How the Plus Pool in the East River Would Actually Work

Plus Pool, the proposal to put a plus-sign-shaped floating pool in the East River, has long felt like a fever dream, taunting us with press releases for a decade. But it looks like the idea, which is the brainchild of designers at L.A.-based Playlab and New York architects Dong-Ping Wong and Oana Stanescu, is finally inching out of rendering territory. Earlier this year, Governor Kathy Hochul committed $12 million in funding to the pool and the city added $4 million (the full cost is $50 million), and a smaller-scale test version is now in the works off of Pier 35. That pilot, which can only fit a few dozen people at a time, is scheduled to open in 2025. Does this all seem like an extravagant expense for a single pool? Probably. Still, on a scientific level, the idea of filtering river water to fill a pool is an interesting one — and for some, it will fulfill their romantic fantasy of swimming directly in the river, like people used to in the past. But how does that technology work, exactly? We reached out to Wade McGillis, a mechanical engineer and environmental scientist at University of Notre Dame who serves as Plus Pool’s science advisor, to better understand the details.

This interview has been edited and condensed for clarity.

How did you get involved in Plus Pool?
I was doing water-quality measurements in New York and the founders came to me when they first started in 2010. I teamed up with them literally during the first Kickstarter. I gave them all of my monitoring equipment. And their idea of making a big Brita filter to swim in — it’s feasible.

Even in something like the East River?
There’s human waste everywhere on our planet. But the levels are really low — when there’s no rain, you can swim there. I have citizen scientists that swim in the river all the time. My students have swum there. Most of the time you can swim in it and, sure, you shouldn’t open your mouth in certain areas — even in a pristine environment. But there’s a long history of regulation. In some states we can’t reuse rainwater for drinking water, but now that’s changing. So our perception of reuse water being yucky is slowly being mitigated because we have better ways to monitor it and things like that.

What are the things that need to be filtered out in the East River?
In the region where this is taking place on Pier 35, it’s mostly seawater. So you really can’t talk about E. coli because E. coli actually gets sterilized in seawater. What we test for is enterococcus, which is just another type of bacteria that comes from human waste as well. But E. coli and enterococcus are fine, it’s the pathogens that are with them — they’re just proxies for saying this is human waste, because they’re so easy to detect. These pathogens can cause intestinal sickness, which comes with everything from diarrhea to fever.

Where does all this waste come from?
A hundred years ago, waste from water and buildings just went into the river. Then they built treatment plants and piped refuse from buildings that they filter and sterilize. But you had such a large growth of inhabitants in New York City that they’re designed to handle a certain amount of flow. Have you ever used a Brita filter? A Brita filter is just like our wastewater treatment plan — you can only put so much water through the filter. When it rains, you basically overfill the filter. When the flow in the pipe gets too high, it goes to the river. That’s a combined sewage overflow.

So how does the Plus Pool’s filtration method work? 
Well, it’s evolved over the history of Plus Pool. The first incarnation was literally putting in a sieve. It was so unique, but it’s hard to manage. It was a passive filtration system where they had walls. So you have river water, and then you have just these layers around the old pool design. The first thing was a screen; the next thing would have been something equivalent to a clay or some synthetic material that kind of scrubs the bacteria. It was a great idea, and I personally tested those concepts on the East River just to show that it worked. The issue was that maybe you would have an event — like a big rainfall — that completely contaminated and overwhelmed the system. You had no control. I think that can still work, but our regulatory stuff just wasn’t ready for it.

So now it’s a different system?
I’m not the designer of the present system, but now they’re pushing for essentially an active filtration where, just like a real pool, you’re pump-filtering water. So Plus Pool is like a wastewater treatment plant that takes water from the environment. The treatment plant for Plus Pool pumps water into a big vessel. When the water has something like baby wipes — people have the impression that they’re biodegradable when they’re basically plastic — the screens take out those big chunks. You’re just separating the big stuff from the little stuff using gravity through a screen. Then it’s pumped through a membrane that takes out the small particles. Then if there’s a little bacteria that passes all of the hoops, you zap it with the same UV-light type of process that zaps bad things that come into your drinking water in New York City. The reason to use UV is that you’re not doing anything bad to it, but with chlorine you could mess up and add too much.

And if it rained, you could control it?
If it rains, they would know to stop actually pumping so you don’t overwhelm the system.

And it would stay clean.
Exactly.

So basically you don’t have any concerns about the viability of the filtering mechanism of the pool.
I don’t. Not at all. Because they’re not using passive filters now, it can be more robust and resilient to weather. And that’s a big deal because the weather cannot be kind sometimes.

If it was raining really hard, could the river overflow into the pool?
No, the whole thing is going to float. So it is always buoyant. Like Noah’s Ark.

And they’re testing this whole system right now. 
They’ve started the testing, and they’re going to do a pilot next summer on a big scale. Plus Pool did tell me that during this testing they’re going to sample six points along that path. So they’re really testing the performance of each individual component. So maybe they have to boost up or make a better membrane if it’s not doing the job. It’s good science and technique to make sure that it works.

Could you tell me about the light system you worked on a few years ago — that lit up blue when it was safe to swim, pink when it was not — to continuously detect the water conditions?
I had a sensor, and I wrote an algorithm that predicted how much rain was in the watershed versus the contamination. And I used real data to link historic rainfall events and how dirty the water probably is. And then I gave that real time to the engineers who made an installation to turn lights pink or blue. The reason it was pink or blue is because the Coast Guard only allowed them to use certain colors. You can’t use red and green because that’s boat navigation.

Is this filtering pool system going to be good for the river as well?Certainly. It will only do good except for where you’re getting the energy from.