Poop-powered sewage treatment system fits in a shipping container

duke University
© Duke University

The Bill and Melinda Gates Foundation's Reinvent the Toilet competition has funded the development of a number of different approaches to dealing with human waste. Duke’s Pratt School of Engineering has received a grant of $1.18 million to build a system that is very different from last year's winner.

The Duke team doesn't really reinvent the toilet at all, but reinvents sewage treatment by packing a very sophisticated processing system into a 20 foot shipping container. In many communities, people empty their latrines into the nearest river; with the Duke system, they either have their cisterns pumped or they carry it to the container. It can handle the waste of a community of 1200 people.

value chain© Gates Foundation

The waste is then subjected to enough heat and pressure that it enters a phase called Supercritical Water Oxidization. They explain:

We are often taught that water exists in three phases: liquid, gas (steam) and solid (ice). However, when heated to temperatures of over 705°F and pressures of more than 3200 pounds per square inch (psi; atmospheric pressure is about 15 psi at sea level), water enters a unique, supercritical phase. This supercritical water flows like a gas but still is able to dissolve things like a liquid. Supercritical water can dissolve suspended sewage, and with the addition of oxygen, one can oxidize (i.e., burn) the sewage sludge, producing a sterile combination of water, carbon dioxide and salts.

Similar to waste incineration, this supercritical water oxidation (SCWO) process produces heat, despite the presence of water. Some of this heat can be used to preheat the incoming sewage to supercritical temperatures, eliminating the need for an external heat source to sustain supercritical combustion once it is has begun. The remaining hot water may be supplied to the community directly or used to power an electrical generator.

This sounds implausible, almost as if they have created an engine that runs on water. However when you read the background studies , it appears that it has been around since the 1980s and is used now to destroy PCBs and chemical weapons, and there is a demonstration plant running in Orlando, Florida. The system actually throws off excess energy:

The SCWO process only requires an external energy source when starting the unit. Energy is needed at start-up to pressurize and preheat the incoming waste stream to a temperature of about 480°F – 525°F, at which point the reaction proceeds quickly and exothermically (releasing energy). Energy is also needed to pressurize the oxygen supply. Once the reaction is underway, the energy released sustains the reaction and can be harnessed to generate electricity or to heat hot water for bathing. Therefore, since an energy input is only required at start-up, it is desirable to run the unit continuously, to the extent that feedstock availability and maintenance requirements allow.

pilot plant© Pilot plant in China

It's a complicated system; a Chinese pilot plant had to deal with problems of corrosion, plugging, and high operating costs. However the idea of combining an almost traditional waste management and pickup system with a technology that turns the stuff into pure water and a source of energy is pretty amazing. More at Duke University.

It should be pointed out that the same kind of collection system could supply an anaerobic digester that produces biogas and compost, a much simpler system that is being used in many places today. However the Duke system is pretty impressive, so I am not going to be supercritical.

Tags: Bathrooms | Toilets


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