What would happen if everyone in the U.S. flushed their toilets simultaneously?

If all Americans flushed their toilets simultaneously, the country's sewer systems would likely be inundated with an overwhelming amount of wastewater.

November 19 marks World Toilet Day, a moment to reflect on the progress made in modern sanitation. In 2005, fewer than 1% of U.S. households—out of over 124 million—lacked a flushing toilet [source: U.S. Census Bureau]. Meanwhile, 71% of India's population, over a billion people, still lacked this basic facility. By the mid-1990s, the U.S. had around 350 million public and private toilets [source: Flushmate], a staggering number by any standard. But what would happen if every American decided to flush at once to mark World Toilet Day?

Since this scenario has never been attempted before, no one can say for sure what would occur. However, our best guess is: 'It would be a disaster,' says Steve Cox, a wastewater treatment facility operator we spoke with.

In the average American home, the sewer pipes are typically about four inches in diameter. These pipes from your house connect to larger systems in subdivisions, which then merge at the street level. Street systems connect to even broader road systems, all eventually leading to wastewater treatment plants. Beneath the streets, the wastewater infrastructure is intricate, resembling a spider's web.

As you approach the treatment plant, the pipes grow in diameter. A four-inch pipe from your home connects to a 12-inch pipe, and this process continues, with pipes in major cities reaching up to 10 feet in diameter. These large pipes can carry vast amounts of water, but could they handle the pressure of everyone flushing at once?

If every one of the 330,584 households in Milwaukee, Wisconsin, flushed just one toilet simultaneously, each expelling gallons, the city's sewer system would suddenly be flooded with 1,157,044 gallons of wastewater [source: NexTag]. Even with the city's newly installed 108-inch pipes, this could cause issues—and this doesn’t even factor in the public toilets around town.

Many cities have uneven terrain, so to address changes in elevation, sewer systems incorporate lift stations—facilities designed to push wastewater uphill towards treatment plants. These stations would be the first to buckle under the pressure of everyone flushing at once. With too much wastewater trying to pass through the pipes simultaneously, the flow would halt. Sewage already past the lift stations would reverse course, and as these stations flooded, the pipes leading to them would back up.

Eventually, the backed-up sewage would make its way back to where it all started—your home. Backflow valves wouldn’t offer much protection. Not only would your toilet overflow, but so would all wastewater lines in your home, including those for your shower, kitchen and bathroom sinks, and even your dishwasher and washing machine.

Outside, the manhole covers scattered along the street would likely flood and spill over, possibly leaving people standing in sewage, potentially up to their ankles. Depending on the population size and the capacity of the sewer pipes in your city, the situation could escalate even further.

However, with low-flow toilets, the situation wouldn't be as severe.

Low-flow Toilets: A Lot Less Mess

If the residents of Milwaukee conducted the same experiment from earlier, but this time using low-flow toilets, the city's sewer system would only be overwhelmed with 528,934 gallons of wastewater. While this would still create some mess, the situation would stabilize much quicker, and the water pressure would normalize more rapidly, too.

Low-flow toilets were never intended to protect Milwaukee from public health risks or environmental disasters. Their purpose is to conserve water. They have become so effective at fulfilling this role that they've replaced traditional toilets in stores. In fact, it is now a federal law that no toilet manufactured in the U.S. can use more than 1.6 gallons per flush.

When low-flow (or low-flush) toilets were first introduced in the early 1990s, many American consumers were skeptical. They frequently clogged, and even when they didn’t, they often failed to clear everything on the first flush. Some people complained that they had to flush multiple times. If a homeowner needed to flush a low-flow toilet three times, they would end up using 4.8 gallons, which is more than a gallon more than a traditional toilet.

Some frustrated Americans even crossed the border into Canada to buy -gallon toilets, since these weren't available in the U.S. at the time. However, low-flow technology has greatly improved since the 90s, and today's models seem to balance better performance with water conservation.

The U.S. Environmental Protection Agency (EPA) recommends that consumers seek toilets with the WaterSense label. This certification is awarded to toilets that use no more than 1.28 gallons per flush, with their efficiency verified through independent laboratory testing.

The EPA estimates that the average American will flush the toilet around 140,000 times over their lifetime. As water availability has become an increasing concern in the U.S., low-flow toilets are helping to address this growing issue. The EPA is not the only government body encouraging the switch: Some U.S. cities are offering $50 to $100 rebates to homeowners who replace their old toilets with low-flow models. If all Americans switched to WaterSense toilets, the nation could save about 900 million gallons of water daily, which is equivalent to approximately twenty minutes of water flowing over Niagara Falls [source: EPA].

The savings alone (around $90 annually on your water bill) are reason enough for many to make the switch. But don’t forget that if everyone adopted WaterSense toilets, it would reduce the mess caused by everyone flushing at once by more than half. Steering clear of ankle-deep sewage benefits everyone.

For additional details on toilets and sewer systems, be sure to check out the following links.