Last Updated: 18 Feb 2014

   Dry Toilets


Topics Covered:

This Page is Related to On-Site Constructed Units.

You may want to jump to one of these related pages:

⇩           ⇩
Why the Dry Toilet? Pros and cons discussed.

Site-Built Units Start with some explanations and decisions.

Base and Structure Construction of base and compartment.

Sanitation, nutrient cycle, re-use, composting, maintenance, and other long-term concerns.


On this page you can skip to:
Urine Diverting Toilet Seats       Urine Channel       Separate Urinal
  Urine Collection        Vault/Bins      Vent Pipe
Insect Mitigation        Info/Instruction Sign  

Urine Separating Toilet Seat:  These have a bowl-shaped section in the front of the seat that captures urine.  Proven models of UD (urine diverting) toilet seats with the width and depth of the urine-collecting part well worked out are now available on the market. These can be acquired or the shape and dimensions of these seats duplicated.

UD seats
(Photo: SSA)

These toilets are not flushed but occasional rinsing of the urine hole with small amounts of water is advisable to keep the passage clear; this will dilute the stored urine, which is desirable. Clean toilet seat regularly with a damp cloth, rag or brush in order to remove urine and possible feces stains. Avoid letting water into the feces vault.

Seat on box built up from floor in hotel bathroom.
(Photo: SSA)
Seat on bench built into wooden cabin.  (Photo: DTS)
Seat on pedestal. (Photo: GIZ)
wood seat uddt Seat on pedestal.
(Photo: SSA)

The feces drop hole should be large so that the sides of the hole are rarely dirtied, reducing the need for cleaning (other than normal cleaning of the toilet seat). (Some models of UDDT seats have lids that cover the hole until the user depresses a lever.) Small amounts of urine (dribble) or menstrual blood entering the feces vault do not cause problems.

The photos above show two different ways that toilet seats are mounted.  One is on a box or bench, which may be the simplest and least expensive way if you are building the structure from scratch, although it may not look like the toilets your users are accustomed to.  The second is the pedestal arrangement that resembles a standard flush toilet, ceramic versions of which are available, but the pedestals may require more clearance from the vault below than a bench arrangement.

An alternate method of separating urine and feces does not require a separating toilet seat, but instead catches the feces and conveys them horizontally to the rear, letting the urine drain from the basin of the toilet.  This approach also allows the feces vault (see later description) to be in the rear, rather than under the toilet cabin, reducing overall cabin height.

FrenchPedal    (Photos: SSA)     FrenchPedal2  One pumps on a pedal to activate a conveyor belt. Urine separation is done by gravity. Urine is collected underneath the toilet and piped, then sent to a container or soak area. The conveyor belt transports feces and toilet paper to a composting chamber built at the back of the toilet.

Although rural units, usually built in structures containing one to three toilets, may have interiors that are strictly utilitarian with exposed wood framing or concrete, many are built for public accommodations or residences that are well-appointed, even stylish.

Double vault UDDT with urinal and shower.  (Note cover over unused vault).
(Photo: SSA)
hotel uddt
Double vault UDDT in Hotel.  (Note cover over unused vault).
(Photo: SSA)
Indoor single vault UDDT in bathroom. (Photo: GIZ)
Single vault UDDT and urinal in residence. (Photo: SSA)

Urine Piping Leading From The Toilet To A Urine Collection Tank Or Container: Odor nuisances from the urine collection system are caused by exposure of urine to air that leads to ammonia emissions which, in turn, dispel nitrogen and reduce fertilizer quality. The only real malfunctions of a urine diversion seat occur when the urine drainage puddles in parts of the piping not having sufficient slope or the urine piping gets blocked. Stagnant urine in the urine outlet results when a user accidentally drops ash or waste products or even defecates in the urine section. Also dried urine (struvite) can develop over time and block the urine outlet.

Urine Outlet From Toilet Seat: The diameter of the urine drain entrance at the bottom of urine bowl should be small (1 cm (0.5”) or slightly larger) in order to impede foreign materials from entering the urine piping (which is of larger diameter beyond this hole). It also makes it more obvious that this is not the feces drop hole (especially for seats where the drop hole is covered with a lid).  A removable coarse sieve at the urine drain entrance will add to the visual impression of “liquids only” and further reduce risk of blockages. Note, however, that such a sieve needs regular cleaning since various materials such as pubic hair, blood clots and urine stone (struvite) can block the sieves.

