Last Updated: 3 September 2014

   Dry Toilets

Sanitation, Agriculture...

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Sanitation          Maintenance          Composting
Agricultural Use          Post-Processing           Insects
Nutrient Cycle           Urine Diversion Pros-Cons

Some History and Perspective.   At this time in undeveloped parts of the world inadequate sanitation methods account for 80% of people's health problems, but the cost of "modern" sanitation systems is too high.  In the developed world where the price can be paid, such systems have been effective in reducing disease but are now considered by many experts to be unsustainable.

The so-called "modern" systems, with flush toilets, are very expensive because they require separate sewer pipes to carry the waterborne excreta and rely on chemically-assisted, sludge-producing treatment facilities that are expensive to install, operate and maintain.  Though considered "state of the art," they remain out of reach for poor regions and are probably unsustainable in wealthier regions because they use too much water and introduce harmful residues into the environment that cannot be managed economically.

Poop and pee once played a major role in European and North American agriculture.  Dating back 150 years and more, human excreta was depended upon  to maintain the fertility of the land.  When used carelessly it was dangerous to human health but nevertheless essential in nourishing food crops.  Where it had been practical to collect excreta in cities and transport it by wagon to nearby farmland, urbanization bought this to an end as cities grew in size and density.   

When the excreta were no longer being carted away, cities began to experience huge sanitation problems which were alleviated by washing or flushing sewage into rivers and seasides.  This, of course, lead to dangerous levels of pollution.  Gradually sewage pipes were built separately from storm drainage channels and treatment plants built.   At about the same time that water-born sewage systems began to be the major method of moving human waste out of the cities, synthetic forms of fertilizer became available for farming.  The nutrient cycle was broken.

Sanitation (pathogen destruction):
  Any toilet system must be sure to contain, then neutralize or destroy microorganisms (bacteria, viruses), parasites (amoebae, worms, protozoa), toxins and other pathogens that can cause health problems or hurt the environment.  Where conditions are favorable (hot and dry), above-ground composting or moldering can humusachieve this with minimum use of water and other resources, avoiding environmental degradation, while producing agriculturally important (nitrogen-rich) organic material.  In cooler climates some form of secondary processing is required to get these results.
(Photo: SSA)

We will review several factors below that, when you read all about them, may sound frightening or difficult to be sure of.  Consider, however, that with care and some attention to detail, you can avoid potential problems.  Keep in mind that what probably contributes most to safety is consistent washing of hands with regular soap and water after defecating or after handling, emptying, cleaning those parts of the system that come into contact with excreta.

Contamination can occur from the following: direct contact with fecal matter; insects, rats, birds that contact feces and track it to food, water or objects that people touch and transfer to mouth; contact with individuals already infected, their wash water or laundry; contact with fecal-contaminated drainage or food.  Parasites (e.g., roundworm) can be transmitted via a single cyst or egg, but bacterial or viral disease requires the transfer of hundreds or thousands of microbes.

Here is a (probably incomplete) list of diseases known to be transmitted through human excrement: amebiasis, cholera, cryptosporidiosis, gastroenteritis, infectious hepatitis, parasite-related disease, salmonelliosis, shigellosis, typhoid fever, and other diarrheal disease (for sources see Sanitation section of Resources and Links page).  Symptoms range from diarrhea and stomach cramps to death and are most severe for children and elderly and in hot climates.

This is a frightening list and so we are rightly motivated to follow the guidance of research showing how the bacteria, viruses and parasites causing these diseases can be contained, neutralized and destroyed.

Primary Processing.  First of all, dry toilets contain the excreta in enclosed bins or vaults to prevent human contact.  While contained, fecal matter begins to dry out and decompose.  Besides pathogens, also present in the excreta are the bacteria that will decompose it.  Pathogens (deprived of where they like to live, namely, inside humans) are depleted because they have to compete for food (carbon, etc.) with other microorganisms, they are dosed with byproducts (toxic to them) of the decomposition process, and they are deprived of water so they dry out.  Given enough time in relatively dry and warm-to-hot conditions the pathogens will die on their own. 

