Urbanizing Deserts Part 2.5 Making it sustainably more on water

At the end of the last part of Urbanizing Deserts I went on a bit of a confused number crunching expedition as I was confused as to how the UN was estimating that people consumed 2000 cubic meters of water per year (528,000 gallons).  It turned out that they were using a method that included the amount of water used both directly and indirectly by humans, producing clothing and food is rather water intensive.  This was good news as it put our desert cities back into the realm of at least passably viable.  Anyways, this section is going to continue on some of the broad strokes technologies and tools that could be used to make it so our cities' citizens get to live comfortable low carbon intensity lives.

Some of the ways our Desert cities can stretch their water supply
Direct Air Capture (Two ways)

Using materials with a high affinity for water, researchers have been able to create a special type of Metal Organic Framework that for each kilogram of capture material they are able to capture 2.8 liters of water, even from air as dry as the Sahara (the Sahara has an average humidity of 25% and the material is known to work at 20% humidity).  To supply the 70 liters of water per person that Cape Town currently limits citizens to you would need 25 kilograms of material per person.  25 kilograms for something that captures water from the air sounds like a pretty good deal, unfortunately the material costs $150 dollars per kilogram (and that's before we have to account for the fact that we would need some extra material as a just in case, in tandem with no idea how long the lifespan of the material is)

More mechanically involved solutions from companies like Zero Mass Water use solar power to dehumidify air and capture the water generated.  According to their faq page their system produces about 6 liters of water a day at a price of around $6,000 US, that means that for each resident of our Desert city, if all of the water came from air capture, the system would cost $72,000 per person, not exactly the most cost effective solution for mega cities**.  These numbers would make it seem like harvesting water using dehumidifier technologies would be prohibatively expensive, what the math I showed previously ignores is the fact that Zero Mass Water is selling their product as a self contained stand alone product.  Their design integrates solar panels, compressors, and other equipment into, more or less, a single box, by doing this they have increased their individual unit cost, but decreased their infastructure costs (well eliminated them actually).  Developing industrial scale dehumidifiers that only do that task, and the power is generated somewhere else would cost less, but we now have an upper bound on costs.

From our basic math, it becomes obvious that air capture on its own is unlikely to provide sufficient cost effective water for our 1.5 billion future residents.  So let us look into water filteration technology (oh yeah we're getting super exciting here, less sarcastically, water filtration makes the world work and will likely be critical)

While there are more than a few ways to produce drinkable water (about 10 according to this wikipedia article) we are going to focus on desalination, the process of removing salt and other materials from sea water for human usage.  Now you might be asking, "hey Obie, I thought we were building in the middle of the desert, aren't we going to be a bit of a ways from the coast?"  Dear reader, you are not wrong, I shall explain my rationale (I hope).

Desalination generally encompasses two technologies distillation and reverse osmosis.  Distillation involves boiling water and then collecting the water vapor.  Reverse osmosis involves using massive amounts of pressure to push salt water through a series of filters to make it clean enough to drink.  Many environmentalists find the use of desalination technologies problematic due in part to the quantity of waste brine that is produced.  Desalination brine is the concentrated left overs from filtering the salt water, according to some estimates, for each gallon of fresh water you produce, 1 to 1.5 gallons of very salty water are generated.  Normally this salty water will be dumped back into the ocean, potentially doing massive amounts of harm to aquatic life near the desalination plant.  Now our desert cities are a bit too further from the ocean than most and they can use that to their advantage.  Instead of dumping brine back into the ocean, or into ecologically critical water tables, city developers could desalinate water, and store waste salt water in evaporation ponds.  These evaporation ponds would allow for less potable water to be evaporated off (it might be possible to capture this moisture, but there would be further economic considerations that are currently beyond my level of understanding).  The products within the evaporation include a range of materials, incluing simple salts and metals.  One major caveat with using desalination and brine mining as a part of our urban development requirements would be developing these resources in such a way that resources can be used sustainably.  If evaporation ponds aren't well built and closely monitored, it would be all to easy for hundreds of thousands of gallons of extremely salty water poisoning natural water sources used by desert creatures and indiginous communities.

For all of the concerns associated with desalination techniques, one concern that our desert residents won't have is the energy necessary to refine all of this water.  Current estimates estimate that drinking water derived from salt water would cost about $3.60-$5.80 for every 1000 gallons of fresh water made.  While these estimates don't account for the cost of transporting our water supplies inland, they also ignore any potential revenue made from brine (ok that's unlikely to really offset costs that much, but I wanted to be positive).  More pragmatically, while the evaporation ponds would be unlikely to produce a massive financial windfall, it would be reasonable, considering the value of water in the desert and how much sunlight there is, to have at least a particular stage of the evaporation pond process to occur in greenhouses that utilize some percentage of the waste brine.


