As It Happens

Engineers create a toilet bowl coating so slippery that 'human waste can't stick to it'

Tak-Sing Wong and his team of mechanical engineers at Penn State have created a new coating that makes toilet bowls so slippery, almost nothing sticks — including, and especially, human feces.

Engineers at Penn State have created a new, anti-stick coating for toilet bowls to save water when cleaning

Tak-Sing Wong and his laboratory team at Pennsylvania State University with a toilet bowl covered in a non-stick coating they developed. (Submitted by Tak-Sing Wong)
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For years, Tak-Sing Wong and his mechanical engineering team at Penn State University have worked to make everything from windshield wipers to pipelines less "sticky." 

But recently, he was approached with a new challenge, one that proved as familiar as it is foul — the toilet bowl. 

Wong and his fellow researchers have created a new coating that makes toilet bowls so slippery, almost nothing sticks to it, including, and especially, human feces. 

He spoke with As it Happens host Carol Off about their findings, which were published this week in the journal Nature Sustainability. Here is part of their conversation.

So what got you into the business of trying to figure out how to make poo not stick?

It all started back in 2015 when we were approached by a research group at Cranfield University [in England]. Back then, they were developing a toilet for the developing world, through the support of the Bill & Melinda Gates Foundation [and its] "Reinvent the Toilet Challenge."

One problem they had back then was that the human waste — particularly human feces — are very sticky to the toilet surface. So that's why they reached out to us to see if we can have a solution, because my research group at Penn State has been known for developing slippery surfaces to solve sticky problems.

Did you think, though, when you got into this business, this kind of engineering work, that you would be asked to try and figure out how to deal with sticky feces?

Not really. Indeed, in the beginning we kind of underestimated the complexity of this problem. And we didn't realize human poop is actually really sticky. 

But now you know that for a fact. 

Oh, now we know that. We actually know many things about poop now.

Besides the ick factor, what is wrong with human poo being so sticky?

I guess everyone wants to have a clean toilet.

But because human feces [is] composed of liquid and solid components, that makes it very easy to stick on pretty much any surface.

And as all of us know, human poop, it comes in different forms. And there's actually the Bristol stool scale that categorizes the poop into seven categories: the Type 1 poop is the solid poop, and the Type 7 is the liquid poop. And anything in between — Type 3 to Type 4 — are the healthy human feces. And those are among the stickiest poop we have.

The special coating repels human waste and bacteria. (Submitted by Tak-Sing Wong/Penn State)

I'm sorry, but I guess I just can't imagine what it is to have to study this much to know that there are seven categories of feces, and that the human stuff is just like glue. It's just a gooey gop, right? That's your problem.

We need to know this. Because, as an engineer, if we wanted to design a surface to repel human feces, we need to understand the physical properties of that. That's why we kind of go into depth just to understand the physical properties of that and how we can test that in the lab. 

Indeed, we didn't start with the actual human feces in our test. We actually used synthetic poop to test our surface, because if I asked my graduate students to test with real human waste, they probably wouldn't be happy about that.

So what is synthetic poop?

We got this recipe to create synthetic poop from our collaborators at Cranfield University, who got this recipe from their collaborators [in] South Africa.

This synthetic poop consists of seven chemical components, which include yeast, peanut oil and miso, among other components. And you can mix them in a specific ratio such that you can emulate the physical properties of the actual human feces.

Tak-Sing Wong is a professor of mechanical and biomedical engineering at Penn State. (Submitted by Tak-Sing Wong)

OK. So you've got your synthetic poop. I guess it doesn't stink and that's another advantage. But ... you're trying to develop a surface on toilets that will be less sticky, right? So you came up with a coating, is that right?

That is correct.

In essence, we created a slippery coating that you can put on ceramic or glass that can repel human waste — including urine, feces and bacteria.

To what effect does it allow a user to use less water in this toilet with this coating?

Human waste can't stick to it, so it wouldn't [leave] any traces of fecal streak or waste. ... And you do not need that much water to clean off the surface compared to [an] uncoated surface.

And applying this coating to an actual toilet — of course, like, a full-flush toilet — you need a certain amount of water to move the waste down the drain line and into the sewage system, and we actually measured that in the actual toilet to see how much volume of flush water is needed to do that. 

So for a typical 1.6 gallon per-flush toilet — which is equivalent to six litres per flush — we find that one would need to take about 50 per cent of the flush volume in order to get the waste down the drain line into the sewage system. So that 50 per cent of water, you cannot save.

However, for the rest of the 50 per cent — which is about 0.8 gallons per flush — you could save most of it, because now nothing is sticking to the toilet surface.  

You're saving half the water of a flush. 

That's correct. 

So you're trying put the toilet cleaning people out of business, I guess.

Well, I wouldn't say that. But I think one of the motivations we have is really to try to minimize the use of aggressive chemicals ... people use to clean the toilet, and also maintaining the toilet self-clean. 

I think everyone will be happy.


Interview produced by Morgan Passi. Q&A edited for length and clarity.

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