Thursday, March 29, 2018

Martini-Glass Energy Banks


Martini-Glass Energy Banks


All about it...

Myself when young did eagerly frequent
Doctor and sage and heard great argument
About it and about, but evermore
Came out by the same door as in I went
Omar Khayyam
as interpreted by Edward Fitzgerald

As you will have guessed, this essay has precious little to do with Martini glasses and even less to do with Martinis. Sorry 'bout that — I suppose.

It does however have something to do with energy banking.

I was thinking about the storage of energy in ways that enable us to rely on renewable energy sources such as solar energy and wind energy without worrying about sunless days and windless nights: simply gather all the energy one can while there is a surplus, save it somehow, and draw on our savings whenever necessary. I already have discussed some alternatives in other essays, in particular:




In some of these I disparaged the use of water as an energy storage medium such as in hydroelectric or pumped storage schemes. One needs to raise too much water to make it worth while, it takes too much water and too much space, and it tends to entail troublesome ecological penalties.

As I have pointed out elsewhere, one can store energy by raising masses and recover it by lowering them again; the more you raise, and the higher you raise it, the more energy you bank. Ten times as high, or ten times as much mass, and you store ten times as much energy. Ten times as much of both, and you get one hundred times as much energy, and so forth. And there are ways to store the water indefinitely with negligible loss and re-use the energy with good efficiency.

Marvellous, not so?

But I also pointed out that this requires dismaying masses of water. I here plagiarise some text I posted earlier:

The fundamental problem is that it takes a lot of mass suspended at a considerable height to store many megajoules.
Oh. What are megajoules?
A convenient measure of energy in any useful form.
Consider: 3.6 megajoules (MJ) = 1 kilowatt-hour (kWh).
You might consume more energy than 1 kWh just in roasting a joint of meat for a family meal, and yet, just 1 MJ is how much energy it takes to raise a 10 Tonne mass 10 metres. That sounds energy-cheap of course, but unfortunately, the other side of the coin is that from a ten tonne mass raised ten metres, you can barely get enough energy to prepare a meal.
So it is hard to imagine the billions of tonnes of water that power utilities need for schemes to store power in elevated storage dams.
One tonne of water occupies about 1 cubic metre, and a ten-metre column of water exerts a pressure roughly the same as atmospheric pressure, and not surprisingly, as already mentioned, if we work with molten lead instead of water, then it takes a column about 1 metre high to exert a pressure equal to one atmosphere.

If we insist on pumping fluids, it would be nicer to use say mercury or molten lead; they are much denser than water.

But they also are a lot more expensive, and have other unfortunate properties, such as toxicity and cost, and in the case of molten lead, also temperature.

And one more time, don't forget that we also are not speaking of producing energy, only of storing and recovering it. We first have to get the energy from other sources; we still will need sun, wind, coal, oil, gas, nuclear or other generators to supply the energy in the first place.

Not encouraging?

No.

So what are the problems?

I hope climate science becomes the big thing.
And then what I want is electrical engineers to solve the world's energy problems, energy distribution problems.
I want mechanical engineers to make better transportation systems.
I want chemical engineers to develop better solar panels, and so on.
Bill Nye

Let us think harder about water. For one thing, in a lot of places, such as inland deserts, we simply do not have enough water to spend on energy storage at all. In other places we don't have the space; you can't plunk down a Boulder Dam just anywhere. Also, we don't want to use fresh water for energy storage where energy storage would interfere with the environment or compete with other needs.

As problems these aren't always and everywhere unacceptable, but more often than not they are serious. Using seawater is good where there is plenty of seawater, but historically we haven't used seawater for much of our power generation, because seawater seldom occurs high up where we can use it to drive generators. And it is more corrosive than fresh water. Granted, there are a few tidal schemes and so on, but they are limited in scope and scattered in location because the necessary conditions are rare.

On the other hand pumping seawater into high towers is a viable, environmentally friendly option almost anywhere on the coast. Often it even is advantageous to wildlife, such as by providing warm water with increased nutrient loads, or creating de facto nature reserves where people aren't permitted to hunt and fish; the real estate around the tower need not be wasted. The seawater simply could run back into the sea when exhausted, or go back into the pumping cycle if it had been necessary to filter it or otherwise treat it for whatever reasons might apply; that would reduce the need to repeat some of the treatment every time much of the water passes through.

But storing billions of tonnes of water and its potential energy takes space; a billion tonnes of water occupies a cubic kilometre, a terrifyingly huge volume.

And if we store the water in a cylindrical tower, the simplest and most obvious option, then the output from a full tower comes out at high pressure, yielding lots of power.

High pressure is good.

The pressure is what we apply to generate power, and the higher the pressure, within limits, the more power we can generate. However, by the same token, the last several percent of the output will yield hardly any power at all because it is under hardly any pressure. The maximal energy we can get is the mass of the water times the height through which it falls. So we want to store our power in the form of as much water as we can manage and held as high as we can store it.

This involves all sorts of engineering complications because of realities of pressures and leaks and foundations and structural design. Not being that kind of engineer, I avoid most of the practical details and leave them to the structural and power engineers. This is a refrain I repeat periodically and unapologetically throughout this essay, which deals with just enough engineering to urge the importance of certain principles.

Firstly, let us repeat, a ground-level cylindrical vessel, or in fact any broad-based vessel, is not satisfactory. As I have pointed out in another article, a large fraction of the water in such a vessel is wasted because when the vessel is nearly empty the water column is not high enough to give you workable pressure. All that the bottom few metres of your expensive water store are good for is to raise the rest of the water column high enough to give good pressure.

One can improve matters by mounting the vessel on a high place, or building it on a tall stem, with the power generation equipment at the bottom of the outlet, far below the reservoir. That amounts to narrowing the base of your energy store. Most big water towers use that trick, though not on the scale that we shall be discussing here, and not using their power for more than delivering the water.

I propose that we consider that trick too, but increase the stem height more than usual, so that the water still delivers a thoroughly usable pressure until the vessel (and even most of the stem) is empty. There should be no technical obstacle to mounting the vessel on a tower say 100-200 metres high. In a typical tall building each floor is roughly 3 metres high so such a stem height would be equivalent to some 30-60 stories; that is high, even impressive, but nothing dramatic. A strong, attractive shape for such a tower would be a hyperboloid, though most towers in that height range currently seem to be cylindrical, because the constant diameter simplifies tower builders' jobs.

But again, such details are for the engineers to specify — get used to that refrain, because I repeat it fairly often.

But those engineering details need not amount to technological barriers.

If our proposal is to be defeated on technological grounds, it will be by the sheer volume of water we would need to suspend up in the sky, not the height of the tower.

Considerations and approaches

We cannot solve our problems with the same thinking we used when we created them.
Albert Einstein

A column of 10 metres of water exerts a pressure of about one atmosphere, so a column 100-200 metres high would give about 10-20 atmospheres of pressure. That would be well within practical engineering ranges, both for storage and for generating power. If we use seawater, which is slightly denser than freshwater, the figures will be roughly 3% better, but still not different enough to affect any of our assumptions seriously, whether for good or ill.

Unless you already know the answer, you should be wondering about why we should want to build the towers so high. Height is expensive and dangerous, surely? And extra water is cheap, especially seawater.

Well yes, but to build the tower twice as high, up to a point, is less expensive than building two towers, and reduces the need for extra foundations, real estate, and equipment in an extra structure. Given the same mass of water a higher tower stores more energy and delivers it at higher pressure, which permits more efficient power conversion. In fact, the same mass of water, twice as high, stores and delivers at least twice as much energy. Of course, there are practical considerations, but decisions about the best design compromises are the responsibility of the engineers. Figures and forms I assume in this discussion are no more than examples to illustrate principles.

Furthermore, it is important to maintain perspective; the first full-scale tower (ignoring smaller prototypes and trial designs etc) will not be the only tower; even if you did build one tower large enough to serve an entire country much larger than Liechtenstein, that simply would leave you with unacceptable problems of power generation, distribution, and maintenance. We may look forward to water-tower "farms", much as we have wind-turbine farms, though it might be better to put isolated towers near to where the water or power is wanted.

If it should prove economical (which I doubt, but again, ask the engineers) to build towers so much higher that the pressure in the effective column of water becomes difficult to control, generators partway up the column could draw power and reduce the working pressure on the lower turbines.

You might wonder whether there is any point to discussing such nonsensically high structures at all, but that is not something to dismiss without serious engineering assessment. Consider: the  Millau Viaduct is a bridge mounted on masts in southern France. Its tallest mast measures 343 metres high, which supports a very heavy road and heavy traffic.

At the time of writing no one is complaining about the viaduct yet.
 

Shapes and sizes

Experts get their expert fun
Ex cathedra telling one
Just how nothing can be done
          Piet Hein

Now, the water vessel itself could most obviously be cylindrical, open at the top and half as high as wide, because that is the most economical shape for an open-topped, floored cylinder of a given radius (after all, why not open-top? Open-top works for hydroelectric dams, doesn't it? And it is cheaper than adding a roof. And at that height above ground, not many birds or bats will be a problem, and no one in a helicopter is likely to drop in to steal our seawater).  

Well, let us examine that idea. We won't want to build a 200 metre tower just to perch a bucket of water; the idea is to store enough water high enough to power a city for a worthwhile interval of time. We might want to do so to tide over a period in which there is no sunshine, wind or wave power, nuclear or fossil fuel power, or whatever forms of power generation might be locally important. The stored water also might be useful in smoothing out power demand so that we could run on base load power supply. When our power demand is too low to consume the base-load power level, we can use the excess power production for adding to our stores of masses of water as high as might be practical. When our demand is too high for economical use of  power from the base-load generators, so that power production becomes very expensive, we can meet excess demand by running accumulated water through the output power turbines.

So, let's think 200 metres high. Ten tonnes falling through ten metres gives one kilowatt hour. Falling through 200 metres the same amount of water gives twenty kilowatt hours, enough to power a typical first-world household for a day or two. A million tonnes would should do for about 100000 families, say a small city. Similarly, a billion tonnes could supply a large country (ignoring problems of distribution etc). Or it could supply enough topping-up power to keep a modest sized country going for a month.

