Saturday, July 22, 2006

Seawater desalination

Prior to 2004, desalination was practiced in Australia with just a few very small brackish groundwater schemes. The Premier of NSW had disparagingly referred to desalinated seawater as ‘bottled electricity’, noting the considerable energy requirements for its production. However, serious consideration of large-scale seawater desalination schemes has been accelerating during the last two years.

In July 2004, the Western Australian Government announced that it would construct one of the world’s largest seawater desalination plants to supply Perth with up to 45 gigaliters per year of potable water.

Soon after, the NSW government announced plans to build a desalination plant for Sydney on the Kurnell peninsular. Following community anxiety, the construction of this plant has been postponed. However, planning continues and construction will begin when Sydney’s supplies dip below 30 per cent of capacity. The plant will initially produce 125 megaliters per day, but will be built with the capacity for further expansion to 500 megaliters per day.

Other cities, including the Gold Coast and numerous smaller coastal towns around Australia have also begun investigating the feasibility of seawater desalination as a component of their overall municipal water supply and management.

To achieve best-quality water production, a number of alternative treatment approaches could be considered. However, in 2006 Australia, reverse osmosis membrane treatment is by far the most energy efficient approach for adequately upgrading both conventionally treated wastewater (water recycling) and seawater.

The fundamental principal of reverse osmosis is the employment of semi-permeable membranes to separate a ‘purified’ component of the water from a waste-stream retaining the concentrated salts. This waste stream is commonly referred to as the membrane ‘concentrate’ or ‘brine’. The sound management and disposal of concentrates has become one of the greatest concerns regarding both water recycling and desalination, and is often a key factor determining the overall viability of a project. The issues involved include technical challenges, permitting problems and high costs.


Concentrate from seawater desalination typically comprises half of the original in-take volume and almost all of the dissolved salts. Accordingly, it is typically double the normal concentration of seawater. Most commonly, concentrates are discharged via ocean outfalls, however the double salinity renders concentrate plumes denser than seawater and thus they sink and can be difficult to disperse. The potential impact of concentrate plumes on marine species in Australian environments has yet to be properly assessed.

Much public discussion has taken place regarding the relative energy requirements to treat municipal effluents and seawater to qualities suitable for reuse. Reverse osmosis technology has developed dramatically during the last decade, decreasing both the energy costs and therefore the financial costs of treatment. However, the major source of energy requirement remains the necessity to overcome the osmotic potential difference across the membrane. That is, the difference in salinity between the purified water and the retained brine.

Seawater normally has a salinity of around 35 grams per litre. Municipal effluent is typically only one tenth of this salinity. This means that the osmotic potential is lower for municipal effluent than for seawater (and a higher fraction of the water can be recovered before the brine becomes too concentrated). Therefore, considerably less energy is required to produce a volume of clean water by reverse osmosis of municipal effluent than for than to produce the same volume from seawater.

Logically, seawater is sourced from sea level (or slightly below). However, most drinking water supplies are stored inland and somewhat elevated. This helps in the gravity-assisted distribution to our homes. Therefore, a second significant energy requirement in most circumstances is the need to pump desalinated water long distances and often uphill.

Some opponents of potable water recyling in Toowoomba have left comments on this blog recomending seawater desalination as a solution for that city. One suggestion has been to allow Toowoomba to extract water from Brisbane’s Wivenhoe Dam and replace that water with desalinated seawater. The pumping costs involved with such a scheme (from the ocean, up the Great Dividing Range to Toowoomba), combined with the treatment costs would make this about the most expensive water on earth.

In addition to these obvious ‘engineering’-type limitations, some more obtuse consequences of desalination are also worth considering. One such consequence is the weakening of the message highlighting the importance of water conservation. When a potential water source is envisaged to be as great as the world’s oceans the urgency to implement water-efficient technologies and practices is reduced. Furthermore, cities that come to rely on seawater desalination rather than conservation or recycling, will also rely on ocean outfall infrastructure for the discharge of municipal wastewaters (as well as desalination brines). As these cities harvest ever-increasing volumes of water from the ocean, they must also discharge similarly increasing volumes to the detriment of Australia’s precious marine environment.

But of course, desalination does have one great advantage over municipal recycling…no yuck factor! And yes, I know…no hormones, RU486 abortion pills, prions, or other yet-to-be-invented chemicals. However, properly treated recycled water will not contain these either…

Whadda you reckon?

18 comments:

Anonymous said...

