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It is interesting that the discussions related to the limitations of our natural resources, including those pertaining to water, do not address the core issue which is that our exponentially increasing population will at some point exhaust our ability to sustain ourselves. Even if breakthroughs in desalination or water reuse occur we will soon have to face the fact that "built out" is a reality. What will we do then? What will be the political and societal consequences then? Has anyone looked to determine what a sustainable population is in the US? I know this is not a popular thing to do but considering that changes in climate are already pushing us to the limit it is a sorely needed research area.

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@Rik Kakareko - Rik, there is very little free hydrogen in the atmosphere with which we could make water. You would probably have to bring tanks of hydrogen to arid villages to make a substantial amount of water, and it would just be easier to bring tanks of water instead... @Ken from Canada - could you post a link to your invention or something?

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I have a very successull water cleaning apparatus that will change the way water is cleaned, economivcally, perfect clean and the worst problemm that I have is getting some company to look at it. The majors are so tied up mentally and $$$ with RO etc that they want us to go away,I will demonstrate my 10,000 gallon per day unit any where any time to anyone willing to help us go forward

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Water is already everywhere on the planet! Water is a great challenge in poor arid places. Water is a very simple molecule all we need is three atoms to create a water molecule. These atoms are readily available in the atmosphere, even in poor arid places. Why wouldn't we consider using solar energy to store and separate atmospheric gases namely Hydrogen and Oxygen, to create water at virtually any location on the planet????

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Rain water harvesting is the cheapest method be it through micro-watersheds or roof-top collection; suitable for rural areas like in India where 70 percent of population is located in 0.6 million villages. Any centralised treatment at village level will be difficult to maintain and operate. POU rather than POS is the best option for treatment;even the cheapest water treatment device is beyond the purse of the poor;solar water treatment raising the temperature to over 65 degrees to kill all pathogens by use of a bamboo basket covered with eating-plates coated with Aluminium foil [total cost rs.50 or one dollar] is the best option for POU; second best is making available Water purifier packets of iron sulphate and calcium chloride [a CDC and P&G innovation] in village shops at Rs.one per packet. The cost can be reduced by use of drumstick/strichnos potatorum for flocculation. The best place to treat water is at point of use and the best agent to do this is the mother.

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In my mind, this is the most important challenge for the engineering community today. I feel this needs to be our top concern due to a Kantian analysis of issues in the world today. I feel the majority of people on the planet have not fulfilled enough of Maslow's needs in order to have much concern over a potential energy crisis, or even finding more "green" ways of growing crops or living in general. The main focus is living. In my mind, Kant would likely argue that it is immoral to have a world where some do not have access to clean drinking water, or life-saving medicines when needed. I feel our major concern should be, then, to address these concerns, starting with water.

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On 9 April there is a hearing in San Diego concerning the development of a desalinization plant in Carlsbad for the purpose of providing fresh water supplies to many communities in the area...to include water for agriculture, fresh drinking water, and other needs. Some years ago, when I lived in Solvang, Santa Barbara had a desalinization plant to provide fresh water supplies to the area, but at some point, they decided the cost was too much, and they shut the plant down. The end result is that Lake Cachuma has all but dried up, and water supplies for that area are piped down from northern California. Are we headed for the same problems in Carlsbad, CA?

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"Clean water" is already a crisis, however, we here in the USA don't feel it & won't until we turn on the tap and there is none. There are simply millions of people in the world with very little water much less clean water. Water conservation (management) is as important as producing it. Imagine If each of us in the USA could just save 10% of what we currently use. We're working on a very simple clean water generating, collecting and distribution system that will work especially well in warmer climates where there are few alternatives. Even nicer it is self sustaining, kinda like a perpetual motion machine. More later.

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A team of five students from around the world is conducting an Open Source Design project to build a Human Powered Water Purifier. We welcome your ideas. To participate, please go to http://www.asme.org/Commu nities/Open_Source_Design .cfm This project is sponsored by ASME.

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A way to produce an abundant supply of clean water and clean energy would be to use geothermal heat to boil salt water and also generate electricity. Microfilters might be used to trap substances in the water to be removed so that pure water is left. We also need to safely reach the fresh water supplies far below the surface. As long as no contamination reaches the supply, we might have enough clean water to supply the needs of over 10 billion people. My Sahara Reservoir Project which would transform much of the desert into the world's largest fresh water reservoir will cost trillions of dollars and take decades to complete even with plasma excavation equipment. But it might be a future source for fresh water for Africa. I also want underground reservoirs to be hollowed out by plasma drilling moles. One in the United States would distribute water to where it's needed from where it's not needed. Too much fresh water is going to waste by entering the rivers and eventually the oceans and gulfs. Excess water in the spring could be collected in a system that contains more water than the Great Lakes and pumped to regions that need the water like Southern California. There would even be pumps in the forests connected to the system to aid fire fighting. Other regions around the world could have their own underground reservoirs to provide plenty of water for everyone.

