Where Does Toilet Water Go?

17:18 minutes

a toilet flushing
Credit: Shutterstock

Many of us have morning routines that use a lot of water. After the alarm goes off, folks may stumble to the kitchen for a glass of water, then head to the bathroom to use the toilet, brush teeth, and take a shower. That very normal part of many people’s mornings is water-intensive. And where does that all go?

For many Americans, it’s a given that when we do dishes or wash our hands, that water is out of sight, out of mind—we don’t have to think about it again. But wastewater and sewage systems are complex and essential networks to our daily lives. And when they don’t work as we expect, whether that’s due to flooding or aged infrastructure, it’s a major problem. 

There’s a whole community of engineers and scientists devoted to improving our wastewater and sewage systems to reflect our changing planet. More people living in cities, and increased rain from climate change are two recent examples of major adjustments that our systems weren’t built to handle. But researchers are now leading projects like New York’s FloodNet, which alerts residents to street-level flooding, while SARS-CoV-2 detection in city wastewater has demonstrated the importance of monitoring these systems.

Joining Ira to talk about the importance of sewer science is Andrea Silverman, assistant professor of environmental engineering at the NYU Tandon School of Engineering.

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Segment Guests

Andrea Silverman

Segment Transcript

IRA FLATOW: This is “Science Friday.” I’m Ira Flatow. I want you to think about your morning routine with me. Maybe it goes something like this. Your alarm goes off, you head to the kitchen, grab a glass of water. Then you head to the bathroom, use the toilet, brush your teeth, maybe take a shower. That very normal part of many people’s mornings uses a lot of water. And where does that waste water go?

Well, for many of us, it’s not something we really think about. We see it go down the drain, and poof, it’s gone, out of sight, out of mind. But wastewater and sewage treatment systems are genuinely incredible. And they deserve our attention. And that’s what we’re going to be talking about today, wastewater treatment.

Here with me to get us excited about the magic of wastewater systems and how we can make them better is my guest, Andrea Silverman, assistant professor of environmental engineering at New York University Tandon School of Engineering, based in Brooklyn. Welcome to “Science Friday.”

ANDREA SILVERMAN: Wonderful, thanks for having me.

IRA FLATOW: I want to give out our number so our listeners can call in and ask questions about it. Because to me, this is a really fascinating subject. Our number, 844-724-8255, 844-SCI-TALK. Yeah, I say that because sewage treatment fascinates me. And I’m going to tell you a secret. I almost opted to go to graduate school in the field, but I opted–

ANDREA SILVERMAN: That’s wonderful.

IRA FLATOW: –I opted for radio instead. How’s that working out? We’ll see. But I’m still really, really fascinated with it. So let’s start with a simple vocabulary lesson, can we? What’s the difference between sewage and wastewater?

ANDREA SILVERMAN: Absolutely. And I should say that wastewater also excites and fascinates me.


ANDREA SILVERMAN: So thanks for having me in this conversation.

IRA FLATOW: You’re welcome.

ANDREA SILVERMAN: Wastewater is a more general term. So wastewater can be thought of any water that results after being used by a process, so that could be domestic, agricultural, industrial, et cetera. Sewage is more specific. So sewage refers to domestic wastewater. And I often use the two terms interchangeably and I probably will throughout the course of this conversation.

IRA FLATOW: And so they do mingle at some point, don’t they? I mean, we’ve always– we’ve heard of sewage systems that are overflowing and mingling with the wastewater system that flows out to the ocean.

ANDREA SILVERMAN: Yeah, well, so sewage is the domestic wastewater and in some municipalities our sewage system is connected with our stormwater drains. And that’s probably what you’re referring to–


ANDREA SILVERMAN: –is combined sewer systems. And so that when it rains, you do have storm water that ends up in our sewage system.

IRA FLATOW: And so what ends up in our wastewater though is– aren’t they the things that we put into– on ourselves, our shampoos, and lotions, and things we put into our system, into our bodies?

ANDREA SILVERMAN: Absolutely. I like to think of it as a couple of things, all relating to what goes down the drain. So the first is anything that goes down the drain from what we put down the toilet to what goes in the sink. So it could be food scraps, oils, chemicals, if we’re pouring paints or chemicals down the drain.

