Dry conditions have persistently plagued the West since 1999, says the National Climatic Data Center. Today, 57 percent of the region is in a drought, and only one county in California may be ready to meet it. Tomorrow, the Orange County Water District will turn on what it is calling the largest water purification plant of its kind in the world. The Groundwater Replenishment System will purify sewer water, leave a finished product similar to or better than bottled water, and fortify the county against persisent drought, predicted water shortages and projected growth. The $481 million project utilizes a three-step process that runs sewer water through two membrane filtration systems -- microfiltration and reverse osmosis -- to remove solids, bacteria, minerals and contaminants, and then exposes it to ultraviolet light and hydrogen peroxide to zap any remaining pharmaceuticals and carcinogens. Half the purified water is than injected underground, where it keeps seawater from infiltrating the groundwater, and the other half is used to recharge lakes in the area. Paid for with a mix of federal, state and local funding, the system will generate enought pure water to meet the needs of 144,000 families, and has already received interest from several cities in California, Florida and Texas.