This paper presents the study concerning long-term effects of a full scale hydrocyclone unit implemented in a continuous flow long sludge age system, on sedimentation, treatment efficiency and sludge morphology. The research concentrates on identifying the mechanisms of sludge behaviour within the system. The gravimetric selection of activated sludge via a hydrocyclone is a recent development for enhancing sludge separation, where heavier flocs are retained in the system, and lighter ones are discarded as waste sludge. The effects of implementing hydroclyclones were analysed with the use of SEM imagining and fractal dimensioning through the frequent assessment of sludge settling capabilities, effluent quality, and floc properties. Over the course of 60 weeks of hydrocyclone operation, sedimentation efficiency varied significantly. Sludge volume index values of 40 mL/g, achieved during the warm season, were not sustained when the temperature decreased and an overgrowth of filamentous bacteria occurred. Good settling efficiency was also observed in batch tests, where settling velocity of experimental sludge was app. 1 m/h higher than for the reference train at the same concentrations. This was confirmed during wet weather, as the experimental train sustained safe sludge blanket height in secondary clarifiers. SEM imaging and fractal dimension analysis revealed that the underflow that returned to the system had a more compact and spherical shape, which led to an increased content of granule-like particles in the reactor. The presence of flocs with a diameter exceeding 900 μm in the underflow, which is not observed in the feed, indicated agglomeration within the hydrocyclone. This is contradictory to most of the literature data coming from laboratory experiments. This phenomenon was attributed to differences in the size and geometry of the used hydrocyclones, and the potential process mechanism was presented.

With an increasing frequency of extreme weather events wastewater treatment plant operators face a need to increase plant hydraulic capacity while continuing to comply with effluent quality requirements. An economically attractive way to meet this challenge is to significantly improve secondary settling tank performance and therefore assure safe wet weather plant operation. The objective of this study was to evaluate gravimetric selection technology conducted in hydrocyclones as a way for improving activated sludge settling characteristics in continuous flow BNR with a long sludge retention time and assess its ability to form granular sludge. Long term operational data and sludge morphology monitoring data were analysed. A significant drop in sludge volume index (SVI) (to values under 50 ml/g) and increase in return activated sludge solids concentration (to values above 20 g/L) was observed. Hydrocyclone installation consistently, selectively retained denser, larger flocs in the system and after a start-up phase large granules begin to appear. Gravimetric selection did not prevent the seasonal filamentous biomass outgrowth and temperature drop at the beginning of winter resulted in rapid rise of SVI and SST sludge blanket height especially after heavy rain. Technology under study proved to be effective under certain process conditions but it needs further research to consistently maintain low SVI values throughout the whole year.

This paper analyses the applicability of radar charts for evaluating sewage sludge dewatering efficiency, based on data for anaerobically digested sludge from a large municipal wastewater treatment plant. Four cationic polyelectrolytes, used alone and with the inorganic coagulant PIX C28 at 0–16 kg/t DS (32 variants), were compared in technological and economic terms. Eight normalization methods were assessed on the basis of the radar chart area variability across 12 geometrically distinct axis arrangements, complemented by the TOPSIS method. Min-Max and percentile normalizations proved most suitable (median coefficient of variation of the area: 0.0042 and 0.0118), while maximum-based normalization showed the distance from the best variant. PIX-assisted systems achieved the highest technological effect (mean TOPSIS score 0.938 vs. 0.660 for polyelectrolytes alone; maximum 0.995), allowing a fourfold reduction in polyelectrolyte dose. However, PIX C28 (€363.37/t DS) accounted for 78–95% of the conditioning cost, lowering techno-economic efficiency by about an order of magnitude. The most favourable effect-to-cost ratio was obtained for Zetag 8190 and Super Floc C446 used alone at 8 kg/t DS, whereas at 4 kg/t DS conditioning without the coagulant worsened the multi-criteria assessment despite reduced capillary suction time. The TOPSIS and radar chart assessments were strongly correlated (r = 0.967). These results confirm the need for multi-criteria evaluation, for which radar charts with an appropriately selected normalization method are a useful tool.