Desalination

Reader Nic White asks for some comments on WA Premier Geoff Gallop’s desalination plan, and I’m happy to oblige, as this is a topic I’ve been meaning to do some work on. We’re talking here about about desalinating seawater or groundwater for human use, rather than schemes for reducing salt inflows to river systems like the Murray-Darling, another big topic in itself.

There are two basic ways of going about this. One is distillation. The most common approach to distillation is to evaporate the water, leaving salt and other pollutants behind, and capture the steam, but freezing and vapour compression. The other is to separate pure water using reverse osmosis or electrolysis. The first approach is the traditional one, but it’s inherently energy-intensive, so unless you have a cheap source of waste heat, it’s becoming outdated. The top candidate at present is reverse osmosis, which involves passing the water through a membrane, and using pressure to reverse the normal osmotic flow from high salt concentration to low. The current energy cost is about 4.5KwH for each Kl of seawater desalinated (the cost increases with the salinity of the source). Electrical energy is required, so this would come in around 25c/kl. The main operating cost comes from the need to replace the membranes.

I found this report (1.2 Mb PDF) which focuses on small scale plants for remote areas, up to 50/kl a day or around 15Ml/year. I assume there are significant scale economies beyond this, but it’s worth noting that, unlike large-scale engineering works, desalination is an incremental option, so you shouldn’t have problems of excess capacity. Operating costs are estimated at 65c/kl for a source with 2000mg/l up to $1.89 for 35 000 mg/l (seawater), for an output salinity less than 500mg/l. I’d guess the optimal way to go would be to accept more saline output and dilute it with fresh water. At a rough guess, I think a larger scale plant could produce water with operating costs of $1/kl

Capital costs are about $1600/kl/day or about $5/kl/year for small scale plants. That’s $5 million for each GL of annual capacity, compared to $10/GL in the unlikely event that the canal alternative could be delivered for $2 billion[1]. Assuming BOOT financing as I did for the canal (a high-cost option, but I want to be fair), the annual capital charge would be around 70c/kl, for a total of $1.70/kl, before reticulation and any additional treatment.

At that price, desalination is a pretty expensive option, and I’d expect to see some fairly dramatic reductions in optional water uses, like watering lawns. Before going to seawater desalination on a large scale, it would be sensible to work through the cheaper options, such as conservation, repurchase of irrigation water and use of groundwater in appropriate locations. This large PowerPoint file has some interesting data.

The availability of desalination as a backstop also suggests we need to take a sceptical look some of the more overblown rhetoric implying that urban Australians are going to run out of water. If we conservatively put the cost of large scale desalination up to $2.50/kl and assume water use of 200kl/person/year (you could manage a suburban lifestyle, including a water-efficient garden but no lawn on half that) it’s still only $500/person/year or $1500 for a three-person household. Not trivial, but cheaper than broadband or cable TV.

fn1. In my post, I suggested staged construction costs of $3 billion (still v. conservative) which gives a capital cost of $15/kL capacity and an annual capital charge of about $2/kl.

8 thoughts on “Desalination

  1. You left out low pressure distilling, including evaporating into a suitable gas with low specific heat (low partial pressure), and freeze distilling. Both use little energy, and the former can use low grade sources of heat like solar energy.

  2. JQ,

    Not trying to nit pick, but had some trouble understanding parts of an other wise interesting article. Did you mean to put a $ sign in front of the energy usage?

    Also is that correct the $10/GL annual capital cost for the canal option or $10 mil/GL, I thought the other article said it was more expensive.

  3. If we planted mangroves we wouldn’t even need to desalinate the seawater for gardening purposes.

    In any case, does anyone happen to know roughly what proportion of national water consumption goes to lawns? Lawns on parks and ovals never see sprinklers these days, but they seem very resilient. They go brown but as long as there’s a bit of rain every few months, they bounce back surprisingly quickly. I never water my lawn, and it looks fine. To the extent that my esteemed fellow suburbanites do water their lawns, then, I predict that their usage would be pretty price elastic. Therefore I recommend a sharp increase in the marginal household water tariff as a first measure, before we build a chain of aesthetically dubious desalination plants from Maroubra to Mona Vale.

  4. Cheers John.

    As Perth is currently having serious power problems, a seperate issue, the energy useage of the two plans is going to be a very important consideration. Do the pumping energy expediture for the canal, as shown in your most recent canal post, outweigh the above energy useage for the desalination?

    One other thing, in the debate Barnett attacked Gallop’s plan on environmental grounds, saying it would cause pollution and major damage to sea life. Is there any merit in this comment at all, and to what extent?

  5. Sorry, but I don’t quite follow the clarification of distilling to include the other modes I mentioned.

    By the way, “freeze distilling” isn’t really distilling at all, just a metaphorical transfer of the term “distilling” to a very different process. The odd thing about sea water is that there is less salt in it than needed to come out with ice, so if you freeze it and let the residual brine leach out, you get fresh water ice.

    Interestingly, you can make really powerful home brew by putting it in a flexible container (or a very strong one) in the deep freeze, then carefully decanting the residual liquid to leave the ice. It isn’t pure alcohol by any means – indeed it contains more of the toxins that true distilling leaves – but it reaches about sherry strength. That’s far stronger than fermentation alone can produce, and it gets into your system better at that strength than pure alcohol would. Since cider is fairly clean and has sugar, this gives you the sort of drink farmers made in Maine winters. Don’t try this unless you are aware of your legal position, though.

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