What Happens After We Flush?

How many times have you flushed the toilet today? Once? Twice? More? Now think about how many times your whole family flushes each day. Multiply that by everyone in your neighborhood, your city, your country—and then the entire world. That is a lot of pee and poo going down the drain!

When you flush, it might feel like your waste just “vanishes”. But it does not really disappear. Every flush adds to a massive river of used water from toilets, sinks, and showers called wastewater. Wastewater flows through underground pipes that carry away everything we send down the drain. The world produces more than 350 billion m³ of wastewater each year, from homes, schools, factories, and farms—enough to fill about 140 million Olympic swimming pools [1]!

Most wastewater travels to treatment plants, where machines and microbes work together to clean it. Treatment removes solid waste, harmful germs, and excess nutrients, making the water safe enough to return to the environment. However, not every region has not every region has enough wastewater treatment plants, and some places have none. Globally, only about half of all wastewater is treated, and about 3.5 billion people still live without safe sanitation—meaning systems that safely collect and treat human waste. Around 8% of the world’s population still has no toilets at all and must pee and poo outdoors. In many parts of the Global South, especially in rural areas, wastewater flows directly into rivers or the ocean without being cleaned, spreading pollution and disease.

Even where treatment plants exist, cleaning wastewater is expensive and uses huge amounts of energy—roughly as much as all the world’s airplanes [2]! So, wastewater is not only a sanitation challenge, it is also an energy problem. Treating water keeps people healthy, but it relies on electricity often produced from fossil fuels, which adds to greenhouse gas emissions. Scientists are now asking whether wastewater itself could help solve this problem.

Is There “Good Stuff” in Wastewater?!

It might sound strange at first, but what we flush away every day may actually be useful! Scientists are starting to see wastewater as a mixture of energy, nutrients, and clean water—three things we constantly need, all waiting to be reclaimed (Figure 1). Instead of spending money and energy to destroy waste, maybe we can recover energy and useful materials from it, turning one of humanity’s biggest problems into a valuable resource and helping to protect the planet in the process.

But how can anything helpful come from pee and poo?!

Bacterial Pee and Poo Power Plants

Wastewater is a mix of solid and liquid waste: poo, pee, food scraps, and other materials washed down the drain. These materials contain organic matter—carbon-based substances that store chemical energy, which bacteria and other microbes can use as fuel. Most bacteria simply use that energy to grow, but scientists have discovered that certain microbes, called electrogenic bacteria, can create an electric current when they digest the energy-containing substances in wastewater (Figure 2A). As they do this, they release tiny, charged particles called electrons. When electrons move through a wire or a circuit, they create a flow of electricity.

Scientists can capture that energy using systems called microbial electrochemical technologies (METs), which collect the electrons the bacteria produce. Inside METs, electrogenic bacteria grow on a metal plate called an electrode, which allows electricity to flow through it. As wastewater passes through the MET, the bacteria feed on the waste and send electrons through the electrode, creating an electrical current—almost like a natural battery that runs on waste.

There are two main types of METs. Microbial fuel cells (MFCs) are METs that make electricity directly from wastewater (Figures 2B, 3A), meaning they can power small devices or lights as the bacteria break down waste through chemical reactions that release electrons [3]. In contrast, microbial electrolysis cells (MECs) are another type of MET. They need a small extra push of power, but in return they perform chemical reactions that produce energy-rich gases such as hydrogen or methane, which can be stored and used later as fuels (Figure 2C). In both cases, as the bacteria feed on the waste, they help clean the water at the same time.

Worldwide, the “hidden” energy in wastewater adds up to roughly one-fifth of all the electricity produced in the United States. If even part of that energy could be recovered, wastewater could shift from being a global burden to a global resource.

Fertilizer From Flushes

Wastewater also contains valuable nutrients such as nitrogen and phosphorus, which plants need to grow. The nitrogen fertilizers that farmers put on their crops are usually made using fossil fuels, and phosphorus is mined from rocks deep underground. Making and transporting these fertilizers takes a lot of energy and can release greenhouse gases that warm the planet. Mining for phosphorus also damages land and leaves behind waste that can harm ecosystems.

If we can recover nitrogen and phosphorus from wastewater, farmers could use these recycled nutrients to grow crops. This could make farming more sustainable in two ways: it would reduce the need to make new fertilizers from fossil fuels and mining; and, if used in places without sufficient wastewater treatment, it could help stop nutrients from untreated wastewater from leaking into rivers and lakes. Excess nutrients in water can cause algae to grow too much, using up oxygen and killing fish.

METs can help reclaim nutrients. As the bacteria inside a MET break down waste and release electricity, they also change the chemical balance of the water. These changes make it easier to capture valuable nutrients. Ammonia (a form of nitrogen) can move across a thin barrier called a membrane and collect as liquid fertilizer. Phosphate (a form of phosphorus) can stick to the metal electrode and build up into tiny crystals that can later be removed and reused on farms.

Clean Water in Dry Places

The third valuable part of wastewater is the water itself. Freshwater is limited—less than 1% of the water on Earth is available for people to drink, irrigate crops, or use in homes and industries. As Earth’s population grows and droughts become more common, finding ways to reuse water safely is becoming more important than ever.

