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Wastewater treatment plant

St. Helena’s wastewater treatment plant.

ST. HELENA — The city of St. Helena is facing a $57,000 fine after its Wastewater Treatment Plant discharged effluent to the Napa River that failed to meet quality standards.

Samples taken in February and March 2019 exceeded limits for cyanide, copper, total coliform, residual chlorine, total suspended solids (TSS), and biochemical oxygen demand (BOD), according to a letter from the San Francisco Bay Regional Water Quality Control Board.

The cyanide is a byproduct of the ascorbic acid the plant uses to dechlorinate treated water — a process that will be less necessary once the plant undergoes a $12.5 million upgrade.

The TSS and BOD violations were attributed to heavy rain and faulty sewer pipes. Heavy rainfall last February and March, combined with old and decaying sewer pipes throughout the city, allowed a high volume of groundwater to infiltrate the sewer system and reach the plant for processing.

That resulted in highly diluted samples with abnormally low levels of TSS and BOD, which are common in sewage but not in rainwater. The treatment plant’s permit requires it to filter out at least 85% of TSS and BOD. With so little TSS and BOD there to begin with, the plant missed those targets.

“Even if it’s mostly rainwater, you still have to meet these reduction targets,” said Public Works Director Erica Ahmann Smithies.

Groundwater infiltration is a problem for aging sewer systems across the U.S., and “increasingly extreme weather events compound the problem,” said Eric Sanders, chief wastewater treatment plant operator.

The plant has been under a cease-and-desist order since 2016, when the Water Board issued a new permit containing more stringent limits than the plant can comply with in its current configuration.

The city is on a Water Board-approved timeline to investigate and implement the upgrades necessary to meet the new restrictions.

The City Council came one step closer to upgrading the plant on Tuesday, when staff presented a conceptual design for the upgrade. The new plant will be funded by debt capacity, grants, impact fees, and wastewater funds.

The upgrade won’t solve the infiltration problem, but the more effective treatment system “will put a dent in and hopefully eliminate the fines going forward,” Smithies said.

The new plant will use ultraviolet light during the cleaning process, with only a small dose of chlorine. By relying less on chlorination, the new plant won’t require the level of dechlorination that created residual cyanide last year.

“That problem should be eliminated with the new plant,” Smithies said.

The plant currently uses a passive treatment system to clean wastewater as it flows through a series of ponds. Under the new configuration, those storage ponds won’t be full all of the time, so they will be able to accommodate heavier flows than the current plant can handle during heavy storms.

The upgraded plant will also be capable of tertiary treatment, producing water suitable for landscaping and other non-potable uses.

The city plans to award a design-build contract for the new plant later this year, with construction sometime between 2021 and 2023. The city is on track to be in full compliance with the plant’s permit by the deadline of March 1, 2023.

Long-term upgrades to the city’s sewer pipes will reduce groundwater infiltration. Consultants are conducting a master utility plan of the city’s underground water, wastewater and storm drain systems to estimate costs and determine which repairs are the most critical.

“We can’t afford to take on the whole body of work in a short period of time,” said City Manager Mark Prestwich. “But this will help us prioritize those investments.”

The fine is part of a tentative order and proposed settlement announced Jan. 7. The Water Board is accepting public comments on the settlement through Feb. 6.

Prestwich said the city anticipated the fines last year and included a $50,000 line item in the 2019-2020 budget.

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You can reach Jesse Duarte at 967-6803 or jduarte@sthelenastar.com.

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