Sunday, July 27, 2014

Climate change, desal and astronaut water

As Eli points out, California has a water crisis, and much of the rest of the country needs to be much more water efficient. Water efficiency is the obvious place to start but then the next-step question comes up.


While plenty of people don't live near the ocean, lots do. Oceanside areas with large populations are going to have wastewater treatment plants (in developed countries, anyway).  These places therefore have two potential sources of new water supply:  ocean desalination or potable reuse of recycled wastewater.

Potable reuse of wastewater is nothing new. Almost any city drawing water from a river which has another city upstream is already doing it; the question is can we do it without lying to ourselves. In the case of astronauts and the International Space Station, they can do it outright, but the rest of us have to catch up.

Or not. Ocean desal actually uses the same technologies that potable reuse requires, either distillation or more commonly through reverse osmosis. The difference is that ocean water has a lot more stuff in it (mainly salt) than wastewater which already has to go through some purification before it reaches your reverse-osmosis system. That means a lot more energy and cost is involved in ocean desalination than potable reuse, so we've got a climate change issue.

The other climate change issue is that the lack of water currently stops a lot of unwise sprawl development, but ocean desal could change that, or maybe even mandate it - a very expensive desal system could be built on the expectation that there will be a lot more development to pay for it. I suppose there's some sprawl risk from potable reuse as well, but because it functions best in an existing populated area, starting at the wastewater treatment plan and then spreading from there, the risk is lower.

Many other factors involved of course, but these are the main climate issues. All but one of the factors weigh in favor of potable reuse. The one factor favoring ocean desal is psychology and political acceptance. People hesitate to drink this water, and that hesitation killed an earlier potable reuse project in San Diego (p. 17).

I view desal and portable reuse as being in a race. Money is limited so communities are going to prioritize. As much as I can I've supported potable reuse and opposed desal.

First step for potable reuse is Indirect Potable Reuse, achieving psychological acceptance by making the treated water sit somewhere for a while before reuse, either in a reservoir or underground. It's good but maximum flexibility and less cost require Direct Potable Reuse, shunting the water to your drinking water plants.

At my water district we've set up a reverse osmosis system. Currently it's just to improve the quality of non-potable recycled water which will help with certain types of uses, but the goal is potable, if we can get public acceptance.



Note:  stumbled across this - Los Angeles actually constructed an indirect potable reuse plant in the 1930s, but shut it down when they acquired Colorado River water. Back to the future, like with electric cars.

Also, desalination sometimes refers to desal of brackish water, usually groundwater. This water is much less salty than ocean water so a lot of the energy concerns are reduced with brackish desal. But brackish water and even potable reuse require a fair amount of energy, just nowhere near as much as ocean desal.


12 comments:

Anonymous said...

I suspect the mining of those under-sea fresh water pools will likely commence, though waste water re-used makes most sense.

Perhaps that's a good problem for solar energy too solve? Especially so, since we don't care if water is processed only during daylight, as long as there is enough for all the boys and girls.

Anonymous said...

If desal could be made less energy intensive, it might be a better source of water for Southern Cal compared to pumping Northern Cal water over the San Gabriel mountains. But query if desal would ever be cost-effective for ag uses?

Bill W

Graydon said...

Or you can go with an actual intermediate step, like running the processed-to-potable water through greenhouses, and pull the potable water off the evaporation side from the greenhouse.

The problem with direct re-use is that it will fail at some point, and it's continuous; lots of people die if e coli gets in the drinking water, and people know that. So they stop trusting tap water and bottled water -- which is certainly *also* an issue, due to the waste bottles, shipping, etc. -- will sell more.

So while re-use, "close the loop", etc., about water is obviously necessary, it can't be direct because direct requires trust of (in many places) semi-private or private monopolies, which really don't deserve that trust, as well as not having it.

Anonymous said...

desal has the problem of what to do with the sal - can it actually make the ocean too salty if dumped back in? I seem to recall so.

Russell Seitz said...

Since Brian does not suffer from delusional fears oof second hand grey water, one hopes he will apply the same criteria to second hand air, as has a Eastern politician:

The compost fire at the former Fresh Kills landfill site on Staten Island is out, but health concerns about the blaze remain.
Many Staten Island residents are worried they have been breathing in toxic smoke from the blaze and now councilmen James Oddo and Vincent Ignizio have asked city agencies to release data from the air monitoring stations downwind of the landfill.