These figures are deliberately simplistic, intended to demonstrate principles, not to propose practical designs. At all times bear in mind that the idea is not to have one tower delivering full power for a whole country all year round. The towers are to accumulate energy when power sources are available at levels greater than consumption, and to generate power to supplement and smooth out the available supplies when the demand is greater than the available sources can realistically supply.

If we think of using a billion tonnes of fresh water, then in many countries we would have problems of supply and disposal, but that would hardly apply to sea water, unless we also wished to desalinate our output, which would complicate matters, because we would need special arrangements to dispose of the salty discarded water. But that is not a complication we need consider here, though salt water at such a pressure certainly should simplify the life of a modern desalination plant. It should in fact be adequate for desalination of brackish water.

For our purposes I shall assume we are sited on the coast and take suitably strained and cleaned water from the sea. That is not as simple as it sounds, but it will do for purposes of our discussion.

But that big cylinder of water up there. Suppose it is 100 metres deep and 100 metres in radius. That would amount to  roughly 3 million tonnes of water at a mean height of 250 metres. However, when one speaks of millions of tonnes of water, one needs to be careful in designing the shape of the container. A flat-bottomed hollow cylinder with a wide base is not a suitable shape to suspend by the centre of its base; it would require massive reinforcing, which would add to the cost and reduce the stem's capacity to support the mass of water. Also, the water pressure inside the cylinder would be a lot greater near the bottom of the cylinder than near the top, and that would have to be taken into account as well. And the water near the top would be more profitable than near the bottom because it would be further from the ground, meaning that it would store more energy per tonne. In fact, in our example the water right at the top would be at a total altitude 100 metres higher, 300 metres. It accordingly would yield 50% more energy per tonne than water at the top of the stem (the bottom of the vessel) at an altitude of just 200 metres. 

Now, there are so many variables in calculating the best shape for the reservoir on top of the stem, that there is no simple optimum; the choice would depend on  the materials in use, the size of the structure, the difficulty of construction, the expected life of the structure (nothing is forever); weather and earthquake proofing, all sorts of things. No single design would equally efficiently suit all climates and situations. So one thing I am sure of is that whatever shape I propose, some engineer who has had the patience to read so far will shoot it down in contempt. That doesn't bother me much, because if he were asked to suggest anything better, some other engineer would shoot down his alternatives as well. (Note that I speak here of male engineers, not so much because I am inclined to expect female engineers to be a bit more tolerant of my amateurish attempts to talk sense, as because of a personal distaste for clumsy PC circumlocution such as s/he.)

There are however several variables of interest.  For instance how high the bulk of the water can be held, how securely, safely and efficiently the shape and material can hold a full load, how efficiently and effectively the water can be extracted from the supply, in our example, the sea, how costly the building process might be, how attractive the design might look — you are welcome to add to the list.

The stem itself could be any of several shapes, of which the most obvious ideas would be cylindrical, conical or pyramidal, or hyperboloidal, possibly cable-stayed, and perhaps in some combination. Cylinders are simple and therefore cheap to build (up to a point), but for very tall structures they offer less strength than the same amount of material in the shape of a hyperboloid, and incidentally they also are less aesthetically attractive. This last point might seem frivolous, but seeing how many people complain about wind turbines, it could become important. And one thing that should apply to all classes of design, and very likely to the vessel on top, would be the addition of helical strakes to shed wind. https://en.wikipedia.org/wiki/Vortex_shedding

Think of the vessel on top of the stem: it must bear its own weight as a box that won't collapse when empty. It must bear the internal pressure of the water that tends to burst it open from within. And it must withstand the piercing stress of the stem supporting it from beneath like a spike.  Also, it must have capacity for a lot of water, and it must not in itself be heavier than we can help, or the stem would have to be too big, dangerous, and expensive. In meeting these requirements the shape of the vessel is very important, because it is one of the major factors determining its strength, the amount of material needed to build it, and the height which it holds the water, which in turn is largely determined by the centre of mass of the full water load. 

It is tempting to think in terms of a spherical vessel, which would hold the most water for a given area of wall material, but both the localised stresses from the stem below, and the internal pressures from water within would complicate that. Besides, if we do not need a roof, we could do better with a hemisphere than a sphere; in fact, a hemisphere would work nearly twice as well, even allowing for giving it a stronger wall. In terms of the volume the hemisphere could hold, its capacity would be two thirds of the capacity of a cylinder of the same depth and radius, and it would be stronger. We also could improve the stresses in a hemispherical vessel at the point where it rests on the stem by narrowing the shell around the bottom to meet the stem top in an inverted teardrop shape. That would provide a stronger base, both for holding up the structure, and resisting the internal water pressure.

There is another variable that could however increase the attractiveness of a spherical reservoir. If it were suitably designed to withstand internal pressure, it could contain a suitable amount of say, nitrogen or propane under pressure. The gas could float above the water, storing energy in about the same manner as a compressed spring, increasing the pressure by perhaps a factor of two or higher, which on the scale that we are discussing, would amount to storing a good deal of extra water in the tower. However, storing energy by compressing gas is tricky if you wish to avoid inefficiency, so here again evaluation of the options would be a job for the engineers.

However, in either a cylindrical or hemispherical container, the wall strength necessary to contain a fluid under pressure is roughly proportional to the diameter unless the wall is very thick, so in a deep container, if the wall is to be of consistent thickness, the internal diameter should decrease linearly from top to bottom, because the pressure increases linearly from top to bottom. Twice the depth gives twice the pressure. This most conveniently gives a right-angled funnel shape, which is a rough approach to the inverted teardrop.

Such a cone has half the capacity of the hemisphere of the same radius, but it also keeps the water higher and is easier to construct than a hemisphere. In an inverted ninety-degree cone roughly 85% of the water will be in the top half of the vessel's height, compared to 50% in the cylinder, and its shape is fairly strong. One could slightly increase the average height of the water mass still further by using an inverted hyperboloidal vessel shape, and possibly the strength of the vessel as well by constructing it in the form of a hyperboloid of one sheet.  (In case you are interested in that option, you might want to read some material at:
https://en.wikipedia.org/wiki/Hyperboloid )

I doubt that the advantage at this point of the exercise would be clear enough to pursue the matter here; the advantages of simply lengthening the stem slightly according to requirements would be more obvious at any rate. Let's leave the fine tuning to the engineers when they get involved; it is not a point to be settled without planning and detailed calculation.

Martini glass?

Be thankful for problems.
If they were less difficult, someone with less ability might have your job.
Jim Lovell
Subject to more work on the point, I would suggest a design in the shape of an inverted ninety-degree cone on a hyperboloidal stalk, or some closely similar structure.

Notionally it would look rather like this (without showing any strakes for controlling wind resistance, nor stays for keeping the structure erect in earthquakes, nor details of wall thickness and other considerations):


Hence the Martini-glass appearance, minus swizzle sticks, cherries or pickles, but with lots of Martini.


Now let's think about the scale of Martini glass we might care to use. Let us suppose the stem to be the same height from ground to its tip, as the depth of the bowl from its narrow tip at the bottom, to the centre of its wide top. That is not necessarily the best in all cases, but it is a convenient assumption, though I suspect that a longer stem would be better. Let us consider some examples.

Suppose we have a tower 100 metres high, carrying a vessel 100 metres deep. It could hold over 1000000 tonnes of water at a mean height of roughly 175 metres above ground, yielding theoretically about 1.75 million kilowatt hours. By the time it has passed through the generators, it should have yielded say 600000 kilowatt hours of electricity at short notice (probably requiring less than a minute to get the turbines up to speed). A cylindrical vessel could store roughly thrice as much energy, but with the disadvantages I already have discussed. By increasing the stem height and vessel diameter and depth by about 32 metres, we could more than make up the difference in volume.

If we still maintain the proportions, doubling the height and diameter etc, we multiply the energy capacity by 16 (eight times the mass of water at twice the height) yielding storage for somewhere near ten million kilowatt hours, enough to keep a sizable city going throughout a night in a temperate region, if all its other energy sources were to fail at once. If base load generators were kept running, such a Martini glass might be good for weeks.

And that is from just one Martini glass.

As for billion-tonne Martini glasses...

It might be worth splitting the interior of the vessel into say three to eight sectors, partly for reinforcement, and partly to permit balancing the load over the centre of the foundation under conditions of high wind or foundation settling. This would not be suitable for rapid response, such as for earthquakes, but could greatly increase the safety and lifespan of the structure in the long term. In quake-prone regions a metal or polymer vessel containing a few hundred tonnes of water could be suspended above the centre of the vessel for rapid sway correction in earthquake-prone regions.  

Real estate and applications

Technology sometimes gets a bad rap because of certain consequences that it's had on the environment and unforeseen problems,
but we shouldn't use it as an excuse to reject our tools; rather, we should decide that we need to make better tools to
solve the problems caused by the initial tools in a progressive wave of innovation.
Jason Silva

The land that such structures would occupy need not be wasted. It could be used for agriculture, for recreation, for solar energy, as a nature reserve, for industry or as residential areas.

If the visual effect of the Martini glass shape were locally unpopular, it could be built invisibly into other structures such as office or residential buildings. Planting vines probably would not be adequate.

Climatically, if the tower used brackish- or seawater, and began to produce enough ice in winter, the ice could be harvested as free desalination.

In hot climates, the inside of the cone could be black, heating the water to provide power via heat pumps, or water vapour to condense for consumption.

Electricity generation need not be the only possible application. Suitable freshwater designs could be used as a source of water for emergencies such as fires, for mechanical power in factories, and for storage of irrigation water and irrigation power.   



So, what are we waiting for?

Snow and adolescence are the only problems that disappear if you ignore them long enough.
Earl Wilson

A few million dollars for prototypes and a few stages of progressively larger installations, such as sized to suit light industry, should be worth exploring.