Urine Pipe or Hose:  Plastic pipes or hose can be of polyethylene (PE), polypropylene, or polyvinyl chloride (PVC) and should be stiff enough to avoid collapsing at bends. In colder climates, consider insulating the urine pipes.  The minimum recommended diameter of the pipes is 35-50 mm (1.75-2”), but the optimum range may be as much as 75 mm (3”) to 110 mm (4”). The larger diameters will help prevent fibers and other foreign materials from blocking the piping.

↑ Flexible tubing used to connect bottom of toilet seat in single vault unit, and bottom of urinal (not visible on left), to urine container that is inside wooden toilet bench (feces container is on right; above it is bottom of white vent pipe).
(Photo: DTS)
Polypropylene urine pipes in brick and concrete double vault unit, one to urinal, one each to toilet seats above first and second vault.  Descends vertically to urine collection in below-ground location. ↓double vault urine pipes
(Photo: WECF)

urine tubing
(Photos: DTS)
plumbing plan

Maximum length should be under 10 meters (4’) to limit the time urine is in the pipes, preventing degradation of urea and possible precipitation. Bends should be minimized as they are points where blockage can occur, and all joints should be properly sealed. A minimum slope of 4% is suggested (4 cm height difference on 1 meter length). This will counteract the effects of chemical precipitation in urine (i.e. struvite and calcium phosphate crystals which also cause viscous sludge).

Urine Odor Seal: The urine piping may benefit from an odor seal (e.g., section of tubing that has a dip in it, or that flattens when not passing liquid, or having tube submerged in storage container below level of liquid) preventing urine odor emanating from the storage tank. But odor seals are not commonly found for outdoor UDDTs because they are usually well ventilated (indoor locations often have electrical fans to expel odors). Fresh air passing into the cabin, down the feces hole, up and out the vent pipe evacuates odor and also discourages insects (see Vent Pipe, below).

A dip in the urine pipe like this would act as a shallow trap.  Locate this segment so it can be accessed if cleaning is necessary. (Photo: SSA)
condom method
A condom tied to the exit fitting of urinal and UD toilet, pierced at the bottom, then pipe reconnected, would reduce air flow from urine pipe and container.  It would need to be changed periodically since debris and struvite will accumulate. (Photo: WECF)

Separate Urinal: It is advised to install a waterless urinal in the same cabin as the toilet for the convenience of men and to reduce risk that they might urinate in the feces hole of the toilet.

urinal     urinal
Inexpensive but sturdy plastic urinals. (Photo: SSA)
porcelinurinal   hilo urinals
Porcelain urinals made waterless by capping the water intakes.
(Photo: SSA)

Urine Collection: If the urine container is located in the feces vault it must be at a distance so the container is not dirtied. Alternatively, it can be located in its own protected space or just outside the vault. The urine container which, with its piping, must be airtight to prevent escape of nitrogen and odor, can be a series of plastic jerry cans or larger plastic water tanks (urine is highly corrosive so metal containers are to be avoided).

Urine container removable from outside the toilet structure.  Being below ground it is out of sight and less likely to freeze in cold weather. (Photo: GIZ)
Jerry cans used as urine containers. (Photo: GIZ)
Two large urine collection tanks buried.
(Photo: GIZ)

An adult is estimated to produce 0.8 – 1.5 liters of urine per day. Graywater from a hand-washing basin nearby could be run to the urine container; a ratio of 7-10 parts water to one of urine makes the result appropriate as fertilizer. A container large enough to accommodate one year’s accumulation is ideal, since any pathogens contributed by ill individuals to otherwise sterile urine will die off in that time.

Urine can alternatively be diverted to a rock-filled soak hole, French drain, seepasoakge bed or trench, or evaporation bed (where flowers might be planted - and will thrive).

(Photo: GIZ)

What Enters Feces Vault? Toilet paper is thrown into the feces vault; no special paper or decrease in usage is required. Kitchen waste, sanitary napkins and other debris should not go in. A separate trash basket along side is essential.

The user should cover fresh feces with a scoop or handful of locally available bulking material or desiccant (dry, carbon-rich and absorbent materials) such as wood ash, wood chips, lime, sawdust, dry leaves or dry soil. It absorbs liquid, adds carbon to the nitrogen, and allows oxygen to decompose feces aerobically (see Sanitation), producing nitrates, phosphates and sulfates which are useful.

If allowed to remain very wet, feces decompose anaerobically producing methane, ammonia and hydrogen sulfide which smell bad and are not useful. A small container of desiccant must be provided beside the toilet with a scoop.

