resulting compost to grow a variety of edible plants.

Since most aspects of composting toilets including design, use, legalities, etc. are addressed in the several books listed at the end of this section, I will cover them only briefly here. During 16 years of designing and using composting toilets I've observed some details which have not been covered extensively elsewhere. These are the primary focus of this section. Since composting toilets require little if any water, they are especially appropriate for use in aridlands and particular attention is given to that aspect here.


First, though, I'll cover the five primary factors necessary to create habitat favorable for the biological agents that destroy pathogens and convert feces and urine into compost. They are nitrogen, carbon, oxygen, temperature and moisture. Problems with composting toilets can often be traced to significant imbalances in one or more of these factors.

Nitrogen is required for microbial decomposition and is present in human feces and especially in urine. No additional nitrogen needs to be added to a composting toilet.

Carbon is also required for microbial decomposition and ideally should be in a proportion of about 30 parts carbon to 1 part nitrogen. This ratio can vary considerably without seriously affecting decomposition. Carbon is supplied by adding a carbon-rich material to the compost. Suitable carbonaceous material includes sawdust, straw, leaves, shredded paper, etc. Shredding or chopping these materials provides greater surface area for the decomposition organisms and also facilitates the absorption of moisture. Whatever the type, this carbonaceous material is often referred to as "bulking agent" although I prefer "cover material" as it emphasizes the aesthetic function described below.  For more information on cover materials, see our Cover Materials webpage.

Oxygen, also necessary for decomposition, is provided either through mechanical means (stirring or tumbling) or by adding coarse carbonaceous cover material which provides air spaces throughout the compost.

The temperature of the composting chamber should ideally be in the range of 80F to 110F, although the temperature of the compost itself may be higher, depending on the type of microorganisms that predominate. Although low ambient temperatures will slow down or even stop the process of decomposition, it will resume when the temperature rises.

Moisture is also necessary for decomposition and should ideally be about the same as that in a wrung-out sponge. One of the most common causes for poor decomposition in composting toilets used in aridlands is lack of sufficient moisture distributed uniformly throughout the compost.

Aesthetics


Attention to aesthetics makes using the toilet a pleasant experience for everyday users. It also challenges the preconceptions of guests, many of whom approach composting toilets with trepidation, expecting the unpleasantness often associated with outhouses such as seeing exposed excrement and toilet paper when the lid is lifted. In fact, it should be no more necessary to have to look at someone else's excrement or used toilet paper when using a composting toilet than is the case when using a flush toilet.

Many commercial designs attempt to resolve this aesthetic issue by segregating the user from the composting chamber, either by some type of moveable baffle or by a long chute between the toilet seat and the composting chamber. Baffles are prone to jamming and chutes to unsightly "skid marks" (black chutes help but don't entirely solve the problem).

Our preference is for simple toilet designs that rely on a modicum of skill and care in use rather than on technological gizmos. Here are three easy steps that we use to resolve the aesthetic issue:

First, to facilitate complete covering of ones deposit, we keep a squirt bottle of water beside the toilet for the purpose of wetting down the used toilet paper. Wetting reduces the volume of the toilet paper and allows cover material to adhere to it better. This makes covering the used toilet paper much easier and requires less cover material to be used. An old liquid detergent bottle works well for this. Less than half a cup of water is required per use.

Second, we use cover material for an aesthetic as well as a biological function, sprinkling enough cover material over toilet paper and feces to cover them completely so that the next user sees only cover material.

Third, over time, feces, toilet paper and cover material will build up in a mound directly under the toilet seat. (This does not tend to occur with bucket toilets.) This makes the covering of ones deposit more difficult since feces and toilet paper tend to fall down the sides of the mound and spread out over the surface of the composting chamber. The mound should periodically be leveled off. Although a hoe or stick can be used for this job, a more effective tool is an inexpensive garden cultivator, the kind that looks like a miniature pitchfork with the tines bent at a right angle to the handle. To further facilitate the covering of deposits, it's a good practice to go beyond simply leveling the surface of the compost pile to actually hollowing out the area under the toilet seat so that feces and toilet paper land in a depression. Covering ones deposits is then much easier. After leveling or hollowing, cover any exposed feces or toilet paper with cover material.

Finally, we've found that keeping the toilet clean often makes acceptance easier.

A clean toilet makes for a happy user!

Insect Control

Insect control is another neglected aspect of compost toilet design and use. Few commercial composting toilet designs feature effective insect control. A composting toilet is a micro-ecosystem that favors biological agents of decomposition.  It is also an excellent reproductive habitat for a variety of insects and other arthropods such as flies, gnats and mites. Regardless of how insect-proof the composting chamber is, or how religiously the toilet seat is closed after each use, these organisms will enter in various ways including flying or crawling in when the seat lid is open or by being embedded in cover materials. By whatever route, periodic insect infestations are likely to occur, resulting in mere annoyance if gnats or mites enter the home when the toilet is used, to potential disease vectors in the case of flies, which are also attracted to human food. Infestations can, to an extent, be prevented by avoiding cover materials such as leaves that may provide a conduit for these organisms. Nonetheless a more effective solution is necessary and often overlooked. 

Insect trap 

Insect trap with parts laid out

When we first began using composting toilets, we occasionally experienced insect infestations. Eventually, we developed the insect trap described here and it marked the end of insects as a significant problem in our composting toilets. The trap is simple, consisting of a transparent container with a funnel at one end through which insects enter, attracted by light passing through the container and into the composting chamber. The funnel is oriented such that the insects can enter the jar easily but find exiting difficult. Success of the design depends on a composting chamber that is insect-proof and in which light enters the chamber only through the insect trap.

We have had excellent results building insect traps using one gallon wide mouth jars with a hole cut in the lid. A funnel is made from the top of a 2 liter clear soda bottle. The funnel is placed in the mouth of the jar, the lid is screwed on and the mouth of the jar is inserted into  a hole near the top of the composting chamber. If the toilet is outdoors, the trap should be on the south side (in the northern hemisphere, that is) to maximize the sunlight entering the trap. Painting the bottom half of the trap black will cause it to heat in the sunlight, killing the trapped insects. If the toilet is an indoor design, some low-wattage bulbs such as LED's should work well for the light source. We've also had good success using 1 quart canning jars and rings and making the funnel from small clear soda or drinking water bottles.  

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