The activated sludge method is most frequently used at the stage of biological waste water treatment. The main reason for improper sludge sedimentation is the excessive growth of filamentous bacteria leading to disturbances in technological processes of sewage treatment plants, such as formation of swollen, flowing out sludge, foam transforming in to a thick surface scum, which consequently contributes to a decrease in the effectiveness of removing pollutions from sewage. There is a growing tendency to make attempts aiming at optimizing the chemical methods used to eliminate filamentous bacteria which often constitute a factor limiting proliferation of those bacteria. Accordingly, research was undertaken to estimate the effect of some iron and aluminum coagulants and oxidizing compounds (chlorine dioxide and sodium hypochlorite) on the elimination of filamentous bacteria in the activated sludge in the aspect of sewage treatment effectiveness. Activated sludge was collected from the mechanical and biological sewage treatment plant situated in the Kuyavian-Pomeranian voivodeship. The study was carried out using a laboratory biological reactor graduated to reflect the technological parameters of the sewage treatment plant. Analyses involved determination of physicochemical parameters (total suspensions, sludge volumetric index, sludge density index) and the microscopic evaluation of activated sludge (floc parameters, the composition of microfauna, the Sludge Biotic Index, the quantitative and species composition of filamentous bacteria). At the next stage of the experiment, water solution of iron chloride, water solution of polyaluminum chloride, chlorine dioxide and sodium hypochlorite were proportioned to the activated sludge. At the same time, samples of activated sludge were analyzed without chemical reagents. After a week, the effect of applied reagents on the quantitative and species composition of filamentous bacteria and the other components of activated sludge biocenosis was evaluated. It was proved that activated sludge subjected to analyses was characterized by poor sedimentation properties, resulted from the excessive proliferation of filamentous bacteria of which the most numerous, dominant group was the species Microthrix parvicella. Best effects of filamentous bacteria elimination were recorded after the use of aluminum coagulant, which selectively affected filamentous microorganisms, effectively limiting their excessive proliferation and at the same time not exerting the negative impact on the other organisms of activated sludge. The preparation affected favorable values of the Sludge Biotic Index, which did not substantially differ from that parameter in the control sample. Polyaluminum chlorite caused elimination of the bacteria Sphaerotilus natans and a considerable decrease in the numbers of type 0041 micro-organisms. At the same time that preparation contributed to improving sedimentation properties of activated sludge flocs, as a result of which the volumetric index values were decreased in successive research samples, whereas the density index increased. The applied sodium hypochlorite contributed to the elimination of filamentous bacteria, and at the same time it slightly decreased the other parameters of activated sludge. It was indicated that chlorine dioxide contributed to a considerable decrease in diversity of activated sludge microfauna, causing a decrease in the number of crawling ciliates on the flocs and sedentary forms of those macroorganisms. Moreover an increase in the number of bacteriocidal free-floating protozoa and small flagellates resulted in a substantial decrease in the IBO value. The highest doses of reagent caused impoverishment of the biocenosis, which resulted in a decrease in the biotic index values by 7 units in relation to the control sample, activated sludge in those samples was qualified as IV class of sludge quality. The best effects in filamentous bacteria elimination were recorded after the use of aluminum preparation and chlorine compounds (chlorine dioxide, sodium hypochlorite).
