There is not enough borehole water on St Marys for the summer population so a desalination plant was put in nearly 30 years ago. The water intake is on the eastern side of the island near Toll's Island. I think they use a flexible pipe because it doesn't operate in the winter and gets taken in. Desalination works by reverse osmosis. In ordinary osmosis, diffusion of water molecules takes them from where the water is most concentrated (ie in a weak or diluted solution) to where the water is least concentrated (strong solution). This occurs naturally and requires no extra energy. To do the opposite requires energy input. In the desalination plant, the sea water in pumped at high pressure through a special membrane which removes salt ions. The natural osmotic flow would create a fluid pressure in the forward direction. For reverse osmosis, the pump must at least match this pressure.
https://en.wikipedia.org/wiki/Reverse_osmosis says that the pressure has to be between 5.5 and 8 MPa but that the fact that there is strong flow in the purified water can be used to recover some of the energy, for example by using a Pelton wheel. The Scilly plant should be able to handle 400 cubic metres a day but the power cable limits the input so that only 280 cubic metres a day can be used, according to an article we found on the island. If it were to operate a 3 kWh energy consumption per cubic metre then that would be 840 kWh a day or on average 35 kW. At 230 V that would be 150 Amps. The pump may operate at a higher voltage.