Why not make use of the desalination plant the Qld Premier is building on the Gold Coast and why not make use of the water grid the Premier is building to shift water around the state?

To get water to Toowoomba, it doesn't have to go up the range that cars travel on to get to Toowoomba. It is piped to the closest of Toowoomba's 3 dams. Yes, there is a lift of water involved. But it is only used in times of necessity so the Council's cost claims of this alternative (based on permanent use) are wrong.

It may be that in the future Toowoomba has a surplus of water which can be shifted to Wivenhoe if needed.

Why not connect Toowoomba to the water grid that the Premier is creating anyway?

Stuart Khan said...

Thanks for this very constructive comment/question, Anonymous.

Piping water “around the state” may sound like a great idea, especially if it involves moving water from high altitudes to low altitudes. But regardless of what “water grid” may be planned, taking desalinated seawater to Toowoomba still involves the same massive energy requirement. The precise route that the water may take makes little difference; the necessary altitude lift is the same: from sea level to the final point of use (Toowoomba’s kitchen taps!).

I agree that water would only need to be transported to Toowoomba in times of need (much like the way Sydney pumps water up from the Shoalhaven). However, while Toowoomba is certainly experiencing a severe drought at present, there is on-going increasing pressure on the city’s water supply. The population of Toowoomba is growing and this additional demand will increasingly exacerbate the need for water in the future. The likelihood of Toowoomba having surplus water to return to Wivenhoe is, in my opinion, remote.

So while your suggestion is certainly possible (and worthy of investigation), I don’t believe that it is a realistic sustainable solution.

mick said...

Cressbrook dam (one of three dams toowoomba uses) is only about 35km away from wivenhoe dam. they are actually rather closer then what people think. that idea is a good idea. The desilation plant when up and running could pipe water in to hinze dam then into wivenhoe dam and where it goes from there is where water is needed. Toowoomba should be included in the grid and not forced too drink it's own waste!! Toowoomba city is being used as an experiment for the other cities in australia too except recyled effulent for drinking purposes. recylcing water should also get the go ahead but for industrial and agricultral purposes only! that would free up a whole heap of drinking water in the dams and thus less drain on the dams! another suggestion is that every council in australia winging about water should just install rain water tanks in every house at THIER OWN cost!!! no one has done anything about water (except blame climate change) which i beleve doesn't exist and is just a scare tactict being used too scare everyone. there hasn't been another dam in toowoomba built science 1988. The Emu creek dam should of been built 6 years ago and toowoomba would of been on level 3 restrictions

Greg said...

You right about Toowoomba's population increasing Stuart - recycling effluent will not be sustainable for very long at all and TCC hasn't even looked at any other options for future water and there a bit like you - they cover up there push for us to drink recycled effluent with somewhat misleading statements like the one from you about the desal water to Wivenhoe to Toowoomba being the most expensive water on Earth - what rubbish! Did you just pluck figures out of the air like TCC and Beattie did on other water option costs for Toowoomba! I am beginning to believe you are just enfatuated with people being made to drink recycled effluent!

Stuart Khan said...

Hi Greg,

I appreciate your scepticism. I didn’t actually quote any figures (thus didn’t pluck them out of the air). However, you are correct to suggest that I was not referring to any detailed study. I am simply assuming that any town or city with good leadership would utilise all other sources of water (including recycling treated effluent) before resorting to desalinating seawater and pumping it up a gradient of more than 1000m. It would be difficult to find any examples where this is not the case…though I would not want to underestimate anybody’s Googling skills! I would be genuinely interested if anyone can show this assumption to be incorrect.

Its true that other sources of water supplies (such as dams) can also be extremely expensive. This is especially true if we are prepared to count all costs including social and environmental costs.

mick said...

I heard somewhere that the lift required for the wivenhoe pipeline is only about 175 metres. a map will show you just how far away cressbrook is to wivenhoe. beleve me you won't need a 1000 meter lift. toowoomba isn't even that height above sea level (625 metres) the pipes are already at cressbrook too pipe water too toowoomba (scince 1988).

Stuart Khan said...

Mick,

Thanks for this information. Assuming that you are correct that the lift from Wivenhoe is only 175 m, then a more viable option could be for Toowoomba to extract water from Wivenhoe (as you, Greg and others have suggested) and use the desalinated seawater to supply coastal regions of Wivenhoe’s service area (rather than pump the water all the way up to Wivenhoe as previously suggested). I assume that this would require significant reorganisation of Brisbane’s water supply, storage and distribution network, but am not familiar enough with the system to really know.

