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The future of wastewater

An engineer explains why purifying “waste” water could be the answer to the world’s freshwater problems.
Close-up photo of a sewage drain.
What are the engineering, sustainability, and public health efforts necessary to create large-scale, safe water resources? | iStock/Blueberries

Guest Bill Mitch says it’s no secret the world is running short of fresh water.

As a civil and environmental engineer, he sees wastewater as a potential solution, if only we can eliminate the impurities. Mitch designs systems to remove toxic chemicals from wastewater to enable its reuse as a drinking water supply. It’s not easy, but it costs half as much as desalinating seawater, Mitch tells host Russ Altman on this episode of Stanford Engineering’s The Future of Everything podcast.

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[00:00:00] William Mitch: What we found is that the reverse osmosis treated waters were of higher quality than our conventional surface waters. So waters you might get from a reservoir, for example, and comparable in quality to what you might get from a groundwater supply, which is traditionally higher quality than what you get from surface water cause there's less organics in other things in groundwater.

[00:00:25] Russ Altman: This is Stanford Engineering's The Future of Everything and I'm your host Russ Altman. If you [00:00:30] enjoy The Future of Everything, please follow or subscribe wherever you listen to your podcasts. This will guarantee that you never miss an episode and it'll help us grow.

Today, Bill Mitch will tell us that the technologies for turning sewage into drinking water have improved greatly and that purified water is coming your way soon. It may even win some taste tests against what you're drinking currently. 

It's the future of waste water.

Before we jump into this [00:01:00] episode, a reminder that if you enjoy the podcast, please rate, review, and follow it. That will help us grow our listenership and it'll make sure that you're never surprised by the future of anything.

Many of us are lucky to have good access to clean drinking water. In fact, the separation of sewage from drinking water was a great public health intervention two or 3000 years ago when our ancestors figured that out and it led to [00:01:30] much better population growth and health outcomes. But access to water should not be taken for granted.

Even with global warming and the melting of the glaciers, not all the water that's being added to the ocean is easily turned into drinking water. One of the opportunities is to process sewage water into purified water that can then be delivered back to our faucets and our showers. 

Bill Mitch is a professor of Civil and Environmental Engineering at Stanford University. He studies the chemical methods for water [00:02:00] purification as well as the engineering, sustainability, and public health issues necessary to create large scale safe water resources. 

Bill, your work in water purification is fascinating. Can we start with a basic review of what's in the water that you're trying to purify, and how good are we these days at getting it out?

[00:02:22] William Mitch: Sure. Well, uh, what we're looking at is reusing and, uh, purifying for potable reuse, municipal wastewater, so [00:02:30] essentially sewage. So... 

[00:02:31] Russ Altman: okay. 

[00:02:31] William Mitch: What's in there? A little bit of everything. Whatever you pour down the drain eventually ends up in sewage. 

[00:02:37] Russ Altman: And, uh, how good are we now at getting out those components and is it a separate kind of chemical process for each type of contaminant, or do they actually all get treated kind of the same?

[00:02:49] William Mitch: Well, one of the difficulties we have to recognize is that we actually don't know all the things that are present in sewage and we can't. Uh, realistically measure them all. So just for [00:03:00] your, uh, for fun, one thing you could try is next time you're taking a shower, look on the back of the shampoo bottle and look at what the ingredients are and see how many of them you conceivably can recognize even if you are a chemist. Most of them I don't recognize. 

[00:03:14] Russ Altman: That's right. 

[00:03:15] William Mitch: So we can't possibly measure the thousands of chemicals we pour down the drain. So the strategy is a little bit different. We don't target treatments to each individual chemical. We try broad, uh, based sort of barrier treatments. It's called the multiple barrier [00:03:30] effect.

And so what we might do is say, all right, in California we use reverse osmosis. Reverse osmosis is a membrane that theoretically, uh, physically squeezes out everything but the water. Uh, so the organic contaminants are rejected into a concentrate stream. That's the eventual waste product, whereas the pure water goes through the membrane.

And so, uh, I, it's a broad-based chemical barrier, but certain chemicals [00:04:00] do we found actually slip through RO membranes. So we follow that with a different kind of barrier, and that's called an advanced oxidation process where we're generating radicals. And radicals are, uh, very potent oxidants, the same, uh, oxidants that, uh, people have heard about when you consume antioxidants to prevent oxidative damage in your body.

