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November 19 is , but if you were able to celebrate it, you should consider yourself lucky. Worldwide, about 2.5 billion people – a third of the global population – have no access to safe sanitation.

This lack of hygiene is linked to the spread of many preventable diseases, such as diarrheal diseases that kill more than 500,000 children under the age of five every year.

Since 2011, a team of U of T engineers under the direction of Professor Yu-Ling Cheng, director of the Centre for Global Engineering, has been developing a solution. They took up a challenge issued by the Bill & Melinda Gates Foundation to design a toilet that could disinfect human waste without connections to water, sewer or grid power. Its total cost had to work out to less than five U.S. cents per person per day, and it needed to be designed for users in the developing world.

Cheng and her team were among the winners at the first Reinvent the Toilet Fair back in 2012. Since then, with continued support from the Gates foundation, they have been working to develop the disinfection process as well as building and refining their mechanical designs. Writer Tyler Irving spoke with Cheng.

Your team presented initial proof-of-concept ideas at the original Toilet Fair in 2012, and have been working since then to develop your ideas. Can you update us on your progress?
Things have progressed a lot over the past three years.  We created a first integrated prototype, as opposed to the proof-of-concept modules we showed at the time of the first Toilet Fair. That prototype was presented at the second Reinvent the Toilet Fair held in Delhi, India in March 2014.

With continued funding from the Gates foundation, we further refined the design of both the process and the prototype, making it smaller and more energy efficient.  We have made a great deal of progress, and have recently been awarded additional support to take us to the point of being able to conduct large scale field trials.

How close do you think you are to a finished product?
I think we’re pretty near completion in terms of designing the technical process. I don’t expect major revisions, but there are things we need to tweak. What we will focus on going forward is to improve robustness and lower costs.  In other words, what happens if we run our system for a long time? Does every part still work?  Can it stand up to people using – and misusing it?  Can we handle variability in user inputs – such as big spikes in the amount of material to process?

How has your design changed since 2012?
At a very high level, the process hasn’t changed very much.  We separate solids and liquids, burn the solids to generate heat, and use the heat to dry new incoming solid waste and pasteurize liquid waste.   But there have been many advances in the details – these have allowed us to reduce the prototype size by a factor of four – from about 4 m3 to 1 m3, and to lower energy requirement by a factor of 5 to about 10 watts on average.

How were you able to shrink the size and the energy requirement?
Our first prototype was based on a batch process.  We collected a whole day’s worth of waste from a household – which for us was defined as 10 people – and then processed it all at once.  

We have since evolved to a continuous process. That means we process small amounts at a steady rate all the time – 24 hours a day, and for as long as we have fuel to feed the process.  A system that operates continuously can be much smaller than a batch system, as long as you control the rate at which you’re feeding in new material.  

In addition, we only need to provide the energy needed to ignite the process at the beginning of the process – which is a big saving in energy. That initial ignition energy cost is amortized over weeks and months. It becomes a very small energy cost.

We have also improved the way we manage heat.  Most significantly, we have eliminated some major inefficiencies in how we dry incoming solids. This sounds like a simple problem, but it has not been easy for us as well as a number of other groups.

Where are you going next?
Going forward, we’re starting to think about how users will interact with it. This is a key challenge: if people require special training to run it, then it won’t meet their needs, so it has to be easy to operate and maintain. It also needs to be robust, and be responsive to changes in input. For a household size of 10 people, we need to process on average about 30 grams of dry fecal mass per hour.  But not every household is that big, and not everyone will be home all the time.  

And we also have to accommodate increased input – parties, weddings, etc. So we have worked out ways to control processing rate over a wide range.

We will do some simulated user studies in our lab in Toronto, probably with the next iteration of the prototype, or something pretty close to it. It’s a squat toilet interface, so not what people here are used to, but it will give us more data on how it responds to variable input.  

After that, the next step will be setting up field trials in India or another developing country. That will involve selecting a field site, making sure we have access to users, community approval, access to machine shops for troubleshooting, access to labs for testing, etc. We are planning those studies for August 2016. 

What drives you and your team to keep working on this project?
What many of my team members have told me is that they like working on this project because it is so purposeful.

The Re-invent the Toilet Challenge set out some very demanding specifications, so it should not be surprising to see some approaches fail. With all the technical advances we have made, I think we have a good chance of succeeding.  

But even if we don’t get to our final goal, I would much rather not quite reach the final goal working on such a potentially impactful project than doing something else that’s less meaningful.

This interview has been condensed and edited.