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In reading over the comments I see two themes emerge. One group promotes potential engineering fixes like desalinazation and weather modification. The other group champions lifestyle changes like composting toilets and raincatchment. Many people with an environmentalist perspective are dubious that we can invent and build our way out of a problem that was caused by invention and building. Now I am not trying to start a fight. Obviously engineering and behavior change are both needed. Perhaps engineering's biggest challenge is to convince people that there is a commitment to make the average person's life better. Many percieve the institution as being subordinated to just profit motive. I see engineering empowering people by developing small scale water and power systems that operate within our existing large grid systems.

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Weather modification is the only practical large water source. Global Rain Enhancement The discovery of a high gain rainfall enhancement mechanism that could reduce drought on a continental scale and may have a significant impact on global warming. 100 years ago weather prediction was considered impossible, today weather modification is so viewed. But we are on the verge.

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Water is primary among the challenges. Without water, everything else is a moot point. We can live without most of the other things on this list, but without drinkable water we will not survive.

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I was pleased to see that you discuss desalination but not pleased that it was dismissed as an option. It is not just rich countries that could benefit from this technology or afford it in the future and the cost is something that might be brought under control with technology by more efficient ways of desalinating seawater. My thought has been ever since I first heard of this in Saudi Arabia was why the US and other countries don't use ocean water as a supplement to traditional sources. I think massive desalination efforts along with an aggressive infrastructure to store and transport water could help drought stricken interior areas of nations. A side benefit could also be to use as much seawater through desalination and offset the rise in sea levels? Cost is an issue with anything but the benefits to the environment and benefits in combatting droughts would be enormous. We could potentially save whole eco-systems that would be damaged by rises in sea levels and benefit dry inland areas by pumping moisture into soils where it would normally receive none. Think of the western US where water is needed desperately due to population explosion. California alone has one of the largest economies in the world and should be able to afford with some federal assistance to build a plant that could pump millions of gallons of water into the dry river valleys and offset the lack of water provided by the Colorado River. We could also build a water pipeline from areas like New Orleans that are fighting the rising seas to the middle of the country to provide water to farmers and communities to reduce the strain on the river systems. Combine some of your other ideas on energy as a way to fuel these large desalination plants like nuclear.

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Water is necessary to life. as we know it. Historically, we have depended upon the Earth to recycle and purify our water, and this works very well if the natural biosphere is left intact. However, we routinely have clearcut all land, destroying the filtration and storage aspects of the natural environment. Where we have done so in the past now exist great deserts. Decentralization is robust and brings immediate results in a very short time. Solar distillation is a practical solution in the Tropics and well into the temperate zone, but it needs to be done on an optimized scale. We should not, and do not need, to live in an urban environment, urban environments always have much greater impact on the environment than the equivalent population in smaller towns. The prime areas in the tropics which are in need of pure water are also places where solar stills are very efficient. Such stills on a village basis are much easier and quicker to construct than the traditional Western water system. Smaller systems make terror attacks much more difficult, educe the need for pipe and are much simpler to maintain.

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After all these technological advances we still don't have clean water. I think this should be addressed first before we try to move forward.

" ...one should consider the more low-tech, cheap, and readily available techniques for home production.   " 

If one were serious about finding alternative and cost effective methods for producing clean water for home use and food crops, then one should consider the more low-tech, cheap, and readily available techniques for home production. The reason for home production is quiet simple...that's where it's needed. Firstly, water has been collected from rooftops with gutters and cisterns/tanks for thousands of years. Secondly, home garden plots, particularly raised beds, have also provided healthy food for families for many thousands of years. The basic technology is thusly: Water and organo-fertilizers fall down, precisely where we need it! Food grows up, again, precisely where we need it,... and each plant even makes more copies of itself! Amazing technology. Apply the appropriate techniques of Nature, and one's genes shall turn around to thank you.

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Providing clean water is an important step to eliminating diseases that debilitate developing countries. With access to clean water, countries can improve the health of their citizens, and thus their workforce. A healthier workforce helps lead to a more stable economy. With a more stable economy, countries can begin to contribute to, and provide, many of the other items on the engineering list. You have to start at the foundation, before you can fix everything else.

