per day per person. In turn, we provide about 160 litres per day per person
of sewage to the water companies. The cost of pumping water around the
country and treating our sewage is about 0.4 kWh per day per person.
At the moment the UK doesn’t spend energy on water desalination. But
there’s talk of creating desalination plants in London. What’s the energy
cost of turning salt water into drinking water? The least energy-intensive
method is reverse osmosis. Take a membrane that lets through only water,
put salt water on one side of it, and pressurize the salt water. Water
reluctantly oozes through the membrane, producing purer water – reluc-
tantly, because pure water separated from salt has low entropy, and nature
prefers high entropy states where everything is mixed up. We must pay
high-grade energy to achieve unmixing.
The Island of Jersey has a desalination plant that can produce 6000 m3
of pure water per day (figure 15.10). Including the pumps for bringing
the water up from the sea and through a series of filters, the whole plant
uses a power of 2 MW. That’s an energy cost of 8 kWh per m3 of water
produced. At a cost of 8 kWh per m3, a daily water consumption of 160
litres would require 1.3 kWh per day.