“San Francisco is unique in California as the only city served predominantly by a combined sewer system. San Francisco collects both sewage and stormwater in the same network of pipes, then treats and discharges the combined flows to San Francisco Bay or the Pacific Ocean. Except for portions of Old Sacramento, all other cities in California have separate sewer systems, which means there are two sets of pipes in the ground. One set of pipes takes sanitary waste to the treatment plant while a second set carries stormwater runoff from street drains directly into creeks, lakes, or the ocean...Many United States cities built prior to 1900 had combined sewer systems. At that time, sewage treatment was not available and sewers simply directed sewage into local water bodies. When sewage treatment became necessary to protect public health, newer cities built separate systems to save on the costs of treating stormwater. Some of the older cities opted to separate their combined systems. San Francisco, already a dense urban environment, decided that separation was too costly and disruptive to the residents. Separating the sewers would have required ripping open nearly every street for stormwater pipe installations. Today, the treatment of stormwater in San Francisco’s combined sewer system helps protect the environment. Many cities that have separate systems are now initiating treatment of urban runoff.” Oakland Museum of CA
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