What happens after you flush the toilet?

(FEBRUARY 3, 2022) An average person makes about 46 gallons of wastewater every day. So what happens to wastewater after it leaves your home, school, or business? Often the local government cleans the water. Here is what happens in Portland.

Wastewater is sometimes called "sewage". It comes from showers, toilets, sinks, washing machines, and other drains. It usually has a mixture of dirt, soap, food, oil, and, of course, human waste from the toilet. The informal words for the human waste we put in toilets is "poop" (solid waste, also called feces) and "pee" (liquid waste, also called urine). 

Portland processes more than 70 million gallons of wastewater every day. Most of it travels by gravity through pipes to the Columbia Boulevard Wastewater Treatment Plant in North Portland. This facility uses a mixture of biology, chemistry, and physics to clean the water. At the same time, it generates energy and recycles nutrients to help the soil.

The water is moving fast when it arrives at the facility. Screens and other equipment remove large things, like sticks, rocks, and litter. Those things go to a landfill. Then the water slows down in large tanks. Solid materials sink to the bottom or float to the top.

Next, the water moves to another tank. There it is mixed with air. This helps trillions of microorganisms (very tiny bugs) eat the waste in the water. After that, the water moves to another tank where it slows again. More solid material are removed. Sensors and cameras are always monitoring the water's movement. Workers often take samples to test the quality of the water.

Finally, the plant uses chemicals to kill bacteria. After the chemicals are removed, the water flows into the Columbia River and out to the Pacific Ocean. All of this happens within about 24 hours of water leaving your house.

Meanwhile, the solid material is taken to another building. It is heated. This helps other microorganisms grow. They eat some of the waste. The bugs make biogas, which is collected and used to produce renewable energy for vehicles. After about a month, the final material (called "biosolids") is sent to Eastern Oregon to help grow grass for animals.

In order to help keep the system as efficient as possible, the city recommends flushing only "the three p's" in the toilet: pee, poop, and paper. People should not flush things like baby wipes, leftover medications, or fats, oils, or grease from the kitchen. This keeps the system flowing smoothly.