Our grandparents' generation has created an amazing system of canals and reservoirs that make it possible for people to live in places where there was not a lot of water. For example, during the Great Depression, they created the Hoover Dam, which in turn made Lake Mead and made it possible for the cities of Las Vegas and Los Angeles to provide water for people who lived in a really dry place. In the 20th century, we literally spent a lot of money in building our infrastructure. In terms of economic development, it was a great investment. But in the last decade, we've seen the combined effects of climate change, population growth and competition for water resources.
This figure shows you the change in the lake level of Lake Mead that happened in the last 15 years. You can see starting around the year 2000, the lake level started to drop. And it was dropping at such a rate that it would have left the drinking water intakes for Las Vegas high and dry. The Third Straw is a waterfall, which is the largest waterfall in the world. The challenges associated with providing a modern city are not restricted to the American Southwest. In the year 2007, the third largest city in Australia, Brisbane, came within 6 months of running out of water. A similar drama is playing out today in São Paulo, Brazil, where the main reservoir for the city has gone from being completely in 2010, to being nearly empty. For those of us who are fortunate enough to live in one of the great cities, we've never really experienced the effects of a catastrophic drought. We have to take the navy showers. We like our neighbors to see our dirty cars and our brown lawns. But we've never really faced the prospect of turning on the tap and having nothing come out. And that's because when things have gotten bad in the past, it's always been possible to expand a reservoir or dig a few more groundwater wells. Well, in a time when all of the water resources are spoken for, it is not going to be possible to rely on this.
Some people think that we're going to solve the urban water problem by taking our rural neighbors. But that's an approach that's fraught with political, legal and social dangers. And even if we succeed in grabbing the water from our rural neighbors, we're just transferring the problem to someone else and there's a good chance it will come back and bite us. already rely upon that water.
I think that there is a better way to solve our urban water crisis and I think that it's open to four new local sources of water that I like to faucets. If we can make smart investments in these new sources of water in the coming years, we can solve our urban water problem and decrease the likelihood that we'll ever run across the effects of a catastrophic drought.
I would probably dismissed you as an unrealistic and uninformed dreamer. I have to say that I have a lot of experience in this area, and I have a lot of experience in this area. The first source of local water supply is the rain water that falls in our cities. One of the great tragedies of urban development is that we have begun covering all the surfaces with concrete and asphalt. And when we did that, we had to build storm sewers to get the water out of the cities before we could flood it, and that's a waste of a vital water resource. Let me give you an example.
This figure shows the water of the city of San Jose when they can harvest the stormwater that fell within the city limits. You can see from the intersection of the blue line and the black dotted line that if San Jose could capture just half of the water within the city, they'd have enough water to get through the entire year. Now, I know what some of you are probably thinking. "The answer to our problem is to start building great big tanks and attaching them to the downsouts of our roof gutters, rainwater harvesting."Now, that's an idea that might work in some places. But if you live in a place where it mainly rains in the winter time and most of the water demand is in the summer, it's not a very cost-effective way to solve a water problem. And if you experience the effects of a multiyear offensive, like California's currently experimenting, you just can't build a rainwater tank that's big enough to solve your problem.
I think there's a lot more practical way to harvest the storm and the rainwater that falls in our cities, and that's to capture it and let it into the ground. After all, many of our cities are sitting on top of a natural water storage system that can accommodate huge volumes of water.
For example, historically, Los Angeles has received about a third of its water supply from a massive aquifer that underlies the San Fernando Valley. Now, when you look at the water that comes off of your roof and runs off of your lawn and flows down the gutter, you might say to yourself, "do I really want to drink that stuff?""Well, the answer is you don't want to drink it until it's been treated a little bit. And so the challenge that we face in urban water harvesting is to capture the water, clean the water and get it underground.
And that's exactly what the city of Los Angeles is doing with a new project that they're building in Burbank, California. This figure here shows the storm park that they're building by hoking a series of storm collection systems, or storm sewers, and routing that water into an abandoned gravel quarry. The water that's captured in the quarry is slowly passed through a man-made wetland, and then it goes into that ball field there and contracts into the ground, recalling the drinking water aquifer of the city.
And in the process of passing through the wetland and explaining through the ground, the water encounters that live on the ruins of the plants and the ruins of the soul, and that purifies the water. And if the water's still not enough to drain after it's been through this natural treatment process, the city can treat it again when they pump if back out of the groundwater fluids before they deliver it to people to drink.
