Monday, March 14, 2011

Wastewater Treatment in San Francisco and the SF PUC

The San Francisco Public Utilities Commission (SFPUC) is a department of the City government that provides: Regional and local water, Wastewater (collection, treatment and disposal), and Power. I spoke with Idil Bereket, Public Relations Officer for the Wastewater division to gain a fuller understanding of the wastewater programs after seeing the “only rain down the drain” signs around the city and chatting up an employee testing wastewater at Saint Francis Hospital.

Treating waste water and collecting storm water are essential city services, but this function also requires that the SFPUC work to educate the public to prevent pollution and limit the trash, contaminants and waste that enters the system. The SFPUC operates and maintains 993 miles of combined sewers, which collect sanitary sewage from toilets and drains in apartments, homes, schools, offices and other businesses, and street runoff throughout the city. Most of San Francisco operates under a combined sewage storage and treatment system. That means for most of the city, all sanitary sewage and stormwater is collected into the same pipes and catch basins and treated in the three treatment plants. There are a few areas of the city that were developed with a separate stormwater sewage system and those are controlled and discharge separately to the Bay.

The SFPUC operates three twenty-four hour a day waste treatment plants and one occasional plant that is used during rainy weather when the combined system collects more water. San Francisco is unusual in that it operates with a combined sewer system that collects and treats both wastewater and stormwater in the same system. Though stormwater is relatively clean, it does wash street pollutants into the sewer system that should not be directly released into the Bay or the Pacific. However, the processes needed to address street pollutants-motor oil, pesticides, metals, dirt and litter are different from the processes to separate toilet paper and sewage, and the pollutants of concern are different. That means the rain that runs off streets and buildings gets treated at the wastewater treatment plants just like sanitary sewage. That is expensive. Rain water is relatively clean and could be treated with a filter and oil and grease separators, but combining the stormwater with the sewage creates a large volume of water that goes through the wastewater treatment plants.

The smallest of the waste treatment plants is the Treasure Island plant that serves only Treasure Island and will be ignored for the rest of the discussion. The two major waste water treatment plants that serve the city of San Francisco are the Southeast Water Pollution Control Plant in Bayview built in 1950’s and the Oceanside Water Pollution Control Plant built in 1993. On a typical day the Southeast Water Pollution Control plant treats about 80% of the wastewater in the city. On a dry day the San Francisco combined sewage system treats about 80,000,000 gallons a day of sanitary sewage, during storms the treatment load can increase to up to 500,000,000 gallons of combined sewage. That six fold increase in water volume is more than can be handled by using the North Point Weather Treatment Plant.

What makes the system so expandable is the system of storage/transport boxes. The storage/transport boxes are huge underground rectangular tanks or tunnels that surround the City, the SFPUC describes it as a moat, a fitting image. The storage/transport boxes catch the combined stormwater and sewage as it overflows the sewer system, but before it reaches the shoreline of the Bay or Pacific Ocean. The storage/transport boxes have a total storage capacity of 200,000,000 gallons and hold stormwater and sewage for later treatment at wastewater treatment plants. If the capacity of the storage/transport boxes should be exceeded, the wastewater including sewage would be released into the Bay or the Pacific Ocean.

The storage/transport boxes provide the fist step of treatment, settling and screening of floatable materials inside the boxes. This step is equivalent to primary treatment that takes place at the wastewater treatment plants. When wastewater reaches the wastewater treatment plant, it passes through a screen used to remove large objects and debris. (You would not believe some of the debris which gets into the system despite the “only rain down the drain” signs.) Then the wastewater enters large settling tanks where heavier solids settle to the bottom, and floatables like oil and grease are scraped off the top.

Secondary treatment is bacterial digestion of organic waste. This process is enhanced by using blown oxygen to increase microorganism population enhancing the speed that the bacteria consume organic material in the wastewater. Afterwards, the wastewater is put into a second round of settling tanks where the microorganisms are separated from the purified water. Treated wastewater, is disinfected before being discharged into the San Francisco Bay from the Southeast Plant and into the Pacific Ocean from the Oceanside plant where the high salt concentration and cold temperatures help to kill any remaining bacteria. Effluent at North Point Facility is also disinfected before being discharged into the San Francisco Bay.

Sludge separated from wastewater during primary and secondary treatment is further processed on a “Gravity Belt Thickeners” to remove more water from the sludge. The thickened sludge is pumped into enclosed digester tanks. It is mixed for 15-25 days and heated at a constant temperature of 95 degrees Fahrenheit to allow anaerobic bacteria to break down remaining organic material in the sludge. Once treated, sludge is transformed into bio-solids. San Francisco produces 160,000,000 pounds of bio-solids every year that are recycled as fertilizer. The methane given off by the anaerobic bacteria is captured and used to power the system heaters.

The wastewater treatment system is not designed to do any more than screen out trash, skim off scum and grease and use bacterial action to digest toilet paper and bio-solids. Pharmaceuticals, pesticides, hydrocarbons and anything else the residents can think to pour down the drain or dump into the storm drains will either clog the system or be released into the Bay or Ocean. Cooking grease hardens when it cools in the sewer pipes. This can constrict the sewer pipes or form blockages that increasing pressure and often causing sewer pipe failure. Pharmaceuticals pass through the system untreated and pollute the Bay and estuary habitat. Pesticides and other chemicals can damage the bacterial habitat preventing adequate treatment of the sewage or be released to the Bay. As the SFPUC points out with their educational campaign, water pollution prevention begins with using your toilet to dispose of the three “P’s” (do you really need me to tell you?), recycling cooking oil, and remembering that everything that goes down the drain goes right out into the bay. A toilet is not a trash can and neither is a storm drain. To optimize the operations of the wastewater system at least expense, utilize rain barrels and cisterns to reduce overall flow to the system, and only allow the intended wastewater to enter toilets and drains and only rain should go down the stormwater drains.

The SF PUC has labeled all 23,000 stormwater drains to remind citizens not to use the storm drains as a trash can or disposal drains. According to Idil Bereket of the SFPUC, these little reminders have been effective in reducing dumping into the storm drains. In addition, the SF PUC runs SF Greasecycle, a citywide project that collects used cooking oil primarily from restaurants and converts it to bio-fuel for biodiesel vehicles. In the past year over 124,000 gallon of used cooking oil were collected and recycled. There is a small program that collects consumer generated cooking oil at several retail outlets and at holiday drop off points. Finally, the Rainwater Harvesting Program, subsidizes rain barrels and cisterns for city residents. Last year they sold 38 cisterns and 192 rain barrels. This diverted over 21,000 gallons of rainwater from the combined sewer system.

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