SF Does Right with (and by) Its CSO System
In the early hours of the morning of a day in February, San Francisco's combined sewer system topped out.
Millions of gallons of raw sewage and storm runoff flowed untreated into the ocean and San Francisco Bay, covering beaches with bacterial and chemical contaminants.
The system had worked as designed.
San Francisco purportedly is the only major city on the California coast with a system built to overflow during periods of heavy or prolonged rainfall.
Although San Francisco suffers the occasional overflow on stormy days, environmental standard bearers commend the city's course—because the city is intent on doing it right. And San Francisco has spent a lot of money to get there.
In March 1997, the city completed a 20-yr, U.S.$1.4-billion system upgrade.
The system comprises an intricate web of pumps, treatment stations, and cisterns that work together to absorb storm flows.
The system's centerpiece is the pair of new wastewater storage cisterns buried along the west side of the city.
The cisterns hold water during storms, giving the treatment plants time to process the millions of gallons running off the streets.
City officials estimate that the new system can handle more than 10 times the runoff volume as previously and that the system catches and treats between 80% and 90% of rainy-day flows.
Despite dramatic system-capacity increases, during the last two years, San Francisco raw sewage has overflowed onto beaches and into the bay nearly 30 times.
And though the average overflow is small, the overflows add up: In the course of a year, between 1,000–2,000 mil gals of untreated overflow bypass the system.
However, "there's no way you can eliminate 100% of all overflows, because there's always going to be a bigger storm," said Chris Phanartzis, a consulting hydrologist with the Public Utilities Commission.
Also, as the city grows and its water use increases, less treatment capacity becomes available for contending with storm-water runoff.
The U.S. EPA estimates that 40-million people in the U.S. are served by combined storm and sewer systems like the one in San Francisco.
However, most West Coast cities channel their wastewaters separately. Sewage is treated, whereas the storm water is simply piped for release to nearby bodies. Such cities generally don't bother about sewage overflows, but their storm water incessantly washes oil, grime, and other waste directly off the streets and into the lakes, rivers and ocean.
The previous case study was adapted from a feature written by Matthew Golec for the March 9 issue of the Los Angeles Times.
Edited by Paul Hersch