A false bottom of polyethylene mesh or window screen suspended (e.g., over “egg crate” lighting diffuser or wire mesh) above the bottom of the chamber or bin promotes air circulation under the feces.

false bottom screen
False bottom facilitates air circulation for this 20 gal. (68 liter) feces bin.  Made with sheet of window screen over sheet of 1/4 inch hardware cloth (wire mesh), raised from bottom by two lengths of 1/2 inch plastic pipe attached with nylon ties. 
(Photo: DTS)

Air circulation could be enhanced with an auxiliary air intake directed at the bottom of the chamber or a vent pipe extending upward from the bottom of the bin into the upper part of the chamber. In cooler climates decomposition of the feces is further promoted if incoming or vent pipe air are warmed, perhaps from a turbine or passive solar arrangement (see Vent Pipe below).

Movable Bins: In a single-vault design with movable bins or containers, an empty bin replaces the full one; by making space available for storage of two or more bins the full one(s) can “rest” and dry out more completely. The single-vault design requires that someone change (the position of) bins one of which still has fresh feces on top, although the exposure is brief, safe, and can be facilitated by placing the bins on sliding platforms or carousels. When a container of feces is dehydrated to a crumbly, dry and odorless material it is removed from the vault for use or disposal.

multi bins
Bins in multi-unit UDDT.  Note vent pipe exiting.  (photos: GIZ)
Plastic-lined bins in single vault with second bin that is already full now "resting." (photos: SSA)
Single vault wood-built unit;  plastic container on left is feces bin; urine container on right. (photos: DTS)
Single vault wood-built unit; feces hole in toilet above feces container.  Note vent pipe entering on right side. (photos: DTS)
← Plastic feces and urine containers for single vault UDDT in compartment located inside suburban residence, below bathroom, opened from outside the house.
(Photos: GWA)

Vent Pipe: Ventilation of the vaults is through a vertical vent pipe that draws fresh air from just below the toilet seat, passing it through the vault, then up and out the pipe above the roof to evacuate moisture, any odor and to promote dehydration. The pipe should be at least 100-150 mm (4-6”) in diameter, as straight as possible, extend approximately 1 m (39”) above the roof line, and be black in color to absorb heat from the sun to encourage convection. 

(Photo: WECF)
(Photo: SSA)
(Photos: GIZ)

Locating the pipe inside the structure, rather than protruding from a wall, may reduce accidental damage but decrease sun warming. Fit a top cover to keep rain out. 

To enhance convection of air up the vent pipe solar exposure can be done in several ways.  vent pattern solar aids
Solar collector "hot box" at vault door.
(Photos: SSA)
vent pipe solar assist
Solar collector (hot box) in vent pipe.  Electric fan could also be inserted here if electricity is available.

Convection can be enhanced with a passive wind turbine device, a “hot box” where the pipe leaves the cabin or at the vault door, a wind-propelled or solar/battery-driven device. Air will be drawn through the toilet opening, but if this proves insufficient, an additional air intake directed at the bottom of the vault may be necessary.

← Known as a "whirligig" this is a small version that will fit on a 4 inch vent pipe.  When the wind moves the turbine rotates and draws air up the pipe.
(Photos: Suppliers)
← Due to the shape of louvers in this stationary pipe top, wind also promotes an updraft.

Insect Prevention: The control of insects in toilets can be achieved by keeping the vault dry, keeping the toilet room dark and by proper construction of a ventilation system. Fly traps, organic insecticides, or diatomaceous earth are not strictly required in climates without many insects but could be used inside the vault or bin. Mosquito netting can easily be fitted to the top of the vent pipe. The dry condition of the feces and the covering with dry absorbing materials (desiccant) means flies and other vectors are minimally attracted in the first place.

If needed a trap can be added.  Here is one that consists 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. Orient funnel such that insects can enter easily but not exit easily. Success depends on a dark vault where insects see no other light than in the trap.

Exploded view of fly trap made from 1-quart mayonnaise jar. → flytrap Mounted on side of dry toilet cabin. →
(Photos: OMK)

Use one quart to one gallon wide mouth jars with a hole cut in the lid or one quart canning jars. A funnel is made from the top of a 2 liter clear soda bottle and 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 on the south side (in Northern Hemisphere.) to maximize the sunlight entering the trap.

An informative sign permanently posted in the toilet cabin should pictorially indicate proper usage of the UDDT. At a minimum the following should be included:

  • This is an eco-toilet that separates urine from feces at the toilet seat.
  • Please position yourself appropriately. Do not pee in the feces hole!
  • Toilet paper is okay, but no sanitary pads, tampons, plastic debris, water or chemicals should go in the toilet. Use trash basket.
  • With this toilet no pollution goes into the environment.
  • After use please add a handful of the dry material from nearby basket.
  • Close the toilet seat lid when finished.
  • Wash your hands.
poster en
(Photo: DTS)
poster es
(Photo: DTS)

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