Fact check: Here's what actually happens to SF's sewage

SAN FRANCISCO (KGO) -- On Wednesday night, President Donald Trump made a new claim about San Francisco: that the city's homeless population is polluting oceans with needles and other debris.

He said the city is "in serious violation" of environmental laws and warned the city will soon be getting a citation from the Environmental Protection Agency.

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President Trump's comments drew swift condemnation from local San Francisco officials, including Mayor London Breed who brushed them off as "more lies."

So, what actually happens to San Francisco's sewage?

We all know there are all kinds of things on city streets that sometimes do go down the drain.

According to the San Francisco Public Utilities Commission website, a few parts of the city (Ocean Beach, Lake Merced, and Mission Bay) are served by a sewer system where stormwater "goes directly into the Bay and Ocean with minimal treatment."

But SFPUC says that most of the city is serviced by a combined sewer system that "collects and treats both wastewater from homes and businesses and stormwater that falls on the city's streets and into the catch basins."

There are two main treatment plants that process any solid waste that goes down stormdrains in those areas.

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We went inside the city's largest water treatment plant to learn this step-by-step process of where it all goes (Spoiler Alert: For that system we didn't find much evidence to prove debris, like needles, can easily make their into the ocean).

Step 1: The first line of defense for waste around the city is catch basins. There are roughly 23,000 of them and anything a street sweeper doesn't clean up will likely go down there.

Step 2: From there, the sewage goes through the 1,000 mile underground sewer system to make its way to one of two water treatment plants in the city. The largest is the Southeast Water Pollution Control Plant in Bayview -- that's where roughly 80 percent of the city's sewage goes.

Step 3: Inside, the sewage is sorted and screened. Any solids -- such as construction material, baby wipes, even needles -- that made it through the catch basins and into the treatment plant are filtered out through two different machines.

Step 4: If any debris makes it through that, there's even another line of defense. "Then we go into the primary tank," Andrew Clark the plant's operations superintendent explained. "The primary tanks are designed to have more detention time to allow settables to settle and floatables to float."

Step 5: Eventually, the filtered solid debris is sorted, taken to a dump, and then to a landfill. The treated water then flows into the bay or ocean.

We asked Karri Ving with the San Francisco Public Utilities Commission if it's different during storms. She said while some debris can get into the ocean during a storm, there's another filtering system in place -- called "primary treatment" -- that stops most of it.

"We're very proud of the system that does a great job separating out solids, including needles," Ving said. "And there are multiple lines of defense before we reach the bay and the ocean."