The mission to build a toilet that utilises human poo

Imagine a toilet that takes human waste and converts it into minerals for fertiliser and clean water, while harvesting energy in the process.

The toilet doesn’t use water, doesn’t need expensive infrastructure of a sewerage system, doesn’t need to be connected to mains electricity and, unlike composting toilets, doesn’t need lots of space and time. If a new multi-million dollar project, the ‘Reinventing the toilet challenge’, is a success, such a toilet may soon become a reality.

The first mass sanitation revolution – the invention of the modern flush toilet and sewerage system - happened in the 18th century and is credited with saving more lives than any other advance in healthcare. However, there are still some 2.6 billion people around the world who have to use pit latrines or defecate in the open.

Such inadequate sanitation causes an estimated 1.5million children to die each year due to diarrhoeal diseases. But with water scarce - 1.6 billion people currently live on less than one thousand litres of water a year and by 2020, experts predict that this figure will rise to between 3.2 and 4.6 billion (the average Briton uses 55,000) - toilets that flush the problem away, using precious water and expensive infrastructure, are clearly not the answer.

Building a low-cost toilet

Last month [19 July], at a conference on sanitation in Rwanda, the Bill and Melinda Gates Foundation announced a package of sanitation funding amounting to $42 million, including $3million in grants to eight institutions around the world to build prototypes of revolutionary ‘off-grid’ toilets (another $5m is for improved pit latrines etc).

Although they will be so high-tech that they will probably need to be controlled by a microprocessor, the toilets must not exceed a cost of 5 cents per person per day.

'Over time, we’d like to bring this cost down to even less than this,' says Frank Rijsberman, director of the Global Development Programme of the Gates Foundation. 'It’s got to be affordable for the bottom billion.' The toilets will be sold, rather than given away, 'for them to be sustainable,' says Rijsberman. 'We’d love to see the ‘mobile phone’ of toilets – something that people aspire to have and want to use,' he says.

All the concept toilets rely on the separation of urine from faeces with the subsequent drying and burning of solids and some form of liquid treatment, such as osmosis or evaporation. Some models meet the requirements for a unit suitable for a single household, others will be community toilets that treat larger volumes of waste, perhaps off-site. The eight teams – mostly universities - have to build prototypes that will be judged in a ‘Reinventing the toilet challenge’ next August.

For example, in South Africa, specially designed toilet pedestals that separate urine from faeces are already fairly common. The project of the University of KwaZulu-Natal in conjunction with eThekwini Water and Sanitation (Durban) will take this technology further by combusting solid waste to a pathogen-free ash that can be used as a phosphate and carbon rich fertiliser. Heat generated in the combustion will be used to dry solids and to concentrate the liquid stream, resulting in a liquid fertiliser after osmosis.

A Swiss team is devising a similar method but one that will treat waste from as many as 2,000 people in one treatment unit. Developing similar urine-separating ‘dry’ toilets (that can be cleaned with water), waste will be collected from those living in high-density informal settlements.

In the project, a simple transport system for faeces and urine will be set up. Liquid waste will be collected using appropriate technology such as a bicycle pump system already in use for emptying pit latrines. For reasons of hygiene, solid waste will be transported directly in the toilet containers.
 
'Conventional wastewater handling is based on mixed wastewater, which is transported in sewers to a central wastewater treatment plant, where pollutants are eliminated to an acceptable standard for treated wastewater, which is then discharged to the receiving water such as a river, lake, or ocean,' says Tove Larsen from Eawag, the Swiss Federal Institute of Aquatic Science and Technology. 

'This is not possible here. We plan to develop a toilet, where three fractions are separated: dry faeces, undiluted urine, and water used for personal hygiene and cleaning of the toilet surface. Whereas resource recovery from urine and faeces is best done at a semi centralised plant, we will develop a technology for treating and recycling the water for personal hygiene and cleaning of the toilet surface directly on site.'

At Stanford University in the US, researchers will develop and test a thermochemical unit capable of processing 100 kg per hour of faecal sludge into a soil-fertilising biochar, a high-grade version of charcoal. The heat energy required for pyrolysis of the sludge will come from burning methane produced from an initial anaerobic digestion stage. There will be no external water or power required.

Researchers aim to test this innovative system in a processing facility in Nairobi, Kenya where it is projected to meet the needs of nearly 5,000 urban residents. In Kenya, it is already common for human waste to be separated into liquids and solids for processing. 'Ordinary sewage systems add water,' said a spokesman. 'These new systems remove water - a completely alternative approach to traditional processing.' 

Making energy from poo

One of the most unusual concepts is that of a Netherlands team at Delft University of Technology. Here, solid waste will be partially dried to remove water (which can be as much as 75 per cent of the content of faeces.) Then, this sludge will be gasified in a 'plasma environment' of hot, ionised air. 'Plasma is chemically very reactive and will be created by a high intensity microwave field in tailor-made equipment,' says project designer, Georgios Stefanidis.

'The gasification process will yield syngas, a mixture of carbon monoxide (CO) and hydrogen (H2). Syngas will be converted to electricity in another unit downstream - a fuel cell. In order for the process to be energetically self-sufficient, part of the electricity produced will be used to generate plasma, while heat recovered from the syngas stream and from the fuel cell exhaust gas (water and carbon dioxide) can be used for drying the waste at the start of the process.'

Although the team intends to build a completely off-grid system, a battery may be needed for start-up. (Other teams’ proposals include solar power and even power harvested from the weight of a user sitting on a toilet.)

An alternative view

Not everyone in the sanitation world thinks that microwaves and microprocessors are an appropriate way forward. Joseph Jenkins, a long-time proponent of ‘humanure’ (www.humanurehandbook.com) - composting of human waste until safe enough for vegetable gardening - says 'the Gates Foundation and most in the international sanitation community are stuck in the old paradigm, which is the belief that our excrement is waste and needs to be disposed of in one way or another.'

'The new paradigm sees excrement as a valuable resource that should be collected and processed for recycling. It's a huge leap in consciousness. When one accepts the new paradigm, then reinventing toilets isn't such a challenge as toilets simply become collection devices. In the new paradigm, every toilet is a piggy bank and every turd is a nickel.'

The Gates Foundation points out that the majority of its sanitation funding is, in fact, on low-tech sanitation projects - basic pit latrines and ‘appropriate technology’ such as urine-diverting toilets and improved composting toilets.

One being developed by the London School of Hygiene and Tropical Medicine, for instance, will use tiger worms to digest waste and, unlike most composting toilets, be water-sealed. But toilets that compost waste are not always suitable for high-density living in urban areas, says Rijsberman. 'It can take six months to remove [pathogens such as] helminth eggs from human waste [by composting],' he says. 'We want a thermo-chemical method that can break it down in minutes rather than months.'

The world eagerly awaits the Gates Foundation’s Sanitation Revolution 2.0.

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