Nonetheless, we are still talking about a massive engineering project with enormous cost to the State of Queensland. Anything is possible, but I would hope that all other (more environmentally, socially and economically sustainable) opportunities would be exploited before taking such drastic measures. Still, I do understand that a significant number of people consider indirect potable recycling to aslo be a drastic measure.

Anonymous said...

Stuart, what could be more drastic than being forced to drink our own shit water? COST IS NOT IMPORTANT!! Peoples lives are at stake here! Bring on desalinization!

Stuart Khan said...

There's an interesting article about desalination in todays Sydney Morning Herald:
Bills and tempers to rise over coast water plans.

mick said...

http://cooltech.iafrica.com/science/769169.htm

good reading

Greg said...

Turning oceans into tap water

Jaun said...

Stuart, why do you insist on a dollar value here, Beattie plans a 900KM pipeline costing FAR more then any propsal for Toowoomba.

Don't use the excuse it will cost more, any value placed on a reliable water source is nothing compared to trying to relaim a drinking supply from a source that is dropping and widly not accepted.

If Beattie can PUMP water 900Km, then he can PUMP water 200Km to Toowoomba. Give us a break with the costs involved, try to make sense without using the dollar value, then weigh up the differences. None!

Stuart Khan said...

Hello Jaun,

I’m fairly confident that financial costs will be taken into account by State Governments and Local Governments when determining which water management strategies will ultimately be implemented. As you suggest, long-term sustainability and public acceptance will also be very important.

The city of Brisbane has a considerably larger population and rate-base than that of Toowoomba, so will be able to justify more expensive solutions. Nonetheless, it is my personal opinion that a 900km pipeline will not happen in the short term, nor in the next few decades, primarily because of the costs involved. Of course, politics is politics and I may be proved wrong…

Stuart Khan said...

Interestingly, the Toowoomba Mayor was quoted in The Australian today, confirming:

"The other option is to set up a desalination plant on the coast and pump water up the range. We've done the number crunching on this, and the severest option is an extra $20 a week on rates."

Jaun said...

Blab what ever like about costs, councils and governments have no problem spending our money when it comes to making a profit from it, but when it comes to use our money on projects that will put a dent in their budget and make our lives better, they opt to use the most cheapest and unrealistic options, then cover it up with bulls...t.

You still havn't answered me about how can recycled water from a supply that is dropping be sustainable? You can't recycle something for ever, the water will eventually run dry if no alternate to recycling is found.

All cities must embrace that recycling will only delay the final outcome by a some months maybe years. But keeping in mind recycling for non-dometic will extend the life of the dams.

I only prey the vote is NO, because I don't wish to live in country that is going to recycle sh.t for drinking.

Stuart Khan said...

Hello Jaun,

You are absolutely correct. Water recycling is not an infinite supply of water. Since not all potable water is returned to the sewers, it is not available for recycling. The best a city can hope for is 50-60 per cent. However, the only source that comes close to being ‘infinite’ is seawater desalination. But seawater desalination is such an energy-intensive and expensive option (especially for a city on top of a mountain range) that it makes sense to exploit other options first. In the end, all city’s have a sustainable population limit and water availability will be a key factor for many of those in Australia.

[george]twba said...

Stuart

As a fellow environmental engineer (who lives in toowoomba) i must admit it is good to read your well written and balanced blog.

As shown by the posts here, the "no" group withuing toowoomba only wants it their way or the highway.

I must admit I have a laugh every time i hear "sustainable bores" or "175m head is nothing" or "desalanisation" or "pipe network". IT WILL BE EXPENSIVE.

My personal view is that the desalinisation plant on the coast is political time wasting to keep people happy. As you said somewhere, its quite a extensive exercise. Anyways i digress.

Most of the so called options are not sustainable in the bigger picture. How much eneregy is requried to pump water from far away to Toowoomba. The greenhouse gas potential is huge. The recharge rate on the aquifers without rain is basically nil, thus we are just waiting for the salt to rise.

For people such as Mick below, who I quote "no one has done anything about water (except blame climate change) which i beleve doesn't exist and is just a scare tactict being used too scare everyone", we will always have to deal with such people and nothing will be good enough except what they want.

Mick, if you truely belive climate change isn't happening go off to the room of mirrors and have a long hard look at yourself. To put it bluntly you have your head where the sun dont shine.

Oh, and use less electricity, its the worst offender in Australia.

Anonymous said...

How can I help? I'm in the USA

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