We're looking at blueberries, blueberries, that sort of thing. We're trying to, uh, go after radicals like hydroxyl radical. Well, in this [00:04:30] treatment process, we're generating hydroxyl radical to ideally tear apart any of the chemicals, many of which we probably don't know that might slip through this membrane.

And so we have these kind of multiple barrier approaches and there are different approaches in other states, uh, depending on their, uh, sort of local geographic problems that we can get into if you want. 

[00:04:50] Russ Altman: So that, yeah, that's fascinating. So I never would've expected that. So be, before we move to the issue of the kind of heterogeneity of the systems, um, I'm [00:05:00] imagining that there's a huge range of chemical, of sizes of things that are in the water. 

[00:05:04] William Mitch: Sure.

[00:05:04] Russ Altman: And down to very small molecules that we don't like, uh, up to I'm sure very big ones. And so you've described these two technologies. Do they, um, do they get down to these very small molecules that you know, that we might not want, that might be carcinogens or whatever and, and, uh, or are they limited in that respect?

[00:05:24] William Mitch: Sure. Uh, great question. So, um, The reverse osmosis membrane [00:05:30] uh, ideally, like I mentioned, only lets through water but it does let through some other, uh, molecules that are small in size. So, uh, we, the reason why we use reverse osmosis membranes in the first place is that they're very good at rejecting charged compounds.

So these are the same membranes, for example, that are used for seawater desalination. So they can, they're very good at rejecting sodium and chloride. Essentially desalting the water. And there are also, of course, plenty of salt and municipal wastewater. So charge [00:06:00] compounds even a very small size you know, sodiums, uh, 23 grams per mole, uh, can be rejected.

But when you start to get to low molecular weight neutral compounds, some of them can slip through membranes. So originally, uh, down in southern California at some of the first full scale plants for potable reuse of municipal wastewater, oops. Uh, the only process was reverse Osmo and then researchers [00:06:30] discovered that certain, uh, molecules called nitrosamines can slip through RO membranes.

So nitrosamines are some of the classic carcinogens that, uh, toxicologists have studied since the 1950s that they show up in, uh, hotdogs, beer, cigarettes. Uh, so if you are having to go to the grocery store and you see a hotdog pack that says, contains no nitrates, that's what they're trying to get rid of are, uh, nitrosamines.

[00:06:55] Russ Altman: Gotcha. 

[00:06:56] William Mitch: So these form by a completely different pathway in municipal [00:07:00] wastewater. But nonetheless, they were discovered and they're very potent carcinogens. And so once those were discovered, that was the impetus to put in these advanced oxidation processes as the second barrier. And those, uh, in the past several decades have been, uh, we look at, uh, the degree to which they can tear apart other low molecular weight neutral compounds that slip through reverse osmosis membranes. Some of those are things like industrial solvents. For example, chloroform, that kind of stuff. 

[00:07:29] Russ Altman: [00:07:30] So that story about Southern California, that goes right to the topic that you suggested, which is, uh, first of all, there's a heterogeneity in the technologies that are used. And now I realize, and I guess it's obvious once you say it, that different locals will have different challenges based on the industries or the lifestyles or whatever. And so that, that might lead to the use of different technologies. 

So tell me a little bit about the variation. You know, now I'm thinking about things like sunscreen, which you're not gonna find some places, but you might [00:08:00] find a lot of it in other places and a lot of other things. So tell me about that heterogeneity and, uh, is it a problem or is it actually a good thing?

[00:08:07] William Mitch: Well, actually, uh, it's a little, the heterogeneity derives from a little bit of a different reason, which is it tends to be inland versus coastal. 

[00:08:18] Russ Altman: Huh. 

[00:08:18] William Mitch: So the challenge is everything that's rejected by a reverse osmosis membrane ends up in a, what they call a concentrate stream that contains all the organic contaminants, all the metals, everything else you don't want [00:08:30] in the water and discharge, and as well as the sodium and the chloride and discharge of that concentrate can be problematic.

So, uh, up till recently it was considered fine in coastal areas to use reverse osmosis membranes because the ocean was, is relatively big and next door. So what if you discharge some salty water to that? 