The most important way to ensure that water is affordable is to use it efficiently.  Desalination, air capture, imported water, and recycled water are all well and good, but the water you don't use could be the cheapest.  For example when most Americans use their toilet, the water that is used to flush the water is just as drinkable as the water that comes from the tap (to be clear we are talking about the water that goes into the tank).  Many regions in the world are now embracing a more local life cycle for water that goes into homes.  Instead of using clean water for every domestic task flushing can now be done using "grey water".  Grey water is the water that is generated from tasks like washing your dishes, clothes, or your hands, basically the water that doesn't have poo in it.  This waste water is stored in local tanks which are then routed to toilets or to irrigation systems.  As the grey water doesn't have too many nasty bugs in it, it is generally safe for people to handle.  After the grey water is used in a toilet it becomes black water, otherwise known as sewage, and at that point you really shouldn't use it for traditional home use.  Current water management tools are limited to clean, grey, and black water, that doesn't mean that as tools get better we won't see further distinctions in water quality that will aid in efficiencies.  Our approach to bathroom design has been pretty good about increasing hygene, but only recently have we started seriously looking into making bathrooms water efficient, it is reaonsable that technologies could devlop that would allow for some amount of bodily maintenance be done with almost zero water useage, and the resultant products get reused as fertilizer creation (yeah this is gross but to make a civilization that makes people feel clean you gotta plan around a lot of wastemanagement)

Ok, I hope that was infomative, any questions and feedback are welcome, next post will look into energy production and storage and how those things might be treated in a modern city built from scratch.




*  (ok so I did some follow up on the number that popped up and it was some promo info for a humidifier supplier advertising in Oklohoma, shows me for using the google immediate response feature, that being said I did find a page that showed the humidity in the Mojave (similar desert different continent) ranged from 10-30% peaking at 50% at night so 25% on average felt reasonable) https://sciencing.com/humidity-mojave-desert-19526.html  my source, anyways, if anyone has a better source please feel free to leave a commenta nd we can fix it
** as we are using the Cape Town water alotment of 70 liters per person per day the work goes this way 70 liters/day divided by 6 liters/(day for $6,000 ) = 70/6= just shy of 12 and I decided to err on the side of caution

*** I didn't plan that paragraph off as well as I wanted 

Urbanizing the World's Deserts Part 2 Making it Sustainably

In Part 1 of Urbanizing the world's deserts we looked into how much land would need to be repurposed to allow for relatively high density cities.  To move about 20% of the world's current population into cities with a density of 6300 residents per square kilometer.  The total area of these new cities would be about 245,000 sq km or a bit smaller than the state of Oregon.  Building said cities in just the Sahara, would require that about 3% of the Sahara's 9.2 million sq kilometers.


No matter how you approach the problem converting 3-15*% of the Saharah Desert in relatively high density cities would be a massive undertaking.  Building these cities in such a way that the impact of building and maintaining them is done as sustainably as possible will be even more complicated.  This article will attempt to highlight some of the challenges and sollutions for developing sustainable cities in water poor regions of the world.

The two most abundant materials in building modern cities are also some of the largest sources of carbon dioxide emissions on the planet.  Concrete and steel production are responsible for between 15 and 17% of humanities carbon emmissions.    Making concrete produces roughly 8% of the world's CO2 emissions, due in large part to the heat and chemistry involved in the concrete fabrication process.  Steel similarly produces 7-9% of global green house gas emissions, where the average tonne of steel requires 1.83 tonnes of carbon dioxide to be emitted.  Efforts to find lower carbon alternatives to concrete and steel will be critical in helping to offset the impact of creating a new city.  A relative new comer to the construction industry is cross laminated timber, similar to plywood sheets but way thicker.  Cross laminated timber has already been used to build structures over 10 floors in height, more than tall enough to achieve a target population density of 6300 people per square kilometer.