The second tap that we need to open up to solve and our urban water problem will flow with the wastewater that comes out of our seawage treatment plants. Now, many of you are probably familiar with the concept of recycled water. You've probably seen signs like this that tell you that the shrubby and the local golf course is being watered with water that used to be in a seawater treatment plant. We've been doing this for a couple of decades now. But what we're learning from our experience is that this approach is much more expensive that we expected it to be. Because before we build the first few water recycling systems close to the sewage treatment plant, we have to build longer and more pipe networks to get that water to where it needs to go. And that becomes prohibitive in terms of cost.
What we're finding is that a much more cost-effective and practical way of recycling wastewater is to turn wastewater into drinking water through a two-step process. In the first step in this process we pressurealize the water and pass it through a reverse osmosis membrane: a thin, permeable plastic membrane that allows water molecules to pass through but traps and returns the salts, the viruses and the organic chemicals that might present in the wastewater. In the second step in the process, we add a small amount of hydrogen peroxide and shine ultraviolet light on the water. The ultraviolet light cleaves the hydrogen peroxide into two parts that are called hydroxy radicals, and these hydroxy radicals are very very forms of oxygen that break down most organic chemicals.
After the water's been through this two-stage process, it's safe to drink. I know, I've been studying recycled water using every measurement technique known to modern science for the past 15 years. We've detected some chemicals that can make it through the first step in the process, but by the time we get to the second step, the advanced oxidation process, we rightly see any chemicals present. And that's in stark contrast to the taking-for-granted water supplies that we regularly drink all the time.
There's another way we can recycle water. This is an engineered treatment wetland that we recently built on the Santa Ana River in Southern California. The treatment wetland receivers water from a part of the Santa Ana River that in the summit consultants almost entirely of wastewater effectiveness from cities like Riverside and San Bernardino. The water comes into our treatment wetland, it's expanded to sunlight and algae and those break down the organic chemicals, remove the nutrients and activate the waterborne pathogens. The water gets put back in the Santa Ana River, it flows down to Anaheim, gets taken out at Anaheim and immersed into the ground, and becomes the drinking water of the city of Anaheim, completing the journey from the sewerler of Riverside County to the drinking water supply of Orange County. Now, you might think that this idea of drinking wastewater is some sort of futuristic fantasy or not common done. Well, in California, we already recycle about 40 billion gallons a year of wastewater through the two-stage advanced treatment process I was telling you about. That's enough water to be the supply of about a million people if it were their sole water supply.
The third tap that we need to open up will not be a tap at all, it will be a kind of virtual tap, it will be the water conservation that we manage to do. And the place where we need to think about water conservation is outdoors because in California And other modern American cities, about half of our water use meets outdoors.
In the current drought, we've seen that it's impossible to have our laws survive and our plants survive with about half as much water. So there's no need to start painting concrete green and putting in Astroturf and buying cuts. We can have California-friendly landscapes with soil moisture detectors and smart pollution controllers and have beautiful green landscapes in our cities.
The fourth and final water tap that we need to open up to solve our urban water problem will flow with desalinated seawater. Now, I know what you probably heard people say about seawater desalination. "It's a great thing to do if you have lots of oil, not a lot of water and you don't care about climate change."Seawater desalination is energy-intensive no matter how you slice it. But that characterization of seawater desalination as being a nonstarter is hopelessly out of date. We've made tremendous progress in seawater desalination in the past two decades.This picture shows you the largest seawater desalination plant in the Western hemisphere that's currently being built north of San Diego. Compared to the seawater desalination plant that was built in Santa Barbara 25 years ago, this treatment plant will use about half the energy to produce a gallon of water. But just because seawater desalination has become less energy-intensive, doesn't mean we should start building desalination plants everywhere. Among the different choices we have, it's probably the most energy-intensive and potentially environmentally harmful damage of the options to create a local water supply.
So there it is. With these four sources of water, we can move away from our reliance on imported water. Through reform in the way we landscapes our landscapes and our properties, we can reduce outdoor water use by about 50 percent, there increased the water supply by 25 percent. We can recycle the water that makes it into the Prophet, whereby our water supply by 40 percent. And we can make up the difference through a combination of stormwater harvesting and seawater desalination. So, let's create a water supply that will be able to withstand any of the challenges that climate change throws at us in the coming years. Let's create a water supply that uses local resources and leaves more water in the environment for fish and for food. Let's create a water system that's consistent with out environmental values. And let's do it for our children and our grandchildren And let's tell them this is the system that they have to take care of in the future because it's our last chance to create a new kind of water system.
Thank you very much for your attention.
Socure: David Sedlak · (Civil and environmental engineer)
David Sedlak’s research focuses the long-term goal of developing cost-effective, safe and sustainable systems to manage water resources.
https://www.ted.com/talks/david_sedlak_4_ways_we_can_avoid_a_catastrophic_drought