Friday, March 9, 2018

Sewage, Healthy and Not so Healthy



A man lived by a sewer
And by that sewer he died
So at the coroner's inquest
They called it sewer-side.

Chorus: Oooohhh, it aint gonna rain no more no more,
                      it aint...

Anonymous bard

Not long ago I linked to an URL in which the importance of dragonflies and damselflies (Odonata if you like) as indicators of healthy water bodies were discussed:
It was a good article with useful links, but afterwards a friend asked me to explain some points that had occurred to her. She said in part:
"...at the [local] human waste sewerage plant you cannot breathe without a mask, I have seen an abundance of dragons and damsels. How does that give any indication of the water quality? I cannot get close enough for photos, my system is not built for that smell."
That was a good, sensible question, and one that is no novelty to ecologists, though it gets asked too seldom in practice. Naturalists have known for years (centuries?) that sewage dams commonly are excellent beats for anosmic birdwatchers and for many other classes of wildlife students as well.
When people speak of an ecology as being healthy, they generally mean a clean, lean situation with nutrients in short supply, and vigorous, stable populations of plenty of kinds of animals and plants. In this they are right as a rule, because when large numbers of species live together and depend on each other then it takes a very bad event to upset the system, and the ecology has a lot of flexibility in adjusting to changing conditions and cleaning up messes. The presence of such large numbers of species in reasonably stable ecological relationships is what we call high biodiversity.
Stable ecological relationships do not mean that nothing dies and rots, that nothing gets killed and becomes smelly, or that there is no manure to tread in, but it does mean that dead plants and animals soon get used up as food, shelter or other resources, by creatures that in turn become resources for others. In fact, for almost anything that dies or grows too plentifully, there will be other things that will clean it up by eating it up or using it up. Usually that happens in several ways at once.
And generally there is much storage of nutrients and other materials in the system. Living creatures accumulate material in their bodies or their homes or in the plants that shelter them, or in the material that surrounds them, such as soil humus that acts as a food store that supports worms on which moles and birds feed.
For example, in large regions of the Amazon jungle the soil is mainly sterile sand that cannot hold onto nutrient minerals. Sand tends to let nutrients wash out instead of holding onto them where they can act as a store of food in the soil. Nearly all soil contains some sand, but most soils also contain a fair amount of clay, and it is largely the clay that holds onto minerals and humus. That sounds marvellous, but not all clays release the nutrients well enough to suit all plants, and some hang onto trace minerals so tightly as to cause plants without mycorrhizae to suffer trace element deficiencies.
It is all a matter of structure. The best soil contains a lot of sand plus finer particles (sometimes called "silt") plus submicroscopic particles we call clay. When the mix is right, we call such a soil "loam". The chemistry of the clay is very important, and it is important that it should match the chemicals in the soil, that the plants need. (Actually, it also might be important to animals too; some animals and birds eat clays as antidotes to poisonous plants that they eat; the clays adsorb the poisons. But never mind that just now.)
Anyway, as a result of the shortage of clays and other suitable minerals in such sandy soils in parts of the South American jungles, the plants and the animals they support have had to adapt. They grab nutrients and water as soon as rotting releases resources from dead leaves or animals. The jungle then supports itself as a sort of inhabited living sponge of humus (or rain forest) on what otherwise would be an eroded sandy desert.
As long as it can go on collecting and juggling its riches without dropping too much, such a jungle is a very healthy ecology, though always hungry, but where you destroy such rain forest, say for agriculture, you soon are left with something that really does amount to a sandy desert. Turning that desert back to jungle does not happen easily or quickly; you have to build it up, and if the denuded region is large, that can take thousands of years. If you begin to destroy such a forest by clearing large areas to establish agriculture, you soon are left with nothing that supports anything stable (unless you call the desert stable, and even that erodes away in a one-way process through the ages).
And that is the general scheme in a healthy ecology. For any living species the rule is: waste not want not. Even if what you can’t use becomes food for something that may shelter or feed or serve you later, that is better than nothing. The squirrel that buries more nuts than it can dig up again to eat, may thereby cause the growth of trees that can feed and shelter more generations of squirrels. This does not imply that the squirrel knows anything about that, just that it presents a clear example to the ecologist. Whatever is conserved might assist some other species to reproduce effectively; what goes around may come around, or it may not, but what you destroy is gone.
Well, if that murderous, grabby, death-eating sort of ecology is how biodiversity tells us what is healthy, then what could be unhealthy?
For one example, whatever causes drastic reduction or extinction of any important part of the populations in the ecology generally reduces the viability and flexibility of that ecology. Such reduction tends to create waste and mess.
If the destruction is bad enough, it can wreck the ecology for thousands or millions of years. We call that we call such a long-lasting event “permanent” destruction, especially if whatever finally replaces it in any healthy way, differs grossly from what had been there before, so that whatever had been good in the past is gone forever, even if good things replace it all millions of years later. If that is what happens, then we have a special type of destruction that is a kind of succession or replacement. Often however, lesser damage can be recovered in a few seasons or decades if things happen in just the right way.
What could cause such gross, lasting breakdown? In the long term it could be continental drift, astronomic events, or major climate change, such as in the Permian. On a shorter time scale one class of disaster could be poisons, either from human activity, or mineral accumulations of poisonous chemicals such as from copper- or arsenic-rich minerals.
Another form of major breakdown may result when harmfully invading species kill entire populations, causing waste and loss of the flexibility necessary for dealing with changes. Other examples include either bush encroachment or bush destruction, disastrous erosion, prolonged drought, and gross pollution. Many such things could result either from human activity or natural events; you name it. The results often turn out to be Badlands of various sorts.
One form that we are familiar with is pollution from human activities. Examples of such pollution include industrial wastes, mine tailings and the like, but some of the most obvious and most widely spread examples are domestic garbage and sewage. Garbage usually has fairly mild, local, but rather persistent, nasty effects, but where disposal builds up for too long, the results commonly are disastrous, making the rivers about you and the soil beneath you offensive, disease-ridden or simply poisonous. If you are curious, read: https://en.wikipedia.org/wiki/History_of_water_supply_and_sanitation plus associated articles.
But small quantities of some kinds of garbage, especially persistent garbage, such as empty food containers, though ugly, may actually be beneficial, in creating some kinds of valuable cover for many species of small animals. Much of that kind of potentially useful garbage however, can choke or otherwise harm cattle or wildlife or sea life.
The fact remains that in the last century or two the garbage or sewage problem has grown so much that it has become very hard to ignore it. Garbage often accumulates so persistently that it forms a sort of fairly stable local ecology of its own type, with scavengers such as rats, insects, gulls, crows, and so on, feeding on selected wastes and often on each other. And such animals tend to be unhealthy, dirty scavengers too.
Although they are temptingly rich in some resources, including food and shelter, such dumps are not really ecologically healthy, because wastes of such types are full of harmful things on which animals choke, or trace poisons that accumulate to cause ill health, short lives, and hormonal or reproductive problems. They also are not ecologically stable, because the things that go in will vary in time and place as the habits and circumstances of the humans that supply the garbage change, which they do all the tine. Still, where dumps of domestic garbage are isolated, they tend to form islands that are unlikely to disrupt major systems. Major disruptions usually occur where human communities spread and intensify to the extent that they swamp or fragment entire regions, causing extensive changes and local extinctions. We see some of that in slums, city dumps and landfills, though encroachment of the cities themselves is worse. 
Actually, human waste dumps are not the only type of waste dump one gets. The precious guano deposits that people used to strip from islands, would have been green oases in the ocean if the birds had not destroyed them with their thousands of years of droppings. However, in doing so, the birds did eventually create ecologically stable guano-based communities that supplied nesting sites and enriched the surrounding water.

We cannot safely sneer at nature's own landfills.

Other kinds of human ecological damage arise where there is extensive forestry or agriculture of single crops over wide areas ("monoculture"). Similarly, urban or suburban regions with lawns, flower beds, or concrete, all lead to a lack of contiguous thickets, and a reduced biodiversity in the resulting patchwork of plants. After all, who wants the thousands of species weeds that grow wild here, when you can buy a dozen species of garden flowers any time you like? In South Africa, especially the fynbos in the far South West, the problem is especially severe, because many pollinators are dependent on particular plant species, which in turn depend on those pollinators. Examples are long-tongued flies (in particular, Acroceridae, Bombyliidae, Nemestrinidae, and Tabanidae) that pollinate many Lapeirousia and many species of Pelargonium. 

You simply cannot grow enough of the tiny, unspectacular, but exquisite, gems of the veld to support enough of the pollinators, because you also would need to supply enough of the prey that their larvae need as hosts. In particular, even if there were enough plants, but fragmented into patches, that commonly is fatal. It takes only the failure of a single link in the chain, either food plant, prey species, or pollinator, and your ecology has lost an entire complex of creatures, and no photographs, no pinned specimens, and no tears will bring them back. 

