Today I took a tour of my local wastewater treatment facility. I recommend everyone does this. Wastewater treatment plants truly are marvels of modern engineering. And you really do need to know what happens to your poop after you flush. When I think about how far we’ve come since the 1800s when we threw our poop out the window into the street, when our waste mixed with our drinking water, and when disease from unsanitary conditions was rampant, to now—it’s amazing how far we’ve come.
Even though no one wants to think about what happens to our human waste, it’s important to acknowledge all the people who make our sullied waters clean enough to be gurgled back into our rivers and lakes again. But wait! The engineers, technicians, and maintenance people working at wastewater treatment plants are doing more than just making sure our communities are healthy and our most precious natural resource is safely recycled. My local wastewater treatment plant is running almost entirely on the power of… poop! That’s right, they’re using the poop to power the plant! It turns out, our poop is a largely untapped resource.
My local wastewater treatment plant is running almost entirely on the power of… poop! It turns out, our poop is a largely untapped resource.
Wastewater itself can have up to 10 times the amount of energy it takes to treat it. It seems incredibly wasteful to think that we’ve been using energy from the fossil fuel grid to process something that already has energy in it. The energy in wastewater comes in the form of organic matter and nutritional elements, like nitrogen and phosphorous, and thermal energy. That energy has been going to waste for a really long time.
There are many ways of turning wastewater into energy (more than I’ll mention here). First, hydropower can be harnessed by taking advantage of the physical flow of the influent (the water coming into the plant) and/or the effluent (the water flowing out of the plant). Or we can use wastewater to power microbial fuel cells. Or (and this is my favorite) if you let organic waste, or sludge, decompose in an anaerobic environment, it lets off methane, aka biogas.
But before we get to that, let’s talk a little about the wastewater treatment process, because before we get to the sludge we have to treat the water. Some municipalities have sewer systems dedicated only to brown water (the water that leaves our homes and places of business), others have combined stormwater and sewer systems. Either way, sometimes plastic, large pieces of paper, and other debris (don’t even think about flushing those “flushable” wipes) gets washed into the system and needs to be removed before the water gets processed. That’s the first step in wastewater treatment.
Then, we filter out our settleable solids. Things like sand and grit that settle out to the bottom of a tank full of wastewater. And we skim off the fats and oils that float to the top.
Now we’re ready to let some good aerobic, or oxygen-loving, bacteria go to work. They feast on our waste, breaking down the organic matter. When they’re done, we’re left with two things: water that is about 85% free of organic material, and sludge—a brown slurry of organic solids and sated bacteria that need to loosen their belts because they ate so much.
That sludge is thickened and sent to the final phase of solids treatment—the anaerobic, or oxygen-free, digester.
When organic waste decomposes in an anaerobic environment, biology and anaerobic bacteria do their thing and we’re left with methane. Methane is a potent greenhouse gas. We can either flare the methane, releasing the less potent carbon dioxide, or harness the methane to do work for us. The methane, or biogas, can be harnessed on-site to power the wastewater treatment plant. And as a bonus, the leftover sludge can be used as a soil nutrient supplement. Smaller facilities can add restaurant grease to supplement the sludge in the anaerobic digester to create more methane. Larger treatment plants can supply energy back to the grid.
To finish off the journey through our wastewater treatment plant we need to disinfect the water that was separated from the sludge before it can be released back into the natural water system. This can be done with chlorine, but wastewater treatment plants are opting for non-chemical ways of disinfecting the water. My local wastewater treatment plant opts for UV light which disrupts the DNA of the bacteria in the water and prevents them from reproducing. Another option is using ozone for the disinfection process.
Wastewater-to-energy systems provide renewable/sustainable energy, economic benefits in terms of savings on purchasing fossils fuels to power treatment plants, and reduced emissions.
The whole process of treating wastewater can, amazingly, be done without using any chemicals. Natural processes, gravity, and simple technology work wonders in these engineering marvels that keep us healthy, safe, and clean. And, with the help of energy from our poop, they could soon be supplying our electricity needs.