Reinventing the toilet – from proof of concept to prototype
Tucked away on the Scion campus is a toilet attached to a microwave reactor. This is no ordinary toilet; this toilet could save lives.
The toilet has been designed by Scion and is one of many projects in the Reinvent the Toilet Challenge, supported by the Bill & Melinda Gates Foundation. The toilet looks like your standard toilet bowl, but when you hit flush, a vacuum pump akin to an aeroplane facility sucks the waste away. From there the waste goes through a process that uses wet oxidation (high heat and pressure with the addition of oxygen) to break down solids and kill bacteria and pathogens.
Scion entered the Reinvent the Toilet Challenge in 2015 based on our wet oxidation technology developed for the pulp and paper industry. By 2016 the team had a created a proof of concept showing that wet oxidation could be an effective way to treat human waste. Since then, they have gone on to design and create an entirely new way of treating the business.
Their journey hasn’t been without challenges. The initial proof of concept device used heating coils located on the outside of the wet oxidation reactor. Although it worked, the process wasted energy as it heated up the exterior of the reaction chamber before that heat passed to the material inside. The prototype needed a more efficient way to provide the heat for wet oxidation. Microwaves were identified as a suitable option.
Mixing also had to be incorporated into the reactor using an oscillating platform. Mixing is essential for mass transfer between oxygen and liquid, but the sloshing moves the liquid all over the void in the reactor and some microwave energy then reflects out of the reactor.
With all the specific requirements in mind, the design of process and equipment also needed to fit within the American Society of Mechanical Engineers, Boiler and Pressure Vessel Code VIII specifications. After all, what good is a new high-tech toilet if no one can safely use it?
Mechanical Design Engineer Rob Whitton explains, “Our work on the reactor pushed up against the limits of the code. It was a potential future stumbling block – because you could create a reactor design that no one would sign off as being safe to use.”
In late 2018, the reactor was officially certified as compliant with the design code, and the project took another significant step forward.
Microwaves are good at quickly and directly heating the waste material, rather than the reactor body itself. The microwave reactor can heat the waste material to 260ºC with speed, yet the technology has not been adopted by any of the other parties in the Reinvent the Toilet Challenge so far. “Using a different and unique technology allows us to add something new to the collective knowledge of the toilet challenge parties. We could potentially provide a solution that might benefit another one of the parties,” explains Dr John Andrews, who leads Scion’s toilet team. “We work collaboratively, because we’re all working on the same problem.”
Engineering to the limits of the design code
However, novelty can be a double-edged sword. The engineering team discovered that microwaves need a lot of fine tuning to be efficient. “Microwaves interact in complex ways depending on geometry and the reactor materials used,” explains Dr Muthasim Fahmy, an Energy Systems Engineer at Scion. “Using them with success requires multi-disciplinary engineering skills.”
The high pressure and extreme heat created operating conditions not easily catered for. The team had no choice but to design a lot of their own equipment. They created various versions of reactors, designed holding tanks with stirrers, made mixing mechanisms, and put it together within a tight time frame. All the while ensuring the system was robust enough to cope with any proverbial goldfish that could get flushed down a toilet.
A particular challenge was to find materials for construction that are compatible with microwaves. Initial tests melted a seal in one of the first stage microwave reactors, and that was just the beginning according to Mechanical Design Engineer Gary Campbell, “We’ve had to specifically consider the material of every component that made up the reactor. Even the sensors had to be specially designed to sit in a port on the outside of the main reactor cavity to protect them from microwaves.”
Support from all ends
Some of the other challenges the team had to overcome were slightly less technical in nature. Sourcing enough human waste to test their treatment was a team effort aided by a few Scion staff, but not without some persuasion.
The team recount some funny moments to balance out less pleasant tasks such as daily emptying of the waste tank to store in the biohazard freezer for later use in testing. A dramatic seal failure once caused a jet of brown steam to leave the reactor, and finding the flushing mechanism was switched off when the toilet needed to be flushed has left more than one team member red faced with embarrassment or laughter. But it has all been part of a wider team effort that has been supported by several local service providers.
Rob says, “This is an unusual project – there’s no doubt about that. But the support we’ve had has been incredible. Local providers have given us great service in time to meet our fast approaching deadlines.”
Some special services that need to be acknowledged for going above and beyond include Tailored Controls, Metalform, Stewart and Cavalier Engineers, Lyle Engineering, Microwave New Zealand and Applied Industrial.
The world needs a new toilet
The end goal is to build a sustainable, cheap to use (less than US$0.05 per user per day) and ‘off the grid’ toilet that is safe for humans and the environment. This technology has shown great promise for treating human waste, whether it be in developing countries around the world, New Zealand’s cities, back country, or in global disaster zones. The team is now in the process of commissioning and testing the toilet and reporting its findings back to the Bill & Melinda Gates Foundation.