San Francisco officials have reached an important milestone in a long-running effort to build a high-pressure water network needed to bring vital firefighting capabilities to the Richmond and Sunset districts — two neighborhoods that have historically lacked direct access to such a system.

After years of planning and deliberations, the San Francisco Public Utilities Commission and the city’s Fire Department have finally picked a design for a new auxiliary water supply system that can supply firefighters with large volumes of high-pressure water needed to put out rampant blazes that could result from earthquakes or other major emergencies.

And in what city water and fire officials say is an added benefit, the proposed 9.75-mile-long system will also be built to carry drinking water, helping to ensure that more potable water is available after an earthquake.

The new system would be a counterpart to the roughly 135-mile-long network of high-pressure pipeline, cast-iron hydrants, reservoirs and other equipment that makes up San Francisco’s emergency fire suppression system. Built after the 1906 earthquake and fire, the existing system is a critical firefighting tool that can be tapped in times when the standard lower-pressure network isn’t sufficient for the job.

“The misnomer is that it’s only used for really big emergencies. We use it on a fairly regular basis,” said Fire Chief Joanne Hayes-White, who said the existing system gets activated eight to 12 times each year.

But the existing system, completed in 1913, was built at a time when most of San Francisco’s population and property were clustered in the northern and northeastern parts of the city. As people spread south and west, that system wasn’t built out to follow them. The new, separate system would fill part of that void.

“If we have a huge fire out there, we will burn,” said Supervisor Sandra Lee Fewer, who represents the Richmond District, where she has lived for decades.

Hayes-White said the Fire Department is well equipped to handle emergencies in the Sunset and Richmond districts. But “certainly, you do have some vulnerabilities,” she said, citing the large number of older wood-frame buildings built close together in those neighborhoods.

In 2010, then-Mayor Gavin Newsom transferred the management authority over the auxiliary firefighting system from the Fire Department to the SFPUC, given the agency’s expertise with water systems. Since then, the SFPUC and the Fire Department have worked together, along with the city’s Public Works Department, to assess the city’s emergency firefighting needs.

“We’ve always had a relationship, but the relationship has changed,” said SFPUC General Manager Harlan Kelly. An initial assessment of the auxiliary firefighting system’s capabilities in 2010 showed some stark results: If a 7.8-magnitude quake hit the city, firefighters would be able to get only 47 percent of the high-pressure water needed to fully extinguish the flames.

The city has undertaken a number of projects since then to boost that figure, primarily funded through two voter-approved Earthquake Safety and Emergency Response bonds. Those projects included installing 30 new cisterns — 15 in the Sunset and Richmond — and making seismic upgrades to water tanks.

Katie Miller, a water systems engineer and a water division manager at the SFPUC, said the agency had considered building out the existing fire suppression system, but quickly discovered that the high-pressure water supply would run out too quickly. The existing system’s water source comes from three reservoirs and two seawater pumping stations.

“When we were looking at the modeling, we realized that the supply fell short and it wasn’t going to work to just extend it,” Miller said. “So we started looking at other supply alternatives.”

The agencies settled on the 90 million-gallon northern basin of the Sunset Reservoir at 24th Avenue and Ortega Street as the proposed water source for the new system, along with a pair of pump stations to help ratchet up the pressure. At 14,000 gallons per minute, that’s enough to keep firefighters supplied with high-pressure water for 4½ days.

The SFPUC has proposed connecting the new system to existing potable-water pipes in the Richmond and Sunset districts in about five places. When there’s no fire to fight, the auxiliary pipes will carry some drinking water to those neighborhoods. And because the new system will be constructed to withstand a major earthquake, drinking water will continue to flow after the fires are put out.

“It’s a win-win for everybody,” said Kelly.

But the SFPUC’s proposal is not without its critics. Tom Doudiet, a retired assistant deputy chief with the Fire Department, questions whether the 90 million gallons in the Sunset Reservoir’s north basin will be enough. He has instead advocated for tapping the Pacific Ocean, calling the Sunset Reservoir proposal “delusional.”

“Ninety million gallons is not going to be adequate to put out fires in the Sunset and Richmond,” Doudiet said. “The solution is the fact that we’re surrounded on three sides by seawater.”

But the SFPUC says drawing ocean water isn’t as easy as it might sound — though it hasn’t ruled the option out entirely. The existing system does rely on backup seawater pumps, but extensive erosion at Ocean Beach has made the agency wary of the idea.

The SFPUC is also strongly considering installing a pump that, in an emergency, could draw on water from Lake Merced, further bolstering the water supply for firefighters. The volume of water in Lake Merced fluctuates between 800 million gallons and 1.2 billion gallons.

The estimated price for the project is $109 million. The SFPUC plans to commit $40 million toward drafting more detailed construction plans and for mandatory environmental reviews. After that’s complete, the agency is prepared to begin putting pipeline in the ground as early as next year and continue work until the $40 million is spent. The remaining $69 million would probably come from another round of bond funding, which would have to be approved by voters in 2020. The SFPUC estimates the city could lay a mile of pipeline each year for the new system.

“We collectively feel it’s worth every penny and that it has the best coverage for those particular areas,” said Hayes-White, the fire chief.

Dominic Fracassa is a San Francisco Chronicle staff writer. Email: dfracassa@sfchronicle.com Twitter: @dominicfracassa