Also things we wash off our bodies, so if you put lotions or gels on your body when you take a shower or bath, they’re going down the drain. A big thing also that we think about with sewage is the things that we excrete, so feces and urine. And in addition to that fecal matter and urine, it’s things that we ingest. So nutrients– we eat a lot of food, there’s a lot of salt as well. A lot of that goes through our body and we excrete them.

Pharmaceuticals is another big class– when we take either pharmaceuticals, or illicit drugs, or whatnot, we’re excreting often the parent compounds or a transformation product, a metabolite of that compound. And then another thing that we might be excreting is if somebody is sick, the pathogen or the organism that’s causing that illness often is excreted as well.

And then finally, the big thing that’s in wastewater is water. We use a lot of water and that water goes down the drain as part of that wastewater.

IRA FLATOW: That is fairly interesting all that stuff that we put into the wastewater.


IRA FLATOW: Let me move on to one of the big reasons wastewater has been in the news recently. And I’m talking about that all over the world, scientists are monitoring sewage for the SARS-CoV-2 virus, the virus that causes COVID-19. Can you explain how this monitoring works for us? Because we’ve heard about it. How does it happen?

ANDREA SILVERMAN: For sure. So the premise behind what we’re calling either sewage surveillance, or wastewater surveillance, or some people are calling it wastewater-based epidemiology, is that some of the compounds or organisms that we excrete can say something about our health, so the pharmaceuticals or these pathogens I mentioned that we excrete.

Second of all, the whole community in sewered systems, municipalities served by a sewer system, the whole community is contributing to wastewater. And so we have had some issues throughout the course, for example, of the COVID-19 pandemic where there have been periods of time where we haven’t been able to test– do clinical testing as adequately as we’ve wanted to.

What’s nice about wastewater surveillance is that everyone is contributing their excreta to that sewer system, so we’re in essence sampling the whole community. So when we do wastewater surveillance, what we do is we collect a sample that we hope is representative of that community. And typically, it’s what we call a composite sample that’s collected over 24 hours.

We then quantify– analyze and quantify that sample for SARS-CoV-2, which is the virus that causes COVID-19. And we’ve seen, in our community working in wastewater-based epi, have seen that the virus concentrations in wastewater correlate to the case counts. And so that we’re actually able to illustrate trends in COVID-19 prevalence through the virus concentrations that we’re able to measure in these wastewater samples.

IRA FLATOW: So it’s pretty accurate–


IRA FLATOW: –in predicting, right?

ANDREA SILVERMAN: Absolutely, yeah. And we’re not able to necessarily say, OK, this many viruses means that this many people are sick, but we are seeing really good opportunity to see the trend. So you see an increase in viruses in wastewater, then you’re expecting there’s an increase in COVID-19 cases, and a decrease corresponds to a decrease as well.

IRA FLATOW: Let me go to our first phone call, Dan in Nellie’s Ford, Virginia. Hi, Dan, welcome to “Science Friday.”

DAN: Hello. I just wanted to see what the prospects are for moving away from waterborne sewage, which was a very bad idea to begin with, because you’re wasting all your nutrients coming from the soil and washing them into the ocean and ways to get it returned. But currently, the contamination in sewage sludge is so bad, it’s basically hazardous waste and can’t go back to the land.

IRA FLATOW: Interesting point, what’s your reaction to that?

ANDREA SILVERMAN: Thanks for the question. One of the exciting research areas in our field is in resource recovery. So how can we take the resources in human waste– and so that might be nutrients, as was mentioned, there’s a lot of energy in human waste, and there’s a lot of water. And I know that we were talking– the caller asked about water free sewage systems, but if you do– water is still a resource in wastewater.

But what are the ways that we can then take these waste products and not call them waste products anymore, treat them as valuable resources, and process them to pull out those resources? So, we’ve done a bit of research looking at non-sewered non water-based sanitation systems. They actually make it a lot easier to recover nutrients in a lot of cases.

One of the challenges is transport. So one of the big benefits of having a water-based wastewater system is that we can get all of the wastewater to our processing facility by gravity because it’s flowing. When you have a non-watered system, you actually have to pick up and transport that solid waste or separated urine, which is one of the challenges.