Reducing the carbon impact of concrete on urban development is more complicated, concrete is one of the most commonly used materials by human civilization, only water is consumed in greater volumes.  Each year humans consume over 4 billion tonnes of concrete, emitting 1.5 billion tonnes of carbon dioxide.  Research is ongoing into ways to make types of concrete that produce fewer carbon emissions but the easiest way to reduce these emissions is to simply not use the concrete in the first place.  Some of the lowest hanging fruit for city planners to eliminate concrete from their cities is to drastically reduce private car ownership.  Estimates for the volume of parking spaces around the world vary, for the United States it is estimated that for our 327 million residents there are 27 thousand square kilometers of parking spaces.  To put that in perspective the US has roughly 1/5th the population of our future mega cities, and consumes over 10% of the land that our 1.5 billion city dwellers would need, just for parking.  Reducing private car ownership and promoting various forms of mass transit could drastically reduce concrete useage.**

Low carbon construction and design are important, but making buildings truly efficient will be critical to the long term sustainability of our desert cities.  As we are building in arid and semi-arid regions of the planet, one primary concern will be water conservation.  In 2018 Cape Town, South Africa was fast approaching "Day Zero" a cut off point where the city's water reserves would effectively run out.  At the height of the crysis Cape Town residents were limited to 13 gallons of water per person in each household, the average American directly consumes between 80-100 gallons per day.  According to the United Nations humans require around 2000 cubic meters of water (520,000 gallons) per year.  The Sahara, on average recieves between 1 and 4 inches of rain fall.  If we assume that our cities are built in the drier regions of the Sahara, so 2 inches of rain fall on an average year, it becomes obvious that the Sahara cannot support 1.5 billion humans from natural percipitation***.  Our future cities will most likely agressively capture and recycle as much water as they possibly can, but realistically some percentage of water used will need to be imported, thankfully enough our cities have a commodity to trade for their water, heat and electricity produced from the 3000+ hours of sulight they recieve each year.

Importing enough water for 1.5 billion people would be an incredible undertaking, so much so that it is likely to be unrealistic.  To provide 2000 cubic meters of water for 1.5 billion people, you would need 3,000 cubic kilometers of water.  For perspective so called super-tankers in the world can transport 320,000 cubic meters of water, which is only enough to provide enough water for 160 people for a year.****  For sake of continuing this series I'm going to act as if the complexities of water use are reasonably solved.

Thanks for reading, if you have any questions please ask, Part 3 will provide a narrative of what life in our cities might look like.

*15% was for the estimation that you were moving all of humanity into the Sahara.


** this article isn't saying that private car ownership within these future cities is an absolute no, I'm just suggesting that reducing private car ownership and promoting mass transit could free up a lot of land for other uses, and ideally be done without requiring as much concrete.

According to UN statistics the Earth is covered in 25.5 million square kilometers of Arid and Semi Arid terrain

https://www.un.org/en/events/desertification_decade/whynow.shtml

*** for those who want to see the math, 2 inches of water is about 5 cm (I'm rounding down but it gets us close enough considering the scope)

that means that to collect 1 cubic meter of water of rainfall, we need to collect all of the water that falls over a year for an area of 20 square meters

0.05cm water/year/sq meter/year *X = 1 cubic meter of water and X = 20 square meters  (ok I'm not showing the right units but I'm like 90% sure the math is good, corrections welcome)

to meet one person's water needs according to the UN we are going to need 2,000 cubic meters, that works out to 40,000 square meters per person

that means that 1 sq kilometer (1,000,000 sq meters) can, at best, with no recycling  but perfect water capture, 1 sq kilometer can support 250 people, less than 1/25th of the population density we require.

**** so it wasn't until I started to write this section that I began to appreciate how difficult it would be to transport enough water into our hypothetical collection of cities, that being said this assumes you're making your megacities all at once and they need all of their water all at once, the more realistic/gradual approach, is that you build smart sustainable cities gradually, building up sufficient water reserves before building new cities would make a huge difference.  Also I'm going to spend some time looking into the 2000 cubic meters per person stat, because when I compared the stat vs American personal water consumption there was quite a gap, ex. Americans consume 80-100 gallons of water per year, ok so that means that every 2.5 days we consume about a cubic meter of water that means in a given year Americans directly consume about 146 cubic meters of water, for things like drinking, bathing etc....

OK so I spent a few minutes looking into this and it looks like the water volume value comes from things like agricultural water requirements, for example, when you eat a donut, it took some amount of water to grow the crops that were used to make the ingredients for our donut, that is where the 2000 cubic meters comes from  https://www.theworldcounts.com/stories/average-daily-water-usage

alright, I will probably talk about updated numbers in part 3

Urbanizing the World's Deserts Part 1

There are a lot of people in the world, over 7.5 billion these days, and that number will continue to climb over the coming decades.  According to UN statistics, in 2018 53% of the world's population live in urban centers, this number will rise to 60% by 2030.  Urbanization, overall has been fantastic in helping to improve the quality of life of the world's population.  Greater density means that resources can be utilized to greater effect, meaning that per capita environmental impacts are generally lower for urbanites than their sub-urban and rural counter parts.  While many modern cities are working hard to further reduce their per capita environmental impact there is one inescapable fact, when a city is built or expands it is often displacing arable land that could either remain more wild or at least used as farmland.  As humans continue to urbanize we should consider the possibility of creating cities in places that cannot be used as farmland.