[00:08:52] Russ Altman: What could possibly go wrong? 

[00:08:53] William Mitch: Exactly, but if you're in an inland location, like Colorado discharge of that brine stream, the concentrate [00:09:00] stream becomes problematic cuz you might be... 

[00:09:01] Russ Altman: ohh

[00:09:02] William Mitch: ...desalting downstream people's drinking water supplies. If you're, say, dumping it in the Colorado River.

[00:09:07] Russ Altman: Okay. So that's the big switch for you is Inland versus, uh, Coastal. 

Now, I did want to go a little bit you mentioned the nitrosamine in cancer, and I know you've written a paper about, um, emerging carcinogens, which obviously must be new molecules that we maybe didn't appreciate were problematic, but are, and even some links to bladder cancer.

Uh, can you tell us that story and is it something that we need [00:09:30] to, that we will be hearing more about things like this? 

[00:09:32] William Mitch: Uh, sure. Well, uh, What we've been trying to do is to figure out what are the toxicity drivers in the water. So the government, uh, tends to focus on a limited set of molecules that they, uh, they monitor and like I mentioned before, there are potentially thousands of chemicals presence to it.

So we can't possibly monitor them all. So we have to choose a few kind of sentinel compounds to monitor. And the question is whether the [00:10:00] government's selection of molecules are the right ones. Because every time you target a select set of contaminants, you're making a choice about a particular treatment technology.

[00:10:09] Russ Altman: Right.

[00:10:10] William Mitch: And so you might be going after, uh, the ones the government cares about, but not necessarily doing a great job with other chemicals. So we wanna make sure that we choose the right ones. 

So what we've been looking at, uh, over the past few years is whether the select compounds that the government focuses on really are the toxicity drivers in these waters, and we don't [00:10:30] believe they necessarily are.

[00:10:32] Russ Altman: Okay. And you found I guess this was in China, if I remember correctly, that there were some, uh, waste work, uh, compounds that were statistically associated with cancers that were, it was a novel association. Is that right? 

[00:10:44] William Mitch: Well, uh, that was a study by a Chinese group that we were reviewing.

[00:10:51] Russ Altman: Okay. 

[00:10:51] William Mitch: But yes, they were looking at, uh, novel sets of, kind of industrial contaminants that are in China that could affect, uh, their water supplies. 

[00:10:59] Russ Altman: Okay, so [00:11:00] thank you for this. Now we all kind of are up to speed on the general technologies. I wanted to ask about a number of things, uh, in terms of whether this technology and I believe you believe it, is ready for prime time.

So what are the energy costs? Because obviously we love everybody loves the idea of pure water. Mm-hmm. That's hard to argue against that. It's right next to motherhood Probably. Uh, But it comes at a cost, I'm sure, both the, um, the, I guess the knock on effects of having a lot of chemicals to treat the chemicals and also [00:11:30] energy.

So where are we now in the energy efficiency of purification? 

[00:11:33] William Mitch: Ah, excellent. So, reverse osmosis is an energy intensive process. Uh, but the, uh, key detail is relative to alternative choices of water supply, namely seawater desalination. It's about half the cost and that's because of the osmotic pressure associated with sewage is a lot lower than the osmotic pressure of seawater.

So it takes less energy to push that water [00:12:00] through the reverse osmosis membrane. So that's one thing, but another, uh, major consideration for utilities is access to the water itself. 

So certainly if you're in an inland area, you have no access to seawater. Uh, but the, even in coastal areas, it's rights to water.

So, uh, should be no surprise in places like California that people are constantly fighting over access to water supplies. So we worry about whether, uh, the pristine water supplies where we have historically [00:12:30] gotten our water will be available due to drought. But also due to competition from other entities. So witness the sort of fights between states over rights to the Colorado River water. 

[00:12:42] Russ Altman: Yes.

[00:12:42] William Mitch: But we, so when we look at something like municipal sewage, it's a reliable supply of water that the municipality controls and the flows approximately match what you need. What goes out the sewer is about what you needed to go in, in the first place to your house.

[00:12:59] Russ Altman: Right, right. So it's [00:13:00] all, it's not quite a closed loop but you could imagine a closed loop. 