In comparison the loss of the Rijksmuseum in Amsterdam, or the Louvre in France, would be trivial.
And apart from the disastrous fragmentation that large, uniform monocultures, such as thousands of square kilometres of field crops, orchards, or timber plantations cause, they are recipes for disastrous explosions of pest populations. They also are some of the worst possible examples of how to breed resistance to pesticides.
One thing that tends to upset people a great deal more than such habitat fragmentation and destruction, is the accumulation of sewage. Even among biologists, it demands special study to understand the biodiversity and ecological structures and aesthetics of undomesticated species. Surprisingly few of them even understand the unpleasant smell, flies, and infections associated with sewage, but at least one can explain stenches fairly easily to the more intelligent specimens, especially if education starts young. They then can understand explanations of the smells and the perceptions of disease and of disease-spreading scavenger insects such as many kinds of beetles and flies.
But sewage contains more than just smells and pests. For one thing, sewage usually contains a great deal of material that in virgin healthy ecologies would be valuable. A bit of dung in the forest or desert seldom lasts more than a day or two in its original form, because too many animals compete for it. For example, go to Signal Hill in Cape Town and look down beyond the seashore — at the time of writing there is a region where the water is brown. A major sewage pipe broke below the sea surface, much nearer to the rocky shore than where the proper outlet led.
The pollution around the break in the pipe got so bad that many species of fishes and other organisms simply could not live in the surrounding water, but many other scavenger species did very well there, and multiplied explosively. After all, with few predators, and huge food supplies, what more could they ask? And only a few hundred metres beyond the break, where the sewage is harmlessly diluted, other species grow fat on the scavengers. Those other species include humans who have learnt that there is no better place to find big, fat crayfish; by eating such scavengers one is in effect feeding on the sewage at a remove of one level.
Whenever we can manage that, it is a far shorter, more efficient recycling pathway than eating the abalone that eat the kelp, that absorb the thinly dispersed and bacterially digested sewage, right?
I do not say that regions too full of nourishing material speak well of human pollution, in fact we have a nasty word for them. We say that they are eutrophied, or even hypertrophied. Eutrophication means something like "full of food" in Greek, and that sounds good, but really, to an ecologist it means something more like "harmfully overfed". A bit like "obese", if you can imagine an obese ecology. Hypertrophication is to eutrophication as pathological obesity is to obesity, or perhaps as sewage is to compost.
Food for one creature is poison for another, and where there is too much food for one species, the excess food probably is killing some other species. For one thing, bacteria digesting the wastes need oxygen, and most of them do not produce their own oxygen, so we find that sewage tends to contain very, very little oxygen. We say that the medium becomes deoxygenated. In deoxygenated soil or water there may be harmful creatures feeding and growing rapidly. Think of fungi and bacteria such as various Clostridium species. They are analogous to the rats and cockroaches in human dumps. You won't find many songbirds or antelope in dumps or sewage.
In that way (and there are many such examples) human eutrophication of various environments may be benign to some creatures even if they are harmful in other ways. Sometimes they only are harmful in very local spots where the muck is concentrated. When that happens, they often are largely beneficial from most perspectives. Under a garden manure pile only some kinds of earthworms and a few scavengers like beetle larvae and other specialist insects can live. And around the pile there is a border of dead earth, but a little further away some tough kinds of plants grow well, and still further away everything is lush for a few metres. Mostly weeds of course, but the very concept of a weed is slippery...
We could do better with our waste than wasting it perhaps, but in places like around the broken sewage pipe, where waves and currents helpfully disperse wastes, it takes bad pollution to do much harm. Examples might be where harmful substances accumulate in the food chains, or take very long to rot and poison or smother whole living communities or choke animals with plastic wastes.
Still, where there is modest eutrophication, pollution often amounts to recycling. Even when the harm does keep some things away from a given area, it thereby might form a nursery shelter for young fishes or other organisms that otherwise cannot breed safely in the open.
Incidentally, there also are subtler forms of pollution that turn out to be beneficial in the end. The Koeberg nuclear power station ejects waste heat in its coolant water into the sea, and the result is what? Wildlife growing enthusiastically in the warm water. Certainly some species can’t survive the warmth, and close to the outlets they die out, but a little further away they do well, each seeking its own preferred level of warmth, and for the most part the effect is stimulating to the community rather than harmful. There are many examples of where industrial areas form de facto nature reserves by excluding hunting, fishing, and other human interference.
Then think of a clear stream with a few dams or lakes along its length. Suppose some community or agricultural activity begins to spill waste nutrients into the water in a few places. A lot of real harm results, possibly because some of the standing water simply turns into sewage, or because some of the bacteria and waste chemicals are lethal to animals adapted to very pure water. Places where we used to swim or fish simply get wiped out. Sometimes whole species become extinct.
We are tempted to say the stream is dead. Certainly the local biodiversity may be seriously reduced, and it definitely has changed. Sometimes it never will change back; you cannot bring back extinct species and it can be very difficult to re-establish local populations once they have been wiped out or even seriously distorted. Most of the things that once had lived in the clean, oxygenated, but nutrient-poor water would be dead or sick or gone away. In a standing dam the dirty water often becomes so rich in nutrients that solid mats of green algae soon cover the surface and smother all photosynthetic species below.
Algal mats may be unsightly, but we might console ourselves with the thought of all the oxygen that the algae produce. However, though that certainly is true, things are not so simple. Some of the algae and cyanobacteria in such mats release dangerous poisons. What is worse, such mats prevent wind or convection from circulating water from the surface down to the depths. None of the oxygen from the air or that the algae themselves produce, can get more than a few centimetres down below the surface. Deeper down, plants die for lack of light and bacteria deoxygenate the water completely. No fish, in fact few organisms apart from microscopic anaerobic life forms such as some bacteria can survive. The dam bed soon is covered with dead material similar to sewage sludge, and most of the water is unproductive and has a low biodiversity.
It is an ecological disaster.
Really.
But some creatures are adapted to flourish in such water bodies. Some live on the huge populations of scavengers, midges, mites, worms and so on, that can deal with low oxygen levels. To some species oxygen actually is poisonous; others depend on lack of oxygen for protection from predators that need lots of oxygen to support the high levels of energy that they expend in hunting.
Some animals that can survive in the green, oxygen-rich layers of the water, feed happily on the algae. Others live on those animals in turn. Many of the species that inhabit the surface algae are larvae that emerge to fly as midges, and in the air above such water dragonflies and damselflies will congregate, because there are so many more midges to feed on than elsewhere. Some of the dragonfly species will lay their eggs here, where the larvae will live in the algae layer near the surface where there is enough oxygen. Many species of dragonflies however, need clean, oxygen-rich water in which they can breed. But even if no dragonfly can live in sewage water, dragonflies from neighbouring cleaner bodies of water sometimes will aggregate where there are enough flying insects over filthy water. And so do many birds, including swallows and swifts.
For breeding, the dragonflies seek out other water, if other water is available, which need not be the case; my friend was asking about a semi-arid region where the sewage dams were almost the only open water in the neighbourhood.
Are such foul waters healthy then, just because we have lots of dragonflies, swallows and other welcome creatures flourishing over them? Definitely not. Whole populations, sometimes whole species, get wiped out in some places, and the biodiversity tends to be low, or at least lacking in clean-water species.
Productivity too, tends to be low compared to what would be possible from the same resources in a healthy environment, because there is little turnover of the accumulated muck that settles out, muck that could take centuries to recycle even after the pollution stops. In a healthy ecology we want balanced input and output, and a healthy rate of throughput. Sewage or a dunghill might be a buffer in which nutrients accumulate, but it commonly is a buffer that dominates the nutrient cycles, and usually we can achieve more things and healthier things with our resources than just piling them up to rot. Ideally materials should be stored mainly in living creatures, such as fishes, insects, tadpoles, water plants, trees, otters and so on. It is good too pass nutrients rapidly from each to the next, conserving the energy from the sun and the minerals from erosion. Simply accumulating material as oxygen-free sewage sludge may attract scavengers and dragonflies, but it does not guarantee ecological health.
Species associated with various classes of ecology, such as those based on nutrient rich or nutrient poor conditions, or on particular poisons or temperature regimes, we often call indicator species. Indicator species are important resources for the ecologist, but the messages they convey, one must treat with insight. This has been known for thousands of years; as Aesop said in effect, "one swallow does not make a summer". Nor does one aggregation of dragonflies make a healthy body of water, or one dead rat an unhealthy garbage dump. It all is part of the immensely confusing, demanding, rewarding and important subject of ecology. One shouldn't just want to know the name of a species but also what it does and how it fits in.
Stop and contemplate; stop and wonder; stop and think; stop and enjoy.





Thursday, March 8, 2018

Water on the Rocks





Many forms of thirst

You should not see the desert simply as some faraway place of little rain.
There are many forms of thirst.
William Langewiesche, Sahara Unveiled
Many an old idea originally seen as impossible, unpractical, or stupidly cranky, has become first possible, then obvious, then vitally, routinely necessary. This has happened repeatedly throughout history and a good deal of prehistory, but as a trend it became especially obvious roughly at the time of the so-called industrial revolution. Significantly, the drive variously to ballyhoo or dismiss, disparage and destroy novel advances remains as compulsive as ever. 

Nearly every genuine advance, whether based on old ideas or new, encounters greater difficulties than expected, but also becomes valuable in more contexts than expected.
And some are more vital than expected. 

The idea of exploiting the planet's largest resources of fresh water, namely our great ice sheets, has been bruited and mocked for perhaps a century, but though it repeatedly has been dismissed as stupidly unrealistic, the simple fact is that it is unavoidable. We will be doing that, like it or lump it, on a scale hardly imagined by either proponent or opponent. People will make and lose fortunes, people will die of it, and people will make livings from it as routinely as sailors sail and technicians build and tend wind turbines and high-tension lines.
For practical purposes this discussion ignores the harvesting of sea ice and glacial ice for the sake of luxury, fashion, science, or whim.  It deals with the older, more prosaic, more visionary, idea of harvesting utility or irrigation water in bulk from sub-polar masses such as sea ice and icebergs.
As a serious suggestion the idea of iceberg corralling dates back to at least the first half of the 20th century, and it has met with understandable derision ever since.  Part of the reason was firstly, that the bulk requirement for water in those days was smaller and less globally urgent, particularly among populations that had the resources and skills to contemplate such giant projects. And most nations had nothing of the type.
Also, until recently the very idea of a major city running dry seemed ludicrous and therefore could be ignored; parched third-world villages were irrelevant because impotent.  They still look irrelevant, and still largely because of their impotence. The poor in their local drought-stricken valleys or villages we have had with us always and we see no prospect of their disappearing.
What is more, before the last twenty years or so of the 20th century the technology necessary for marine ice collection was not at a level that encouraged private enterprise to harvest ice for water; our shortcomings in this undeveloped technology still are something of a challenge.  

