Prior to wide-spread use of drinking water disinfection techniques in the early 1900s, thousands died annually in the U.S. of infections by water-borne micro-organisms such as Cholera, Hepatitis A and Typhoid Fever. Since the adoption of good wastewater disposal practices, source (watershed) control and protection, and chemical treatment of water supplies, outbreaks associated with drinking water have been rare. In the San Francisco Bay Area there have been no outbreaks associated with drinking water.

While treatment effectively reduces the risk of disease associated with water-borne microorganisms, drinking water is not sterile and waterborne diseases still exist. Water-borne microorganisms may be naturally present in drinking water sources or they may be introduced through cross-contamination with wastewater. Not all water-borne microorganisms cause disease. Of those that could pose a risk, none are present at levels that would cause disease in healthy water consumers. Some water-borne microorganisms can form biofilms, which act to create a suitable environment for propagation of the organism within the distribution system.

The San Francisco Public Utilities Commission (SFPUC) water sources are the Hetch Hetchy, Alameda and Peninsula watersheds. The Hetch Hetchy Reservoir provides for approximately 85 percent of the system’s total water needs. The U.S. EPA has deemed this water of such high quality that only treatment and not filtration is required. The Alameda and Peninsula watersheds provide the rest of the total water supply. Water from these watersheds is both treated and filtered as they are vulnerable to contaminants associated with wildlife and to a limited extent human recreational activity. Despite this, the levels of contaminants in the Alameda and Peninsula watersheds are very low.

Water-borne microorganisms can be divided into three different families: protozoa, bacteria, viruses. General information and health effects for water-borne microorganisms are found in the following fact sheets*: