25 Mar 2015

Is desalination a boondoggle or helpful backstop?

JH emails:
I’m from Melbourne, and I was wondering if you could tell me more about you opinion on desal plants.

Melbourne only built the desal plant when our water levels looked dire due to a long drought. I was just finishing high school at the time, and remember how the newspaper began printing water reservoir levels in the daily.

In retrospect that the desal plant looks like an bad decision, but at the time I don’t really recall that there was significant objection to its initiation (there was a lot of political noise later due to its cost and delays etc.). Once the drought broke there was clearly no need for it, and it was a colossal waste of money. But what if the drought hadn’t broken? In that case perhaps the government that built it would have looked like geniuses.

Do you think desal plants have a role in preventing catastrophes if extreme droughts end up exhausting existing supplies, or do you think that with correct management and pricing this really shouldn’t be happening?
I replied:
The answer is "endogenous," i.e., there's no need for desal IF management keeps demand in line with supply, but that depends on supply "behaving." In the case of Melb, Sydney, etc., there was a doubt on whether supply would return to "historic norms" or if it was changing in an unprecedented way (i.e., Australia's 12-year drought), such that desal was necessary.

This doesn't mean that many cities and regions must go to desalination first. There are better ways to improve supply buffers for droughts (aquifers, then reservoirs), but desalination has the advantage of "making" water. The question is the appropriate scale given that big plants are more expensive. San Diego's $1billion plant can only meet about 6 percent of local "needs" that include lawns and pools -- so you can see that demand -- at 500+ liters/capita/day -- is still inflated there.

I think Aussies made the right choice (Sao Paolo is experiencing the results of the wrong choice), since it's better to have less money than no water. That's a hard point to make when rain arrives for "free"...
Bottom Line: It is useful to add desalination to the urban supply portfolio after all other demand and supply options have been implemented.


Anonymous said...

What about the impact of desal brine management on the environment? Will coastal desal plants have a negative impact on the ocean disposal sites?

David Zetland said...

They can (e.g., Persian Gulf) but need not (e.g., long outfalls with many holes for diffusion). It costs more, but some governments don't care.

John Troughton said...

Australia has $6B of stormwater untapped in cities. Will put off need for desal for 30years.

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