The San Diego Union-Tribune Editorial Board used to be among the skeptics who maligned “toilet to tap” — the purification of sewage for regular water uses — and questioned a proposal by local officials on health and cost grounds. Then six years ago we changed our minds with an editorial headlined, “The yuck factor: Get over it.” The science is clear that such water is safe. And history is clear that California is deeply vulnerable to droughts, and that the San Diego region must diversify supplies, given the capricious history of its main water supplier, the Metropolitan Water District of Southern California.