The very ideas that visionaries espoused, towing icebergs for example, were non-viable.  
Another reason is our lack of any currently adequate infrastructure; one does not go out on a whim to collect billions of tonnes of water and transport them thousands of kilometres, simply relying on the principle of "what could possibly go wrong?" The sheer scale of any viable project of that type beggars the imagination of Joe Average and Percy Politician, and so does the variety of obstacles to overcome; if so much as the thought is to be worth while we must think in terms of delivering, not millions, but billions of tonnes, not annually, but pretty nearly daily.  
Still, the times they are a-changin' and by now it is a matter of religious conviction that climate is a-changin' with them; and climate stable or climate unstable, human populations are growing and water resources are shrinking. 
It no longer is too early to think once more of harvesting ocean water and ice. Few people realise how unstable climate really is, with or without modern human culpability. Before our "industrial revolution" of the last three centuries or so, at least a dozen "civilisations", or at any rate established urban communities, were ruined or dispersed by droughts that lasted for decades or centuries. Collapses of that type have happened on every major land mass where cities of any sort existed. The biblical seven lean years were altogether believable as a very mild instance. The very Sahara as we know it, ancient and dreadful as it seems to us, is a relatively young desert, having been largely savanna just a few thousand years ago. It is so recent a development that apparently a few immemorial Atlas cedars still survive where rare underground water sources happen to have sustained them.
Nowadays the idea of letting a large modern city die of thirst is practically inconceivable, so it is pretty certain that some drastic modern technological and infrastructural advance must be developed to supply the water they need, no matter how ridiculous such measures seemed in quite recent past.  Certainly desalination with even greater sophistication and efficiency than what we can manage at the moment will be one objective. Just a few decades ago our current desalination technology would have seemed miraculous. Improved reticulation and redistribution of fresh water already is a must. Gathering and recycling fresh water from as yet unexploited sources will be crucial.  
Various possibilities often complement each other, for instance desalination works best when the input water is only slightly impure, brackish, such as one might be able to gather from estuaries or from sea ice.  One way or another though, we need to explore and develop as many sources of water as we can, if only because different solutions will be required in different circumstances and different regions.
And we have the thirsty with us always.
Discussion of the potential technologies that I propose here were inspired by a drastic drought where I live in South Africa, though it is far too late for immediate application of this idea to that drought, and it probably is too early to take the ideas seriously at the time of writing. Anyway, we have had droughts before.
But never a drought that so threatened modern cities, and with every prospect of more and worse threats in future. 
And one must start somewhere.

How much fresh water would we need?

Think of a number…
We consume a lot of water already, domestic, industrial, and agricultural. No one knows just how much, but it definitely is beyond the untrained imagination; in fact I suspect that even the trained imagination manages only on the principle of "shut up and calculate". People in the know cannot afford to let themselves be distracted by the boggling of their minds in the face of realities. To begin now would be none too soon for rationality, though far too soon for the politicians and business powers that be. 
A rough guesstimate at current global human consumption might be four teratonnes: four million million tonnes of fresh water — per year.  
That alone is nearly as much as the entire flow of the Amazon River. It amounts to a few thousand cubic kilometres, and our rate of consumption is not decreasing; in fact many millions of people, from pole to equator,  remain desperate for fresh water all their lives, and their frantic demands for entitlement grow hoarser with anguish as the ages roll, and so does their rage.
In our calculation or planning in this discussion we may ignore those desperate people; they cannot afford to do much about their situation, so a more practical beginning would be to provide a few billion extra tonnes of water per year for the typical large, affluent metropolitan region.  A billion tonnes of water occupy about one cubic kilometre, just a large drop in the global bucket. However even that drop is beyond the imagination of most people, and what is more important, the necessary scale of the engineering is far beyond the imagination of Jane or Joe Average, who seem to think that all you need to do for all the water you want, is to install a tap.
Or, more typically, get someone else to install it.
It would be funny if it were not so infuriatingly tragic: even more tragic than thirst. 
Unless you happen to be really, urgently, thirsty. 
Bulk water engineering and the design of new bulk water handling technology are not for amateurs; we already get water from many sources that Joe Average hardly dreams of. And if he did dream of it, he would be grateful to forget the dream on awakening. Most of us don't even realise that we get a lot of our water from underground, let alone from sewage when there is enough sewage. Not many people realise that many of our underground water sources are limited; the exploitation of many of them amounts to mining of water that accumulated over thousands of years. 
Such supplies eventually peter out when the water mine is exhausted, or is ruined by damage to geological formations that hold it. 

Many also don't even realise that much underground water is unusably salty, often saltier than seawater, and that when rainwater percolates through to salty groundwater, it becomes unusably salty in turn.  
So don't look to underground water for indefinite supplies, unless you happen to live in a blessed region where rainfall comfortably replenishes your clean water mine indefinitely and you can hope that it will continue to do so. 
And dammed water, say from giant schemes like the Three Gorges dams, not only is a limited resource at any given time, but is wanted for more than relieving drought; apart from producing potable water, it has to produce power and support transport. Such applications are not fully compatible; and they compete more and more severely as the water reserves run lower.
So don't get too optimistic. We already have seen the mighty Colorado River reduced to a polluted trickle; the Nile is threatened by each of the many countries it passes through...
Watch this space; even the Amazon and Congo are not infinite.

Why the poles?

We must always remember with gratitude and admiration the first sailors 
who steered their vessels through storms and mists, and increased our 
knowledge of the lands of ice in the South.
                                                Roald Amundsen
In the sea we have more water than we can use, though not more than we can pollute; and our planet-wide problem is not a shortage of water as water, but how to separate the water from the salt. The sun provides the planet's largest still, purifiers, and condensers, but only a fraction of the usable water goes to where we want it on land, in rivers, and in lakes. Even less ends up where we can use it for domestic and industrial purposes. 
To add injury to insult, the condensed water often arrives in the form of floods, always wasteful, commonly harmful, and often deadly.  
And most of what we do receive on land soon flows uselessly back into the ocean, where it once again is lost, carrying valuable soil and nutrients with it.
What drives that cycle is the fact that the sun heats the water, evaporating it. Sooner or later the vapour cools till it condenses or freezes.  To use the water, we must intercept it at suitable stages between salt and salt.  Sometimes this interception is easy, but by ignoring some of the more difficult options, we lose a great deal of water that otherwise could have been useful.
On this planet some of the largest and most dramatic regions of accumulation of fresh water are sub-polar. That is where warm air from temperate and tropical regions meets cold air, and dumps its water burden as ice in various forms. More importantly, sub-polar regions also are where seawater freezes. As it freezes it more or less abruptly extrudes brine from between crystals of pure ice. 

These regions are remote from where many people live and work, and their ice is largely in unfriendly forms. Accordingly we have paid them little attention as sources of fresh water, but that must change. Just as we have been grubbing for oil in ever more incredibly difficult circumstances in recent decades, so we shall need to look at sourcing water from the more challenging, but more rewarding, less ecologically harmful, sources.
Almost amusingly, we can reflect that within remote living memory, before refrigerators were standard household appliances of the affluent, the seasonal harvesting of ice was a major industry in regions such as much of North America and parts of Europe; entrepreneurs would cut blocks from frozen lakes or the like. They would warehouse those blocks, and during summer they would transport the ice to major cities such as New York. For decades the iceman was as familiar a figure as the milkman in such cities. Domestic ice boxes were designed to hold standard-sized blocks, and a suitably insulated block might keep food fresh for perhaps a week or more. 
Extravagant people even made ice cream, producing the necessary sub-zero refrigeration by adding salt to crushed ice. The ice pick was such a standard household utensil that it figured in many murder mysteries. In real life even Leon Trotsky was reported murdered with one, though technically in his case it was an ice axe, and Trotsky himself never was reported to have insisted on the distinction.
Anyway, the sub-polar regions are textbook examples of where there is plenty of fresh ice at all seasons. It just happens to be regrettably inconvenient to collect and transport the permanent ice and convert it into usable form where it would be most welcome.
Oil and gold they say, are where you find them, and the places where you find ice in paying quantities, quantities large enough to slake the thirsts and cool the fevers of cities and nations, are places where you find cold.  
For example in sub-polar regions.
Nor is that all; cold is not only where you can find ice, it is where you can make ice. 
Surprise.
So what are you waiting for? Fetch!
But the question of what to do with your ice once you have found it or made it, leaves you with whole ranges of problems. As I shall point out, the fact that there might be problems does not prove that ice is not worth finding, but it certainly means that we cannot expect half-baked ideas to solve all those problems.  We shall have to work hard to learn what to do with what we gain.

Some Practical Problems

...The secret fountains to follow up, waters withdrawn to restore to the mouth,
    And gather the floods as in a cup, and pour them again at a city's drouth
Rudyard Kipling, The Sons of Martha
Among all the practical problems we consider, we might as well start with the sheer scale of the need and the supply. Just moving the equivalent of a supertanker of fresh water is no joke. Let’s suppose it carries half a million tonnes — nothing special when one is carrying only water; convenient dimensions for a full load might be something like 420m long, 60m wide by 20m high.  Nothing special, as I said:  barely six stories high; you could hardly get four football fields on top of it and you could walk round it in less than 15 minutes.
Now, if you really think that is nothing special, try lifting or dragging a single cubic metre of ice. If you use a crane, see what it does if you drop the block or bash it accidentally. Tonne masses are big and unwieldy, but million-tonne masses of ice are unbelievably worse. Try standing by a twenty-metre ice cliff (which would be very modest; Antarctic ice shelves rise some 50m above the sea surface, and extend about 450 below). Then try to imagine how you should go about loading that into your tanker or dracone. (If you need to know about dracones,  look up "dracone barge" on Wikipedia at https://en.wikipedia.org/wiki/Dracone_Barge .) 
Simple to load? Just scoop out the ice with mechanised giant ice cream scoops?
Not really so simple.  
Cold ice is quite a strong material, and the rate at which you would have to scoop it would be mind-numbing if you were to stop to think how much you would have to move to make it worth while; remember that to be worth collecting, water must be cheap and plentiful, and we need to think in terms of at least hundreds of thousands of tonnes per load, not single tonnes. And even "warm" ice — ice that is just about melting —  is a lot harder than ice cream.
And ice cliffs tens of metres high are shockingly treacherous, especially when warm. You can't just park next to them with your bulk carrier and start loading; you and your ship could be kilometres under the sea, crushed, your bones being picked over by hagfish shortly after the first unexpected calving dumps a hundred thousand tonnes of ice on you.
And if you think that towing a cargo of realistic size is a doddle once you have it loaded on a barge or in a dracone, forget it. Ocean-going tug work is some of the toughest and most hazardous on the water. It also is expensive. And slow. For towing any mass of the order of a large cargo vessel the hawsers are huge and may be kilometres long, deliberately being dragged through the water to prevent disastrous consequences if one breaks, which even giant hawsers quite easily do if there is a sudden change in tension. 
Well then, instead of towing, why not load up ice into your tanker and speed directly up to the customer city? 
Also not as easy as it sounds. What do you do with your ice when you arrive? Unloading will need to be fast if you don't want to go bankrupt with your ship in port, and you will need to park your payload somewhere practical as you unload it. If you want to melt it, that will take huge energy, and even if it had melted en route you would need pumping machinery of huge capacity to deliver it to your reservoirs. And reservoirs cost money too. And not just any harbour can accommodate large bulk carriers.
Might as well give up. Just sit in the corner and cry about how unfair it all is. 
Actually all these problems have solutions, some more promising than others. I shall discuss some of them, but all will need proper engineering and proper economic consideration before they deliver anything. 
And they won't be cheap either.

