Not many people like to talk about it, but we all need to go to the toilet. In the developed world the vast majority of us have access to some form of flushing toilet connected to a sewage system that carries our waste away to be processed-usually in remarkably ingenious processing plants that we are happy exist, but few people want to live near. In the developing world the situation is not so rosy. Billions of people have no access to any form of toilet and for those that do the toilet is little more than a pit. This lack of sanitation leads to misery, disease and death. The Bill and Melinda Gates Foundation has recently begun to tackle this problem head-on through a series of initiatives.
A team from Loughborough University has been involved in one of these - the Reinvent the Toilet Challenge[1]. This challenge aims to develop a toilet system that can process the waste from humans in a self-sufficient way (off grid), producing safe high value products (clean water, salts, ammonia, etc.), in about 24 hours. Alongside this technical brief the toilet also must provide a safe and pleasant environment for the users. All this must be done for the challenging sum of $0.04 per user per day. When I first heard of the challenge I thought that it sounded like the Foundation was proposing to install something like the toilets used in the International Space Station across the developing world. It turns out that space toilets don’t come very close to meeting the challenge (even if the cost requirements are ignored). Science and technology would have to try harder to solve this problem. To meet this challenge the team at Loughborough encompasses Civil Engineering, Chemical Engineering, the School of Design, and the Department of Materials. We think we have got a good solution. At the end of the first stage of the challenge process our work was awarded second prize ($60000) at a technology fair held at the Foundation’s headquarters in Seattle in August 2012. This fair brought together research sponsored by the Foundation, representatives of governments from around the world and various international development agencies and generated a lot of worldwide media interest (and headlines with toilet based puns) at the time.
Our design is based on a process called hydro-thermal carbonisation. In this the solids and liquids are treated by heating to around 200°C under pressure so that the water remains in a liquid state. The main chemistry in this process is a dehydration of carbohydrates[2]. This process is not new has been researched for many years as a way of converting biomass into coal or fuel oil. Our research has shown that under these conditions the faecal material is converted into a brown coal like material in about fifteen minutes. Our calculations show that as long as care is taken with the amount of water in the system then the process should be self-sufficient in terms of energy - the process is exothermic and the end products can be used as a fuel.
So far the solution sounds like an example of chemical/civil engineering, so where do materials fit in with this? It turns out that materials science is going to play a key role in ensuring the project's success. Let's start with the "user interface". We decided early on in the process that we would not go for urine separation and ideally have a low flush volume water trap system. Two key factors in this decision were: urine separation is bound to go wrong at some point and water traps are an excellent way to keep smells out. In order to keep the amount of flush water used in the system to a minimum we need to minimise the amount of material that sticks to the bowl. Teflon-like coatings are one way to go but are prone to wear from the cleaning processes and damage by foreign objects. The chemistry of these coatings means that repair is not something that can be done in the field. Instead we have investigated using a self-assembled responsive polymer coating developed by the U.K. company Chamelic[3]. This coating creates a wetting layer on a surface so that water spreads out on it and any solids on the surface are lubricated away. The coatings were developed for applications like keeping photovoltaic arrays or cars clean, however we have shown that it is well suited to our toilet application and unlike teflon coatings the material can be easily reapplied with a simple spray that can be part of the cleaning routine.
Inside the processing system there are also significant materials challenges. The continuous flow reaction chamber that our design will use will need to be able to withstand the abrasive qualities of the solids as the conversion progresses as well as the chemical effects of the superheated water and the dissolved ions in the reaction mixture. The Department of Materials at Loughborough has a strong background in studying the microstructures of steels and coatings particularly for the power generation industry and this knowledge will be used to guide the materials choices and analyse how they behave in the system.
As well as the prize we have been awarded a second stage of funding for this work and are now part way through this. In a few months we will be at another fair and time we will have a working toilet system competing to get funding to produce the system in sufficient numbers that a useful test programme can be run in locations around Africa.
References
[1] http://www.gatesfoundation.org/What-We-Do/Global-Development/Water-Sanitation-and-Hygiene
[2] A. Funke and F. Ziegler, “Hydrothermal carbonization of biomass?: A summary and discussion of chemical mechanisms for process engineering,” Biofuels, Bioproducts and Biorefining, vol. 4, no. 2, pp. 160–177, 2010.
[3] Chamelic Ltd, UK: http://www.chamelic.co.uk/
Author
Dr. Simon J. Martin is a Lecturer in the Department of Materials at Loughborough University.
Contact
S.J.Martin@Lboro.ac.uk