Wastewater map, via SFWater.org
While most people know the source of our delicious drinking water, few know what happens when we’re done with it. Now is also the time of year to think about rainfall, and the effect that water has on our design decisions. In San Francisco, we have a combined sewer system, which is to say we don’t discriminate between sewage and stormwater. Of course most of this infrastructure is also quite old. San Francisco’s first sewers were built over 100 years ago, but the brick construction holds up quite well.
Originally a cost-cutting measure, San Francisco is unique in having one pipe for all water. Now it’s actually quite beneficial to the environment when we treat everything before dumping it in the bay. The only problem is during extended periods of rainfall, the system gets backed up. The current infrastructure is quite amazing – several large storage tanks under our largest streets, simply to store water prior to treatment. Rainfall gets treated over the course of several days, before heading out to the bay or ocean. Even though the 3 treatment plants can process about 500 million gallons a day, we end up dumping water about 10 times a year. Investments into stormwater absorption could greatly help alleviate the sewer issue, but it also raises other problems.
The other aspect is water that is absorbed into the ground. This is always preferable to treating wastewater, but we have to acknowledge our limitations. San Francisco was once populated by many creeks and tributaries, beginning on hilltops and carving our many valleys. While the surface ones are almost all diverted, many still run underground. But of course they’re not gone, simply forgotten, ignored, or rerouted. We only have a rainfall of about 20″ a year, but we have a large collection of springs and other unexplained sources of water to power these streams. The Hayes River is powerful enough that BART has to pump 2.5 million gallons of water from the Powell Station each day. Many other buildings have to continuously worry about inundation while planning their basements or foundations. One project I’m working on is undergoing Discretionary Review partially because people are worried about the change in water flowing through the underground river. Changes in hydrology can cause landslides and flooding if not properly accounted for, just one more unique design challenge in San Francisco.
Subsurface water storage, via SFWater.org
Every day we live with the decisions of the past, and our water infrastructure is no exception. Sewers and stormwater are the same system, and now we have to pay extra fees for each toilet or sink that we build. Culverted streams carry a large source of potable water directly to the bay, instead of being used for irrigation or drinking. We don’t live in a desert, but we’ve designed our city to be completely independent of the weather. Each rainy season we should think what our priorities are when rain drains pollution to the bay, but we’re worried about Sierra snowmelt for drinking. We need to design all of our buildings and infrastructure to be ready for changing climate and rain patterns. Relying on old systems will no longer be enough – it’s time to make sure each addition can stand on it’s own.
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