Determination of Toxicity Using the Activated Sludge Respiration Inhibition Test and Identification of Sensitive Boundary Conditions

Toxic pollutants in wastewater can negatively affect biological processes in WWTPs, leading to operational failures and environmental harm if undetected. This study refines the ISO 8192 method for enhanced toxicity detection in WWTPs, meeting the critical need for rapid and reliable monitoring. The novelty of this study lies in the optimization and identification of sensitive boundary conditions (e.g., temperature stability, consistent mixing speed, and immediate aeration), which are crucial for ensuring the accuracy and reproducibility of the method. The optimization results indicate that sludge prepared via settling yields EC50 values comparable with those achieved with sludge prepared via centrifugation. For total oxygen consumption inhibition, the EC50 values were 9.22 ± 0.21 mg/L (centrifuged) and 9.42 ± 0.16 mg/L (settled), with nitrification inhibition values of 1.92 ± 1.24 mg/L and 2.17 ± 1.5 mg/L, respectively. The verification results obtained using samples from the Graz and Trofaiach WWTPs demonstrate EC50 values of 12.38 ± 2.05 mg/L and 12.25 ± 0.62 mg/L (total oxygen consumption inhibition), 23.31 ± 4.62 mg/L and 19.32 ± 0.29 mg/L (heterotrophic inhibition), and 1.54 ± 0.94 mg/L and 3.48 ± 2.65 mg/L (nitrification inhibition), which are all within the accepted ranges of validity of the method. The optimized ISO 8192 method provides a robust and efficient tool for WWTPs, enabling rapid responses to toxicity events and supporting regulatory compliance. These refinements enhance the practicality and reliability of the method.