On Monday, thousands of beaches in Los Angeles banned swimming. The night before, 17 million gallons of wastewater from the city’s largest sewage treatment plant flowed into Santa Monica Bay.

A mechanical failure “at the Hyperion plant last night caused untreated sewage to be discharged into the ocean,” Los Angeles County Supervisor Janice Hahn said on Twitter. “Water samples are being tested and I’m getting more information about the scope of the problem. Beaches from El Segundo to the Dockweiler RV Park are closed for swimming.”

Closed signs were posted on El Segundo and Dockweiler State Beaches and the area around the Grand Ave. storm drain, and officials urged visitors not to enter the water. The county public health department said at a press conference that these beaches total about 4 miles and will remain closed until the water samples test negative for bacteria. The Hyperion plant is the oldest and largest wastewater treatment facility in the city and has been in operation since 1894. The plant is designed to process 450 million gallons of water per day.

“The facility became inundated with overwhelming quantities of debris, causing backup of the headworks facilities,” said Executive Plant Manager Timeyin Dafeta.

“The plant’s relief system was triggered and sewage flows were controlled through the use of the plant’s one-mile outfall and discharge of untreated sewage into Santa Monica Bay,” Dafeta said.

The statement said that about 6% of the facility’s daily load was discharged as an emergency measure to prevent the plant from closing and dumping more untreated sewage. “Bacteria and viruses in raw sewage are extremely dangerous to people and can carry a variety of diseases,” environmental advocacy group Heal the Bay said. “Debris such as tampons and plastic trash, when released into the Bay, can harbor bacteria and can cause entanglement of wildlife, but it seems in this case those debris were successfully filtered out of the spill before it made it to the Bay.”

“I understand that the plant was able to prevent an even larger spill, but we are going to need answers about how and why this happened,” Hahn said on Twitter.

Dafeta told the Los Angeles Times that the mechanical failure was resolved on Monday morning. The largest spill in the city’s history occurred in 1998 when more than 30 million gallons of sewage were leaked during the El Niño storm. Another notable sewage leak occurred in 2016 when a damaged sewage line leaked approximately 2.4 million gallons of untreated waste into the Los Angeles River and forced the closure of all county-level beaches in Long Beach and Seal Beach. In 2018, a sewage leak at the Dodger Stadium forced the cancellation of the last game between the Los Angeles Dodgers and the Los Angeles Angels.