• Are funds limited for purchase of the more expensive manufactured units?
• Is it important to use local products or materials?
• Are wood (lumber) locally available, also concrete, dry carbon bulking agent (ash, wood chips or sawdust, leaves, hay, straw, cut grass, dry soil)? 
• Can you acquire the appropriate toilet and urinal fittings (they are not very expensive but the types needed are seldom available in hardware or plumbing supply shops and probably must be ordered online)?
• Is there a desire for local involvement, labor, design input?

(Photo: MTU)                                                                                                                            (Photo: SSA)

If you answered yes to most of these questions then building on-site is indicated and you need to figure out how many you need.

If you are going to build on-site, evaluate whether the more expensive but lower maintenance multi-chamber design is feasible or desirable.  A single-chamber or removable container design will take less space and be less expensive but require somewhat more continuous maintenance; look here.

Finally, will waterless methods, including urine diversion, be found acceptable to your community of users?  Look here.

Do You Want to Build More Than One Toilet?  If you have more than four or five people who depend on your dry toilet during the same time period, more than one toilet will help.  But (as queuing theory shows) the number you need for, say, 40 people is not double what you need for 20.  Here are some high and low estimates:
Number of People⇨   5       10      15      20      40      60      100     150
High Nr of Toilets ⇨   1        2        3       3        4        5         6        7
Low  Nr of Toilets ⇨   1        1        2       2        3        3         4        4

(Photo: SSA)

(Photo: SSA)

Will You Build Single Vault or Double Vault?  The "vault" in question is the compartment below the feces hole of the toilet into which poop drops.  An optimum size for this vault is approximately one square meter because a group of 4-6 individuals may fill it in approximately one year.  Then what? 

(Photo: SSA)                                                                 (Photo: SSA)                                                              (Photo: SSA)                             

If the poop falls into a removable bin within the vault, that's one thing.  If it falls on the floor of the vault then it must be shoveled out in order to continue to use the toilet.  This is where the "double vault" design comes in.  After one vault of the double vault is more or less full, the toilet seat is switched to the second vault (which is entirely walled off underneath from the first), the first vault is sealed off from the user compartment with a cover, and the contents desiccate (becoming far less obnoxious) for the next year before anyone has to mess with it.

Many of the photos and diagrams on this site are double-vault designs and you can see that they require more space, labor and materials to produce - but they are less maintenance. On the other hand, single-vault designs are simpler.  But they are not really practical unless some kind of bin or container is inserted into the vault, under the toilet so that, when almost full, the container can be removed, transported, and emptied as appropriate.  This requires more and regular maintenance.  Try to choose based on space, labor, cost, and maintenance considerations.

Will You Build a Urine-Diverting Toilet?  With flush toilets we are accustomed to everything going down the drain to the same wet place.  However, with a no-flush toilet, the dryer the excreta can be, the better.  This is because it is the combination of urine and feces together that makes so much bad smell.  You may be very surprised to find out that feces with no liquid, together with TP and bulking material (e.g., sawdust), smell very little. 

This is where urine diversion enters our story.  Although many composting toilets use conventional toilet seats that send the urine and feces to co-mingle (see Sanitation page regarding composting), many others use the re-designed seats that separate the urine and send it to its own repository (see Fittings page regarding UDDT seats).  Be sure to review the Pros and Cons of UDDTs.

(Photo: GIZ)

The UD (urine diversion) seats are used pretty much like any toilet seat, but they look different and may cause some users to hesitate.  People will need information and reassurance that problems are rare.  With familiarity, UD seats are widely accepted by users, male and female, young and old.

This decision is more than about choice of toilet seats.  It also influences the project in other ways relating to storage space, plumbing, etc.  So read the relevant parts of the links just mentioned and make a decision.

Not considered here are the various "honey pot" designs (poop-and-pee bucket under a toilet seat) found in use at some homesteads, allotments, camps or other rural projects (and, due to high liquid content, can become unpleasant to use, burdensome to empty and to maintain).  

Six Basic Functional Elements of The Dry Toilet:

1. Toilet shelter or superstructure.
2. Toilet seat (either conventional or urine-diversion, as decided above).
3. If urine diversion, urine piping leading from the toilet (and urinal) to a urine collection container or soak area. (There is no flush water; only occasional rinse water is used.)
4. One, two or more (as decided above) feces collection bins or vaults that remain dry, protected from rain or runoff.
5. Ventilation pipe from the feces vault operating by convection, which can be enhanced with solar features or by battery-powered or solar-powered fan.
6. Bucket with dry covering material (desiccant) such as wood ash, wood chips, sand, lime, leaves, compost, dry earth, saw dust, rice hulls. (Also a second container for trash, plastics, wipes, sanitary pads and liners.)

Examples:  It is worthwhile to examine units built in several countries to specifications drawn up by a number of architects, NGOs and sanitation planners.  They are shown at various points on this website.  Although all are flushless (waterless), they are not strictly dry toilets if urine and feces co-mingle; but in such cases we can see what difference this makes in the designs and how the designs might be specifically modified to divert the urine.

As is preliminary example, here is an owner-built design used in Australia by Eric Tawnee, a U.S. Peace Corps volunteer, adapted from toilets in Tonga, Kiribati and Fiji.  It is a double vault unit without UD (urine diversion).
                        

(Photos: MTU)