The plant will use the reverse osmosis method. Interestingly, the decision by the California Coastal Commission opens up the possibility of constructing some 20 other plants on the California coast.
From the Sand Diego Union-Tribune.
Interesting, and a good idea, but am I right in reading between the lines that the average household uses about 446 gallons of water a day? That seems awfully high even for a family of 4-6 people.
That includes water for washing machines, dishwashers etc.
My water bill from April says that our average water usage is 8,692 gallons per month. That’s 290 gallons per day, for a 4-person household. Perhaps San Diegoans use more than I do. 
Water use statistics also vary by country and region. Some of these can be looked up on sites such as the US EPA and the American Water Works Association.