Researchers around the world are working to reinvent the toilet, bringing toilets to the 2.5 billion people worldwide who don't have a safe place to relieve themselves. But there's a slightly gross problem—how do you test a toilet in a sanitary and, ahem, repeatable way?

Enter "fake poop," my preferred term for what scientists call "synthetic sludge simulant." Yes, this is a material meant to simulate fecal matter, and it has to have properties very similar to real fecal matter—minus all the pathogens, odors, and grossness. For this year's Reinvent the Toilet Fair, a new recipe was developed by the Pollution Research Group at the University of KwaZulu‐Natal, South Africa. Their recipe was inspired by a research paper on simulated fecal matter used to test space toilets ("Simulated Human Feces for Testing Human Waste Processing Technologies in Space Systems," SAE Technical Paper 2006-01-2180, 2006, doi:10.4271/2006-01-2180). The Pollution Research Group developed a series of recipes, finally settling on the ninth one. Here's the breakdown of what's in "Synthetic Sludge Recipe Number 9":

1. Instant Yeast

Plain old store-bought yeast packets you'd use to make bread.

2. Psyllium Husk

Seed husks that provide mucilage, described by Wikipedia as "a thick, gluey substance produced by nearly all plants and some microorganisms." You can often buy this in the bulk aisle at grocery stores.

3. Peanut Oil

Just what it says on the tin.

4. Miso Paste

The fermented seasoning used in many Japanese recipes. Available in packet form from grocery stores.

5. Polyethylene Glycol (PEG) 400

PEG is used in all sorts of applications (including skin creams and toothpastes), but if you've ever had a colonoscopy, you'll recognize it as that gloopy stuff you have to drink to, ahem, clear out your system. Well, technically, the colonoscopy prep material is PEG 3350, the number referring to the material's molecular weight as measured in daltons.

PEG 400 is actually available on Amazon, and it's an ingredient in many non-fake-poop products, including inkjet printer ink.

6. Inorganic Calcium Phosphate

Often used as a leavening agent in baking, Calcium phosphate is also used in some cheese products. If you're looking to follow the recipe exactly, you'll want to buy this from a chemical supply company.

7. Cellulose

The recipe calls for cotton linters, a byproduct of cotton harvesting and an ingredient in paper manufacturing. The recipe also includes plain old paper tissue, shredded.

8. Water

What's a recipe without a little water? (Actually, a lot of water -- see the mass numbers below.)

What it Looks Like

The resulting product was "very sticky," according to a person close to the matter. It reportedly has a "vinegar-yeasty smell." Let's just look at a photo of the fake poop being bottled and leave it at that:

Photo courtesy of The Bill & Melinda Gates Foundation.

Putting it Together

Here's the breakdown when it's all put together:

Ingredients % Wet Mass Mass for 1kg % Dry Mass Instant yeast 7.3 72.80 32.49 Water 77.6 776.10 -- Psyllium 2.4 24.30 10.84 Peanut oil 3.9 38.80 17.31 Miso paste 2.4 24.30 10.84 PEG 2.7 27.20 12.14 Inorganic Calcium phosphate 2.4 24.30 10.84 Cellulose (half cotton linters/half shredded tissue) 1.2 12.40 5.53 Total Mass 100.0 100.20 100.00

This year's fake poop was manufactured by Unilever and donated to the Reinvent the Toilet Fair: India. Unilever also makes the toilet cleaner Domex, and is holding a Domex Toilet Academy aiming to build 24,000 toilets by 2015.

Fake urine, on the other hand, can be less complicated. If you're just testing fluid flow in a toilet, plain old water will work. (For applications involving chemistry, well, let's just say that's a whole different story.) Here's an image from the Reinvent the Toilet Fair showing a little fake urine going in the loo:

Photo courtesy of The Bill & Melinda Gates Foundation.

For a bit more fake poop fun, check out this TEDx Talk by Professor A.J. Johannes, who didn't use this recipe for his fake poop. He said, "Mashed potatoes, curiously enough, are very, very similar [to human feces]. I know, I know."