Analysis of the Application of Radar Charts in the Evaluation of Sewage Sludge Dewatering Process

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.