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.

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