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Asked by longitude_prize to Water on 17 Jun 2014.
Question: Question from @swiftjon on Twitter: Would investment in water recycling be a viable alternative to mass desalination?
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Water Challenge answered on 17 Jun 2014:
The short answer to this question is, sadly, no. Yes, it is possible to reduce the amount of “new” water required by recycling, but the amount is quite small compared to the overall water need.
If we look at the three major consumers of water, they are for agriculture which typically takes 80% or more of the resource; commerce and industry which takes around 15% and domestic water supplies which consume around 5% of the total. Even if everything that is supplied domestically is recycled, this only represents 5% of the total resource.
This probably means that mass desalination is the only viable alternative and the trick is to find how we can do it with minimum energy together with an acceptable system to dispose of the salt. Any suggestions ?
Comments
Moderator - Shane commented on :
Only 5% is domestic? That’s puts into context my efforts to get the kids to turn off the tap while brushing their teeth.
Are those percentages for UK or global averages? How does it differ between water rich countries like the UK and water poor ones like Spain?
Water commented on :
In 1977/78, I worked for the same group that had designed the dams which were implemented by The Bureau of Reclamation, and they were, at that time, regarded as a world authority on water supplies. Lillienthal who was the founder of the group, had designed a series of huge dams on the Karun and Marun Rivers in Iran and was invited back to run a joint venture with the Iranian Government to implement the Iran National Water Plan. Lillienthal had amassed quite a bit of data on water use because the Iran National Water Plan sought to look at all uses of water in all river basins in Iran, including quantity, quality, location and timing so that coherent plans could be developed. The figures I quoted for agriculture, commercial/industrial and domestic were those we used. Interestingly, having completed the work in July, 1998, the Shah was deposed within seven months but when I went back to Iran to help with setting up refugee camps in 2003, all Iranians I spoke to remembered the Plan and they were planning to use it in one form or another.
dan-botham-3 commented on :
I would have thought recycling/reducing waste would always be worth while.
How much avoidable water waste is there in Agriculture, Commerce, and Industry?
What if crops were only watered at night to reduce the water lost through evaporation?
(I have posted my idea for desalination on the FB page)
Water commented on :
If you thought I said that it wasn’t worth recycling or saving water, then I think you may have misunderstood me. It is ALWAYS worth saving waste. My answer was to indicate that even if we recycled every drop of water that was originally supplied in the domestic sector, it would make very little difference to the availability of water because on average, domestic water accounts for only about 5% of all water use.
Since agriculture uses very much more water than domestic (80% against 5%, meaning that agriculture – on average – uses 16 times as much water as domestic water supplies), it should be possible to save water here. Indeed, watering at night can be helpful. Also, where water is very precious, drip irrigation to the root zone has been used to cut down on evaporation. However, when one looks at crop-water demand, there is huge variation between different plant types so that choosing plant types to grow can theoretically reduce water requirements. The trouble is that the crops that need less water are rarely the ones that people want to eat ! Also, while rain-fed agriculture is possible, to obtain high crop yields usually requires irrigation.
Looking at commerce and industry which – on average – use around 15% of the total water resource, indeed there are ways of saving water. The trouble here is that fresh water is generally priced so low that nobody cares to make savings. When the price of water rises, people are much more inclined to save every drop they can.
Money can be a powerful incentive to change people’s behaviour. For instance, the best way to reduce fossil fuel use is to price them so high that people will avoid unnecessary journeys; they will look to buy the most fuel efficient cars; or they will convert to all-electric cars charged by photo-voltaic cells on the roof. Likewise, if you price water the same way, people will find ways not to use water or to be very much more efficient in its use.
Your comments are very perceptive. Are you planning a career in water supplies ?
dan-botham-3 commented on :
I wasn’t planning on it, but you never know what lies in wait around the corner.
Water was the challenge that I voted for so I have been thinking about different ways to tackle the problem (Not just desalination).
Water commented on :
Dear Dan:
I note that you have put your ideas for desalination on FB (FaceBook) but as I have declined ever to join FaceBook because of their lack of data security, I am unable to see them. I wonder if it would be possible for you to post your ideas as a “Comment” on this website ? That way, I – and all participants interested in the subject – will be able to see them.
dan-botham-3 commented on :
Is there any way of adding photos to this site?
