Urine-diverting dry toilet

A urine-diverting dry toilet (UDDT) is a type of dry toilet with urine diversion that can be used to provide safe, affordable sanitation in a variety of contexts worldwide. The separate collection of feces and urine without any flush water has many advantages, such as odor-free operation and pathogen reduction by drying. While dried feces and urine harvested from UDDTs can be and routinely are used in agriculture (respectively, as a soil amendment and nutrient-rich fertilizer—this practice being known as reuse of excreta in agriculture), many UDDT installations do not apply any sort of recovery scheme. The UDDT is an example of a technology that can be used to achieve a sustainable sanitation system. This dry excreta management system (or "dry sanitation" system) is an alternative to pit latrines and flush toilets, especially where water is scarce, a connection to a sewer system and centralized wastewater treatment plant is not feasible or desired, fertilizer and soil conditioner are needed for agriculture, or groundwater pollution should be minimized. There are several types of UDDTs: the single vault type which has only one feces vault; the double vault type which has two feces vaults that are used alternately; and the mobile or portable UDDTs, which are a variation of the single vault type and are commercially manufactured or homemade from simple materials. A UDDT can be configured as a sitting toilet (with a urine diversion pedestal or bench) or as a squatting toilet (with a urine diversion squatting pan). The most important design elements of the UDDT are: source separation of urine and feces; waterless operation; and ventilated vaults (also called "chambers") or removable containers for feces storage and treatment. If anal cleansing takes place with water (i.e., the users are "washers" rather than "wipers"), then this anal cleansing water must be drained separately and not be allowed to enter the feces vault.

Some type of dry cover material is usually added to the feces vault directly after each defecation event. The dry cover material may be ash, sawdust, soil, sand, dried leaves, mineral lime, compost, or dried and decomposed feces collected in a UDDT after prudent storage and treatment. The cover material serves to improve aesthetics, control flies, reduce odor and speed up the drying process.

Terminology

Alternative names for UDDTs include "urine diversion" for the "UD" part (instead of "urine-diverting") in connection with terms like "dehydration", "dry", "drying", "desiccation" (for the second "D") which gives quite a high number of possible mutations of the terms that UDDT can stand for. The "T" always stands for toilet. These variations in wording place different emphasis on the amount of drying that takes place in the feces vaults. Probably the most advanced drying takes place in UDDTs that feature two vaults or in UDDTs that have a built-in electrical fan. As a general overarching term the adjective "dry" is better than "dehydration" as it applies to a larger range of UDDT types. UDDTs are often considered synonymous with "ecosan toilets" (ecological sanitation) or with composting toilets. However, neither of these associations is correct. The term ecosan is in fact much broader and not limited to this type of toilet. Not all UDDTs are used in conjunction with the practice of reusing excreta. There are also many other ecosan technologies that hygienically return nutrients to the food chain, such as subsurface artificial wetlands that produce fodder for cows. In addition, there is usually little composting taking place in the UDDT vaults (instead, it might be carried out in a secondary composting step which is external to the toilets). Urine-diverting composting toilets are similar to UDDTs in that they also collect urine and feces separately. Treatment is achieved through the composting, a process quite different from pathogen removal by dehydration. In Malawi, UDDTs are also called Skyloos.

Appropriateness UDDTs are an interesting alternative for many special situations. For example, when water is scarce, a low-cost solution is required or the soil conditions are such that it is difficult to dig pit latrines. Areas that are frequently flooded are another suitable application for UDDTs. UDDTs can also be built for schools (many examples exist for example in Kenya, Rwanda and Uganda). However, these school UDDTs suffer from the same issues as any other type of school toilets: if clear responsibilities and a dedicated budget for the maintenance of school toilets is lacking, then the toilets may easily fall into disrepair, for example with blocked urine pipes or feces vaults that are not being emptied.

Design considerations

Fundamentals Urine diversion takes advantage of the anatomy of the human body, which excretes urine and feces separately. In a UDDT, the urine is kept separate and drained via a basin with a small hole near the front of the toilet bowl or squatting pan, while feces fall through a larger drop-hole at the rear. This separate collection – or "source separation" – does not require the user to change positions between urinating and defecating, although some care is needed to ensure the right position over the user interface. Female users may find that some urine may enter the fecal vault during normal operation. This is typically a small amount and does not significantly affect the functioning of the toilet. The urine separation function can also be achieved by designing the toilet bowl in such a way as to take advantage of the Coandă effect. In this design, users must direct their urine stream against the interior walls of the bowl (usually assisted by a sloping front section) so that it can be collected via a small trough that is recessed into the base of the bowl. Designers and users of UDDTs should strive to keep the feces pile as dry as possible, as this is the most important factor in the prevention of odor and flies. During storage, the feces’ natural moisture slowly evaporates and exits via the vault's ventilation system, or is absorbed by the dry cover material. This process is called drying, dehydration or desiccation. A UDDT consists of several functional elements. The main ones - which distinguish a UDDT from a pit latrine - are the urine diversion toilet seat or squatting pan, and one or two feces vaults (above ground).

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