FOLLOWING SEWAGE’S LONG PATH

Millions of gallons of sewage flow daily from the toilets, sinks, garbage disposals, showers and washing machines of San Diego-area homes and businesses. On the average, each San Diegan dumps 85.6 gallons of waste water into the sewers every day, according to city water utilities officials.

Sewage is 99% water; the rest is suspended materials such as human fecal matter; food grindings from garbage disposals; paper products, such as toilet paper and facial tissue; condoms; plastic products; tampons; dust rags; pieces of wood; grease and oil; soap and detergents. About 20% of the suspended materials is silt--seeds, rocks, sand, clay and soil.

The region’s relatively small industrial base, coupled with a special pretreatment program to catch industrial waste before it is dumped into sewers, means that there are only trace levels of dangerous metals--nickel, chromium, cyanide--in local sewage, say city officials. Other industrial wastes are found in levels below federal requirements, they say.

The Metro Waste water Treatment System, which is operated by the City of San Diego, handles sewage from 16 cities and water districts.

Gravity carries the sewage from residences and businesses through small pipes called laterals, which are in turn connected to larger sewer mains that run underneath city streets. As the increasing volume of sewage surges toward the treatment plant, the pipes become correspondingly larger. Sewage from East County and the South Bay area, for instance, eventually flows to a huge interceptor pipe that runs along the shore of San Diego Bay.

Because of the region’s undulating topography, gravity alone can’t be expected to move all of the sewage. Thus, pump stations are used to move the sewage over hills. There are 68 pump stations in the Metro system alone. The most notorious one is Pump Station 64, which has malfunctioned and spilled millions of gallons of raw sewage into Los Penasquitos Lagoon 60 times in the last 10 years.

Eventually, the river of sewage travels through underground tunnels downtown. From 60 million to 70 million gallons a day are pumped through a large force main that passes close to the El Cortez Hotel, at 7th Avenue and Ash Street. This flows to a final pump station on Harbor Drive, near Lindbergh Field, where it is joined by 110 million gallons coming in from North City. Screens at the pump station remove the larger debris coursing through the pipes: rags, pieces of wood, dollar bills. The remaining waste is pumped through a huge main to the Point Loma Wastewater Treatment Plant.

The Point Loma plant handles 180 million gallons of sewage a day. As the torrent of sewage feeds in through a 114-inch main, it goes through a line of four rotating screens. These screens catch smaller debris. The items caught on the screen are dumped on a conveyor belt and eventually buried in a landfill.

The next step is the grit chambers, where the flow of waste water is slowed so that sand, egg shells, rocks, etc., can settle out. Meanwhile, air is injected into the sewage to keep the organic matter, which includes human fecal matter, afloat.

After it leaves the grit chambers, sewage is then injected with ferric chloride and polymers. This chemical process is what gives Point Loma the ability to perform advanced primary treatment, which ultimately removes 75% of the suspended solids from San Diego sewage. The chemicals bind the floating organic matter into larger particles so they can settle out more easily.

The sewage is next put into one of eight sedimentation basins. Each measures 60 feet wide, 240 feet long and 16 feet deep. The flow of waste water is slowed dramatically to encourage further settling of the suspended materials. Oils and grease--scum--float to the top and are skimmed off; material on the bottom of the basins--sludge--is removed for further treatment.

To control odors, fans take the fumes from the sedimentation tanks and draw them into large white chambers. The fumes are sprayed with chemicals to control the smell.

After being pushed through one last screen, the treated sewage is then sent out a two-mile-long outfall, where it is shot through a Y-shaped diffuser into the ocean at a depth of 220 feet. At this depth, city officials say, the sewage is beneath the thermocline, a layer of the ocean which separates the warm and oxygen-rich water near the surface from the cold and oxygen-poor water below it. The thermocline, in effect, traps the sewage and keeps it from washing up on beaches.

Point Loma does not offer secondary treatment like other plants such as those in Escondido and Oceanside. San Diego will be required by the federal government to upgrade its facilities to secondary treatment.

While the advanced primary treatment described so far is mainly a physical process, secondary treatment relies on an additional biological process to remove even more of the suspended materials from sewage.

During the secondary process, primary sewage discharge is put into yet another series of tanks, where oxygen is injected. The sewage sits for a matter of hours and bacteria begin to eat the solid material. The sewage is then put through another series of sedimentation tanks, where the bacteria settles out. This process takes out 90% of the waste from sewage.

Material that settles out during sewage treatment is called sludge. At Point Loma, it is pumped into large digester tanks, which are heated to body temperature. The sludge is kept in the tanks between 20 and 30 days. This process stimulates the bacteria to begin feeding on the sludge. The bacteria reduce the sludge by 50% in volume, and also give off methane and carbon dioxide gases. The remaining sludge--700,000 gallons a day--is then pumped 7.5 miles through an eight-inch pipeline to a site on Fiesta Island in Mission Bay, where it is dried for burial or use as a fertilizer.

Traditionally, gases created during sludge digestion are burned off. But San Diego City officials have diverted the gases to run two 2,000-horse power generators. Those generators provide all the power to run the Point Loma treatment plant; surplus power is sold to San Diego Gas & Electric Co. for a profit of $40,000 a month.