What are the options being considered for a desalination plant in Sydney?
The State government has (so far) mentioned two options for the size of a potential Sydney desalination plant. A
100 ML/day plant and a 500 ML/day plant. Sydney's usage is about 1500 ML/day. The desalination plant
(or plants) would be placed in the eastern suburbs of Sydney and provide water to the
eastern suburbs primarily through existing pipes. The energy minister, Frank Sartor, said that the plant
would look much like a Bunnings Warehouse. The drawbacks of a desalination plant are its high start-up cost
and high electricity running cost.
How much water would be in our dams now if we already had a desalination plant in Sydney?
The following table assumes that Sydney's usage and evaporation are the same with a Sydney desalination plant as without one.
We consider the following Scenarios: The "As is" conditions (i.e. what has actually happened), the result of a 100 ML/day desalination plant running continuously,
the result of a 500 ML/day desalination plant running continuously and the option of a 500 ML/day desalination plant which is only switched on whenever
Sydney's dam levels are below 50%.
|Hypothetical since 11.2001||Available water (GL)||% full all dams||1year change GL(%)|
|As is || 2500.5
|100 ML/day|| 2551.25
|500 ML/day|| 2552.85
|500 ML/day when levels < 50%|| 2826
What does this show us?
We can see that a 100 ML/day plant does not solve our water problems by itself. Such a plant would need to be run continuously and we would still be under
water restrictions with such a plant. A 500 ML/day plant which is only switched on when dams fall to 50% of capacity would save electricity (given the history
of our dam levels) and allow for a much more severe drought because it can provide a lot of water when we need it and be moth-balled almost all of the time. Of course,
if the current drought is a sign of climate change rather than a temporary lack of rain then only a 500 ML/day plant will allow us time to take action. It is also
interesting to note that dam levels would still have dropped under a 500 ML/day desalination plant in Sydney, so if climate change is occurring then this may not be enough!
Can't we just cut down our usage a little more instead of going for desalination?
We could cut down our usage some more, but if we run out of water anyway then this does not help! In the following table and graph we analyse some different usage
scenarios (assuming no desalination plant). They compare current usage (under the water restrictions that have been in place) with what the water levels would have
been if our usage had been (for example 20% less) than what it has actually been. Note that level 3 water restrictions reduce usage by about 3%-10% over level 2 water
restrictions, so a 20% drop in usage is very significant! If everyone had rainwater tanks for their gardens, usage could drop by around 20% on level 1 water restrictions.
|Hypothetical usage since 11.2001||Available water||% full all dams||1 year change GL(%)|
|20% More|| 1430.6||55.41
|As is || 2500.5
|20% Less|| 2551.8||98.84
|50% Less|| 2553.2||98.89
From this data we can see that, provided the current drought does end in the next couple of years, using less water can be sufficient. However, given that a
further 50% drop in usage would be required to prevent the dams falling in the last year, a Herculean effort would be required for Sydney to survive on
its current rainfall and usage patterns if this drought is merely climate change and representative of a more permanent change in rainfall. Hence something more than
this, such as a desalination plant, may be required.
What about the Shoalhaven scheme, and deep pumping from our dams?
Whilst deep pumping does help once each drought, it is not a continuous supply of new water. Only dams which often overflow, even during this drought, such as
Tallowa Dam, could make use of such water more than once each drought. The Shoalhaven scheme is more promising, however. A desalination plant does have the most
continuous source of water.
You can see the history of the Tallowa Dam water levels. In this you can see that Tallowa Dam often overflows. The Shoalhaven
Scheme upgrade that is planned for the next few years is designed to transfer more water from Tallowa into the main Sydney system. The two stages of this upgrade
are estimated to provide 80,110 GL/year. The effect of this (with and without a 500 ML/day desalination plant) is shown here:
A 500 ML/day desalination in addition to the Shoalhaven Scheme upgrade does appear to be sufficient to keep dam levels close to steady in the current drought.
What about options to save water other than desalination
Several other options are under consideration. Water recycling is feasible for new developments, but would be expensive to introduce new pipes to
existing suburbs. This would also take a long time to introduce (which we may not have if the current drought continues as fiercely as it is). Grey water, rain water tanks, water efficient appliances will all help, but not by the 50%
figure we would need if this drought does not end. Of course if Sydney was happy to accept the mixing in of recycled water into its drinking supply then the introduction of large scale recycling could be done as quickly as a desalination plant. Unfortunately at this relatively late stage, time remaining is important... and a desalination plant, as environmentally unfriendly as it may be, is a relatively quick option.
Would the western suburbs of Sydney still be relying on dam water while the east gets a nice guaranteed supply of perfect desalinated water?
Yes and No! The current plan by the government does have the eastern suburbs getting the desalinated water, while the western suburbs uses dam water. However,
it is not the case that the eastern suburbs will no longer need to obey the same restrictions as the rest of us. Each litre of water that the easter suburbs
saves in water from the desalination plant means that the desalinated water gets to travel a little further west, reducing the pressure of our dams. If our dams
were to run dry, extremely harsh water restrictions could be used to spread the desalinated water over Sydney.
Water quality is a different issue... the desalinated water should be of generally higher quality than water from our dams. This is particularly the case as
our dam levels drop (decreasing slightly the quality of our water). The hardness of the two sources of water will also be different. The area of Sydney that
is likely to receive the most difficulties with water quality is not the areas that rely solely on dam water, but the area that is in between that
(during low water usage periods) sometimes uses desalinated water and (during high usage periods) reverts to dam water. For this area of Sydney the hardness of
the water (and hence ph, as well as other factors) may change dramatically from day to day. This can make it difficult, for example, when keeping fish it is desirable
to keep conditions fairly stable. A drop in water hardness can mean greater ph fluctuations and the possibility of a carbon crash. Perhaps this changeover area
between water from the dams and desalination plant can be in the cbd where there are less plants to suffer, and aquariums are often managed professionally.
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|by Peter Davies @ 10 Jul 2005 10:51 pm |
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