Composting and Moldering:  As mentioned, the bacteria that will decompose pathogens are naturally present in the excreta.  Different bacteria are effective in different temperature ranges.  Mesophilic bacteria operate where the temperature is less than 38-40ºC and are effective over time in destroying fecal coliform (considered a principal contaminant and indicator of other microbial contaminants), reducing it to safe levels.  If the excreta pile is well aerated, temperature in the range of 42° to 67°F (5-20°C), and less than 40% water content, Mesophiles will cause the excreta to desiccate and decompose in 12-24 months. This is sometimes referred to not as composting but "moldering" (Brit: 'mouldering') and is discussed more below.  Unfortunately, certain other dormant pathogens survive long periods and might become harmful once moisture is introduced and they are re-hydrated.

Thermophilic bacteria operate above 113ºF (45ºC) as discussed further below.  Their activity produces enough heat to sustain temperatures as high as 160°F and they can decompose the excreta in approximately 6 months at this temperature.  Such active composting requires aeration to supply oxygen and 40-70% water content (more than that and leachate or residual water will pool and drown the aerobic bacteria). Carbon-to-nitrogen ratio, pH, and air circulation are other important variables influencing rate of decomposition.  (More detail in Sanitation section of Resources and Links page.)  When successful, this active composting kills the pathogenic Mesophiles which live best at 45-67ºF (7-20ºC).

If temperature, water content, and Carbon-to-Nitrogen ratio are maintained within certain limits a natural cycle of three phases will occur:
  1. First the mesophilic phase of approximately 25 days where biodegradation of organic compounds increases temperatures to 40 - 50ºC and the population of microorganisms increases. The nature of pathogen decomposition in this phase is...
  2. Then the thermophilic phase from approximately 30 to 110 days during which temperatures up to 65ºC kill off mesophilic while promoting thermophilic microorganisms; maintaining the temperature at about 60ºC by turning the mass, ventilating and watering it prevents the heat rising so much as to kill off the thermophilic organisms (which would, in turn, result in decreased decomposition rates).  The nature of pathogen decomposition in this phase is...  Besides consumption of carbon content and decomposition of microorganisms, the thermophilic microbial activity and the temperatures reached in this phase destroy harmful fungi and their spores.
  3. Finally a cooling phase lasting from about week 12 through 17 (90 - 150 days) that results from depletion of organic materials; temperature will return to about 30ºC.  The nature of pathogen decomposition in this phase is... Fungi can begin to grow again during the milder conditions of this phase.

While composting can be encouraged in temperate climates by addition of passive solar features or electric heating devices, allowing extra time for excreta to dry out and desiccate will break down the material, helping it reach a sanitary condition.  

In climates with sub-freezing winters, composting toilets maintain a low average temperature, not hot enough for active composting by Thermophiles.  In this case, 

Aerobic decomposition by moldering seems to work well in hot, dry climates.  So how well does it work in temperate climates or regions with cold winters?

Bacteria become inactive once moisture has evaporated from feces but, if the temperature is not high, all pathogens may not be destroyed.  For example, in very dry conditions some pathogens take the form of endospores that can survive many years, as is true with some parasite eggs; adding water later, for example in agriculture, could revive these pathogens.  Feces and urine collected in such systems should be handled with due caution, observing routine safety measures. (Of course total destruction of all pathogens is not assured for various reasons, including local climate, unexpected weather conditions, mistakes or misuse.)

Post-processingdrying bin (secondary processing) or drying or desiccating excreta by removing it  from feces vaults and placing it in large, shallow and flat bins exposed to hot sun (20-35°C) for several months or a year is the practice in some countries. 

