Mill Valley will spend $34.5 million to maintain its sewer system for the next decade.

The City Council unanimously approved the capital improvement program on Monday to monitor, clean and renovate its network of 55 miles of sewer pipes, 1,481 manholes and three pump stations that collect and send wastewater to a regional treatment plan.

The capital improvement program will increase annual monitoring and maintenance costs by $1 million to roughly $3.5 million.

“We absolutely do not want to have any issue,” said Andrew Poster, public works director. “There’s large health and safety issues.”

Mill Valley spends about $10.5 million a year to operate the system, he said, noting it is one of the city’s largest operating expenses. Residents pay for the costs through user fees.

“This is a 34-and-a-half-million-dollar program,” he said. “We really want to know what’s going on with our system. We want to plan ahead for it. Things take years to design, to build, to procure, and this, together with other studies, really serves as the basis for the program.”

Ben Schick, an engineer with Schaaf & Wheeler, the city’s consultant, discussed what was known and unknown about the city’s sewers.Much of the system is 50 years old or older. Some pipes are clay, which is no longer used today. Older manholes are brick. Roughly a third of the pipes have been examined by video cameras. The city has older inspection data on another third. The final third of pipes are unexamined.

The public works office divides the city’s system into five zones. One zone per year is cleaned. The city also monitors “hot spots” that are regularly examined for infiltration and leaks. Its pumps include one station that pushes wastewater up and over a hill to the treatment plant.

“The city rehabilitated over 11% of the sewer system through annual sewer improvement plans,” Schick said, adding that the public works department schedules these repairs before repaving streets.

“The plan basically consists of gathering existing data and compiling it to figure out what the current condition of the sewer system is, and then also extrapolating that into a plan so that we can come up with improvements to basically maintain the sewer system over the next 10 years,” Schick said.

The city’s records and video inspections have found “broken pipes, root intrusion, offset joints, sags in the pipes, poor lateral connections” and other problems, he said.

“Ultimately, the goal of this is to reduce inflow and infiltration,” Schick said. “So, reduce the amount of stormwater that enters the sewer system, which ultimately would have to be treated, which is a large capital expense. Reduce the risk of pipe failures.”

City Council members asked about the prevalence of hot spots; whether the system could handle runoff from major storms and expected new housing; and what rate increases would be necessary to fund the program.

Poster said the list of hot spots has shrunk from more than 100 to 55. He also said the system could handle the 865 new dwellings that the state expects Mill Valley to create by 2031.

City Manager Todd Cusimano predicted a rate increase of “maybe 3%.”

“Everybody in the Bay Area is basically in the same exact situation that the city of Mill Valley is in,” Schick said, “and that is your system is 50-plus years old and it’s starting to fail and needs to be rehabilitated and replaced.”