[00:13:02] William Mitch: Yes, exactly. Uh, and it is, uh, that's, uh, when you speak about closed loop, uh, that's an excellent point because that's essentially what's going on in our field is historically we had the once through motif we would get water from distant sources.

Hetch hetchy reservoir up in Yosemite, use it once, clean it up just enough so you could throw it out in the bay and a safe fashion uh, and that's perhaps no longer tenable given the [00:13:30] sort of droughts we've experienced over the past few years. 

And so now we're moving to a circular water, uh, cycle. So, uh, there will be some makeup, no doubt, from pristine water supplies, but we're increasingly looking at, uh, gathering the water, using it and re reusing it, uh, in a potable fashion.

But beyond just extracting the water, looking at sewage as a source of vital resources. So some of the things, uh, that we're also looking at, 

[00:13:58] Russ Altman: huh, 

[00:13:58] William Mitch: elsewhere on the [00:14:00] Stanford, uh, campus along with collaborators in my department is, uh, you have dissolved organic matter. Uh, no surprise and sewage. And historically what we would do with that is oxidize that dissolved organic carbon to CO2 using bacteria, pumping in oxygen. Oxygen's, poorly soluble in water, so that's a very energy intensive process. 

Instead, could you ferment those organics, make methane and harvest that to run the wastewater plant? So on the Stanford campus, [00:14:30] there's a 

[00:14:30] Russ Altman: Wow, wow. So I'm just gonna stop you because that's a really exciting idea. So the old way was use energy to get oxygen in there, grow bacteria, have the bacteria basically eat and transform the, uh, the organic waste.

But now there's the idea of harvesting the waste and not letting the methane become a greenhouse gas, obviously this would be very controlled, so you could just use it as fuel to I guess to, uh, process the other water 

[00:14:57] William Mitch: Exactly. And so [00:15:00] harvest the... 

[00:15:00] Russ Altman: unbelievable 

[00:15:00] William Mitch: from, uh, and run an essentially an electricity generation plant onsite to help run the plant.

[00:15:07] Russ Altman: Have these been prototyped? Do they look like they'll work?

[00:15:09] William Mitch: Well, exactly. So, uh, on the Stanford campus there's a research center called the Codigo Center, where we have a pilot scale facility that is scalping sewage. From the campus sewer and is running this anaerobic process generating methane.

And, uh, we're not currently, uh, we don't [00:15:30] have an electricity generation plant, but we're monitoring methane yields and we can model how much wood form 

[00:15:35] Russ Altman: Right 

[00:15:35] William Mitch: uh, Redwood City or the, what's called Silicon Valley clean water. It's sort of the wastewater treatment plant that services Redwood City, Menlo Park, and those sorts of areas on the peninsula has an even larger pilot scale facility where we are, uh, that we're helping operate, and, uh, we're taking the methane out of that wastewater and current calculations indicate [00:16:00] that for the first time this wastewater plant is energy positive. 

So instead of putting energy in to just 

[00:16:06] Russ Altman: Yup 

[00:16:06] William Mitch: treat clean up the wastewater, this would just be for disposal to the bay. 

[00:16:09] Russ Altman: Yes. 

[00:16:10] William Mitch: But that we can now run it as an energy generation system. 

[00:16:13] Russ Altman: This is The Future of Everything with Russ Altman. More with Bill Mitch Next.

Welcome back to The Future of Everything. I'm your host, Russ Altman, and I'm speaking with [00:16:30] Bill Mitch of Stanford University. 

In the last segment, bill told us about advances in technology for turning sewage into drinking water, and he ended with these intriguing ideas about gathering energy from the sewage water so that the power for purification comes from the water itself.

In this next segment, he'll tell us about other things that can be pulled from wastewater and he'll also talk about the challenge of public acceptance of drinking water that used to be sewage. [00:17:00] 

Bill, you tell us about this, what you called anaerobic style, where instead of having bacteria use oxygen to break down these contaminants, you're actually using different processes to create methane, which can be used to actually fuel.

So, Is that the main thing you're getting out or are there other things that you can also extract from the wastewater? 

[00:17:22] William Mitch: Well, so we can extract the water through the reverse osmosis membrane. We can extract energy in the fashion we were just talking about [00:17:30] fermenting the organics to and then harvesting the methane. Uh, and then the third thing that people are targeting is harvesting nutrients. So nitrogen and phosphorus out of the wastewater. 

