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Demystifying Sewers: How They Work Beneath Our Feet

How the sewer system works

how sewage flows from your home or business through the sewers and to the treatment plants.

sewer pipeline largeSewage (often called wastewater) most often enters the sewer pipeline network from kitchen, bathroom, and laundry drains in our homes and businesses. Also, some industries and larger businesses have special permits allowing them to place wastewater from their operations into the sewers.


Like small creeks that feed larger streams that turn into rivers, sewers become larger and larger as the sewage flows further and further “downstream”.


Sewage first flows through small diameter sewers called lateral pipelines that take the sewage away from a home or business and into sewer mains that run under streets, alleys, or other rights-of-way.


Mains connect to collectors and trunk lines, all larger and larger pipes that eventually become interceptor sewers – the largest pipes in the City’s system. The interceptors are often over eight-feet in diameter and carry millions of gallons per day.


These huge pipes convey the sewage to the City’s treatment and water reclamation plants.

pumping plant 2 large pumping plant 1 largeMost sewers are designed so that gravity alone carries wastewater to the treatment plants. Low-lying areas such as some beach communities or valley locations need pumping plants to push wastewater through pipes that are under pressure (force main sewers) so that the sewage can reach a gravity sewer. The City operates 48 pumping plants.

drop structure largeDrop structures are like waterfalls. They quickly move sewage from a higher to a lower elevation. In the past they have been used in hilly parts of the City. However, the falling wastewater causes turbulence at the bottom of the structures and this can produce sewer odors. The odors are being treated by a number of methods including air treatment facilities.


This drop structure model is being used to test hydraulic flow.

siphon largeSewer siphons are used to move sewage underneath freeways, rivers, or other potential obstructions that could disrupt the sewer’s route. The sewer dips under the obstruction and then comes up on the other side and ties back into the sewer at about same elevation where it began the dip. Like a siphon used to take gasoline from a car, the sewer siphon is powered by the pull of wastewater on its downstream side.

clarifierss largeSewage that begins in your home or business eventually flows into one of four City of Los Angeles wastewater treatment plants. Three of these plants have advanced treatment facilities that produce reclaimed water for many non drinking water uses. This potentially saves billions of gallons of drinking water each year. The treated wastewater (effluent) that is not reclaimed is piped into the Los Angeles River, Santa Monica Bay, or Los Angeles Harbor. The effluent meets or exceeds stringent state and federal water quality standards in order for it to be compatible with these aquatic environments.

new coverMost everyone has seen the metal plates that cover maintenance holes (formerly called man holes). The maintenance holes give the City access to the sewers for periodic inspection and repairs. The City’s wastewater collection system contains 140,000 maintenance holes.