Birds, bugs and bacteria control flies around biosolids drying beds at the Dos Rios Water Recycling Center in San Antonio.


The air around the biosolids drying beds at the Dos Rios Water Recycling Center used to be thick with flies.

They were a nuisance to neighbors and staff, and the pesticides used to kill them contaminated the biosolids and interrupted work for several hours. Today, flies aren’t a problem. Tad Eaton, the manager in charge of the biosolids program at Dos Rios, found a better way.

“Natural is always better,” he says. He’s been using some strategies for years, but has learned more about sustainable ways to control what can be a problem at plants using outdoor drying beds.

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The 125 mgd Dos Rios facility in San Antonio, Texas, produces 160,000 wet tons of biosolids per year. Some is dewatered on belt filter presses, but much of it goes to 132 sand drying beds that, at 84 by 100 feet, occupy 25 acres. “I’m trying to manage the solids and the flies. It’s quite a feat,” says Eaton. “It makes it a whole lot easier to work out here, but we do it mainly for the neighbors.”

He started with birds for fly control in 1995 at Leon Creek, another San Antonio Water System (SAWS) facility, where he was in charge of composting biosolids. “The very first thing I did was introduce purple martins, and that worked really well,” he says. The birds are voracious, eating thousands of flying insects every day.

Knowing predators

Over the years, Eaton has developed a well-rounded, three-pronged attack on the flies at Dos Rios, where he has worked since 1992. When he got there, the drying beds were used very little because of the flies. The city fogged the beds with pesticides.

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“That’s bad for the biosolids and the water,” Eaton says. “We’re here to protect the environment, not make it worse. It also hampered the guys who were trying to work out there because when we fogged, we had to be out of the area for about four hours.”

Today, the natural approach still starts with the purple martins. Dos Rios has 50 purple martin houses, each with 12 rooms, for a population of about 600 breeding pairs. But they only live in Texas from late February to July, leaving Eaton without a solution for much of the fly season. So, he found other natural tactics.

“I did some research and found out about parasitic wasps that directly attack the flies,” he says. “I introduced those and we put them out during the hot season, from about March to October.”

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Every week, the Dos Rios plant puts out 60 units of the wasps. With 80,000 to 100,000 in each unit, that’s 4.8 to 6 million planted throughout the drying beds. Eaton gets the pupae from a company that breeds them commercially: “I receive the parasitic wasps unhatched. They’re ready to hatch a day or two after I get them.”

There are three species of the wasps. Once hatched, the tiny wasps, about the size of a pinhead, seek out fly pupae and lay their eggs; the wasp larvae consume the fly pupae.

This year, to protect the wasps from ants and birds before they hatch, Eaton built 600 protective houses. That solved a problem he had been struggling with for a long time.

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“It’s a four-inch PVC pipe about 20 inches long with about 8 inches of sand and a cap that lets the wasps in and out and keeps out the ants and birds,” he says.

The drying beds are also a breeding ground for fungus gnats. Eaton hasn’t found a cost-effective predator for them other than the martins, but he uses Gnatrol, a natural soil bacterium. He spreads it throughout the biosolids, and the gnats consume it while feeding. It forms a crystal in their stomach and kills them.

He also uses Strike Ultra (Central Life Sciences), a product certified for use around wastewater treatment facilities and ponds whose active ingredient keeps insects from developing into adults.

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Not much labor

Eaton says his pest-control methods don’t take much time. Gnatrol powder and Strike Ultra liquid are injected into the biosolids from a chemical batch tank; preparation takes about five minutes. Planting the parasitic wasps takes about an hour a week and has become part of the normal work schedule. “We usually get them on Wednesday and plant them first thing in the morning on Thursday or Friday,” Eaton says. The martin houses are cleaned and disinfected every year after the birds move on.

There are still some flies, of course, but Eaton considers the problem well under control. That sometimes forces the martins to look for food elsewhere. “At one point this year for about a month, we just didn’t see them,” Eaton says. “They were taking off early in the morning and would come back once or twice during the day, but they weren’t flying in swarms around here like they normally do.”

Eaton uses fly traps to check on his success. He used to buy them but now builds his own. “We put them around to monitor where the fly activity is,” he says. “We have them out there right now and there’s nothing in them. That’s not to say you can walk around and not see flies or gnats, but I remember years ago when you could put out a fly trap and you’d have thousands of them in there by the end of the day.”

More evidence of the methods’ effectiveness is the lack of complaints from neighbors. “We meet with them now and then,” says Eaton. “We have good relations with the neighbors. We work hard and we care about what we do out here.”


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