So the traditional way farmers get, uh, nitrogen is through the Haber Bosch process, which is very energy intensive, essentially breaking the triple bond in nitrogen gas to make ammonia.

[00:17:51] Russ Altman: Yep. 

[00:17:51] William Mitch: We use it once. It goes out the wastewater plant and fuels algae blooms in places like the Bay. So a lot of the [00:18:00] utilities surrounding the bay are awaiting regulations on the discharge of ammonia to the bay and how are they gonna deal with that?

The conventional way of dealing with that is putting a lot of energy into the system. To oxidize that ammonia to nitrate and then a separate process to reduce it back to nitrogen gas, but a very expensive process overall. So rather... 

[00:18:21] Russ Altman: Yes. It sounds like several chemical transformations and some of those chemicals are not your favorite safest chemicals, it sounds like. 

[00:18:28] William Mitch: Well, it's the, these are all for the [00:18:30] most part biological processes that are... 

[00:18:32] Russ Altman: okay 

[00:18:32] William Mitch: ...doing these transformations of the nitrogen. But it still involves a lot of energy and, uh, there are some associated chemicals as for... 

[00:18:39] Russ Altman: yes 

[00:18:39] William Mitch: ...part of these processes. Um, but the overall, uh, scheme though is that we're putting energy in the front end to make the ammonia, using that ammonia once, and then putting in a lot of energy at the back end just to safely dispose of this water to the bay. So could we instead harvest the ammonia, which is ultimately what the farmers want directly from the wastewater.

And here's where some of the [00:19:00] synergies come in because ammonia is a low molecular weight charred substance, kind of like sodium, ammonium. So, as it hits the RO membrane it's rejected into this concentrate stream ,so that when we, uh, conduct reverse osmosis for wastewater reuse, we essentially pull out about, uh, six sevenths or about, you know, 85% of the water comes out as clean water, but all the contaminants are now [00:19:30] crunched down into something that's about one seventh of volume. So we get a sevenfold concentration factor, essentially of the ammonia for free 

[00:19:38] Russ Altman: water. 

[00:19:39] William Mitch: Yeah. Which makes it much better to essentially harvest this, these nutrients.

[00:19:43] Russ Altman: Yes. I was gonna say, is it then relatively straight harvest, straightforward to harvest the ammonia and deliver it to the farmers and what not? 

[00:19:50] William Mitch: Well, that is actually the scheme people are working on now, not necessarily just me, but other people on campus are looking at different technologies to [00:20:00] extract alimony.

It's certainly not, uh, even at pilot or, and definitely not at full scale yet it's most, these are mostly lab scale schemes. So we can take advantage of the concentration but how do you then extract and purify the ammonia from all the other contaminants that are in that concentrate stream. 

[00:20:16] Russ Altman: Great. 

[00:20:16] William Mitch: So to kind of summarize, For the circular water economy, we're already at full scale for extracting the water. Uh, these plants are prevalent throughout Southern California and one's under development right now at the Palo [00:20:30] Alto Wastewater plant. So it'll be coming soon to, uh, Stanford essentially becoming part of the Stanford water supply ultimately.

Uh, and, uh, for the sort of harvesting methane we're at pilot scale in places like Redwood City and on the campus, but harvesting the nutrients is gonna be, uh, further down the road, shall we say.

[00:20:49] Russ Altman: Gotcha. Got it. Is it a similar story for you mentioned phosphates as well? 

[00:20:54] William Mitch: Phosphates, uh, are also of interest that the immediate need, uh, from certainly in [00:21:00] utilities around here is nitrogen removal because there's a regulatory impetus for them to have to remove the nitrogen, putting all this energy in the, there are no, uh, at least in this area, uh, measures on the part of the regulators to, uh, require phosphorus removal.

So there's less of a regulatory driver to fall 

[00:21:21] Russ Altman: I see. 

[00:21:21] William Mitch: To find an alternative. But ultimately, yes. Uh, the, we can't keep harvesting phosphorus or mining phosphorus in Florida forever. [00:21:30] So.

[00:21:30] Russ Altman: Great, great. So I wanted to go to the issue of public acceptance of these technologies. You know, there's a,and I'm sure that you've come up against this, people talk about recycled water and they get nervous.

