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
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
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