Thursday, 5 April 2012

How much energy to desalinate water?

Conserve water by using  stored rainwater in the garden
Given that much of the UK is in a drought again, I thought I'd review how much energy we use on supplying and treating water, and how much more we would use if we had to desalinate sea water instead of using fresh water sources. Despite the Environment Agency's efforts to persuade us to use less, and the water saving features going into new homes, it seems we are quite stubborn in our habits and in 2009/2010 we were still using an average of 149.8 l/p/d (litres/person/day) in our homes, up slightly from the year before which was a more promising 147.4 l/p/d. (Metered households use less and one water company reported an average of just 118 l/p/d.) Also, the energy used to supply and treat this water has been increasing: the energy used per unit water supplied was up 5% in 2009/2010 over the previous year. We are told this is due to increasing standards of water quality and treatment. However, considering how important water is to us, the energy used is a very small fraction of our overall energy consumption.

 Taking the average usage as 150 l/p/d, and allowing for leakage of 25% (actually it was 24.6%, down by 0.4% on the previous year), the energy used by the water companies in supplying this water and treating the sewage is just 0.28 kWh/p/d (kWh/person/day) [1]. However, this is just the domestic water use - taking into account commercial and industrial consumption as well we used 213 l/p/d water requiring 0.4 kWh/person/day. Compared to the average UK person's energy consumption of 81 kWh/p/d [2] this is just 0.5%.

 By the numbers

  • UK average domestic water usage 150 l/p/d
  • UK total water usage 213 l/p/d
  • Add leakage 25% total 283 l/p/d
  • Water treatment energy 0.4 kWh/p/d
  • Desalination would add 1.1 kWh/p/d
  • cf total UK energy use 81 kWh/p/d
If we ran out of fresh water, we could make drinking water from sea water by various methods of desalination. Most desalination plants work by repeatedly distilling the water by evaporating it at low pressure (the low pressure lowers the boiling point). This is fairly energy intensive. The most energy efficient method with current technology is reverse osmosis, whereby water is squeezed through a semi-permeable membrane which lets the water through but not the salts. This requires between 0.003 and 0.0055 kWh/litre [3]. Call it 0.004 kWh/litre. Allowing for the 25% leakage rate this adds 1.1 kWh/p/d, giving a total of 1.4 kWh/p/d, or 1.8% of current energy use.

In practice, we would not need to desalinate all our water and the requirement would mainly come in the summer time which means we could expect to get a reasonable proportion of this energy from renewable sources such as solar panels. If we were desalinating one quarter of our water, in July, using energy from solar panels, a city the size of  Cambridge would need about 45,000 square meters of solar panels, or 4.5 ha - around 6 football pitches[4].

Cambridge is not near the sea. It would be a great deal more sensible to reuse treated waste water. For many people this concept has a high yuk factor but in fact it is already happening, albeit indirectly, in some places. For example the Langford Water Treatment plant in Essex pumps cleaned waste water into the River Chelmer 4km upstream from the point where water is abstracted for the Hanningfield Reservoir [5]. Treated waste water is often higher in quality than river water [6].

[1] Water UK Publications: Sustainability
[2] Energy and Carbon Emissions: the way we live today by Nicola Terry
[3] ENERGY REQUIREMENTS OF DESALINATION PROCESSES (Encyclopedia of Desalination and Water Resources)
[4]  Based on 120,000 people, solar panels efficiency 15%, total July insolation 5 kWh/m2/day
[5] Langford Recycling Scheme
[6] Efficient re-use for potable water supply Environment Agency 

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