EFFECT OF FLOODING ON MOTH FLY LARVAE AND ZOOGLEA IN A SEWAGE TRICKLING FILTER SYSTEM

Populations of larval flies, particularly Psychoda alternata, are considered to be important to efficient BOD removal in sewage trickling filter systems. This importance arises from their grazing upon the zooglea which coats the filter substrate with the result that the zooglea is kept in a growth phase. Filter flooding is carried out to control the resident population of adult flies. It was hypothesized that the flooding routine also removes significant proportions of the larval fly population and the zooglea - effects which could be detrimental to the efficiency of a filter. In order to assess the effects of filter flooding on the fly larvae and the zooglea, preflood and postflood samples were taken in such a filter in Athens, Georgia. On each of three dates before flooding and on the 3 days immediately after flooding, the filter was sampled for larvae and zooglea at 30. 48 cm intervals to a total depth of 121. 92 cm. Comparison of the preflood and postflood samples showed that both the larval fly population and the zooglea were significantly reduced by the flooding. The data from the samples were extrapolated to the total volume of the filter bed. Losses from the filter attributable to flooding were estimated to be 1. 6 multiplied by 10**8 individual larvae (more than 59% of the preflood total) and 660 kg of zooglea (approximately 47% of the preflood biomass. 7 refs.