(Photo: GIZ)

Use in Agriculture: Results for agriculture seem to be satisfactory, although questions remain about how much nutrient value is available to plants when the material is only dried but not biologically decomposed (as occurs in Thermophilic composting).  Agricultural application factors have already been mentioned above.  In general, the end result of composting is a dirt-like, inoffensive organic humus that will retain moisture, is chemically stable and, though it should always be handled cautiously as described above, can be used to build up soil structure, support beneficial organisms, and steadily release nutrients that plants need. (See references.)

What seems unknown is whether endospores and parasite eggs, dormant after drying, are entirely destroyed by such post-processing.  What must be avoided when the end product is used in agriculture and after re-hydration, is that the desiccated material again becomes pathogenic, endangers workers, or is carried back by pests to become a threat to other humans.

Alternatively, secondary- or post-processing that consists of taking the partially dried and desiccated excreta from the toilet vaults or bins and putting it in enclosed composting bins may produce a superior result.  Such Thermophilic aerobic composting should be done in containers (preferably insulated) with approximately equal amounts of non-fecal, easily degradable materials such as kitchen compostables, garden cuttings, etc., and some, but not too much, hydration.   Properly managed and in temperate seasons the composting mass can reach or exceed 50°C and require as little as seven days to reduce pathogens to a safe level.  During winter or cold periods 50°C is not attainable and longer composting periods will be required.

Contrast the composting or moldering methods just described with what happens in a septic tank or mainstream sewage system.  There, the excreta is immersed in liquid and fermentation by anaerobic bacteria working at a lower temperature and in the absence of free oxygen.  The process produces smelly, difficult to use sludge that (due to lower temperatures) may retain active pathogens.

Maintenance: This includes emptying waste, transporting it to an appropriate location, and, depending on how long the waste has composted or moldered in place, providing secondary treatment (such as inclusion in existing, maintained compost pile) or storage (warm, dry and emptyingprotected from pests).

(Photo: WECF)


Water and Nutrient Cycle:  [to be completed]

Urine Diversion - Pros and Cons: Reduction and elimination of pathogens from excreta can be accomplished whether urine remains with the feces in a single container (which will produce objectionable odor), or it is diverted via separate urine channel or pipe (little or no odor).  Either method can meet the need for good fertilizer (18-23% nitrogen and phosphorous).  (Descriptions of methods for urine diversion are found in Fittings.)

The arguments for letting the urine go to a common container with the feces include:

  • Simpler construction.
  • Toilet bowl looks like the typical flush toilet found in developed countries, familiar to users.
  • By keeping urine and feces together the resulting compost is rich in nitrogen that won't so easily evaporate as ammonia (so the nitrogen remains in a form most available for plants).
On the other hand the arguments for urine diversion (see photos in Fittings) include:
  • The feces pile is kept dryer which prevents fermentation (anaerobic decomposition that produces hydrogen sulfide, with its noxious sewage odor, and flammable methane).
  • Almost odorless aerobic decomposition.
  • Reduced amount of carbon bulking material that would otherwise have to be added to excreta in order to nourish aerobic bacteria and soak up liquid.
  • Urine is sterile and channeled to a soak hole, French drain, seepage bed or trench, evaporation bed (with decorative plants), or to a separate, closed container, valuable for agricultural use later.

If urine is not diverted, that is, urine is not separated from the feces and they both go into the same vault or container, the combined urine and feces may contain too much nitrogen (and may be too wet) for effective composting.  This can be reconciled by adding more carbon material (that is also absorbent) such as sawdust.  The added carbon material can balance the carbon/nitrogen ratio, absorb excess liquid, and cover excreta enough to eliminate odor.  This prepares the pile for thermophilic composting, but will probably require a considerably larger bin to accommodate excreta plus the added filler material.  (See Composting in links/references.)  Probably this will require a considerably larger bin.

An example of this appears to be the Crapper.

Insect Infestations and Mitigation:  In many environments flies or other insects will find their way to the excreta vault or bin and must be eliminated.  See Fittings.

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