For weeks the staff at the Clarkston Wastewater Treatment Plant battled an aggravating overpopulation of Daphnia in the secondary clarifiers.

The tiny crustaceans, often called water fleas, were reproducing in such numbers that they were clogging the screens. A team member suggested putting fish into the clarifiers to eat the Daphnia — and it worked. The Daphnia disappeared. Then, surprisingly, so did the fish.

Daphnia are an indicator of good water quality, so their presence in the clarifiers wasn’t a sign of a treatment issue. But in large numbers, they can cause problems, and that’s what was happening in 2018 in Clarkston, in the southeast corner of Washington, when the water in the clarifiers took on a reddish color.

What’s that sheen?

“One operator came in all panicked,” recalls Kevin Poole, Clarkston’s public works director. “He thought somebody dumped some kind of fluid in there. Another operator told him, ‘Fluid doesn’t move out of the way when you put your hand next to the water. You’ve got Daphnia.’”

The Daphnia were so thick in the water it looked as if automatic transmission fluid had been dumped into the clarifiers. “When the sunlight shines through the Daphnia, you kind of get a red sheen,” Poole says.

“We probably would have tried to put up with it and see if the bloom would eventually take care of itself, but we were eating up so much manpower cleaning the screens to our utility water that we decided we needed to take some form of action.”

Clarkston (populations 8,000) is on the border of Washington and Idaho at the confluence of the Snake and Clearwater rivers. The treatment plant has a design capacity of 1.2 mgd and an average flow of about 1 mgd. The two clarifiers affected by the Daphnia were each 60 feet in diameter and 16 feet deep.

Enter the bluegills

Adding fish to the clarifiers turned out to be relatively simple from a regulatory standpoint, compared to chemically treating the clarifier with chlorine or some other chemical.

“When we talked to Department of Ecology, they said we’d have to note any chemical we added to the clarifier,” says Poole. “We’d have to note an exception in our operating procedure. When we asked them about fish, they said we just need to note it in the operating log. We were already testing for suspended solids, and that was the only thing they would be curious about, if the TSS went up or down.” The fish had no impact on TSS.

The state Department of Natural Resources required any fish added to the clarifiers to be native to the Snake River, the plant’s receiving stream. The fish also had to come from a supplier certified to stock fish in the state’s waters. The DNR provided a list of suppliers.

The Clarkston staff chose bluegills, which are native to the Snake River. They purchased about 1,400, each about the size of a quarter.

The fish had an immediate impact on the Daphnia population. “Within a couple days the problem went away,” Poole says. The staff expected the fish continue thriving in the clarifiers after they took care of the Daphnia, but that’s not what happened.

“There are probably still Daphnia in there and probably residual things along the edges of the clarifiers,” Poole says. “We thought there would be plenty of food for the fish.” But when the clarifiers were drained for routine maintenance several months later, staff members were surprised to find no fish. They knew some had escaped over the weir, but expected to have to dip net bluegills out of the tank.

Mass escape

“We drained those clarifiers down to where we could get a person in there, and we looked around just because we wanted to see if we could find any bluegills,” Poole says. “We didn’t see any. We did see a couple of them in the trench after the UV light, and that was a couple months after we put them in.”

Apparently, all the fish had gone over the weir and washed out to the river. Poole thinks it’s unlikely any of them survived going through the UV channel. “The people we picked the bluegill up from told us that they naturally swim toward the sound of flowing water,” Poole says. “They said they were pretty good at finding escape routes.”

Clarkston hasn’t had a problem with Daphnia since the bluegills cleared them out: “We still have a few, but not as aggressive a bloom. I think at the time we just had the right conditions for an over-aggressive bloom. And it persisted until we put in the bluegills.”

If the plant were to have another Daphnia bloom, Poole wouldn’t hesitate to use the same technique again. And why not? The fish worked quickly, they didn’t cause any regulatory issues, and they managed to clean up after themselves. From a sustainability point of view, it was practically perfect.