SACRAMENTO — In Sacramento, even an inch of rain can lead to a much more complicated problem underground.
The city is one of only two in California and four on the West Coast still operating a combined sewer system—a century-old design where stormwater and sewage flow through the same pipes.
As climate challenges intensify, modernizing and maintaining the system is a 24/7 job.
Sacramento's combined sewer system (CSS) stretches across 7,500 acres in neighborhoods like downtown, east Sacramento, Oak Park and Land Park. The system serves 300,000 residents, at least ten times the population when it was built.
"These systems were never meant to keep up with the type of rainfall we're seeing today," said Carlos Eliason, a spokesperson for the city's sewage operations.The system dates back to 1914 and has been added to over the decades. The first sump station was originally built on Riverside Boulevard and still functions today. Stage 2 was built in 1938, and an additional pumping capacity was added in 2000.
The city stopped building combined sewers in 1946, but maintenance and new technology allow the City of Sacramento to run the system up to full capacity, if needed, during big storms.
"Anything can go wrong. Sometimes it's the littlest thing that can cause the biggest problem," said Philip Meyer, supervising plant operator for the CSS with the City of Sacramento utilities.
Meyer is a veteran of the system with nearly two decades of experience and says he is confident in the combined system, even with its age, noting that the options to separate the pipes into stormwater and sewage water would be costly and disruptive.
What happens when the sewer system fails?
In heavy storms, Sacramento's CSS can reach its capacity quickly. When that happens, combined sewer overflows, or CSOs, can spill into the streets and, in rare cases, the Sacramento River.
Videos posted online from 2021 from residents in east Sacramento show flooding that included sewage after a storm. To understand the system's flaws, it's best to understand how it works.
The system includes two major treatment sites — Pioneer Reservoir and the Combined Wastewater Treatment Plant — capable of processing over 60 million gallons of water per day during rain events. Still, when overwhelmed, partially treated or untreated water is discharged into the river.
Approximately 11,300 acres within the city contribute flows to the CSS. This total includes approximately 7,500 acres within the downtown, east Sacramento, and Land Park communities, which contribute sanitary sewage and storm drainage flows to the CSS. Approximately 3,700 acres within the east Sacramento and River Park communities, as well as Sacramento State, contribute sanitary sewer flows only, and the remaining 100 acres contribute storm drainage flows only.
The CSS drains to two pumping stations to the west, pump station 1/1A and pump station 2/2A. The two pumping stations transport flows to treatment facilities and eventually to the Sacramento River.
Based on the city's contract with the Sacramento Regional County Sanitation District, the city can convey a maximum of 60 million gallons per day to the Sacramento Regional Wastewater Treatment Plant prior to discharge to the Sacramento River. When the flow rate exceeds 60 million gallons per day, the Combined Wastewater Treatment Plant and Pioneer Reservoir are utilized to provide treatment and disinfection for an additional 130 million gallons per day.
According to the California Water Boards, direct, untreated discharges into the Sacramento River are rare. Over the past decade, the system averaged five overflow events annually, with only two being untreated — both back in 2013.
"In the event that we surpass all of our storage capability. That is when the treated water is then discharged out into the Sacramento River. That happened four times this year [and there were] no untreated discharges this year," Meyer said.
With the combined system, water pours into a sump station and the treatment process begins with removing solids, suspended solids, and trash. The water is treated with a disinfectant before it is discharged into the rest of the system for further treatment.
"Because we also disinfect, you get that added layer in the combined system that you don't get at a conventional storm station. We feel that the combined system does a really good job," Meyer said.
City leaders for decades have taken on the question: Can the system be separated?
"To separate the storm and sewer systems would cost billions and triple utility bills," former Sacramento Councilmember Jeff Harris warned. "It's just not feasible."
Instead, Sacramento is investing in what it calls optimization, and that means massive underground water vaults. The McKinley Water Vault, buried beneath a park and sports field, is the most visible (or invisible) example. It's designed to handle a 10-year storm and divert excess water to avoid surface overflows.
"These vaults are essential," a city engineer said. "They're out of sight, but they protect entire neighborhoods from flooding and sewage backups."
Federal funding is also playing a part. A recent $3.5 million investment will help build two new underground reservoirs near W and 25th streets and at 24th and K streets in midtown that are set to break ground in 2026, according to Eliason.
Organizations like the Natural Resources Defense Council (NRDC) are calling for even more proactive efforts, especially water conservation. Using less water reduces strain on the CSS, making room for stormwater during wet events.
"We're asking Sacramento to prioritize conservation where it counts most," the NRDC wrote in a recent letter to state regulators. "If we wait for the system to fail, the solutions will be far more painful."
What you can do: Water conservation and awareness matter
Every flush, every faucet, every rainstorm — how we use water matters. While Sacramento's infrastructure may be rooted in the past, its future depends on planning, innovation, and public awareness.
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