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Paso wastewater too high in chemicals, says water board 

The city of Paso Robles needs to clean up its wastewater.

That’s the opinion of the Central Coast Regional Water Quality Control Board, which says the city struggles to meet state wastewater standards for trihalomethanes (THMs), a disinfectant chemical byproduct that can be harmful to the environment and human health at certain levels.

“While progress has been made, consistent disinfection without the formation of THMs has not yet been achieved,” read a July 14 Regional Water Quality Control Board schedule order to the city, signed by executive officer John Robertson.

Paso Robles discharges 2.7 million gallons of treated wastewater daily into the Salinas River, which is also a water source for the city. 

“We are actively working to reduce the discharge of THMs to the Salinas River,” said Matt Thompson, Paso Robles wastewater manager.

The city made more than $40 million in upgrades to its wastewater plant in 2015, adding a biological nutrient removal system and other improvements. Dick McKinley, Paso Robles public works director, told New Times the city’s old plant was, “really old, terrible, and low tech,” and produced toxic levels of ammonia.

As part of the recent upgrades, the plant switched to a chloramine-based disinfectant system for the wastewater. Chloramination hasn’t consistently been able to thwart the presence of THMs.

“It’s a picky, super sensitive process,” McKinley said. “It’s really hard to get it right. It’s a constant moving target.”

Harvey Packard, a manager at the Central Coast Regional Water Quality Control Board, said the THM issue is not an uncommon one in wastewater plants. THM standards are strict, based on the effect of the chemicals on fish.

“I’d say [Paso Robles wastewater] is not a huge threat right now,” Packard said. “The city meets the THM standard most of the time but isn’t 100 percent in compliance.”

The city’s solution to THM is its tertiary water treatment project, which will use ultraviolet light to disinfect the water and provide recycled water to be used across the city.

The water board schedule order was made to accommodate the city’s tertiary water treatment project, which is 90 percent designed and is estimated to cost the city another $15 million. 

“The only way to [eliminate THMs] is getting off of chlorine,” Thompson said. “That’s the purpose of the time schedule order.”

Paso Robles has until Oct. 10 to submit a THM pollution prevention plan and then until 2021 to come into compliance. The city will be held to interim THM standards about six times lower than the state standards until then.

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