Tote that Berg!

And one who licks his lips for thirst with fevered eyes shall face in fear
The palms that wave, the streams that burst, his last mirage, Caravan !
And one — the bird-voiced Singing-man — shall fall behind thee. Caravan !
And God shall meet him in the night, and he shall sing as best he can.
                        James Elroy Flecker    Song of the East Gate Warden

The traditional scheme for farming ice, was to cut slabs out of surface-frozen lakes, and some of that still gets done, but it is not an approach of much interest to our topic. It has nothing to do with transporting water to places that need it in quantities that could quench the thirst of cities and agriculture. Its very objective is not the same; those people sold ice as ice, not as water. 
The needs they met were real needs, and they met them respectably, but they are not the needs we are confronting.
We really want to deal with sources that could in principle produce billions of tonnes annually, sustainably and without harming the environment; possibly even helping to conserve the environment. And we might be more interested in water than ice as such.
The earliest proposals were to tow icebergs to places like Arabia. It was the obvious option, but a non-starter in practice. Only small icebergs are towable, and they have inconvenient shapes and their behaviour under tow is very, very bad: they topple and yaw and split and all that. Also, they cannot be towed quickly, and not only do they melt continuously en route anyway, but moving them through the water really melts them quickly. To get some idea of how quickly, treat yourself to a soda containing a few sizable ice cubes. Select a well-behaved cube, push it under the surface with a drinking straw and gently suck up the liquid so that it must pass over the ice on the way into the straw. Within just a few seconds you will erode a hollow, and in less than a minute you can melt a hollow so deep that the cube can't slip away and you can drink freely. Pretty soon you can become adept enough to drill holes through more than one cube per glass of soda.
Now imagine what liquid seawater would do to any mass of ice passing through it day after day and week after week, such as would be necessary to deliver the ice over distances of thousands of kilometres. 

This is not my personal fancy, please note; people have experimented. The idea is a non-starter. 
In summary, most regions in need of water, let alone in need of ice, are hopelessly too far from any realistic route to be served by towing icebergs.
But even if you could show up off Rabat or Beirut or Kuwait with your modest little million-tonne iceberg, what now? Could you pipe it ashore before it all melted? Even if you could lift it out of the water and dump it on land, how would that get it into the water supply? 
Such practical problems are unending. Not that they aren't soluble in principle, but in practice many of them simply are not worth solving. 
Especially because, as I explained, a million tonnes of water is just a sip; you would have to keep the sips coming pretty fast if the infrastructure is at all to be worth developing.  

Let's think bigger…

Ocean-going tugboats are built for two purposes: to tow huge 
inanimate objects across the ocean at a snail's pace or to slam 
ahead at full speed into the teeth of a gale to come to the assistance 
of a vessel in distress. Of the two, it is hard to say which is the most 
exciting. Personally, I found the long slow trips towing a dry-dock, 
a dredger or even a whole factory in the shape of a tin-dredger a 
more exacting experience than the salvage business. For, during 
the long trips, the officer of the watch develops a tendency to gaze 
astern instead of ahead, which he will find a difficult habit to lose. 
When, later, he is on watch on any other ship's bridge, pacing up 
and down at the comfortable walking speed that is the secret of 
relaxation, he will often experience a sinking feeling in the pit of 
his stomach on seeing the empty wake. 
Jan de Hartog   A Sailor's Life 

The water from a billion-tonne iceberg would represent a significant addition to the water supply of even the largest city, but in every way would represent a challenge, both at the source and at the point of delivery... 
Now, there are other approaches to towing icebergs, really large ones, in particular tabular bergs from the Antarctic, but all of them present problems of their own. The process would be difficult and so slow that precious little of the payload would get anywhere useful.  It has been suggested that by sticking to cold currents such as the Benguela, that move from sub-polar to sub-tropical seas, we could improve the parameters. 

True. 

We certainly would be fools not to take advantage of them, but that is not nearly sufficient on its own. Even the coldest currents would not be cold enough to prevent ice from melting, especially when it is being towed through salt water, and as it approaches regions where the water is wanted, the air temperature would rise dramatically, causing faster melting above water level.
 
The natural lifespan of a large tabular berg in sub-polar water, if it has a mean diameter of kilometres, is a few years, which is too short for towing even if towing were practical, and what is more, if we put everything we had into towing it, its lifespan probably would drop to less than a year. We would be putting all that effort into re-dissolving nearly all that water back into the sea instead of delivering it to a thirsty land.
Not attractive.
We could improve the balance sheet dramatically by coating the underside of the berg with sheets of plastic, using automated, remote control underwater craft, but the sheer scale of the effort would be sobering. For a comfortably sized chunk of floating shelf, say some ten kilometres across, we would need over 100 square kilometres of material.
That is not in itself an unrealistic investment in material, but as an engineering feat it would be shocking, and recovering the material afterwards, or ensuring that it would recycle harmlessly into the ocean in the form of innocuous fish food, would also be demanding. 

Ensuring that the jacket would survive the trip long enough would be a serious challenge in itself; wave erosion and impact are shockingly powerful forces. Amylose film rather than non-biodegradable plastic plastic jacketing might do the trick, but I am not sure that any realistic plastic jacketing would last well enough on the business leg of the trip. At present amylose certainly would be too costly, but one never knows...
Still, it does open tempting lines of thought. We may return... 

What is so special about towing?

We need to quit arguing about whether the glass is half full
or half empty — and instead acknowledge that there's not
quite enough water to go around.

                                        Kate Brown

When you come down to it, the idea of towing icebergs is na├»ve. Towing them certainly does save all sorts of complications and ships, with no problems other than the loss of practically all your payload and the need for special facilities at the delivery end. 
But with problems like that, who needs droughts and disasters? 
Well then, that doesn't sound encouraging, but do we have alternatives? 
Yes. 
Two forms of alternatives at least. 
Firstly we could collect the water at the source (meaning mainly ice shelves in Antarctica, and glacier calving areas in the Arctic). 
Alternatively we could let a berg drift as it pleases, but begin by selecting masses suitably situated for currents and winds to deliver them efficiently; we then could proceed to parasitise them while they travel, benefiting from the distance that they drifted spontaneously. To do that we could accompany them with ships and equipment with which we could carve them up or speed up their melting. 
What would that achieve? 
To begin with, any payload that we could get on board a suitably designed ship or barge, no matter how slow and cheap that ship might be, we could move much faster and more efficiently than would be possible by towing the berg, and the ship could be far less dependent on wind or current. Once ice or water were loaded aboard, it would be delivered almost loss-free, without any race against melting into the sea. 
We also would need no more infrastructure at the discharge end than pumping and storage facilities. No magic iceberg-handling would be necessary. If the payload were conveyed by dracone, it wouldn't matter whether much of the ice had remained unmelted or not; the towing vessel simply could deliver the dracone as temporary storage. After delivering the dracone, the ship could prepare for the next voyage and leave with empty dracones as soon as refuelling etc were complete, possibly even before unloading the payload had begun.
In short, no towing of any iceberg would offer any attraction unless there were reason for positioning or rotating a berg. For example, a berg in a region of water too cold to encourage melting as required, might be towed, or at least nudged, towards warmer water, or if selective freezing were required, to colder.

What Sort of Vessel Should Convey the Payload?