Especially, you know, they're fine using it perhaps for watering their plants and they're fine with a bunch of uses. But then when you tell them that it's gonna be part of the drinking supply, my guess is that people start asking questions. And so tell me about where we are with that? Have you experienced that? And how receptive are people? To, [00:22:00] um, the arguments, uh, in favor of this kind of closed loop that we've been talking about.

[00:22:04] William Mitch: Sure. I think there's a been a sea change in people's attitudes in the past couple of decades. So when I was in grad school back in 2000, there was a full scale plant that Dublin, San Ramon, uh, an entity near us across the bay, had developed for potable reuse. And they never turned it on because of, uh, the public, the media got ahold of it and people got afraid of essentially [00:22:30] drinking toilet water.

[00:22:30] Russ Altman: Right.

[00:22:31] William Mitch: A similar story happened down in San Diego about two decades ago. They were planning to put in a full scale potable reuse, and eventually had to shelve it due to consumer concerns. Uh, that's changing and now, uh, San Diego is putting in uh, one of the largest potable reuse facilities in California and it eventually will supply about 50% of San Diego's water supply. So 

[00:22:54] Russ Altman: so let me ask, sorry to interrupt. What changed? Is it the technology that changed or the [00:23:00] communication and consultation plan with the community? 

[00:23:02] William Mitch: Uh, primarily the ladder. So the, uh, San Diego and other entities that have engaged in potable reuse have, uh, put a lot more effort into the public relations campaign.

So they've realized it takes a decade or more, uh, to sort of bring in school. So what they typically do is we'll build a demonstration scale facility for the technology where they can bring people in, school groups, etc and have them walk through the plant and notice [00:23:30] that it's not a filthy conventional wastewater plant, but looks like a drinking water, uh, facility.

They can see the product water in some cases down in, say, orange County. They've actually bottled the water and they can sell it.

[00:23:42] Russ Altman: You can have a tasting room. It'll be like going to a brewery.

[00:23:45] William Mitch: Exactly. So, uh, they have those already, uh, down in Southern California and, uh, our own valley water in the San Jose area as one of these demonstration scale facilities that tours go through, tour groups go through, etc.

The other [00:24:00] thing is, uh, they've brought on board other entities that can benefit from the potable or the whole circular water cycle. Uh, so for example, surf Riders Foundation, people who surf, 

[00:24:12] Russ Altman: uh, 

[00:24:12] William Mitch: traditionally are surfing off the coast where there might be a wastewater outfall. So, uh, and to them it's having, uh, reuse of various kinds is nice because it diverts contaminants away from their surfing areas.

[00:24:26] Russ Altman: Right. 

[00:24:26] William Mitch: Similarly for various, uh, sort of [00:24:30] ecosystem groups, you know, nature conservancy, that sort of thing. Because if you're, uh, recycling the water there's a lower demand on the pristine water supplies. 

[00:24:39] Russ Altman: Yes . 

[00:24:39] William Mitch: And so there's more water for fish, etc. So looking at these kind of multiple benefits, plus the more obvious, uh, driver of drought in most... 

[00:24:49] Russ Altman: right 

[00:24:49] William Mitch: ...people's minds have realized that, hey, um, that plus seeing some of these full scale facilities that have been up in operation now for decades in Southern California and not hearing disaster stories of [00:25:00] people getting sick and that sort of thing, people have learned to accept this. 

[00:25:04] Russ Altman: So this is great. And, uh, a couple of quick questions to follow up cuz you gave a lot of interesting news there. 

First of all, when you look back at these plants from 20 years ago or 30 years ago, uh, was the technology, was it actually gonna be fine?

In other words, if those hadn't been stopped by the public, would everything probably have gone well or in retrospect, yeah, that technology wasn't ready? 

I'm just wondering about that kind of post talk [00:25:30] analysis. 

[00:25:30] William Mitch: They, uh, have not changed dramatically in the past two decades. 

[00:25:34] Russ Altman: Okay. 

[00:25:34] William Mitch: The technology is essentially the same. And one of the things we did recently was, uh, to sort of address some of these potential consumer concerns is to say, to what extent is the effluent water from these plants. How does it compare in terms of toxicity to our conventional drinking water supplies?