“There’s no health issues here, you have fires all the time, smoke goes off into the air,” Bloomberg said.

Bloomberg said the fire shouldn’t pose a health risk because what burned was mostly old Christmas trees and trees that were knocked down by Tropical Storm Irene.

-- The Yeshiva World News, April 11, 2012

Para a Posteridade e mais Além said...

that depends usually groundwater have salt diapir's near by in many many places .... This groundwater is fossil water and only a small % have more than 0,5% of salt or 5 parts in 1000 or 5,000 ppm of ionic compounds ...some groundwater water is much more salty than ocean water 35g per liter or to thousand g's

but so a lot of the energy concerns are reduced with brackish desal.....why? you can build a solar desalinator and desal---easier in warm hypersaline waters

and you have the salt of danakil to prove it ....

solar energy is free

brackish water .....not

Fernando Leanme said...

We have a reverse osmosis unit in the city where I live. I have a reverse osmosis unit in my apartment because city water is a bit too hard.

The city has a huge underground water storage tank on the hill by my apartment, and i heard from a neighbor they have an interruptible supply deal with the power generator so they get cheap electricity when the wind turbines are running, and can be cut off as required to help keep the grid safe when the wind dies.

I suppose this system we have works just fine, but our water bill is high. And in the summer when the city fills with tourists the water does get a bit saltier. This means many hotels set up their own reverse osmosis units.

My draw is to use reverse osmosis, build a lot of fresh water storage, make a deal to get interruptible electricity for the reverse osmosis unit pumps, use sea water and deliver medium hard water to the city. But if we start using recycled water the tourist business will suffer. And the city makes a lot of money from tourism in the summer months.

Dano said...

If I may, our potable reuse here in the semiarid Intermountain West is having negative consequences on our urban landscapes.

The salts are too high in our alkaline soils and the confers are starting to die.

Best,

D

imback said...

So wait a minute. You're saying you want us to put water on the crops? Water? Like out of the toilet?

Brian said...

First anon - some of the brackish water is under/near the sea, like here in the SF Bay area.

Bill W - there's a physical limit to how energy-efficient desal can be. I'd be surprised if it's better than pumping over Tehachapis, especially because you can recover 80% of the power on the downhill side.

Graydon - failure's possible with any system, but PR water is safer and less risky than the river water we'd mix it with.

2d Anon - yes leftover brines are a problem with desal. The same is true for PR, but to a much reduced extent.

Russel - I have no fear of second-hand air, but I do fear second-hand gray air full of microparticles. Those natural smoke particles are dangerous.

Fernando - water storage can certainly help with energy issues, although storage has its own costs too.

Dano - sounds like recycled water, not RO systems which should have pretty low salt content.

Imback - yes, but cleaned up. Like we do right now without thinking about it, and doing it better.

Aaron said...

In the old days in California, rain came down sat in ephemeral wetlands and recharged aquifers. Now, much of that water is diverted into drains and discharged as storm water.

In the old days the ephemeral wetlands warmed in the spring, producing water vapor that was blown east and which condensed on the Sierra to top off the snow pack.

In our subdivision, we have clay soils and all lots including lawns and garden areas have drains, so that any rain of any significance produces standing water which is drained away through architectural drains. The result is that we can have a good rain and a couple of days later the soil is dry and people are watering again -- rain water that should have become soil moisture became storm water runoff because of landscaping practice.

My estimate is that half of the total rain fall that lands on lawn and garden area goes off as storm water run off. Add in run off from roofs and streets and likely 2/3s of the total rain on the 200 acre subdivision goes off as a storm water drainage. We get just under 2 feet of rain /yr so call it 240 acre ft/yr.

And that is for one subdivision with ~800 lots in it.

One acre ft/per year is the approximate use of one household.

California has gotten into the habit of pouring nice fresh rain water down its drains. We have drains everywhere. And, these surges of water down concrete channels that sit dry between rain events do not really help fish and wildlife.

Rain water is cheap, and it is already in concrete channels for collection. The problem is that storm water in concrete channels comes as surges that are hard to store.

or dumped in the roadsin winter time said...

the brines can be used in several industrial process'es