We forget that the water cycle and the life cycle are one.
                                    Jacques Yves Cousteau

Especially in the early days of gaining experience with polar water transport, we could experiment with second-hand tankers or tugs, but as experience accumulated, we might consider designing dedicated vessels for any of at least three options, all of them on a very large scale. 
Buoyancy should be no problem, because fresh water floats on seawater, and so does ice. Nor does fresh water in trivial quantities such as a few trillion tonnes pose any significant pollution risk even if spilt wholesale. So, laden or not, neither sinking nor pollution should pose any special risk.
Dracones, giant sausage-like balloons containing thousands to hundreds of thousands of tonnes of fresh water or ice, have certain attractions. The filled dracones could be connected in chains many kilometres long, and towed by tugs. This is no novelty; dracones have been used in similar ways for other types of cargo for many years, though on smaller scales. They have many advantages in flexibility and low overheads. This approach could solve the storage problem at the port of delivery, and also problems of necessary delays for melting residual ice so that it could be pumped ashore after docking; once delivered and secured, a dracone could be left behind for as long as desired, while the towing vessel immediately proceeded with its next task.
All the same, towing of any gigantic load of liquid in any form probably would be slower and more costly in energy than conveying the same load in a rigid vessel. Possibly some sort of resonantly contracting design of dracone would be specially efficiently towable through water, but that remains to be demonstrated, and the dynamics almost certainly would be complex. It is not clear for example, whether large fluid-carrying dracones should contain internal baffles to control resonant internal sloshing, or they should have exterior contours or appendages to improve control and attitude in the water. The practicalities and economics would have to be assessed in each context. 
Tankers for carrying massive cargoes of fresh water could differ from any previous designs of tankers in several ways. Unlike the largest of oil supertankers, they would demand little precaution against pollution, because only the ship's fuel would be a serious hazard. A few million tonnes of fresh water spilt into the open sea would be a major monetary loss, but no more of an ecological disaster than a rainstorm at sea. Accordingly, double-walled construction and similar precautions need not be considered unless it were thought worth the extra expense and weight to reduce the risk of loss of the vessel. 
Again, the size of the ship need not be limited by anything other than the available facilities at the designated ports of delivery and the hazards at the points of collection. In contrast to oil tankers, freshwater tankers carrying volumes exceeding a million tonnes could be routine. Any baffles, separated tanks, cargo containers, and leakage protection within the ship would be matters of detailed design, rather than basic problems. 
A third option would be giant barges. In essence they would amount to the equivalent of the tankers, except that they would largely be unpowered except possibly for manoeuvring, pumping, and crew accommodation. They variously might be manned or unmanned. They might or might not be connected in strings for towing, and they might be used for storage as well as transport, but independently of such considerations, they ideally should be very, very large, much like the powered tankers. 
The advantages of each option would depend on the nature of the collection mechanism. For example, parking a quarter-million-tonne ship or barge anywhere near an iceberg or ice cliff, or even aggressive sea ice, might be suicidal. It might prove more practical to collect the ice or water with fleets of small foraging vessels that capture it and process it before passing it on to the transport vessel. 

   

Why ice?

People say that if you find water rising up to your ankle, that's the time
to do something about it, not when it's around your neck.
                                                Chinua Achebe

Ice has all sorts of disadvantages compared to water — harder to load, less dense, further to fetch, needing heat to make it usable, dangerous in large masses, melts inconveniently, can't be piped — the list goes on.
But some of those just can't be helped; if fresh water simply were available in any desired quantity wherever wanted we certainly would not consider prospecting for it in the form of far-off ice. As it is increasingly at a premium however, or simply unavailable, we must go out of our way to get more, or stop moaning. 
And from a different perspective some of those problems look more like opportunities.
Harder to load? Well, in some circumstances ice certainly is hard to load; it is not easy to beat efficient pumping of liquid water. All the same, as we shall discuss, suitable preparation and equipment can be developed for collecting sea ice, crushed ice, and slabs of ice. We still have a lot of infrastructure and technology to develop, but new infrastructure and technology are necessary for exploiting any new opportunity. And once properly loaded, ice can't slosh about and endanger the tanker, which can be quite a problem with liquid water.
Dangerous in large masses? Too true; terrifyingly dangerous and in various ways too, but liquid water in large masses is no less terrifyingly dangerous and in various ways too. Each needs its own techniques and precautions — there is no value to whingeing about it; buckle down and earn your winnings.
Less dense? Meaning that your ship can't hold so much? That also means greater buoyancy, which means that the larger storage or transport ship may cost no more to build than a ship designed to carry the same mass of water. And the stiffness of a mass of ice can be exploited to reinforce a suitably designed ship rather than decreasing its stability the way that sloshing liquid or slush would. 
And the lower density means that it can burst pipes when it freezes? True, but that is more of a problem in buildings on land than ice freighters at sea. Routine problems like that are easily dealt with after the first few sinkings have taught us the first few lessons.
Further to fetch? How sad. But that only is relevant when nearby fresh liquid is available. Where there is none nearby, even distant ice can be worth fetching.
All these simply are realities to be approached intelligently and positively in context. That is what engineering is for.
Mining or farming sub-polar ice also offers vital advantages over exploiting seriously inadequate freshwater resources in temperate or torrid regions. The ice is plentiful and is constantly renewed whether we love it or loathe it, both in the Arctic and Antarctic, and it offers hopes of dealing with increasingly ominous threats of global warming.  For example, removing sea ice cover increases the rate at which cold air can produce ice, and accordingly also increases the production of surface brine that contributes to the natural cycle of cold water and carbon dioxide to the depths.

Such capture of carbon dioxide is regarded as very, very important, fresh water or no fresh water.

Ice types and harvesting strategies

           If you don’t think too good, don’t think too much.
               Yogi Berra


Not all sub-polar ice is the same. In this topic the main categories from our point of view are firstly drift ice, ranging from perhaps 10 cm thick up to say three metres thick. 
Secondly there are ice shelves and glacier calvings floating out to sea as they lose contact with land. 
Thirdly there are the large, irregular icebergs typical of those in the Arctic; they too are from glaciers, but under circumstances different from those in the Antarctic. 
Instead of mass ice, drift ice several years old could be precious, being easier to harvest. 

Preferably we would look for sheets a metre or two thick, or fairly undistorted floes. Sheets of drift ice could in fact be so precious that prospecting for them by satellite should be rewarding. Such ice is fairly salt-free and could be collected by fleets of harvester vessels. 

The design of the harvesters could be based on modifications of ice-breaker principles; unlike the traditional icebreaker, that breaks through floating ice by riding up on it and breaking it, a harvester could invert the process by sliding beneath the crust and raising the ice in strips a few tens of metres wide and stacking them on board till the load reached capacity. It then could return the booty to the dispatching facility for processing while the harvester returned to its floe nibbling.
Other cheaper utility shuttle vessels could scavenge free-floating blocks small enough to fish out of the water, a few tonnes or tens of tonnes at a time. One cannot always expect ice to break neatly. 
Because old ice is so much more valuable than young, it probably would be worth maintaining satellite surveillance of young ice fields until they were ripe and had thickened and shed their brine, rather than attack them while still green.
Harvesting such well-formed sheets of ice should be relatively safe and profitable. A fairly small field, say a hundred kilometres square and with a mean thickness of about 1 metre, could yield about ten billion tonnes of relatively pure ice in manageable form. Of course, that would take some thousands of ships to collect all the harvest and deliver it to the client countries and cities. The clients in turn would need facilities to handle the imports, but those need not be any more demanding than damming and treating the water of major rivers, and a good deal less costly. 
Young sea ice, one or two years old, generally would be less valuable, because it is saltier, but it still is much less salty than seawater. So where there is no convenient pure ice, it still could be worth harvesting young ice. 

The big lumps

              When the going gets tough the tough gets going.
                                              Anon

Calving glaciers and irregular icebergs might well prove too dangerous to be worth harvesting, but their substance is tempting, being large masses of practically pure water. Practically all the major bergs originate on land, from snow and other precipitation, so they don't contain significant amounts of brine. Even those that originate from collisions of exceptional masses of drift ice generally are old enough to have lost most of their brine. 
So if we could find efficient and effective ways of cleaving them into harvestable slabs, that would be nice. I have no firm suggestions as yet, but explosives or injection of compressed gases or seawater might be used to smash bulk ice into harvestable sizes or to induce calving. ANFO probably would be the cheapest and safest explosive, but liquid oxygen mixed with fuels such as propane might have advantages of speed and cleanness. 
Instead of drilling into mass ice, we might find that light artillery specially designed for shooting charges into dangerous ice masses, might work rapidly and efficiently enough to be valuable. After all, they would never need to work at ranges of more than a couple of hundred metres. The propellant could be a gas/air system such as propane. A suitable gun based on such a principle would not need any propellant cartridge; Diesel-type compression could render ignition unnecessary. 
Ice shelves attached to land might best be avoided for reasons of safety and possibly ecological considerations, but detached or detaching floating shelves of pure drift ice that simply would melt uselessly as they drifted towards the equator, should be worth intercepting in time for harvesting. A fairly realistic tabular iceberg with a harvestable thickness of 100 metres and an area of 250000 square metres (say 5 km square) should yield about 25 million tonnes of fresh water. How to harvest it is a more complicated matter, because one cannot simply skim it like cream or sea ice. 
Really large floating ice shelves, such as those of the order of 10 billion square metres would be very valuable in principle, but it would hardly be possible to harvest more than a fraction of such a shelf before it broke up. 
All in all, sea ice seems to be the most promising large scale resource. 
Still, it might be possible to harvest enough shelf ice to be worth while, especially in very high latitudes, where freezing winter cold alternates with summer temperatures above freezing. As we shall see, such alternation could be exploited in various ways. 
As already noted, explosive nibbling designed to detach harvestable chunks in huge quantities should be practical, and particularly valuable where the ice is weak and starting to melt faster in warmer water. 
It might even be worth exploring options for tethering major ice shelves that threaten to separate and drift wastefully away. Nowadays we usually have several years of warning of the separation of huge shelves. If we could slow down their separation enough to match the maximal rate of nibbling at the seaward margin and melting from beneath, that would pay for quite costly tethering.
But, tethered or free, where the shelf still is cold enough, thick enough, and stable enough it might be worth installing our equivalent of ice-cream scoops, though the resemblance to your familiar retail ice cream scoops would be remote. Devices like the leviathans used in open cast mining of coal could strip huge areas of ice onto transport facilities for loading onto barges or into dracones. As the upper surface was stripped from a shelf of say, 100 to 500 metres thick, the ice sheet would float progressively higher till the shelf in the mined region became too thin for stability. Then the whole lot could up sticks and move on to thicker ice. 
If it proved practical and worth while, the final residual layer could be broken up into suitably sized blocks for direct loading. 

After retrieving the valuable equipment for the next sheet of course. 

