But by the time it was completed in 1598, the Guadalquivir River had begun to silt up, causing all New World trade to move to Cadiz and the building became a rather opulent filing cabinet instead.

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Ian Barclay, a project manager on the silt scheme, said it would be good to add the extra barrier to stop the silt building up again so quickly.

BBC: St Aubin

The theory is relatively simple: Seawater and freshwater, filtered to remove silt, are fed into pipes which lead to a membrane system, made up of spiral coils to maximize surface area.

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