SURVIVAL OF THE MODEL HELMINTH ASCARIS-SUUM EGGS IN THE SLUDGE DRYING BEDS OF SEWAGE-TREATMENT PLANTS

In the period between June 1992 and May 1993 the survival of nonembryonated eggs of A. suum was studied in two sludge drying beds of sewage treatment plants (STP) under different climatic-geografical conditions: STP Michalovce in the East Slovak Lowland (elevation 111 m above sea-level) and STP Poprad in the submontane area of the Poprad valley (elevation 695 m). Sludge drying beds of both sewage treatment plants (STP) showed different survival of eggs (Fig. 1). In STP Michalovce we detected a rapid reduction in viable eggs from October through December (from 80.4% at the initiation of the experiment to 19.8% in December 1992). Later this decrease became less rapid and at the end of the experiment, after 240 days only 5% of eggs were viable. In STP Poprad the viability of eggs was reduced rather gradually, and after 320 days of exposure 36% of viable A. suum eggs were still recorded. Sludge dry matter in STP Poprad increased from 2.2% to 14.2% and in STP Michalovce from 4.1% to 19.2% at the termination of the experiment. Sludge pH showed no marked variation in both STPs, ranging between 7.1 and 7.8. The percentage of sludge organic matter was higher in STP Poprad, ranging from 55 to 75%, than in STP Michalovce, with 34-31%. We studied the correlation coeficients (Tab. I) of exposure time, air temperature, sludge drying bed temperature at 10 cm depth, pH, dry matter (Figs. 2 and 3) to the viability of model A. suum eggs. The most important factors reducing viability of eggs in STP Poprad were exposure time and dry matter (P < 0.01). Other factors showed no statistically significant influence. In STP Michalovce, in addition to exposure time and dry matter (P < 0.01) also sludge pH, drying bed temperature and air temperature statistically significantly (P < 0.05) affected the viability of eggs. Percentage of organic substance in the sludge of both sewage treatment plants had no effect on the devitalization of the model pathogens.