IRA FLATOW: Yeah. Let’s talk briefly then about how wastewater is traditionally treated. Give me a moment, for example, in New York, where you work. How is the wastewater treated here? Give me the process it goes through. Because I drive by West Side Highway and I see these treatment facilities and stuff. I want to know what’s going on in there.

ANDREA SILVERMAN: Absolutely. So a traditional treatment system has a couple of steps. The first is a screening step. So we have a relatively wide mesh screen that removes the big stuff in wastewater. So you can think about wet wipes, and leaves, and condoms, unfortunately, and tampons, sometimes money or fish, whatever people flush down the drain that’s pretty big or goes down the storm drain.

After the screening step, there’s a settling step. And one of the goals of wastewater treatment is to remove particles from wastewater. And so in that settling step, we have these big tanks that settle out the particles by gravity.

After that step, the wastewater then goes to a process that’s called activated sludge. And the goal of activated sludge is to degrade the organic matter in wastewater, which is another goal. There’s a lot of organic matter. And if that organic matter goes into a natural environment, it could suck up all the oxygen in that environment because oxygen is needed to break it down. So in activated sludge, we rely on bacteria to eat up and break down that organic matter. And to do that we aerate, we provide them the oxygen they need to do that.

After activated sludge, there’s another settling step to settle out everything that was created in activated sludge. And then the final step is disinfection. And typically is chlorine that’s used. And that disinfection step really is trying to prevent transmission of waterborne pathogens to the environment.

IRA FLATOW: So when that’s all done, it just gets flushed out into the river if you’re living in a city where there are rivers or out into the ocean.

ANDREA SILVERMAN: Absolutely. So as mentioned, there’s a lot of water and it doesn’t just disappear. So we have to put that effluent, that water effluent into whatever surrounding water body is nearby. And one thing I do want to note is that sometimes those water bodies become the drinking water sources for other communities. And so that’s why wastewater treatment is so important. It’s one of those barriers that we use to prevent that transmission of illness or disease to another community.

IRA FLATOW: In this whole process I did not hear a step for taking out all those drugs and pharmaceuticals we put in the water through the toilet.

ANDREA SILVERMAN: Absolutely. There’s a couple of things that traditional treatment facilities aren’t able to remove. And it’s not because we don’t care about them. It’s just that when these systems were designed they just– we weren’t thinking about certain things. One of those things is pharmaceuticals.

And so we don’t have– some pharmaceuticals are removed, but there’s a number that the processes just aren’t in place for their removal. And another is nutrients, traditional wastewater treatment facilities do not do a great job at nitrogen removal, for example. And so to be able to target these compounds what you’d have to add on is what we call advanced treatment processes. And there are some facilities that do have these advanced processes in place, but it’s not it’s not all of them.

IRA FLATOW: It’s expensive.

ANDREA SILVERMAN: It’s very expensive.

IRA FLATOW: Yeah, that’s–

ANDREA SILVERMAN: Unfortunately.

IRA FLATOW: When I was studying this in college it was called tertiary treatment.

ANDREA SILVERMAN: It still is, tertiary or advanced.

IRA FLATOW: And it always had the big money signs around it. That’s why we don’t have them in very many places. Let’s go to Jeff in Wichita. Hi, Jeff, welcome to “Science Friday.”

JEFF: Hey, how you doing?

IRA FLATOW: Hi there, go ahead.

JEFF: Yeah, so I live in Wichita, Kansas and I bought a kayak once about six years ago and tried to recreate in the Kansas River. And it was just like a sewage pond. It’s just total waste. And then I lived in Europe and you can go to these major rivers and they’re clean. How can we clean our rivers in America, with all the runoff and the sewage that goes into them.


JEFF: How can we do better?

IRA FLATOW: Yeah, thank you Jeff. How do we do that?

ANDREA SILVERMAN: Wastewater treatment is really one of the important barriers there. So in the United States we have the Clean Water Act. And that’s the regulation that’s meant to protect our water resources. And it’s done an incredible job.

So a lot of our wastewater treatment facilities were built in response to that act that was put in place in the ’70s. I don’t know exactly what’s going on in Wichita and why the treatment facility is not working, but I do think that wastewater treatment really does help provide that barrier.

