In Praise Of Desalination


As desalination projects have been completed in Sydney and Kwinana, with others under way in Melbourne and Adelaide, community awareness of their costs and impact increases. With this comes increased scrutiny, and then resistance, from environmental and community lobby groups looking to stall or completely shut down these projects. The recent concerns over sewage infiltration in Sydney and spying allegations in Melbourne highlight the high-stakes nature of these concerns.

But, are these fears misplaced — and do we have any choice but to use desal? The answer to the first question is yes — and no to the second.

On the matter of choice, like it or not, capital expenses are determined by elected officials: state and federal ministers. Their ability to deliver or otherwise on their promises is judged at the ballot box and so their decisions are largely governed by matters of risk and certainty. As far as desalination projects are concerned, it is the risk that we might run out of water — and the certainty that any water minister on watch when we run out of water will lose their job and their seat.

There are broadly four ways to supply potable water to big cities: dams, desalination, sewage treatment and re-use (which is broad) and stormwater harvesting. Water efficiency isn’t on the list — while I am a vigorous supporter of efficiency in resource use we will never reduce demand to zero.

Despite assurances from Herald Sun water expert Miranda Devine, more dams will not solve the certainty of supply problem and nor are environmental concerns the prime reason that more dams are not being built. Rainfall is difficult to predict in both time and space and there are a multitude of studies showing that rainfall patterns have changed since human settlement and will likely change as the effects of climate change make themselves apparent.

Stormwater harvesting already occurs to some extent in Sydney, but is often referred to with the delightful epithet of "sewer mining". In these projects sewage (which often contains large amounts of stormwater even though it shouldn’t) is removed from the pipe between customer and treatment. At this point the water is treated to a rudimentary level then used to replace potable water in irrigation and industrial applications. These projects are only minor in scale and do not produce potable water, so are therefore only ever going to be efficiency projects.

Large-scale, stormwater harvesting is technically difficult and as a result extraordinarily expensive, particularly in areas with peaky rainfall. Stormwater systems are designed with one goal in mind; getting rain out of the area in which it falls quickly so that no one drowns. Drains are sized for 1 in 100 year events and so spend much of their time with a tiny trickle of water in the bottom. This is obvious in the open drains of Canberra. During dry weather flows, there’s virtually no water to harvest, which means that these storm events need to be captured and stored.

Consider the difficulties of getting the water out of the suburbs quickly, yet stopping it where it meets the sea, and pumping it all to some sort of holding area to supply the city in between rain events. Then imagine installing all of this and watching it not rain for seven years. Again, certainty of supply not solved.

This leaves desalination and recycling sewage as supply options. Despite appearances, these two processes are largely similar; both rely on removing impurities from water, to a very high standard of cleanliness. The choice between these two is largely one of cost. For a sewage treatment plant (STP) to produce potable water it must be built from scratch with this in mind; they can not be effectively retrofitted. Using Sydney as an example again, that would mean replacing over 20 treatment plants. If they can not be demolished and rebuilt on the same site with no disruption to processing, then the sewage catchment network will also need to be reshaped. This is a very big job, particularly as the largest plants in Sydney are at Bondi, North Head and Malabar. That’s some expensive real estate to acquire right there.

Desal, on the other hand, has the enormous advantage of being able to be built virtually anywhere where there is access to sea water and electricity. It’s essentially a big white-good, which you just plug in and use to make water.

But what of the environmental concerns and limitations of desal?

Community objection to desalination has spread around the country, with a national body, CADI and regional collectives such as the Phillip Island Conservation Society. There are three main concerns specific to the process: energy use, brine disposal and pathogen infiltration.

Desalinating water requires (pdf) up to 4 kWh per kilolitre of seawater and recycling sewage consumes (pdf) slightly less than that at up to 3.7 kWh per kilolitre.

However, these figures only include the purification process energy use, and neglect any energy costs associated with collecting sewage or transporting water. In some situations placing a desalination plant close to the population could supply water for lower energy intensity than recycled water, delivered from a plant further away.

Plants planned and built in Sydney, the Gold Coast, Perth and Melbourne will have their emissions offset through the purchase of Renewable Energy Certificates and other state based mechanisms. The Sydney plant is powered (pdf) entirely by the Capital Hill wind farm at Bungendore, with the possibility of this linkage being used for innovative electricity demand projects which more directly track load and electricity generation.

Brine disposal is a bit trickier and depends strongly on manifold design and the prevailing water currents. The output from a desal plant is water roughly four times saltier than sea-water and about one degree Celsius warmer. Obviously, dumping this into a bay would be a disaster and would likely sterilise it. To address these concerns the plants need to meet strict environmental protection guidelines, including regular monitoring of the impacts brine disposal has on the local environment.

Australia is blessed with strong ocean currents all along the eastern seaboard and have used them to good effect for generations. The Sydney coastal STPs treat water to primary stage (essentially just solids removal, in the order of 30-50 per cent effective) then pump it out to sea to use the mechanical action of the current to finish the treatment. It sounds terrible, but it’s been working for years and will likely continue to do so.

Linked to the above is the issue of pathogen infiltration and the recent revelation  that the Sydney desal inlet manifold is but 2.5 km from an STP outlet. (Interestingly, the plant nearby is Cronulla STP which has the highest standard of treatment on the Sydney coast). A closer look at the desal process shows why this revelation should not be a cause for concern.

Desalination removes salt through a process of filtration; using what are usually Teflon membranes to filter dissolved salts out of water. It does this extremely well, as the final product is so clean that minerals are added back into the water. Salt ions are essentially just atoms, so this filter screens atoms from water molecules. All of the pathogens are made of atoms, many of them in fact, so any filter that can remove salts is definitely going to remove pathogens. What if the membrane tears? Then normal monitoring of salt content will spike ridiculously and the water will be embargoed until the hole is fixed. One does not need to test for pathogens if one is already testing for salt.

Desalination is not without drawbacks, but the question is not "is desal good?" but rather, "if not desal, then what?".

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