San Francisco’s water supply relies upon the Hetch Hetchy Reservoir and the Tuolumne River. And like beer: We don’t buy it; we just rent it. (Courtesy photo)

San Francisco’s water supply relies upon the Hetch Hetchy Reservoir and the Tuolumne River. And like beer: We don’t buy it; we just rent it. (Courtesy photo)

San Francisco’s underground river

Ever hear that adage: “You don’t buy beer; you just rent it”? Well, it’s true with water, too.

In San Francisco’s water system, the brief moment from faucet to drain is a tiny fraction of our water’s journey from high atop the Sierras to miles out in the Pacific. So how does it all work? And should we worry the well may run dry?

Oddly enough, our entire drinking water system is predicated on environmental decisions that would probably never get approved today. Exhausting its local supplies a century ago, San Francisco somehow got Congressional approval, via the 1913 Raker Act, to dam a valley in Yosemite National Park and transport its water 167 miles west.

Hetch Hetchy is simply the reservoir that dam created. Our water comes from the Tuolumne River.

Gravity brings the Tuolumne’s water west, through 280 miles of pipelines, 60 miles of tunnels, 11 reservoirs, three hydroelectric powerhouses (generating enough electricity for 200,000 homes) and two treatment plants. In the Bay Area, 2.6 million people drink from this tap, about a third in San Francisco and two-thirds outside. (The San Francisco Public Utilities Commission, which manages our water systems, sells water to four other counties.)

We actually drink a blend of 85 percent Hetchy and 15 percent local water, because the reservoirs along the way accumulate rain. Hetchy water is not filtered, coming from a pristine “granite bowl” in Yosemite, but is disinfected near Tracy. Local water is filtered and treated before joining the party.

When it reaches The City, water is distributed via 1,250 miles of pipes, 17 pump stations, 10 reservoirs and eight tanks. All told, we can house a five-day supply.

San Francisco uses 65 million gallons of water per day, about 100 Olympic swimming pools. Residents use 45 gallons per person per day, one of the lowest rates in the state. Not having large yards helps; our conservation efforts help, too. And the biggest water user in the house? It’s typically the toilet.

Which brings us to the next step in our water’s journey.

If you’re on the westside, your sewage goes to the Oceanside plant by the zoo. If not, it goes to the Southeast plant in Bayview, where, owing to The City’s eastern density, about 80 percent of sewage ends up.

One thousand miles of sewers, some as old as the Gold Rush, carry wastewater on an hours-long journey to the plants. If it’s raining, your sewage is joined by all the storm runoff because San Francisco, unlike effectively every other city in the state, treats the two together. This keeps oil, metals and litter out of the ocean but also requires a much larger system.

On dry days, Oceanside processes 17 million gallons, and Southeast 67 million. During storms, the system can treat up to 600 million gallons and store an additional 200 gallons in The City’s “moat,” underground boxes lining The Embarcadero and Great Highway. The treatment process is not always as thorough during such conditions though, which is why you occasionally see signs at beaches advising against swimming. Heed those signs.

Wastewater is first screened to remove sand, grit, rags, trash, tampons, condoms, leaves, lumber, drowned rats and baby wipes. By this time, the toilet paper and boom boom (technical term) have largely disintegrated.

The sewage then enters primary treatment tanks, where solids settle to the bottom and are scraped away, while floatables rise to the top and are skimmed away. The water in the middle flows to phase two. In secondary tanks, bacteria and oxygen are added. The bacteria feed on dissolved organic material and settle to the bottom for removal. It’s a biologic process, like a giant human gut.

At Oceanside, wastewater goes through one more separator tank then gets pumped 4.5 miles out in the Pacific. At Southeast, subject to tighter regulations because it flows to the Bay, water is treated with bleach to kill pathogens then sodium bisulfite to neutralize the bleach. It’s then discharged by Pier 80.

Everything that settled or floated out along the way goes to digesters, the big tanks you see. It’s heated and broken down by anaerobic bacteria. This generates methane, which is burned for electricity, making Southeast about half self-sufficient. The finished product, called “cake,” is hauled to farms or landfill.

In a matter of 11 hours for the liquids and a few weeks for the solids, everything is “given back to God.” We only rented it.

El Niño stabilized our water supply. Growth and climate change will strain it. But for now, San Francisco’s underground river keeps flowing.

Conor Johnston is the chief of staff to the president of the Board of Supervisors, London Breed, and co-founder of the East of Twin Peaks Neighborhood Association.Conor JohnstonHetch HetchySan FranciscoSFPUCTuolumneWATERYosemite

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