Why we should recycle instead of Desalinating!

So previously I discussed the links between energy and water in modern society, in particular the costs of desalination.

Today I just wanted to give a short comparison between recycling water (and yes recycling sewage) and desalination. Basically the only reason that Australia's major cities have desalination plants is because people are a little squeamish. Or to put it another way because people are stupid. If you tell someone that the resulting output is cleaner and less contaminated than regular water they still won't touch it, the problem is that it is psychologically dirty. People think that it is dirty and can't drink it.

The problem is people don't think further than that. The reality is that if you live in a country town (or Adelaide for that matter) then you'll probably be drinking 'recycled' water and not really think about it. The difference here is that somewhere further upstream some towns "output" has been cleaned etc and then squirted into the river, then the downstream town pumps this out of the river, processes it etc and pumps it around town as its water supply.

The difference with places like Sydney and the Gold Coast is that that process would be short circuit the "psychological cleansing" that shoving the water into a river and pumping it out again is removed. What people don't understand is that it makes no difference. What is equally silly if you think about it is that this is what is happening in Sydney anyway, they pump the shit out the "deep water outlet" and then suck in sea water for the desalination plant....

I'd love to think that if people knew a few things about it they would want to chose recycling water over desalination anyway. One it is a heap more efficient to recycle water than to desalinate it and two the recycled water is actually a lot cleaner.

To take the first point the efficiency of recycling lets consider Singapore, arguably the most advanced country in terms of both desalination and water recycling technology. Their most efficient desalination plant uses 3 MWh/ML where as their most efficient recycling plant uses 0.7MWh/ML to produce drinking water.

To the second point, again using Singapore as an example, several Silicon processing companies have relocated operations to Singapore because of the reduced costs that surround using the recycled water output. Silicon processing, specifically the production of wafers for semiconductors, requires very pure water. Singapore supplies this companies with output direct from their recycled water plants that is so clean they can vastly cut down on the amount of processing that they do on the water before it is used in their processes.

Finally, and this applies to both Sydney and South East Queensland the power requirements of desalination place such an additional burden on the power systems that we will not be able to meet it without considerable investment in power supply and generation infrastructure. When you factor in the increased water bill to subsidize the construction and then operating costs of the new water plant and the increased power costs for the infrastructure and new generation capacity there will be a serious impact of the desalinized water on the consumer.

Perhaps this hip pocket impact will make people rethink the "Desalination is King" mantra and reconsider both recycled water and storm water capture technologies.


SOURCE: IEEE Spectrum 6.10

Water vs Energy: The Balancing Act

Water and Energy are very tightly integrated in modern human society.

You can not create energy without water, whether it is directly driving hydroelectric turbines or being boiled to drive a steam turbine or more likely being used to cool the systems.

You can not get water without using energy, if you're lucky enough to live next to a pristine stream you're going to need to pump it out, otherwise you have to process it, or pump it great distances.

A great example of this in Australia, that has received a lot of attention is desalination (particularly if you are a Sydney sider). Desalination requires A LOT of energy and when I say a lot I mean a lot! Currently it takes around a top of the line efficient reverse osmosis filtering plant around 5 MWh of electricity to generate 1 ML of water, or around 1.25 Wh to get your glass of water filtered. And if you consider an old school thermal desalination plant it takes around 80MWh/ML or around 20 Wh per glass of water.

Now when you consider that the majority of energy in Australia is generated using coal fired power stations, cooled with cooling towers then we need to factor in the water cost of generating the electricity it takes roughly 1900 L of cooling water to generate a MWh of electricity.

Further to the water cost of the electricity we need to consider the waste water from the desalination plant. The most efficient RO plants have water usage efficiencies of around 48% (which means 52% of the water that enters a desalination plant leaves as waste water) and generally the efficiency is more likely to be down in the mid to high 30s. So if we take the most efficient plant around then for every ML of clean water that is produced there is 1.083 ML of dirty water produced.

So basically for your glass of water you'll be using 1.25 Wh and 273 mL if you're getting it via desalination. If you don't happen to live at the desalination plant then there'll be additional energy that is required to pump the water to your location and further if the desalination plant isn't co-located with the power plant then transmission losses of around 20% need to be factored into the equation as well.

SOURCE: IEEE Spectrum 6.10