top of page

After the FLUSH: Sewers via the Yonkers Wastewater Plant

Updated: Jan 29

One of the most common questions we get from the public when we attend events is, "What happens when I flush my toilet??" The beauty of this question is that it depends on where you are and how your system works! When we think about how toilets generally work, we often point to the sanitation value chain to explain it simply (see image below). But it may not help you understand how exactly your toilet works.

After the flush, toilets should generally go through this step-by-step process. (Credit: Unknown Author under CC BY-SA)

So, FLUSH wants to help explain what could happen after your toilet…flushes. In our new blog series, "After the FLUSH", we will explain how your poo and pee are managed by looking at specific places to make it easier to understand.

This Edition: Sewers & Treatment Plants

In our first in this series, let's see what happens when your toilet is connected to sewers with a treatment plant. As we have explained in previous blogs, sewers are simply systems of underground pipes that take your flushed waste from your home to (ideally) a treatment plant. Sewers largely work through gravity, with a few pumping stations to help move waste too far down back up to continue pushing towards the plant. But once we are out of the sewers, what happens??

Recently, FLUSH got to join our friends NYC H2O for a private tour of the Westchester Wastewater Treatment Plant (WWTP) in Yonkers, NY (US)! The plant's team showed us the different steps they go through to make the waste from our sewers into clean water that can go back into the Hudson River.

The Yonkers WWTP has a lovely view of NYC and the GW Bridge. (Credit: FLUSH/K Worsham)

Intro to Yonkers & Westchester

Westchester County in NY state manages seven WWTPs for nearly 1 million people. The Yonkers plant is the biggest out of the seven, serving about 500,000 people and receiving about 145 million gallons per day (MGD - but can take up to 285 MGD). It's a publicly-run plant, which means that the staff there are all civil servants.

Yonkers receives only wastewater received from county sewer lines. If a household in the area uses a septic tank, emptying trucks can empty into a maintenance hole for treatment.

Don’t dump stuff into the storm drains, please. (Credit: FLUSH/K Worsham)

The system does have combined sewer overhaul (CSO) systems, which we won't get into for this blog. Yonkers has a CSO system that treats rainwater excess simply before pushing it out to the river.

How the WWTP Works

Generally, wastewater treatment plants go through processes that rely on gravity to separate wastewater water from the solids. The ultimate goal is to clean the water from wastewater enough to put it back into the environment and, thus, the natural water cycle. Below is a simplified process for how the WWTPs function – though more involved, these are the main elements.

A simple WWTP system. (Credit: FLUSH/K Worsham)

Let's briefly walk through these steps by seeing how the Yonkers WWTP works!

STEP 1: Screening

The first three steps are the same for all incoming wastewater. First, an initial screening process pulls out all of the big bits of trash that can gum up the treatment system. This is where all of those wet wipes, t-shirts, and chip bags that shouldn't be in the sewers get caught and removed.

Wet wipes are a growing concern; though they are still labeled flushable, they really should never be flushed because it mucks up the whole system. But, don't worry – there are efforts to stop "flushable" labels.

The screening process takes out stuff you definitely shouldn’t have put down your toilet. (Credit: FLUSH/K Worsham)

The screened-out stuff goes to a landfill out of state for this plant.

STEP 2: Primary Treatment

After screening out big things, wastewater goes to the primary treatment process. Here, heavier solids settle at the bottom and separate from the water. Plant operators also skim the scum floating on top – such as grease and fats. At this plant, they use aeration. Some powerful microbial "bugs" eat contaminants in the water while the larger solids settle and get removed. Yonkers covers their primary treatment to reduce the smells and capture gasses for safe management to be good neighbors.

Yonkers wants to be a good neighbor, so they put a lid on their primary treatment. (Credit: FLUSH/K Worsham)

STEP 3: Water vs. Solid Treatments

After, we see two different treatment systems– one for the water and the other for organic solids. There are places where these separate treatments still intersect (you can see some dotted lines in the visual above). Still, two distinct processes are happening – water gets further cleaned and put back into the river. In contrast, solids get treated and safely disposed of in a landfill.

STEP 3a: Water Treatment

Water gets put into a secondary settling and aeration system. Here, the water gets thoroughly clarified – further removing solids as much as possible. More microbial bugs eat organic contaminants (such as amoebas) and other solid organic matter that still need to be removed. The bugs eat away at these contaminants constantly, taking breaks only when they're heavy and full and put back into the treatment system when they're hungry again.

The leftover solids get put into the solid treatment process.

Secondary treatment puts oxygen back into the water, while bugs munch on contaminants. (Credit: FLUSH/K Worsham)

At the end of this step, the plant chemically disinfects the water before pushing the treated water back into the Hudson River. Don't worry – they remove the disinfecting chlorine before it is released, so the fish don't get sick.

STEP 3b: Solids Treatment

Now, what about the solids? This includes the poo from the toilet and food bits that get into your drains.

First, the solids must be dewatered (is exactly as it sounds), ensuring that the treatment process has as little water as possible (remember, we want that water back in the river!). At Yonkers, the treatment process makes the solids three times less watery!

Then, the digestion process starts. Digestion works a lot like our body's digestive tract –anaerobic bacteria further break down the organics and remove the pathogens while harvesting some energy, such as the methane gas that naturally gets emitted when organic solids decompose. This plant goes through two different digestion processes - the primary digestion system happens for 15 days, and secondary digestion happens for another 30 days. These digesters are kept around 98F to ensure biological processes without killing the good bacteria. Much like your stomach, there needs to be a temperature balance to ensure the good bacteria maintains digestion. When things are out of whack, the plant operators can coin that the digesters "get sick," much like we do when we mess up our digestive tract.

Biodigesters process the solids and pull out those nasty pathogens. (Credit: FLUSH/K Worsham)

The Yonkers plant has two key outputs from digestion – biogas, and biosolids.

Happily, the plant harnesses most of their generated biogas (such as methane) from digestion to fuel some of their engines. For now, they flare the rest, but they continue to integrate methane into their energy needs and plan on using it all soon.

Their biosolids are 70% water. Biosolids can have less water (there's a cool article about this in The Source's July 2021 edition), but regulations have certain requirements that make this amount of water ideal. Also, processes that would make biosolids drier would be expensive and change their operations. So what do they do with those biosolids? Right now, they send their biosolids out to landfills upstate, like 22% of all biosolids in the US.

Ensuring It Works

Yonkers makes sure their treated water keeps the Hudson River beautifully clean. (Credit: FLUSH/K Worsham)

The plant uses laboratories throughout treatment to constantly guarantee that its processes are working. To avoid river pollution, they ensure treatment removes microorganisms and removes various chemical elements. They also use the laboratory to make sure the digesters never get sick, which would mean they stop working. Sometimes they find out that their treatment process is cleaning up pollution from other industries upstream. They like to figure out what is going on to prevent it in the future.

They've also started wastewater surveillance and laboratory testing for viruses like COVID. They test the wastewater biweekly to see how viral loads trend over time. The amazing part is that they can see viral loads surge a week before testing and hospitals notice. People have started using home testing without reporting their results to government health agencies. This becomes essential to track public health.

It's Essential Work

One of our guides reminded everyone that, while most of our offices shut down at the end of the day, WWTPs never stop running. They have a team working around the clock to make sure that they can ensure our water and air stay safe without us even having to think about it. We often take this for granted, which in some ways shows just how essential their work is to keep us healthy.

Special thanks to NYC H2O for inviting us to this tour, and the Westchester County Yonkers WWTP team for showing us around their amazing site!

337 views0 comments

Recent Posts

See All


bottom of page