[00:25:50] Russ Altman: Right. Right. 

[00:25:51] William Mitch: Because from the utility point of view, it's the opposite of what the consumers are thinking because they're saying, well, in your conventional water supply there are for sure [00:26:00] contaminants, and we're putting in much less sophisticated technologies to clean it up. What we're doing at these potable reuse facilities is essentially making deionized water. 

[00:26:09] Russ Altman: Right.

[00:26:09] William Mitch: So perhaps the water is actually of higher quality. 

[00:26:12] Russ Altman: Right. 

[00:26:12] William Mitch: Uh, so we wanted to uh, sort of compare side by side conventional drinking water and potable reuse waters and see how they compared with at least one toxicity assay. And what we found is that the reverse osmosis treated waters were of higher quality than [00:26:30] our conventional surface waters.

So waters you might get from a reservoir, for example, and comparable in quality to what you might get from a groundwater supply, which is traditionally higher quality than what you get from surface water.

[00:26:41] Russ Altman: Right. 

[00:26:41] William Mitch: Cause there's less organics and other things in groundwater. 

[00:26:44] Russ Altman: So that's fantastic. And I guess one kind of random question that I wanted to ask is, cuz I know you've written a little bit about this, which is this, um, when sometimes when you, when we drink water, especially if we're traveling to a new area, um, it, there can be a chlorinated taste to it.

[00:27:00] And I wanted to ask, and this is just a basic question, should I feel reassured by that chlorinated taste? Or is that a red flag? 

[00:27:07] William Mitch: Uh, well, uh, that's an interesting question. So, oftentimes what you are tasting is a difference between the final disinfectants in your water. So historically people use chlorine, essentially Chlorox bleach. Uh, and that doesn't have much taste itself, but it interacts with organic matter and makes products like chloroform that are [00:27:30] volatile and so they can 

[00:27:31] Russ Altman: Oh 

[00:27:31] William Mitch: go up your nose and you can smell it, which is also part of taste. Uh, but in areas like the Bay Area people use chloramines as the final disinfectant. So it's essentially you inject pneumonia, it reacts with the chlorine and makes a less reactive oxidant that's still a disinfectant called chloramines. Uh, but they tend to be more volatile, so you can taste them more essentially. 

[00:27:52] Russ Altman: Gotcha, gotcha. 

[00:27:53] William Mitch: Because again, nose and, 

[00:27:55] Russ Altman: and the final question I want to ask is, I've had other guests, uh, on this podcast [00:28:00] talking about how it might in some point be useful to have separate infrastructures for potable water versus water that's used for other purposes because of the expense, uh, and challenges and I'm wondering, given our discussion about the closed loop, do you see that as part of the future or is that just, uh, like kind of a logistical nightmare that won't likely happen? 

[00:28:20] William Mitch: Uh, it's actually in most cases it's the ladder. So in the early days of reuse due to consumer concerns, most of the larger utilities such as [00:28:30] valley water down in San Jose installed separate infrastructure, what they call when you see, uh, purple pipes.

It's the non-potable reuse water, the 

[00:28:38] Russ Altman: Oh 

[00:28:38] William Mitch: water you used to irrigate golf courses, but the cost of the treatment upgrade to make potable quality water is a drop in the bucket compared to the cost of installing separate pipelines. So most entities, including Valley Water, are moving towards potable reuse as a, uh, more efficient system. That doesn't mean they're gonna shut down the purple pipe system because [00:29:00] once you have the infrastructure... 

[00:29:01] Russ Altman: it's right. 

[00:29:01] William Mitch: It is cheaper to treat the water fashion. But yeah, but for the most part, they're moving towards potable reuse. 

[00:29:08] Russ Altman: Thanks to Bill Mitch, that was the future of wastewater. You have been listening to The Future of Everything podcast with Russ Altman.

If you enjoy the podcast, please consider subscribing or following it so you'll receive news of new episodes and never be surprised by the future. 

Tell your friends about the podcast too, and definitely rate and review it. [00:29:30] We have more than 200 episodes in the archives, and you might want to check those out because there's a lot of good stuff.

You can connect with me on Twitter @RBAltman and with Stanford Engineering @StanfordEng.

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