The Auxiliary Water Supply System (AWSS, though often referred to on manhole covers and hydrants as HPFS for High Pressure Fire System) is a high pressure water supply network built for the city of San Francisco in response to the failure of the existing emergency water system during the 1906 earthquake. It was originally proposed by San Francisco Fire Department chief engineer Dennis T. Sullivan in 1903, with construction beginning in 1909 and finishing in 1913. The system is made up of a collection of water reservoirs, pump stations, cisterns, suction connections and fireboats. While the system can use both fresh or salt water, it is preferential to not use salt water, as it commonly causes galvanic corrosion in fire equipment.
The large, white oversized hydrants that are supplied by the AWSS/HPFS, of which there are 1889, are visible throughout the city. The hydrants have painted tops that are color-coded as to zone:
The Twin Peaks Reservoir acts as the backbone of the AWSS system and is located in San Francisco's Twin Peaks hilltop. It is made up of a 10.5-million-gallon storage reservoir made out of 6-inch-thick (150 mm) reinforced-concrete slabs. Fresh water is delivered from the city's domestic water system by two 750-US-gallon-per-minute (2,800 l) centrifugal pumps. For safety, the reservoir is broken up into two tanks, and each tank can be emptied separately so that in case of a pipe breakage only half of the reservoir is lost. The tank is set at 758 feet (231 m).
The Ashbury tank has a direct connection to the Twin Peaks reservoir and has a total capacity of 500,000 US gallons (1,900,000 l). The tank is set at 494 feet (151 m) and, when combined with the Jones street tank, can provide hydrants with 214-psi pressures. It is located in the city's Ashbury Heights neighborhood.
The Jones street tank has a direct connection to the Ashbury tank and has a total capacity of 750,000 US gallons (2,800,000 l). The tank is set at 369 feet (112 m), providing hydrants with 160-psi pressures. It is located in the city's Nob Hill neighborhood.
There are two emergency pumping stations present within the AWSS.
Both stations are capable of pumping 10,000 US gallons (38,000 l) per minute of salt water at a pressure of 300 psi with on-site generators.
To supplement any outright failure of the pumping stations or reservoirs, three fireboats can be utilized to deliver salt water into the system.
As a final measure to counter any outright failure of the AWSS piping, a network of 177 independent underground water cisterns is present. Sizes vary from 75,000 US gallons (280,000 l) to over 200,000 US gallons (760,000 l) depending on location with a total storage capacity of over 11 million gallons of water. These cisterns are easily spotted at street level by red brick circles complete with manholes labeled CISTERN S.F.F.D