METs can help clean water as the bacteria break down the waste that makes it dirty. While METs cannot remove every pollutant on their own, they can greatly reduce the amount of solid waste and harmful substances, making the remaining water much easier and cheaper to treat. Cleaned wastewater can be reused in many ways: to water crops, flush toilets, or cool power plants. After careful treatment, it can even become drinking water again.

In places with little rainfall or where rivers are drying up, reusing water helps communities save precious freshwater and stay better prepared for dry seasons. Using METs to clean water could make safe water reuse more affordable, especially in areas without large treatment plants.

From Toilets to Technology: Putting Waste to Work

While many METs are still being developed in labs, several are being tested in real life. One of the best-known examples is Pee Power^®, a project that uses microbial fuel cells to turn urine into electricity. At the Glastonbury Festival in the United Kingdom, a large Pee Power^® urinal let thousands of people help make power each day—just by emptying their bladders! The electricity was used to light the toilets at night, charge mobile phones, and even run a “Pee to Play” station where visitors could power old-school video games with their own pee! At the same time, the system cleans the urine and produces fertilizer as a by-product. The same technology has also been introduced in schools and community toilets in Uganda, Kenya, and South Africa, making sanitation safer and brighter in places without reliable electricity (Figure 3).

Some projects combine METs with hydroponics, which is a way of growing plants without soil [4]. In these systems, bacteria in the METs break down waste in the water, turning it into simpler nutrients that plants can use. The water then flows to the plants, which take up the nutrients and help remove any remaining impurities. Plants also release oxygen through their roots, which supports the bacteria that keep cleaning the water. The electricity produced by the METs can even power small lights to help the plants grow. This partnership between microbes and plants can produce both clean water and fresh food.

A similar idea is used in shallow, man-made ponds called constructed wetlands, where plants and microbes work together to clean wastewater in much the same way.

In countries that already have wastewater treatment plants, METs might have the most potential as a part of these existing systems. By adding METs early in the treatment process, engineers can recover some of the energy stored in wastewater before it goes through later cleaning steps. The energy that METs produce can replace some of the power these plants normally use, helping them save money and protect the environment.

What is Next For Wastewater?

Together, these examples show how wastewater can be treated not just as a problem to manage, but as a resource that can provide energy, nutrients, and clean water through the work of microbes. Turning pee and poo into power might sound amazing—and it is—but these technologies still have a long way to go before they can be used everywhere. Right now, METs are expensive to build, and the electrodes need to be tougher so they can last for years in harsh, waste-filled water. Scientists around the world are improving designs, testing new materials, and finding ways to lower costs.

The amount of electricity METs produce is still small compared to the huge amounts of energy needed to run large treatment plants. For now, METs work best in smaller or special situations—like places without electricity that need low-cost sanitation or lighting. METs could be especially useful in many parts of the Global South, where reliable power and sanitation are often limited. There, METs could provide clean, well-lit toilets and make these facilities safer to use at night, especially for women and children.

The idea of recovering energy, nutrients, and clean water from wastewater is part of a bigger vision called a circular economy—a system where nothing is wasted and everything can be reused or returned safely to nature. It will take time and teamwork between engineers, biologists, and communities to make this future real, but every new experiment brings us closer. One day, the water we flush away might help power homes, charge devices, and grow food—all thanks to the tiny microbes that clean up our mess.

Glossary

Wastewater: ↑ Used water that comes from toilets, sinks, showers, factories, and farms. It contains waste, germs, and nutrients that must be removed before the water can be safely reused.

Sanitation: ↑ Systems and services that keep water and environments clean by safely collecting and treating human waste, helping to prevent disease and protect health.

Electrogenic Bacteria: ↑ Bacteria that can release tiny charged particles called electrons while feeding on waste, which can be captured to create an electric current.

Microbial Electrochemical Technologies (METs): ↑ Systems that use electrogenic bacteria to break down waste and release electricity, helping recover energy, nutrients, and clean water from wastewater.

Microbial Fuel Cells (MFCs): ↑ A type of MET that makes electricity directly from wastewater as bacteria feed on organic matter.

Microbial Electrolysis Cells (MECs): ↑ A type of MET that uses a small boost of electricity to turn wastewater into energy-rich gases like hydrogen or methane, which can be burned as fuels to produce electricity.

Hydroponics: ↑ A way of growing plants without soil, using nutrient-rich water to feed the roots directly.

Circular Economy: ↑ A way of making and using products that keeps materials in use for as long as possible. In a circular economy, things are reused, recycled, or safely returned to nature.

Conflict of Interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

I wish to thank Dr. Susan Debad for providing me with a first draft and continued collaborative input as co-author. I would also like to thank the coauthors of the original manuscript: Falk Harnisch, Elizabeth Heidrich, Ioannis A. Ieropoulos, Bruce E. Logan, Dibyojyoty Nath, Deepak Pant, Sunil A. Patil, Sebastia Puig, Jason Ren, Ruggero Rossi, Amelia-Elena Rotaru, and Annemiek ter Heijne.

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