Control of nuisance chironomid midge swarms from a slow sand filter

The pyrethroid insecticide permethrin was evaluated for controlling the emergence of chironomid midges from slow sand filter beds. The hydrodynamics of the slow sand filter were studied using a chemical tracer, and mesocosm experiments were undertaken to examine the effects of permethrin on the filter bed micro-fauna community. A single treatment dose of 96 µg/l permethrin was applied to a slow sand filter. Permethrin rapidly dispersed in the water and accumulated in the surface layer (the ‘schmutzdecke’) of the filter, attaining mean maximum average concentrations of 8.3 µg/l in water and 2.3 µg/g in the schmutzdecke after 1 and 6 h, respectively. Concentrations then rapidly decreased to below detection limits after 7 days in water and 48 h in the schmutzdecke. After 28 days the filter bed was drained and core samples were retrieved for analysis of permethrin. Permethrin was not detected in the out-flowing water at any time or in any of the filter bed core samples. These data suggest that all the permethrin was adsorbed and/or degraded in the water column and schmutzdecke. The single treatment was effective in eliminating chironomid midge emergences from the filter bed for a period of 1 month. Furthermore, there were no apparent adverse effects on other major components of the filter bed micro-fauna community.