IRA FLATOW: OK. This is “Science Friday” from WNYC Studios talking with Andrea Silverman, assistant professor of environmental engineering at NYU Tandon School of Engineering. And let’s go to– I think we have time for a really interesting– another call. Let’s see if we can get this one in from Nick in Kentucky. Hi, Nick.

NICK: Hi. I just wanted to say I’m an operator here in Kentucky. And I just wanted to say that– give a thanks because a lot of operators don’t get a thank you for what they do. And I was told in my class to get my operator license that we’re protecting the environment and protecting the people from diseases and stuff.

And I just wanted to say a lot of our job is dangerous, like especially when it comes to having to change out chlorine tanks and sulfur tanks, because the sulfur takes chlorine out of the water. And it’s just a dangerous job all around. Just kind of wanted to say thank you to other people.

IRA FLATOW: Let’s throw some love to Nick. Thank you, Nick.

ANDREA SILVERMAN: Yeah. And thanks for calling. I’m so happy that you called in. I really am so appreciative of operators. You guys are incredible. You are protecting public health. And like you said, I don’t think enough people know the hard work that you do to allow us to live the way we do.

IRA FLATOW: Let me see if I can get a quick call in from Brendan in Rhode Island. Hi, Brendan.

BRENDAN: Hi, how are you?

IRA FLATOW: Quickly.

BRENDAN: Good I’m a graduate student at the University of Rhode Island for the hydrology department, so I absolutely love what you guys are doing. I appreciate it. The question I wanted to bring to the table was, what do you guys expect for future considerations when we have increased populations? If there’s going to be an increased load on the nutrients, then what do you guys expect going forward?

IRA FLATOW: Good question, I see why he’s a student. Yes.

ANDREA SILVERMAN: Yeah, that has definitely be taken into account. So when thinking about population growth, you definitely have to plan your facilities in that way. And I agree, I think nutrients are going to be an ongoing challenge for us. And so there are certain facilities that are upgrading to have what we call biological nitrogen removal, to be able to target nitrogen specifically, to try to protect our waterways.

IRA FLATOW: I’m going to give you a blank check question, which I don’t have the money, but I’ll make believe I do. If you had a blank check and you could spend it on upgrading all these sewage systems, is there enough money in the world to upgrade all the sewage systems that need to be upgraded? And what would you do en masse for most of them?

ANDREA SILVERMAN: I think there’s enough money. And I think what I would do is shift the focus to resource recovery. So can we create energy products? Can we create fertilizers so that we don’t have to be making synthetic fertilizers? And can we reuse the wastewater, especially in places that are water limited. I think that resource recovery really is such an exciting opportunity for the next generation of wastewater treatment facilities.

IRA FLATOW: Do you need to develop new filtration systems?

ANDREA SILVERMAN: Yeah, so as mentioned, there’s a lot of things in wastewater we need to manage. So there’s a lot of research going into things like advanced oxidative processes that can help break down pharmaceuticals and viruses so they don’t become a health risk. Membrane filtration to try to also remove potential contaminants to make sure that we’re doing this in a safe way.

Because still remembering that direct reuse of, let’s say, wastewater, there can be some constituents that can cause health impacts. So we do need to keep working on research to come up with efficient ways to remove them.

IRA FLATOW: And quickly, what can we do at home to help you?

ANDREA SILVERMAN: Don’t flush things down the toilet that shouldn’t be flushed. Anything that you’d imagine– like when you flush big things down the toilet, the first– I mentioned the screen is the first thing in the treatment facility– it gets stuck on a screen and then the operators have to physically remove it from the screen and throw it in the garbage and it goes to the landfill. So just imagine, anything that you put down the toilet is eventually going to a waterway and if you don’t want it to go in that waterway–


ANDREA SILVERMAN: –don’t flush it down the toilet.

IRA FLATOW: All right, great words. Thank you, Dr. Silverman, for taking time to be with us today.

ANDREA SILVERMAN: Thanks for having me.

IRA FLATOW: Andrea Silverman, assistant professor of environmental engineering at New York University Tandon School of Engineering in Brooklyn.

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