The storage/transports are huge underground rectangular tanks or tunnels that ring the City like a moat. During rainstorms, the storage/transports prevent untreated shoreline discharges. The storage/transports have three functions:

• Capture the runoff. At the City’s perimeter, the storage/transports catch the combined storm water and sewage as it overflows the sewer system but before it reaches the shoreline of the Bay or ocean.
  
• Storage. The storage/transports hold storm water and sewage for later treatment at wastewater treatment plants. (Total storage capacity is approximately 193 million gallons)
  
• Treatment. The storage/transports provide treatment consisting of settling and removal of floatable materials prior to shoreline discharge when wastewater flows exceed the system’s total storage capacity. Such exceedances occur an average of one to ten times per year depending on location. These discharges consist of approximately 6% sewage and 94% storm water, and have received primary-level treatment. Most rainstorms do not fill the storage/transports and all the wastewater is held for later treatment. The solids retained in the storage/transports are flushed to the treatment plants after storms.

The following figure depicts the functions of such a transport/storage box.