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The water in our area is contaminated with chemicals and minerals deposited into our streams and lakes by coal companies. Our water supply comes from these streams and lakes. We need a way to clean these lake and stream sources and prevent future contamination.

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I am very worry about water supply

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Some day, someone will invent a way to economically desalinate water in agricultural quantities. I will say "Why didn't I think of that?" Meanwhile we should spend the $750 billion per year USA war budget on useful projects like more dams and canals, energy efficiency, solar power.

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-that water be a public good; not privately owned, not bought and sold- almost a human right , -in poorer countries even simple technology of clean wells and pumps. -in poorer countries technology and devices that use water sparingly; capture what rainwater there is. -in richer countries ways to use less; not to use drinking water for lawns and garden; technology for homes to use less water for toilets. - An example: I would like to use the water that washed my clothes to wash my vehicle. Restrict car washes in some way or use "dirty water" in some way to wash vehicles.

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yes yes yes to much contaminated future what future ???????????

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I don't know why we are waiting for a crisis to implement desalination in Los Angeles. We could be charging underground sources now. San Diego, likewise.

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Since all humans need water, and water distribution seems to be an increasingly crucial problem, I would think that this is one of the first problems on this list that should be solved. Perhaps combining the efforts of providing energy with providing water could essentially resolve both problems simultaneously; power could be easily supplied and water would be accessible to even those places where it is the most scarce.

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Give tax credits to every home that installs composting toilets. Solar power can power the electrical needs. No need for water flush; saved water. No need to "treat" the water; saved energy. Unit pricing is comparable to Kohler W.C. Give tax credits for gray water conditioning units. Water from showers, baths, dishes can be "conditioned" and used for watering lawns, gardens, washing the car, etc. Softer, friendly footprint for our Earth.

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To sustain humanity water supply is the most important without which we cannot go further. Drink first then Food, without water all agriculture will cease. All matters are formed from water.

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Supplying water, like other necessities such as health care, is not so much a technical problem but a distribution problem. The focus needs to be on applying approiate technology rather than developing new technology. A strategy that requires implementation is increasing the storage of water in its virtual form; food commodities. A program needs to be started to store vast amounts of food that would be sufficient to cover shortfalls enhanced by climate change. In conjunction a virtual water storage project would be a commitment to end hunger globally. Corn needs to be stored not converted to fuel so we can push around some terrestial tin.

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The lack of water is, of course, paramount in the aforementioned Engineering Efforts. Water IS Life. This has the most direct and immeadiate impact on our continued survival. The civilized world is now witnessing what could be termed as "peak water". This peak is, as the term implies, the maximum deliverable level of clean water. This peak will coincide along with peak oil and the soon to be realized peak food crises. The problem of water is inseperably linked with the remaining 14 stated engineering problems. Along with these we will witness peak oil, peak health, peak wealth, peak mobility, peak food, peak CO2, peak desertification, peak life expectancy, peak government, , peak greed, peak fear, peak war, and of course, peak population. One could neither engineer nor politic a "specific" solution for one alone without aggrevating or straining the remaining parallel crises. The solution must be "global" to all the mentioned efforts. We must firstly realize that we owe our daily existance to the planet we call Earth, and of course, ALL Earth-Life, including each other. The aforementioned "global" solution was meant literally. We HAVE such global solutions. Be Earth-Kind, share, and waste not. Simply let the trees and ALL seedlings and younglings grow in their natural, organic, wild, divine, unaltered, undivided, unfortified, unspliced, unpoisoned, and unhindered state! They already know how, just let them. They ARE the "global solution". It is already "engineered" into the divine seed, and in each and every heart on this planet.

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Just as one of the goals set forth is to make solar energy affordable, an equally important goal would be to make desalinization cost effective to provide copious amounts of water throughout the world.

"We need a paradigm shift in the way we relate to the physical environment..." 

In the desert south west we are facing a shortage of water; period. But we are also suffering from a lack of clean air and within cities, open space. We soon may be facing a shortage of petroleum, fish in the sea, copper, helium and soil just to name a random assortment of the many natural resources that consumerism is helping us to chew through at an alarming rate. It strikes me that picking a subset of problems caused by our fundamentally unsustainable way of life will not lead us to a fundamental solution. We need a paradigm shift in the way we relate to the physical environment - which admittedly is a challenge that falls outside of the field of engineering. But it worries me that by picking a few "grand challenges" to which we will apply a band aid, we mislead the public about the nature of the problem. Our problems are interrelated, integrated and global - we need to think about solutions in the same context.