Imagine flies so unbearable that sitting outside of your house on a warm summer evening would be impossible. This was the case several years ago for residents near the Nelson Complex, a wastewater treatment facility in Johnson County, Kansas.
Two species of filter flies, the Psychoda alternata and Telmatoscopus albipunctatus, were emerging in high numbers from trickling filters at both Nelson Complex plants, Mission Main and Turkey Creek. Residential areas, a city park, and an industrial park surround the complex so the flies had become an issue with area residents throughout the seven-month filter fly season that normally runs from April through October. The fly season is weather-dependent and can vary dramatically.
Even though filter flies do not cause a public health concern or affect wastewater treatment to any extent, the flies are a nuisance to residents and plant operators. Residents took their fly concerns to the Mission City Council. In turn, the city requested that the countys Unified Wastewater Districts and Environmental Department address the problem.
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Most standard wastewater treatment manuals suggest controlling filter flies by flooding and chlorinating filter beds, but this technique can cause problems.
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"A chlorine overdose can kill the biological activity in a trickling filter, which will hurt the treatment process," said Phil Wittek, director of the Johnson County Environmental Department. "Trickling filters cant be chlorinated and flooded on a daily basis."
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In the past, the Wastewater Districts fogged the plant grounds and neighborhoods with a commercial insecticide to help control the flies. Fogging insecticides control adult flies only upon contact, so this method failed to prevent flies from reproducing in the filters. Some neighbors disliked the pesticide treatment so fogging was discontinued. The department tried alternative pesticides, which also failed to control the filter flies.
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Wittek had experience with the active ingredient in Strike? Professional Midge Control and thought that it would work in this situation. Strike contains an insect growth regulator (IGR) from Zoecon, a division of Wellmark International, that prevents the emergence of adult pests. The IGR imitates natural biochemicals in an insects body, disrupting the insects normal development and preventing the emergence of adult flies.
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The active ingredient in Strike, a water-based liquid concentrate, is approved by the Environmental Protection Agency and has no effect on the treatment plant biomass or the quality of wastewater effluent.
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Prior to using the control system, the staff at the Johnson County Environmental Department needed to prove to the state that its use would not be toxic to the receiving stream. In 1996, the department conducted an extensive literature search as well as testing and biomonitoring. The pilot program began in October 1996 and was a success.
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IGR is applied with a metering system in which a diluted mixture is combined with the filter influent. The suggested application rate is five ounces per one million gallons of flow. The amount applied is adjusted according to the normal daily flow. For example, the Mission Main and Turkey Creek plants have an effluent that combines at the dechlorination basin. The combination flow averages 15 million gallons per day, so approximately 75 ounces would be applied per day. The addition rates may depend upon individual situations. The metering system assures that the precise amount of product is continually delivered. This rate is used for two weeks to provide an initial knockdown of the flies, then is lowered to a maintenance rate to prevent re-infestation.
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"Persistence is the key," says Dan Shelby, operations supervisor for the Johnson County Unified Wastewater District. "Its important to maintain the recommended rate."
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"Last year we chlorinated and flooded one of the trickling filters to prepare for our Strike regime," Wittek said. To flood a filter, the effluent side of the filter is plugged to allow chlorinated wastewater to fill the basin for 24 hours. The filter fly adults drown or leave the beds, while millions of the pupa and larva float to the top of the filter to be skimmed. Flooding the filter depopulates the filter bed and helps control the rate of adult production.
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"Its important to check the system twice a day and to keep good records of your dosing, maintenance schedules and anything out of the ordinary," Wittek said. The mixing tank, pump and lines should be checked for proper operation. The filter beds also should be inspected on a regular basis for growth that could inhibit the effectiveness of the IGR.
Added Benefits
"Due to a state requirement, we dechlorinate the treated and disinfected wastewater before it is released into the stream," said Mary Coakley-Pegg, plant superintendent for the Johnson County Unified Wastewater District. The dechlorination allowed midge flies to develop in the Turkey Creek receiving stream, where the Nelson Complex discharges its treated water. The chlorine had controlled midge flies in the past, but the dechlorination requirement allowed them to thrive.
Added Benefits
"After using Strike to control filter flies, we discovered that the midge flies in the receiving stream also were being controlled," Coakley-Pegg said. Even though Nelson Complex does not actively treat for these midge flies, some residual carries through the treatment process into the receiving stream and knocks out the midges.
About the Author:
Ann Wells is a public relationsw account executive for Nelson & Schmidt Marketing Communications Inc. in Milwaukee, Wis.
