Trickling Filter Flies Get the Bum's Rush at an Alabama Facility

Operators use mechanical ingenuity and science to overcome the annual infestation of trickling filter flies at an Alabama plant.

Trickling Filter Flies Get the Bum's Rush at an Alabama Facility

Jeff Adams, superintendent of the Cullman (Alabama) Wastewater Treatment Plant, with the modified primary trickling filter in the background.

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From May through October, trickling filter flies (Psychoda alternata) and odors plagued the Cullman (Alabama) Wastewater Treatment Plant and residents of a 50-home subdivision across the fence line.

At a January 2019 City Council meeting, homeowners said mild and intermittent odors had always wafted over their neighborhoods. Since 2017, however, the odors had become more frequent, the stench was putrid, and swarms of flies drove them inside on warm evenings.

Jeff Adams, superintendent of the multistage 4.75 mgd (design) trickling filter plant, was desperate for help. He called Tony Glover, the county extension coordinator. Glover called Ron Trygar, a senior training specialist in water and wastewater at the University of Florida TREEO Center.

“Ron ran a few calculations based on our plant data and suggested slowing down the trickling filter’s dosing rate,” Adams says. “The dosing rate is given as inches per pass of a distribution arm and quantifies the depth of water it applies over the surface of the filter before the next arm passes. That’s what we implemented by modifying the plumbing, and it’s been fantastic.”

Initial efforts

Adult female flies laid their eggs in the organics in the primary trickling filter and ignored the secondary trickling filter. Operators used a synergized permethrin formula to control the insects until they seemed to develop an immunity to it. The seasonal increase in BOD loading also contributed to several other products, including a chemical fogging machine, not working as effectively as they had hoped.

In 2019, Adams found Essentria (Zoëcon Professional Products), a natural product containing oils of rosemary, wintergreen and peppermint. “In tests, it killed the flies in 20 seconds,” Adams says. “I prefer it over releasing chemical pesticides into the environment.”

He also consulted Fudd Graham, Ph.D., an entomologist at the Auburn University. Graham collected some flies to identify the species. “He was a big help,” Adams says. “If we knew the exact species, we’d know which chemicals were the best match.”

Making changes

Gaining some control over the infestations was a step forward, but Adams wanted to end them. Trygar’s suggestion to reduce the dosing rate from 28 seconds per revolution to four minutes per revolution had potential to drown the air-breathing midges, or fly larvae, maturing in the effluent.

The operators first considered putting a wheel with a brake on the end of the distribution arms, but the basin with standing columns wasn’t perfectly round. Adams envisioned mounting spray nozzles on the back of the arms to create drag. His operators and Whit McCurley from maintenance went to work.

Beginning the week of July 22, 2019, John Gaines, assistant superintendent, and McCurley removed the first five of 33 nozzles at the end of each arm and inserted 2-inch stubs of 1.25-inch PVC pipe with a stop nut into the orifices. Then they chemically welded sections of PVC pipe into J-shapes, with the short leg fitting over the stub. The long leg, passing beneath the distribution arm, received the spray nozzle and rode 12 inches above the surface grating.

Happy neighbors

“The trickling filter’s normal flow was 1 inch of water column, but 4 inches flooded it,” Adams says. “The unknown was how many minutes it took to apply that much effluent. We tried five reverse nozzles on each arm, but they didn’t slow it down enough.

“A month of adjustments and 60 reverse nozzles later, we achieved 12 minutes per revolution. That’s still not long enough to hit our target of 4 inches water column, but we’re almost there. What makes it tricky is that the two primary arms are 2 inches lower than the secondary arms.”

By early September, the vicinity was 98% free of flies. Operators continued to fog to play it safe, but one morning Adams didn’t see a single fly. “The mayor, council and neighbors are smiling from ear to ear,” he says. “One man told me he can drink two cups of coffee on his front porch now instead of one, and there are no flies floating in it.”

An unexpected advantage of the higher water column has been increased BOD removal. “It’s been fantastic, dropping from 95% three months ago to 97% in September,” Adams says. “Our effluent BOD went from 6.4 mg/L to 4.3 mg/L, despite influent BOD increasing over summer from 135 mg/L to 158 mg/L.”


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