Miguel Barcenas, City of San Luis Obispo Interim Utilities Deputy Director of Engineering Kathi Robinson, Mimiaga Engineering Group/Carollo – Construction Management Services Chris Lehman, City of San Luis Obispo Water Resource Recovery Facility (WRRF) Supervisor

The SLO Mystery: What Caused a Sudden Drop in Nitrification?

Nitrification Loss at SLO Raises Questions About Causes and Cures
Jim Force, Resource Recovery

Read our second story, a Q&A with HDR experts on QACs.

Last September, nitrification came to a sudden halt at the Water Resource Recovery Facility in San Luis Obispo. While plant staff has come up with a couple of potential solutions, and the plant is nitrifying again, the real cause and cure remain somewhat of a mystery. As a result, Chris Lehman, SLO facility supervisor, advocates a broader discussion among plants experiencing similar issues.

“This was different from nitrification issues we’ve had in the past,” says Lehman. “Something happened. We went a whole month without nitrifying, and we’ve never gone that long before. The biology simply stopped working. There was some underlying issue we couldn’t explain.”

The culprit could be quaternary ammonium compounds (known as QACs), which reside in disinfectants the community may have used to combat the COVID-19 pandemic. And, since SLO is home to California Polytechnic University, the episode seemed to correlate with the return of the student population to San Luis Obispo.

QACs are known to impact wastewater treatment processes. In aerobic treatment, they can cause biomass foaming, diminished BOD removal, and loss of nitrification.

SLO Treatment Plant

The SLO Water Resources Recovery Facility treats an average daily of flow of 3.5 mgd, using a trickling filter followed by conventional activated sludge, filtration, and disinfection. A portion of the effluent is reused for irrigation. The plant accepts wastewater from the city, the Cal Poly campus, and the county airport.

Primary effluent from the plant is pumped back to an onsite university research station where it is treated in a series of shallow algae ponds. The small campus plant serves as a research and testing facility for faculty and students and may have provided a warning to Lehman’s operation.

“They saw the impact before we did,” he says. “They lost nitrification, too. It was a heads up for us.”

That theory aligns with the experience of Carollo Engineers, the firm providing oversight of the current MBR-UV upgrade at SLO. Carollo’s Bryan Coday, who has published extensively on toxic interferences relatead to past SARS and COVID 2 events, says his firm has seen similar events at plants in Colorado, especially those in the 2-20 mgd range. Not all occurrences are in college towns, says Coday, but the incidents correlate directly with stay-at-home orders and widening use of disinfectants in homes, businesses, and restaurants. “It’s almost a direct overlay.”

Adds Carollo’s Steve Waller, “This is happening, and a lot of people don’t know what it is.”

Unanswered Questions

To get a better handle on the issue, SLO ordered a Hach colorimeter test kit, calibrated specifically for QACs.

“We conducted testing weekly of the influent both at our plant and the effluent of the university,” says SLO laboratory manager Matt Anderson. While the testing confirmed the presence of QACs in the influent as well as plant processes, samples sent to Hydro Solutions, a commercial laboratory in Louisville, KT, reflected numbers traditionally viewed as being not high enough to explain the nitrification loss, according to Lehman.

“We haven’t been able to definitely prove QACs are the culprit.” he says.

Nonetheless, Hydro Solutions lab coordinator Julianna Isaac says the numbers from SLO influent were some of the highest her lab has ever seen. She explains that QACs accumulate in the solids at a treatment plant, attaching to and killing micro-organisms. There are also “free” or soluble QACs which can disrupt nitrification at levels as low as 2 ppm.

Isaac says her company is seeing QACs “across the board,” adding that their levels correlate directly with cleaning and disinfection associated with COVID-19.

Lehman says the inconsistencies may be due to the batch nature of the Quat flow, and that grab samples may not provide a complete picture.

“The numbers we saw shouldn’t have been causing the problem. There have to be other factors,” he suggests.

More Questions

There are also questions associated with combatting the QACs once they’ve entered the treatment process.

Working with the treatment plant at Goleta down the coast, SLO experimented with anti-QAC agents such as CounterQuat and NeutraQuat(tm). Results were positive.

Lehman says nitrification “recovered completely in a couple of days” after dosing with chemicals. But the products are expensive, he notes.

At Goleta, plant operations manager John Crisman says episodes of toxic interference go back a few years, due to cleaning chemicals coming from the county airport and a nearby bus depot. More recently, the sources include industrial surfactants and consumer products.

“We had a pattern where our floc would fly apart and turbidity would increase every Sunday,” Crisman says. “We tried CounterQuat and saw results almost instantly.”

Crisman says they sent some CounterQuat to SLO to try, and since then Goleta has been exploring other chemical additives.

Carrying high level of solids in the plant may be another way to reduce the impact of QACs.

“Maintaining a higher MLSS and keeping enough solids in the system may be a successful strategy,” Lehman suggests.

He cites similar experiences at the Paso Robles plant nearby, where consistent maintenance of a higher MLSS, above 3,000 mg/L, has resulted in no significant loss of nitrification with similar QAC concentrations.

Time to Talk

Questions like these call for a wider discussion of QACs and their impact on wastewater treatment plants, says Lehman.

“Other places are experiencing the loss of nitrification, especially small plants. We have a couple of solutions in our back pocket now if we need them, and we’d like to share our lessons learned with other facilities. We’ve learned a lot, but we don’t have a good baseline, so research in ongoing.”

“We need to keep gathering data, developing best practices, and sharing ideas with each other,” he says, much like the cooperation between SLO and Goleta.

“The story’s not over.”