To keep household taps flowing even in dry years, Napa is launching the most expensive public works project in city history, more than doubling the size of its Jamieson Canyon water treatment plant.

The $48 million expansion will help Napa to meet its water needs through mid-century, said Phil Brun,cq general manager of the city’s water department. The expanded plant will be named in honor of former Napa City Councilman Ed Barwick, who made securing Napa’s water supply a top priority when he served in office.

When the project is completed in mid-2010, Napa will be able to pump more water from the State Water Project, saving local supplies for the driest years. The city should have enough water to survive droughts without severe hardship until almost 2050, Brun said.

Consumer taste buds will benefit as well. Water arriving at the Jamieson Canyon plant from the Sacramento-San Joaquin Delta will get additional treatment, improving both taste and safety, Brun said.

Currently, the city draws 55 percent of its annual need from two local reservoirs, Lake Hennessey and Lake Milliken, with 45 percent coming from the State Water Project through Jamieson Canyon.

After the plant expansion, 75 percent of the city’s supply will come through Jamieson Canyon in a typical year, Brun said. The city will reserve Hennessey and Milliken for peak summer demand and dry years.

Built in 1967, when Napa had less than half today’s population, the Jamieson Canyon plant is woefully undersized, Brun said. Napa cannot take full advantage of its State Water Project entitlements, which are increasing annually, he said.

The 40-year-old plant can process 7,000 acre-feet per year. (An acre foot is enough water to cover one acre of land in one foot of water, or about 326,000 gallons.) The city’s state entitlement is 15,350 acre-feet, growing to 20,800 acre-feet by 2021.

Since annual allocations are usually less than entitlements, the plant’s new capacity of 16,000 acre feet will allow Napa to treat all available state water most years, Brun said.

Napa treats up to 1,500 acre-feet of state water at Jamieson Canyon for Yountville and Calistoga. Plant improvements will increase water quality and reliability for those Upvalley cities as well, Brun said.

The city pays $4 million a year to be part of the State Water Project, regardless of how much water it receives through Jamieson Canyon, Brun said. Napa will get more for its money after plant expansion, he said.

The State Water Project draws from Lake Oroville, in Butte County. The massive man-made lake dumps into the Feather and Sacramento rivers, then the water enters the Sacramento-San Joaquin Delta. Napa extracts its share from Barker Slough, southeast of Travis Air Force Base, through the North Bay Aqueduct.

“Delta water is challenging to treat,” with taste issues that must be overcome, Brun said.

When modernized, Jamieson Canyon will use ozone — a naturally occurring, electronically charged form of oxygen capable of eliminating pollutants, bacteria and unpleasant odors — to treat the water. This will reduce the need for chemicals later in the treatment process, Brun said.

The water will taste better, Brun said. City water can sometimes have an earthy taste, especially in summer, he said.

Ozone treatment will help the city deal with “emerging contaminants,” such as society’s heavy use of skin lotions and aspirin, chemical traces of which find their way into water supplies, said Joy Eldredge, the city’s senior water engineer.

Health officials are concerned about trihalomethanes and haloacetic acids, byproducts of using chlorine to disinfect water. The presence of both will be reduced when plant improvements are finished, she said.

Expansion of Jamieson Canyon is part of a program of system improvements recommended by consultants in 1997. In recent years, the city has invested $25 million to build three water tanks and modernize lines.

To pay for these upgrades, the city raised water rates 5 percent a year for three years. Rate increases are now pegged to the Consumer Price Index.

The city has issued $48 million in water bonds to pay for the Jamieson plant upgrades, which include seismic strengthening. “This is our generation’s contribution to the overall integrity and reliability of the system,” Brun said.

A contract for the Jamieson Canyon plant, to be renamed the Edward I. Barwick Jamieson Canyon Water Treatment Plant, is scheduled to be awarded in December.