Biological stability and phytotoxicity of sludge generated from extended aeration activated sludge systems

Activated sludge technology is widely used for treating municipal wastewater. In particular, the extended aeration activated sludge (EAAS) method shows longer sludge retention time (SRT), which may enhance the biological stability of the resulting sludge. In this study, the biological stability of secondary sludge without dewatering (SS) and dewatered secondary sludge (DSS), dried in a conventional drying bed for 25 days, were examined using two full-scale EAAS systems operating under varying environmental and operational conditions (EAAS 1 : 24 °C; 926 masl; SRT 16.8 days and EAAS 2 : 16 °C; 2513 masl; SRT 23.6 days). The volatile solids to total solids ratio (VS/TS), volatile solids reduction (VSR), and specific oxygen uptake rate (SOUR-20) were assessed as indicators of biological stability. Germination tests (phytotoxicity) using radish seeds were conducted with soil-to-sludge mixtures (0:100, 25:75, 50:50, 75:25) and controls of soil only and distilled water. SRT showed a strong correlation with the biological stability of sludge. The EAAS systems did not ensure biological stability for SS, DSS did achieve biological stability, indicating that the dewatering stage, in addition to reducing moisture, contributes to the biological stabilization of the sludge. Heavy metal concentrations in SS and DSS were within regulatory limits, but pathogen levels exceeded them. Phytotoxicity in the SS and DSS was observed at higher sludge concentrations (0:100, 25:75), but the 50:50 mixture showed no toxicity, and the 75:25 mixture even enhanced germination. The results showed that DSS from EAAS systems have agricultural potential; however, their use is contingent upon additional treatments to eliminate pathogens. • Secondary sludge without dewatering did not achieve biological stability. • Dewatered secondary sludge achieved biological stability after 25 days of drying. • Sludge retention time showed a strong correlation with biological stability of sludge. • Dewatered secondary sludge has potential for agronomic use.