字幕列表 影片播放 列印英文字幕 In 2015 a series of droughts started to dry up this dam, the source of almost half of the water available to Cape Town, South Africa. In this satellite time-lapse you can literally see the stored water decrease month by month. In 2018 the city was approaching "day zero". Shorthand for the day the taps run dry and people would have to queue to get water rations. Cape Town was the first major city to risk running out of water, but it's not going to be the last. Jakarta, London, Beijing, Tokyo could all face their own day zero in the coming decades. “Most parts of the world at least for a month a year are experiencing some water stress or water scarecity." “The gap between demand and supply of water is narrowing down.” Our planet is awash with water. More than one billion trillion liters to be precise. The problem is that 97% of Earth's water is salty and most of the freshwater is frozen in ice caps. Less than 1% of the Earth's water is drinkable. That makes one solution especially promising: Desalination seems a pretty straight forward solution: you take that undrinkable salt water, you remover the salt, and you have an unlimited supply of freshwater. So, why are we not building more desalination plants? Desalination is a natural process that's been known for millennia. As already Aristotle noted, the sun dissolves ocean water into vapor, which then condenses again and falls back as rain. His compatriots took note. Greek sailors boiled seawater for long trips. Romans used clay filters to trap salt. These are still the two basic principles used today. Thermal desalination uses heat. Salt's boiling point is a lot higher than water, so if you boil salt water, only freshwater will evaporate, leaving all the salt behind. Membrane desalination uses pressure. Saltwater, here colored in red for clarity, is pressed through a membrane that is only partially permeable. Freshwater can pass through, here colored in blue, but the salt is trapped on the other side. The technology didn't improve much until the 19th century, when industrialization and population growth encouraged more research. “Population growth is the main driver for increasing water scarcity.” Manzoor Qadir is an environmental scientist with over 30 years’ experience in water management. “Say, for instance, the Middle East and North Africa region. That region has a population of about five percent of the world's population, but just has one percent of the global water resources.“ And soon, another factor could make desalination even more crucial: As the climate warms more water will evaporate. And as Aristotle noticed, more vapor equals more clouds, equals more rain. But that rain won't fall evenly. This map shows how precipitation will change as the climate warms. Regions in purple will get more rain, those in orange less. Now compare it with this other map. The red dots here show areas that already today don't get enough water. Dry areas like California and the Middle East will see even less rainfall. Other countries like India will have more rain in the monsoon season, but less in the dry season, when people need it most. This will make desalination even more popular. "And that has really started to explode, let's say since the late '80s and '90s. But especially in the last 20 years, you've seen a big acceleration." Edward Jones is a Ph.D. researcher who has put together a "state-of-the-art outlook on the status of desalination." "Nowadays we have around 16.000 desalination plants which are producing more or less 100 million m3 per day.“ But take a closer look at this map. "If you look at how much desalinated water we produce on this globe, currently, 71% is produced in high-income countries.“ That's because desalination is very costly. Boiling billions of liters of water takes a lot of energy. “In the Middle East, the availability of oil and especially fossil fuels makes the thermal processes cheaper, but for other types it can be 25 or 30 times more expensive.” But that energy doesn't have to come from fossil fuels. A startup in Berlin has a sustainable alternative. “My name is Ali Al-Hakim, I am the co-founder and CEO of the company Boreal Light. I moved from West Germany to Berlin... Let me just drink a glass of water. Do you also want to drink something?“ “Your water?“ “If you like.“ “Yes, please!“ “So the water comes from the borehole tank to the system. And after that, it's going through the buster pump. With 40 bars of the water is pressured to the membranes. It's clean, desalinated water, with green energy.“ Green energy: that's the key to the company's success. You are looking at of their plants in Kenia. These solar panels keep the cost of water low in villages like this, where electricity is not available. “We get the water for free, we get the electricity from solar and wind for free, so we can now produce one thousand liters for fifty cents. This price is actually competitive [with] clean water from rivers or from boreholes.“ But there's another problem. What do you do with this water? “We forced all of this salt out of the water to produce our freshwater. But now the salt is still contained within our substance, but it's just in a smaller volume, so it's more salty.” This water is called brine. “At the global level, we produce more brine than we produce desalinated water.“ “You've got your pipe that's coming out of the desal[ination] plant. You're discharging hypersaline water here, and as it flows out it will sink because it's more dense.“ “The salinity and the temperature can also deplete the oxygen available and this is what's causing the organisms more damage, just the lack of oxygen. They are basically suffocating.“ Brine can also contain chemicals harmful to sea life. “There needs to be a better plan for the industry of dealing with this brine. We are producing more waste with no plan.“ But what if this waste could become a resource? Tomatoes, seaweed, and certain fish can tolerate high salinity. Boreal lights uses brine to cultivate them in tubs like this. "There's also the opportunity for salt recovery and for metal recovery." "At the moment technologies are available for brine management, but those are on a very small scale. The challenge is how we can transform those small-scale technologies into a large-scale operation.“ Desalination is not a magic formula. The process must become more efficient before low-income countries can afford it. Plants must convert from fossil fuels to renewable energies to limit emissions. And the whole industry needs to come up with a plan to deal with this brine. But plants like this are already a lifeline for many communities. “It's very important to realize that desalination is here to stay. We really need to work towards solving the challenges of desalination.“ “This is a gradual process this will not happen overnight. But I can see that there is a push there is a willingness to harness the potential of desalinated water.“ Today Cape Town is doing a lot better and the dam is almost full. The city was rushing to build desalination plants to avoid day zero. But the solution was not desalination or any other technology. Citizens became water-wise. They radically changed their water use, and valued water for the essential and irreplaceable substance that it is. Fun fact: The amount of desalinated water we produce every year is comparable to half the water that falls off the Niagara Falls every year. That's it from us. As you can guess from this quick time-lapse, these videos take a lot of time. If you liked it, please give it some love. Share, subscribe, press the like button. And stay tuned. We have more videos like this coming out every Friday.