According to the United Nations over 17% of the world's land mass is either desert or semi desert (hyper arad and arad), land that historically has made very little sense for large human populations to gather.  What would it look like if humans were to begin moving their population into these desert regions by building sustainable cities of the future?

Before we look at the technologies and potential urban planning solutions that could be used to make these proposed desert cities sustainable, let us first look into the math of estimating how big these new urban centers would need to be.  Our first assumption is that these cities are unlikely to get their food from traditional farmlands located within the bounds of the city (this would still allow for either food imports, vertical farming, a mix of the previous two, or something entirely unimagined by this author).  The next assumption has to do with how many people will relocate to these new urban centers, the lower bound is obviously zero, for any number of reasons people just don't want to move to a manufactured urban center, the upper bound is almost 100% of humans, where authoritarian regimes have forced all people to live in these new cities, and only a few hold outs remain outside city limits.  The last variable (for the basic calculation) is the population density of our new cities.

Population densities can have a tremendous impact on how efficiently resources can be shared by residents of a city, but it also can impact your quality of life.  While it might be super efficient to have 50 people share a single dorm style bathroom many would consider that a less than desireable compremise on quality of life. Kowloon Walled City is considered one of the most densely populated urban environments in human history, at its height Kowloon WalledCity had an effective population density of 1.2 million people per square kilometer (I say effective population density as the city only covered 0.026 square kilometers)    To put things into perspective, if all humans lived at the density of the Walled City, you could fit every human being into an urban environment just about the size of Delaware  (this sounds like it could have a decent chance of being rather unpleasant).   On the other end of the spectrum we have the city of Anchorage Alaska, with a population density of about 60 people per square kilometer, at the population density of Anchorage, we would need to create a city that covered 128.3 million square kilometers (the entire Earth only has 510 million square kilometers of land)  A sweet spot might be something like the population density of a place like Boston Massachusetts, with a density of 5100 people per square kilometer, you would only need 1.5 million square kilometers of city to house all of humanity.  At this time you might be questioning the statistic used for the population density of Boston, as you have likely seen photos of Boston, with its collection of sky-scrapers and ability to play act at being a big city.  The reason for your confusion stems from the fact that much of Boston is devoted to things like office space for people who come into the city from other communities to work.  North of Boston in Cambridge the population density is much higher at 6300 per square kilometer, with Somerville taking the cake with an average density of 7100 residents per square kilometer, all without resorting to large tracts of skyscrapers (this may change over time).

For sake of argument we will assume our desert cities will strive for a population density like Cambridge, Massachusetts.  We will now assume that roughly 20% of the world’s population could be convinced to move into these new urban centers that means we are going to be making cities for 1.5 billion people, that is more than the population of either India or China.  These new cities will require roughly 245 thousand square kilometers (95 thousand sq miles or just shy of the area of the state of Oregon) of land to be converted into cities.  Now this amount of desert being converted into cities seems like a lot and in human terms it is, for comparison, the Sahara Desert is 9.2 million square kilometers (3.5 million square miles).  That means that our new cities would cover less than 3 percent of the entire Sahara, let alone the other large deserts of the planet.  It also means that if we decided to move all humans into the Sahara, we could do so with land left over*.

Thanks for reading part 1 of the desert development outline,  Part 2 will focus on some of the technologies and policies that might be useful to ensuring that people actually want to live in these new Desert Cities

*7.7 billion humans / 6300 humans/sq km = 1.2222 million sq kilometers

1.2 million sq kilometers/9.2 million sq kilometers of sahara *100%= 13.3 % of the Sahara Desert

Reduced per capita environmental impacts are beneficial, but we can do better.

In most instances when a city needs to expand, the city willwill do so by acquiring neighboring land and converting it into a part of the city, by building more dense structures.  This is natural and not unreasonable, unfortunately there are times where that expansion is done at the cost more ecologically sensitive regions

As humans continue to urbanize many urban planners and architects are working hard to make their cities as sustainable as possible for future generations

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