This week the South Australian opposition leader proposed a desalination plant for Adelaide, more detail here as well.

The model for the proposal is the recently constructed desalination plant in Perth. This plant has some excellent features, it uses reverses osmosis which is the most energy efficient method that we currently have for desalination, and it is powered by renewable energy (a wind farm). So why don’t I agree with the proposal?

The motivation seems to largely be political point scoring – trying to be populist by saying that they can get rid of the water restrictions. The overall goal has to be sustainability in our use of water resources. Our supply of fresh water is limited, so it is very tempting to make some more from the vast expanse of salt water nearby, but we have to realise that there is an energy cost involved in this. Often evaluations of whether a desalination plant is worthwhile compare the monetary cost of water from different options, but this fails to take into account that the cost of the energy used in desalination may increase. Really it is a matter of shifting from one resource (water) to another (energy), and while there may be instances where this is a sensible move (perhaps in Perth, but I don’t know enough about the situation there. The small scale plant at Rottnest Island certainly sounds reasonable). The opposition’s motivation here ignores the need for sustainability, rather they want everyone to continue with our current water usage – in particular they mention keeping our gardens green – but we have to think about whether sustaining gardens consisting of plants unsuitable for our environment is a sensible use of our energy resources.

If a desal plant was to be powered by renewable energy then why would this be a problem? We have to consider our overall energy usage. To deal with global warming we need to greatly restrict our CO2 emissions, and realistic models on how to do this involve a combination of the use of renewable energy, and a decrease in energy use via increased efficiency. Is it efficient using desalination to provide water? At the moment we are very wasteful in our use of water resources – and this is exactly a problem that water restrictions address. Rather than desalinating seawater to water our gardens we should reconsider what sort of gardens we should have. We should use rainwater tanks, and recycling of grey water, recycling of stormwater using wetlands (as is done by the City of Salisbury) and so on. We should make the most of the water that comes naturally first. Essentially I think that the task ahead of us in dealing with our energy usage is difficult enough as it is, without compounding it by using it to solve our water problems.
It may appear that some water recycling methods / tanks etc are more expensive, but these are long term solutions for achieving sustainability, rather than the band-aid solutions of desal which relies on our uncertain energy future, and if used to enable continued inneficient usage of water will eventually lead us back to the same point anyway – so why not look at reducing usage now, rather than getting more water. As we are forced to cut emissions energy will be more expensive, and any economic advantages of desal may be eroded. We can’t just keep getting more water by using more energy – this is not sustainable!
I should point out that in the News Ltd article linked above, it is mentioned that the Liberals’ policy also includes recycling of grey water and rain water tanks – this is commendable.

It has also been suggested that this plan would help the Murray River, however the impact of taking 45 GL per year from seawater, while a large amount compared with the 119 GL per year we take from the Murray, is very small compared with the 12903 GL taken in total from the Murray-Darling system (source, pdf). This is not to say that reducing our usage from the river is not a good thing, but I don’t think that Adelaide has a large enough impact to use it as justification for a desal plant, rather we should efficiently use the rain that falls in the Mount Lofty ranges and Adelaide plains.

There is one other aspect that I would like to consider briefly – is there a significant detrimental environmental impact from a desal plant? At least one Adelaide marine biologist thinks so. It is easy to think that the sea is so huge that we can’t do that much damage, but I imagine that when people started building smokestacks they didn’t imagine that we would have a significant impact on the atmosphere. The problem here is that there could be some rather large local effects. As anyone who has been to an Adelaide beach would know, the Gulf St Vincent isn’t exactly the open ocean, so the salt removed might stay in the area. This could affect local ecosystems and the long term operation of the plant if its intake was increasingly saline. These problems aren’t necessarily insurmountable but could present some extra hurdles.