It is not clear that this strip-mining approach would compete successfully with collection of thin drift ice or nibbling at the edges of shelves with explosives, especially in the early years of the industry, but exploration of those options we may leave for future generations. 
The fact that ice shelves are melting from beneath and from above suggests other approaches. In the warmer sub-polar regions suitable pigments on ice shelves could collect sunlight to create ice lakes kilometres across and many metres deep, well worth pumping directly into barges and dracones. No ice breaking involved. Just spray your pigments, such as carbon or dark clay or soluble dyes and wait till next season to start pumping.
Irregular bergs probably could not be scooped easily enough to justify the installation of equipment in the same way as big shelves, but they might well reward explosive carving or smashing into blocks for loading. Similarly, the edges of tabular bergs could be cleaved vertically or nibbled into loadable blocks. 
The fact that fresh water floats on salt has other promising implications. Barriers of biodegradable polymer foam could be extruded by robot underwater craft, forming fences beneath ice shelves in suitably chosen locations. The barriers' buoyancy could hold them against the underwater ice ceiling. Any molten fresh water would remain pressed against the underside of the shelf by its buoyancy. Alternatively, the craft could carve hollows beneath the ceiling by directing seawater jets upwards. In either case, holes drilled from above down to the underwater domes could enable freshwater melt to float upwards for collection. 

Because of contact with the seawater, much of such molten ice would be brack, but as noted elsewhere in this article, even reasonably brack water is valuable. Depending on the most profitable options, it either could be delivered to market directly, or pumped into ponds on the shelf surface to freeze into plates of usable purified ice in winter.

Deliver ice or water?

Diarrhea, 90 percent of which is caused by food and water contaminated 
by excrement, kills a child every fifteen seconds. That's more than AIDS, 
malaria, or measles, combined. Human feces are an impressive weapon 
of mass destruction.
                                                            Rose George

Delivery of either ice or water has its attractions and each presents its own problems. Water is easy to pump aboard and ashore, and it is compact, either in itself or if we use it to fill the gaps between ice blocks in storage vessels' holds. But it also needs special precautions to handle at sea and it cannot be stacked like solid ice blocks. Also, its capacity for storing cold is small, compared to that of ice. In industry concentrated cold can be just as valuable as heat. That is why we spend money on freezers, heat pumps, air conditioning and the like. 
Accordingly, massive ice delivered to warm regions where suitable infrastructure is established could be valuable in cooling and drying air, and in the process it could collect a fair amount of condensed water from the warm, humid incoming air. The cold also could be used in heat pumps to freeze seawater from which pure water could be collected. 
Again, seawater warmed by solar power or heat pumps could be used to humidify air that could melt or carve mass ice while yielding an extra profit in desalinated water.  Air that had been heated and humidified by passing it over ponds of sun-heated sea water, then cooled and dried by passing it over ice, could be used for air conditioning, much as one can use waste heat from power stations for combined heat-and-power schemes. 
Ice delivered in such masses rather than in loadable blocks would be difficult to get off large ships and therefore would be something of a liability if it took months to unload. This would be a good reason for using low-value dracones or barges for transport. Such ice-laden vessels would serve as storage buffers while the load thawed and got pumped ashore as required. 

Meanwhile the tugs could have returned polewards, taking previously emptied vessels with them. They might well have fetched a few more full loads on successive journeys by the time their previous load had been consumed. 
Brackish ice might be expected to produce more saline water from its lower levels as it melted, the upper levels being effectively pure water, while the brack material could be desalinated and the brine dumped. Some brine would remain after desalination of brackish water until it became too concentrated to be worth further desalination. In contrast to the residue from desalination of seawater, the volume of brine from brackish water might be too small to be worth attention, so there would be advantages to desalinating it to no higher concentrations than the local seawater could accept without special treatment. 
For one thing, that would remove the problem of disposal, because the brine then could be dumped anywhere into the sea without special precautions. 
The technology of managing such processes economically could become quite sophisticated. 


Why icebergs at all?

We buy a bottle of water in the city, where clean water comes out in its taps.
You know, back in 1965, if someone said to the average person, 'You know
in thirty years you are going to buy water in plastic bottles and pay more for
that water than for gasoline?' Everybody would look at you like you're
completely out of your mind.
                                                                        Paul Watson
Icebergs, especially large tabular icebergs, are very concentrated sources of water, and in suitable circumstances are less seasonal than drift ice. They also tend to be very low in salinity. This suggests  that it should take less energy to collect them than it would take harvester craft to retrieve drift ice. 
It also might be easier and cheaper to deliver potable water from icebergs than from thawed young drift ice that might be expected to be more saline. Accordingly it might be worth breaking large bergs into manageable blocks with explosives, as already suggested, after which the blocks could be loaded or broken up mechanically into loadable sizes.


Why not make the ice instead of collecting it?

The society which scorns excellence in plumbing as a humble activity 
and tolerates shoddiness in philosophy because it is an exalted activity 
will have neither good plumbing nor good philosophy: neither its pipes 
nor its theories will hold water.
                                                                        John W. Gardner
The primary objective is to deliver water to the thirsty clients, and for them to receive it exactly when required and pump it to where it is required is the obvious option. However, the entire operation is costly in energy, infrastructure, and human resources; the source is largely seasonal, and to consider every material saving and gain is in the enlightened interest of all parties.   
Polar ice is in many ways valuable, but it does not come presented on a tray ready for consumption, and the most convenient forms of drift ice often contain more salt than we would like, though only a fraction of the concentration in seawater. That does not mean it is valueless; we might well ship brackish water to willing clients for further processing, but at the same time, where local circumstances are suitable, we might have better uses for large quantities of such water than dumping it at sea. 
For example, imagine a giant tabular ice sheet near Antarctica, either still landfast or not yet about to fragment or proceed rapidly north. Imagine that we had excavated a large, deep hollow into it. During summer we could dump our marginal harvest of brack water into the hollow, where it would melt high quality, low salinity ice, increasing the volume of brack water, especially in autumn. During the winter its surface should freeze thick, say 30 cm to 1 metre or so, and the ice in such a situation should be of high purity and undiluted by seawater: Nature's free desalination service — or nearly free anyway. Come the harvest season in spring, such effectively pure ice would be easy to collect from the surface and load onto transport craft.
Given that once the ice harvesting industry had matured, there should be a large volume of dracones and barges continually available, but not yet fully loaded, or laden with brack water. They could be left for the winter to freeze, and their deeper layers would concentrate the salinity while their upper layers would improve in quality. Come springtime each vessel could be assessed for appropriate treatment, either discarding brine or preparing it for further treatment. Meanwhile it could fill up the spare space with good water pooled from other sources.


Why not pipe the ice instead of shipping it?

I love the sounds and the power of pounding water,
whether it is the waves or a waterfall.
                                    Mike May

This obviously sounds unattractive for clients near the equator, and there is no question of anything of the kind in the short term, but the need for water world-wide shows no prospect of decreasing. Meanwhile the population is increasing. There certainly will be an increasing need for global water reticulation. The necessary infrastructure will be far too huge to pop into existence suddenly. One of the early forms will very likely be large, long-distance ducts for shipping ice and slurry from the sub-polar regions to consumer regions.
Transoceanic ducts could be collapsible modular submarine pipelines of suitable tough plastic, each say a few kilometres long, with control and communication modules at one end or both, designed to control their buoyancy at each unit’s  appropriate depth.  They could very likely be used for communications and power transmission too, and maybe for certain types of material transport as well, very likely containerised. 
The water ducts could be say a few metres in internal diameter, at least partly driven by wave power. Modules would serve simultaneously as ducts, processing units, and buffer storage. Suppose an internal cross sectional area of ten square metres; then a kilometre unit would have a capacity of 10000 tonnes of water. A 1000 km pipeline could act as a buffer store for holding ten million tonnes. Not huge, but enough to matter. 
If a portion of the capacity were reserved for air to be fed into the warm end, it could be used partly as a source of water of condensation, and partly to melt or at least warm ice at the cold end. Where it is released at the cold end its excess pressure could be used as a source of power.  For instance it could contribute to ice breaking and loading.

Brack, schmack!

Seeing that our thirst was increasing and the water was killing us, while
the storm did not abate, we agreed to trust to God, Our Lord, and rather
risk the perils of the sea than wait there for certain death from thirst.
                                                Alvar N. C. de Vaca
As already mentioned, one thing we cannot expect is to get pure water from sea ice. Old sea ice generally has shed most of its brine, and ice shelf and glacier ice are largely old snow deposits, so such classes of ice generally have very low salinity, but in practice we must expect pollution from contact with seawater; for example shelf ice may soak up a lot of seawater because it is porous, and some of our most easily harvested ice will be just a year or two old — it still contains too much brine anyway. To shed much of that brine would take another year or two of warming and cooling by the seasons and of massaging by storms and swells. 
The old ice often is drinkable when melted, but not really up to standard for heavy irrigation and commercial potability. Really young sea ice is barely drinkable in times of desperation if at all; it might contain say 1% or so of salt. Seawater usually is somewhere between 3% and 4% salt, and realistic desalination is practicable up to roughly twice that concentration. 

However, the cheapest, fastest and most sustainable desalination is the purification of large volumes of slightly brack water. It certainly is better and cheaper than trying to desalinate seawater. 
Firstly, one does not simply chuck weak brine into the machine and get out pure water plus concentrated brine. Different strengths of brine require different treatments with different membranes and pressures and energy consumption, and disposal of concentrated brines is more difficult than disposal of weak brine. 
Accordingly, if one cannot provide the client with pure water, he might well be willing to pay for say 0.3% brine, about ten times weaker than seawater, so that he only discards about 10% of the output instead of 50%. He can do so without precautions against pollution, because at that rate the discarded brine is pretty close to the concentration of seawater. For many purposes a solution about fifty to one hundred times weaker than seawater is practically usable as is, with no more discarded output than from processing most kinds of fresh water. 
Mind you, do not take these figures too seriously; they are just to give some idea of the major principles. In practice desalination is not as simple as it sounds; cleaning, backwashing and so on make for some waste as well. 
The upshot is that the ice harvesters need not insist on pure-water ice, but would need to assess every major item the attacked or brought on board. They would not mix relatively strong brines with more or less potable water, and they would price the water according to its intended purpose and the amount and nature of purification or dilution it would need.  No doubt such things as dates of delivery, contracts, and special circumstances would affect prices too. 
Future markets in water promise to be a very interesting field of study and practice.