[Comparative studies of the kinetics of hygienically relevant microorganisms in activated sludge]

Several samples were taken regularly from 14 different activated sludge ponds in 11 municipal wastewater treatment plants (Table 1). During 7 days, the samples were supplied with oxygen continuously by stirring and the following parameters were determined daily in the filtrate and also in the insoluble sludge: total bacterial plate count grown at 36 degrees C; additionally the oxidase-positive bacteria; the gram-negative bacteria grown on MacConkey agar; also the lactose-positive bacteria of these; C. coli; aerobic endospore-forming bacteria; and protozoa. Furthermore, salmonellae were determined in some aerated and non-aerated samples. The sludge index (SI) as the quotient of volume and dry weight of sludge, in addition the optical density of the sludge-free filtrate was measured. The total bacterial plate count, the count of gram-negative rods, and the lactose-positive enterobacteria were found to be reduced in 12 of the 14 investigated series. The amount of E. coli diminished in as many as 13 of the 14 series and the oxidase-positive bacteria dropped in 11 of 13 cases. The concentration of aerobic spore-forming bacteria remained constant or increased. A propagation of salmonellae was observed only under anaerobic conditions in sludge (Fig. 4). An analysis of the bacterial dynamics shows that the propagation of all bacteria occurs mainly in the free water. Also the reduction of bacteria is more in the free water than in the insoluble flocs of sludge (Table 2). Hence it follows that the flocs are not the centres of microbial activity although most bacteria (medium 76%) are attached to them. Moreover, we found characteristic relations between SI and optical density of the filtrate for the different wastewater treatment plants. There is also a positive correlation between the frequency of protozoa in sludge and the SI (Fig. 2). The SI is correlated positively also with the reduction rate of the most groups of bacteria, excluding the aerobic spore-forming bacteria (Fig. 3). This fact shows a conflict of two possibilities. On the one hand, the reduction of bacteria in wastewater is being improved by increasing SI but the deposition of such sludge is poor. On the other hand, sludge with a lower SI is settling better but the removal of bacteria by protozoa is diminished. Altogether, the sedimentation of flocs is the most important step of elimination of fecal bacteria from wastewater. The process reduces the concentration by the factor 10. Therefore, the successive sedimentation of raw sewage and activated sludge is about 10 time more effective than the simultaneous sedimentation.