There are many versions of the twin-pit composting toilet and they have been constructed in large number across many Asian countries. An Indian organisation, Sulabh International, has been particularly instrumental in the spread of this technology and it is now being adopted in a number of African countries.
The twin-pit pour-flush composting toilet consists of two leach pits to which a pour-flush toilet pan is connected via two soil-pipes. A minimal amount of water is required to flush the pan. The effluent from the toilet is directed first to one of the leachpits which are permeable and thus allow liquid to pass into the surrounding soil, while solids remain in the pit. As a ‘rule of thumb’ a family of seven takes two years to fill the first leach pit, at which point, the effluent is directed into the second leach-pit, leaving the contents of the first leach-pit to develop into compost which can then be removed with a shovel and safely used to fertilise crops. See also information on the Akvo.org website.
The twin-pit design I am most familiar with is one developed by the Auroville Building Centre and introduced in many of the Tibetan refugee settlements spread across a number of Indian States. In this design, most of the components, from leach pits to superstructure, are all manufactured of ferrocement – see photos.
Since an NGO called ApTibeT first introduced this design to Tibetan refugee settlements in India in the 1990s, it has proved consistently popular. Whereas the toilets were initially introduced with financial support, by 2009 they were regularly purchased by householders in the Tibetan settlement of Doeguling in Karnataka and there was frequently a waiting list. Returning children and youth who had been educated away from the settlements in schools and colleges with good sanitation facilities, reportedly drove the healthy market in twin-pit toilets. This ferrocement design is a sustainable technology, provided the ferrocement production is carried out by trained personnel and quality control maintained.
As awareness of the need for healthy habitats increases, this fuels a steady demand for low cost sanitation solutions. There are many and varied alternatives to ferrocement. For example, the pits may be consructed from bamboo, or bricks, while the superstructure can be made from bamboo or timber poles and canvas, or stabilised compressed earthblocks. The roof covering may be of thatch, metal or corrugated sheet. The production of these toilets has the potential to quickly generate income, once basic capital costs are covered.