It’s basically a cross between a solar still and a solar power tower like PS10 near Seville in Spain.
http://www.power-technology.com/projects/Seville-Solar-Tower/
Mod - Shane commented on :
Sorry Dan, we can’t handle photos in the comments.
Water commented on :
Dear Dan
I’m aware of the Seville Solar Tower and it’s obviously the forerunner of more like it in areas of intense sunlight. To use the energy in a solar still is imaginative but may only make a relatively small contribution to the total need. That is not to say that it isn’t useful, but I believe that the Longitude Prize is looking at the bigger picture where we need to be able to achieve considerable economies of scale. My own experience of a solar still was in Gwadar, southern Baluchestan (Pakistan) which had the sunlight but the still used glass panels which seemed to mist up (possibly from small amounts of salt) rendering the system inoperable over a couple of years. Besides, the yield was very modest.
Personally, I feel that we might need to look at the problem by combining all factors we know about saline water, and see whether there is something that might provide a way of stripping out the salt from the water using low energies for large volumes. So what might those factors be ?
First, looking at how people in the Tharparker Desert in Sindh Province, southern Pakistan, obtain their water, we see that they skim fresh water which rides over the slightly denser saline water in the acquifer. It’s not perfect but uses an attribute which exploits the difference in densities of the two qualities of water. Secondly, I think of how Sir James Dyson used cyclones to separate (heavy) dust from (light) air, and I wonder if something like this could be applied to a denser fluid (saline water) to separate the two qualities. Perhaps a cascade of cyclones could do it if a single one couldn’t ? Thirdly, knowing that salts carry a small charge, I was wondering if a stream of saline water were to pass through an electrical field, would it not tend to concentrate either the salts or the fresh water ? Fourthly, if we can produce a much less concentrated saline, it would be possible to “polish” it to produce drinking quality water by reverse osmosis, using one of the recently developed plastic films which require rather less energy that the traditional ceramic filters.
I’m sure I have missed quite a few other attributes of saline solutions, but my guess is that there is a MUCH less energy intensive way of producing quite large quantities of fresh water at an affordable price.
What do you say to these ideas; and do you have any other better ideas to add ? Please feel free to share them so that others can chip in and have their say, too. Earlier I suggested crowdsourcing – isn’t this discussion a type of crowdsourcing ?
If you are with me on this line of thinking, I feel we should not forget to deal with the considerable volumes of salt that are likely to be generated. Just a passing thought on this matter – doesn’t sea water contain trace amounts of iodine, that vital ingredient to prevent IDD (iodine deficiency disease) ? A pity we can’t sit over a table and discuss these matters face to face !
Water commented on :
Dear Dan:
Further to my earlier comments, I think my suggestions look at the properties of water and salt.
They look at the weight of salt in solution; they look at the charge of salt in solution; and they look at the size of salt molecules (catching the molecules during the reverse osmosis process). I missed one feature – that icebergs are always made of fresh water. This last feature may not give us the key we seek because to tow icebergs to where they can be used is impractical and to freeze huge quantities of water to eliminate the salt would take far too much energy.
What other characteristics of salt have we missed that could lead to an innovative way of stripping the salt out of sea water ?
We need a Newtonian “apple moment”, don’t you think ? What can it be ?
Water commented on :
Obviously, there will be variations depending on rainfall, soil types, industrial development and standard of living. About not running the tap while brushing your teeth is good because one can extend it to not running the shower the whole time, but wet yourself, then switch off, soap yourself, switch on again and rinse. This “saving” mentality should pervade everything we do. Don’t leave the lights on if they’re not needed. Replace all incandescent globes by LEDs. Buy only a car for which you pay no road tax (ie low carbon footprint), and only ever have one car per two person family. Sounds familiar ?
Water commented on :
Apologies. Typographical error. The Shah was deposed in 1979, not 1998 ! This means that the Iran National Water Plan was completed in 1978.
Water commented on :
Dear Dan:
It might be worth investigating how common salt in solution behaves with different catalysts. The Wikipedia article on catalysis includes the statement: “The production of most industrially important chemicals involves catalysis.” Isn’t “pure” water an industrially important chemical ??
If, for instance, it can be shown that salt in sea water can be concentrated using a catalyst, then the “waste product” would be fresh water. After all, in the UK we use large quantities of salt on our roads during winter, so that local production of salt – if economic – might have advantages as it would benefit from the combined economy of scale of two industries.