What Happens To Water After You Flush?

20842725342_ac19f12973_zNYC Treatment Plant

Most of us take water for granted. We turn on the tap, and there it is. But the truth is, by 2025, 63% of the world’s population will be living in water-scarce areas.

Water is rapidly becoming the most scarce human resource on the planet. Cities like Los Angeles and Phoenix already experience sporadic water shortages, and experts say problems like this will only get worse.(1) In fact, the UN predicts that by 2025, 63% of the world’s population, some 5 billion people, will be living in water-scarce areas.(2,3) But the truth is, where our water comes from is only half of the equation that needs to be considered when we’re talking about our water supply.

It’s becoming increasingly clear that we must be just as focused on managing the outflow side of water – where it all goes after we consume it or use it – in order to ensure that we continue to have clean water down the road. Every time we shower, flush, wash our cars, clothes, or dishes, America’s water infrastructure network, which spans more than 700,000 miles, takes over.(4)

How this water is treated and rehabilitated for reuse is complicated, but there are things we as water consumers can do to assist the process, ease the burden on our water systems, and help maintain a healthy water environment. I’ll talk more about what each of us can do in just a moment, but I think those points will make more sense if we first discuss how water treatment systems came to be and how the wastewater treatment process really works.  

I’d like to use New York City as an example. Let’s start  with a bit of history. In the years prior to the development of the city’s first upstate reservoir system in 1842, citizens depended on ponds, streams and wells for their water. There was no system for disposing of garbage, sewage, or human or animal waste. So the frequent epidemics and outbreaks of cholera and yellow fever were predictable. The first wastewater treatment plants were built in New York City in the 1890s.(5)  

By the early 1900s, as New York City’s population reached 3.5 million, the discovery of the link between bacteria-laden water and human disease stimulated a need for a master plan for the city’s water. In 1904, the Metropolitan Sewage Commission was formed and a progressive plan was drawn up. By 1968, there were 12 water treatment plants in the city, and while conditions were better, they were by no means perfect.(5)   

It was not until 1972 that the federal Clean Water Act created minimum standards for wastewater treatment.6  This forced New York City, then managing more than1 billion gallons of wastewater a day but removing only 65% of pollutants, to upgrade its entire system to reach the new standard of at least 85% pollutant extraction.(5) 

Today, some 8 million city residents consume about 1.3 billion gallons of water a day and release 1.4 billion gallons of wastewater. Since 2001, many New York waterways have been deemed suitable for bathing and there have been no beach closings.5 How does our water get so clean? To answer this question, we have to go behind the scenes and underground.  

Once it’s used, New York City’s water travels into the sewer, which is made up of 6,000 miles worth of pipes. From there, the entire water treatment process is overseen by the Bureau of Water Treatment, which has 1900 employees and an annual operating budget of $262 million.(5) The group’s task?  To convert all the water we use in our homes, schools factories and businesses, combined with rainwater and street runoff, into high-quality wastewater that’s suitable for release into waterways and eventual reuse.  

How does the process work? In New York City, once the water hits the sewer, it meets up with runoff rain or snow and a variety of materials caught in street-side catch basins. It all travels underground to 14 plants around the city, and then goes through a seven-hour series of five steps that rehabilitates the water to a quality that would take weeks to be achieved in the natural environment. This five-step process, which I’m about to go into, eliminates up to 95% of pollutants and converts sludge to a biosolids form that can be used as a fertilizer or soil conditioner.(5) 

Step one occurs several stories underground. Everything that makes it to the sewer hits screens that capture large pieces of trash and other material. The garbage is collected and transported to landfills. The remaining sewage is pumped up to ground level surface plants.(5)  

In step two, the wastewater enters sedimentation tanks and over a period of one or two hours, the solids settle to the bottom while lighter materials float and are skimmed away. The settled solids are pumped to sludge-handling facilities. The remaining liquid flows on to step three.(5)  

In step three, air is pumped into the liquid to stimulate the growth of oxygen-using bacteria. These micro-organisms consume most of the remaining organic material that’s polluting the water. This takes three to six hours and leads to a second set of settling tanks that allow solids to settle to the bottom as secondary sludge and be pumped along to sludge-handling facilities.(5)  

Step four is disinfection. The water is mixed with sodium hypochlorite, the same material found in common bleach, and it purifies the water so it can be released into local waterways.(5)  

Step five is sludge treatment. Untreated sludge is 99% water. The sludge is heated to encourage additional bacteria growth and consumption of organic materials. Methane gas is captured and used as an energy source for electricity for pumps and boilers. Spinning centrifuges then remove 90% of the water and the addition of a polymer substance converts the material to a bio-solid “cake.” The biosolids are converted into fertilizers and soil conditioners that are used in parks and golf courses throughout New York.(5) 

This is a 365-day-a-year process, and it will require ongoing investment. In fact, the U.S. Environmental Protection Agency estimates that infrastructure costs for the U.S. water and wastewater systems may be as much as $1 trillion over the next 20 years.1 That investment will ensure safe, clean drinking water for U.S. citizens, and safe, effective management of wastewater, plus disease control and clean recreational waters that are capable of supporting healthy fish and animal species. 

What can we as citizens do to help out and lessen the burden on our water systems? 

First, conserve water. The more we use, the more we must manage. Second, do not put hazardous substances down sinks, storm drains or toilets. Third, don’t litter. Litter ends up with wastewater in treatments plants. Fourth, dispose of household items, like grease, properly. Don’t throw it down the drain. And finally, use mass transportation and practice energy conservation. Toxic chemicals from air pollution end up in wastewater and have to be removed.

The job of wastewater management is large, but it becomes much more manageable if each of us does our part. 


1. Deutsch CH. There’s Money In Thirst; Global Demand for Clean Water Attracts Companies Big and Small. New York Times. August 10, 2006. http://www.nytimes.com/2006/08/10/business/worldbusiness/10water.html?ref=business&_r=0

2. United Nations Population Fund Press Release. “Investments in Reproductive Health Can Reduce Water Insecurity, Says UNFPA Report.” March 19, 2003. Available at: http://www.unfpa.org/news/news.cfm?ID=193&Language=1

3. Magee M. Healthy Waters. New York. Spencer Books. 2006. 

4. DrinkTap.org. “Straight Talk on Water Infrastructure.” Available at: http://www.drinktap.org/consumerdnn/Default.aspx?tabid=198

5. New York City Department of Environmental Protection. “New York City’s Wastewater Treatment System.” Available at: http://www.nyc.gov/html/dep/html/drinking_water/index.shtml

6. U.S. Environmental Protection Agency. “Safe Drinking Water Act (SDWA).” Available at: http://www.epa.gov/safewater/sdwa/index.html