The earliest documented dry toilets with urine separation were installed in multi-storey houses in Yemeni towns and were, until recently, used continuously for hundreds of years. The UDDTs with double dehydration vaults that we know today were originally designed around 1950 in the Kanagawa Prefectural Public Health Laboratory in Japan and further developed in Vietnam in the 1960s as a means of increasing the hygienic safety of excreta reused in agriculture.1
Since the 1990s, modifications of this design have been promoted in countries like Mexico, Guatemala, El Salvador, India and Sweden. Ventilation pipes in the faeces vault were gradually integrated and allowed for the installation of UDDTs inside houses. More recently, prefabricated ceramic or plastic UD squatting pans and pedestals have become available on the market, generally increasing the durability and perceived prestige associated with the system. The design was further adapted in India and West Africa to accommodate anal cleansing with water, by including a separate anal cleansing pan with a drain to divert wash water into a dedicated disposal or treatment system.
As early as 2000, the bench UDDT, which is a design that allows the user to sit directly on the vault, began to be promoted in Ecuador and Peru. This model is easily incorporated into existing housing structures and has emerged as a popular design for indoor installations (see Section 10.2). UDDTs have also been commercially produced in Sweden since the mid- 1990s. These commercial products are often installed in locations where piped sewerage is not available, such as remote summer cottages.
In 2001, the EcoSanRes Programme was initiated at the Stockholm Environment Institute (SEI). This was followed shortly thereafter by the establishment of the ecosan program at GIZ. Together these two government-funded programmes have helped to disseminate knowledge of UDDTs and have triggered the promotion of this technology in developing countries and countries in transition. The exact number of UDDT users is impossible to determine, but a rough estimate based on known projects in 84 countries puts the number at approximately 2 million worldwide.