Alex Rappaport

Alex Rappaport did not set out to spend his life on membranes. Before any of this, he was a river raft guide, teaching whitewater stand-up paddleboarding to campers aged 7 to 15 on the Potomac River outside Washington, D.C., and it was those summers in the water that pulled him toward environmental engineering in the first place. He studied it at Tufts University, where, by his own admission, he was "not the student that they might have expected" to end up in environmental technology, mostly because his second love at school was entrepreneurship. He ran the undergraduate entrepreneurship club, founded the first venture incubator on campus, and started thinking of engineering as the medium and entrepreneurship as the force, which is more or less the sentence the rest of his career runs on.

Alex Rappaport co-founded ZwitterCo in 2018 with Chris Drover, a membrane veteran who had been pushing the boundaries of the field at Oasis Water on forward osmosis, and together they went after filtration's Achilles heel. Here is the problem, decoded: a membrane is a very fine sieve that cleans water by letting water molecules through and holding everything else back, and the curse of that whole industry is fouling, which is just the polite word for the membrane clogging up with oil, grease and organic gunk until it stops working. Most of the time, the dirtier the water, the faster the membrane dies, which is exactly why the world's hardest wastewater (think food and beverage plants, oil and gas, bioethanol) usually never gets cleaned and reused at all. It is sent down the drain because nobody had a filter that could survive it.

ZwitterCo's answer is a material called a zwitterion, which is a molecule that carries a positive and a negative charge at the same time, and that double charge makes it cling to water with unusual force. Rappaport's membranes use a zwitterionic copolymer as their active layer, and because that surface is so thirsty for water, it holds onto a microscopic water shield that keeps oils and organics from ever touching and sticking to the membrane. The result is a filter that shrugs off the streams that would have clogged a conventional one in days. "In 5 years," Rappaport told me, "we've never found a fluid that permanently fouls the membrane," which for an industry that plans its budgets around how fast membranes clog is a genuinely different proposition.

Alex Rappaport has turned that into one of the better-funded stories in water technology. ZwitterCo has raised about $98 million across four rounds, and the pace tells you how investors read it: a $4 million seed in 2021 led by filtration giant MANN+HUMMEL, then a $33 million Series A in 2022 led by DCVC, which was reported at the time as the largest Series A ever in water tech, and a $58.4 million Series B in 2024 led by Evok Innovations. The money is paying for the unglamorous part that actually decides whether a deep-tech company survives: a 30,000-square-foot innovation center in Massachusetts where ZwitterCo makes its own polymer, spins it into membranes, and tests them in-house, so a new formulation can go from the lab to a finished element in under a month.

Alex Rappaport is also unusually honest about why he is doing it, and that is most of what makes him worth listening to. His ambition is not really a unicorn label. What he wants, in his words, is to prove water as an investable thesis, to turn ZwitterCo into a lighthouse that pulls more money and more hungry young founders into a sector that has spent decades being overlooked, and to get industries to a point where they run on their own resilient supply of water even as scarcity and climate change bite. It is the rare founder pitch where the business case and the public-good case are genuinely the same sentence, which, if you have heard enough of these, you know is harder to pull off than it sounds.

“What would be so, so motivating to us is to prove water as an investable thesis. I would love for this to become a beacon that helps bring more dollars, more hungry young minds, into this space. And I could not care less whether it's labeled a unicorn at the end of the day.”

Rappaport came on (don't) Waste Water in 2023 to walk through how the zwitterion trick works and how he built a record-setting raise somewhat against the odds. The full conversation is below, and ZwitterCo's own progress is worth tracking on its site.