ORINDA – The East Bay Municipal Utility District’s newest $325 million addition to the Orinda Water Treatment Plant centers around a high-tech plan to use ultraviolet light as the primary decontamination strategy to combat rising organic matter found in the Mokelumne watershed.
The project seeks to protect against future increases in run-off, flooding and water temperatures that lead to more pathogens, viruses and natural organic matter. When organic matter is treated with chlorine, it can create harmful byproducts, known as trihalomethanes (THMs), which can impact human health.
The Orinda Water Treatment Plant was the first building commissioned by EBMUD in 1935, and it has historically treated raw water from the Mokelumne watershed using chlorine decontamination to kill bacteria and viruses, according to EBMUD Senior Engineer Tim Karlstrand. However, this process can create chemical byproducts that can be harmful to human health.
In the 2010s, EBMUD sensors detected a rise in THMs from environmental changes, and officials adopted a plan to implement a new water-quality system that could safely protect the East Bay’s supply for the next 100 years.
“This is our first water treatment plant that will have an ultraviolet disinfection,” said Karlstrand. “We’re making the largest dollar investment in our infrastructure ever as part of this project.”
EBMUD officials devised an annex to the Orinda Water Treatment Plant that would use UV light as the primary decontamination method – a highly-regarded practice among international water agencies that was first developed for municipal use in 1910 in Marseilles, France. UV decontamination “instantaneously” destroys pathogens as raw water passes through a pipe under fluorescent lights, Karlstrand said, where it’s bombarded by ultraviolet rays that break down the DNA of bacteria, viruses and other organic compounds. Chlorine will still be used as a secondary disinfectant to prevent bacterial outbreaks when sitting in pipes, Karlstrand said, but the primary use of UV decontamination will limit the byproducts of chlorine.
“It’s a little bit more expensive than chlorine, but we benefit a lot from using ultraviolet light,” Karlstrand said. “With chlorine, we’ve got to store the chemical and there are precautions and hazards associated with storing and using the chemical. Ultraviolet light, on the other hand, just uses electricity, and nobody’s exposed to it.”
Some three stories below the surface, workers are constructing the UV decontamination basin, pouring concrete that will seal the basin forever. On a recent visit to the facility, construction workers hung from rebar-covered walls while others walked across wooden platforms, making their final measurements.
To date, the project has used 1.65 million pounds of rebar and 13,000 cubic yards of concrete, according to engineers. At the bottom of the construction site, Karlstrand walked through a 7-foot hole between the rebar walls that will eventually connect waters from the Mokelumne watershed to customers in the East Bay.
“We’ve been out here two years building the underground portion,” Karlstrand said. “The project has every imaginable complexity in civil engineering you can imagine. There’s a challenging supply chain, especially for electrical equipment, which can take two years, so any delay is significant.”
When EBMUD activates the new facility in 2026, it will process up to 200 million gallons of water per day for 800,000 customers across the East Bay, Karlstrand said. Yet for all its complicated planning, switching over to the new disinfectant plant requires a simple turn of a blue valve.
“The underground infrastructure is what everybody takes for granted; it’s what the world never sees,” Karlstrand said. “The project that we’re building is gonna last us another 100 years. And that’s really exciting, because that outlives me, that outlives you and that provides the reliable drinking water supply for future generations.”