Carlsbad State Beach is a Southern California idyll. Palm trees adorn the cliffs above the sand, and surfers paddle out for the waves. From the beach it is impossible to tell that a huge desalination plant not half a mile away is sucking in seawater to produce 50 million gallons of new drinking water each day. It is the largest in America—for now. Soon it may share that title with a proposed sister plant 60 miles (97km) north in Huntington Beach. But only if that one is built.

Poseidon Water, the developer that also built the Carlsbad plant, first proposed the Huntington Beach facility in the 1990s. But it has taken the company more than two decades to persuade Californians of the plant’s necessity.