No matter who you are or where you come from, you are most definitely interested in the slightly disgusting bodily functions of our flawed, weird, fascinating species. Fortunately, so is science, and a bit of mathematical rumination sometimes reveals some startling pieces of information.
We at IFLScience have already shown that peeing in the shower may help save the environment somewhat from exorbitant water waste. Continuing on this thread, is there anything we can do with our poop in order to help the environment? (No, not that.)
Sometimes, it’s used in composting, but what would happen if our poop was shipped off to a combustion engine and burned in order to produce energy? Man-made climate change is here and it’s messing everything up, from the natural world to the global economy. Would burning our own biological waste assist in alleviating this problem?
Let’s dive into the science of poop to find out – but it’d be best to hold your nose as we go.
What’s in your poop?
Feces, the semisolid metabolic waste product of the human digestive system, come in many forms, but on average, they contain mostly water (75 percent of the total volume). The rest is comprised of dead and living bacteria, protein, fiber, cellular linings, fats, salts, mucus released from the intestines, and some additional extras – perhaps paper still attached to the sweet you hastily unwrapped and consumed.
As you’ve probably guessed, burning water is somewhat tricky, and any poop needed to be ignited needs to have this water removed. In order to conserve energy, let’s assume that it is sun-dried, at no additional energy cost.
The average human adult produces anywhere between 100 and 250 grams (0.22 and 0.55 pounds) of fecal matter per day. Children produce less per day, but it’s not clear what the average is, so for the general human average, we’ll take the lowest number, 100 grams, for our calculations. This means that 25 grams (0.055 pounds) of each poop is solid matter – the stuff we will eventually be burning.
Don't breathe this. Credit: fluke samed/Shutterstock
Stealing the sugar
So what of this solid brown mess can be burned to produce decent amounts of energy? Technically, burning anything produces energy, but some things produce more energy than others, and the amount of energy expunged depends on how efficient the combustion is.
Carbohydrates, many of which are found in poop, are made of carbon, hydrogen, and oxygen. Our body converts carbohydrates into glucose (sugar), which is primarily used to power cells. Ultimately, it is the high-energy-density glucose that we want here.
Carbohydrates also take the form of starch – many sugar units bonded together – and fiber, the latter of which only comes from plant-based foods. In the body, unlike other carbohydrates, fiber cannot be converted into sugar. Chemically speaking, however, if this fiber is made of cellulose (and much of it is), then it can be – it’s a type of polysaccharide, which means it's a molecule made of many interconnected glucose molecules.
However, almost all starch and sugar will be used by the body and not pooped out. Any glucose not immediately needed is converted into fat for long-term storage, and some is excreted as poop, but it cannot be converted into glucose again, and the body uses it as a fuel in a very different way. Conversely, dietary fiber isn’t digested by the human body, so all of that is excreted. For our poop combustion engine, then, we can use this expunged fiber.
So what proportion of our poop is made of fiber? Out of our 25 grams of solid matter per daily poop, as much as 30 percent is made of fiber. So that’s 7.5 grams (0.017 pounds).
Dietary fiber mostly comes from food like this. Credit: Oksana Shufrych/Shutterstock
Burn baby burn
Let’s say that in our hypothetical system, we have an artificial pancreas that produces enzymes that are able to break down dietary fibers into sugar molecules. Our poop-extracted fiber will, for the sake of simplicity, be completely converted into burnable glucose by these enzymes.
So, at this point, this means that we have 7.5 grams of glucose per poop. Glucose burns in a plentiful oxygen supply to produce thermal energy, water, and carbon dioxide. Per gram, it produces 14.2 kilojoules of energy. This amount of poop-extracted goodness will therefore produce 106.5 kilojoules of energy.
The current population of the world is 7.125 billion. If they all had a 100-gram poop, the 7.5 grams of fiber could be removed and converted into glucose, and if this glucose was burned, this would produce a total of 759 billion kilojoules of energy every single day. In a year, this would total roughly 277 trillion kilojoules of energy.
Based on information given by the US Energy Information Administration, the annual electricity consumption for the average American home was 39.4 million kilojoules in 2014. This means that our poop-powered electricity engine could power 7.03 million US homes every single year.
That may sound like a lot, but considering that we’re using all of the human poop in the world here, that’s not much at all. There are roughly 125 million households in the US alone, so the global poop production would fuel just 5.6 percent of all US households.
There’s another way
There are some obvious problems with this method of energy generation, aside from the fact that it doesn’t produce that much energy at all. Many assumptions have been made, and the final process will likely produce less energy and cost more energy to set up and run.
You may have spotted earlier that carbon dioxide is produced during the combustion of glucose. This is the problem with all fossil fuels and biofuels – carbon dioxide is a common byproduct, and this is the primary driver of man-made climate change. Nuclear, hydro, wind, solar, and geothermal power have a near-zero carbon footprint and are far better at generating energy than burning poop.
There is an alternative method of producing energy from poop, and some places around the world already do this. In the absence of oxygen, certain microbes break down organic material (such as poop) into carbon dioxide and methane (among other things), a mixture known as biogas.
This methane can then be burned and used for additional energy, but this also produces carbon dioxide. Hopefully, the carbon dioxide can then be stored and kept from escaping into the atmosphere, but this is an entirely different kettle of fish.
If, however, we want to mitigate man-made climate change and produce cleaner energy, we should do two things – focus on using nuclear and renewable energy sources and investing in nuclear fusion research. Burning your poop, we’re afraid, just isn’t going to cut it.
That's the sort of wind power you should invest in. Credit: zhangyang13576997233